Fundamental Handbook of Nuclear Physics and Reactor Theory

This is a useful handbook for understanding the basic of Nuclear Physics and also about reactor theory. The book is devided into two major modules. The first module is explaining the basic characterisics of atomic and nuclear physics which introduces concepts of atomic physics including the atomic nature of matter, the chart of the nuclides, radioactivity and radioactive decay, neutron interactions and fission, and the interaction of radiation with matter.
Next on the second module mainly as an advance theory than first. The second module provides information on reactor theory and neutron characteristics. Includes topics such as neutron sources, neutron flux, neutron cross sections, reaction rates, neutron moderation, and prompt and delayed neutrons

Overview

The Department of Energy Fundamentals Handbook entitled Nuclear Physics and Reactor Theory was prepared as an information resource for personnel who are responsible for the operation of the Department's nuclear facilities. Almost all processes that take place in a nuclear facility involves the transfer of some type of energy. A basic understanding of nuclear physics and reactor theory is necessary for DOE nuclear facility operators, maintenance personnel, and the technical staff to safely operate and maintain the facility and facility support systems. The information in this handbook is presented to provide a foundation for applying engineering concepts to the job. This knowledge will help personnel understand the impact that their actions may have on the safe and reliable operation of facility components and systems.

The Nuclear Physics and Reactor Theory handbook consists of four modules that are contained in two volumes. The following is a brief description of the information presented in each module of the handbook.

Source :
D O E


DOWNLOAD :

DOWNLOAD MODULE 1
DOWNLOAD MODULE 2




Read More ..

Robotic vocabulary building using extension inference and implicit contrast

TWIG (“Transportable Word Intension Generator”) is a system that allows a robot to learn compositional meanings for new words that are grounded in its sensory capabilities. The system is novel in its use of logical semantics to infer which entities in the environment are the referents (extensions) of unfamiliar words; its ability to learn the meanings of deictic (“I,” “this”) pronouns in a real sensory environment; its use of decision trees to implicitly contrast new word definitions with existing ones, thereby creating more complex definitions than if each word were treated as a separate learning problem; and its ability to use words learned in an unsupervised manner in complete grammatical sentences for production, comprehension, or referent inference. In an experiment with a physically embodied robot, TWIG learns grounded meanings for the words “I” and “you,” learns that “this” and “that” refer to objects of varying proximity, that “he” is someone talked about in the third person, and that “above” and “below” refer to height differences between objects.

Follow-up experiments demonstrate the system’s ability to learn different conjugations of “to be”; show that removing either the extension inference or implicit contrast components of the system results in worse definitions; and demonstrate how decision trees can be used to model shifts in meaning based on context in the case of color words.

Introduction
Robots that could understand arbitrary sentences in natural language would be incredibly useful—but real vocabularies are huge. American high school graduates have learned, on average, about 60,000 words [29]. Programming a robot to be able to understand each of these words in terms of its own sensors and experience would be a monumental undertaking. To recognize the words in the audio stream is one thing; to recognize their referents in the real world, quite another. The messy, noisy nature of sensory input can render this difficult problem intractable for the programmer. It would be far more useful to have a robot that is able to learn the meanings of new words, grounded in its own perceptual and conceptual capabilities, and ideally with a minimum of user intervention for training and feedback. The present paper describes a system that is built to do exactly that.

According to the developmental psychology literature, children can learn new words rapidly [7] and in a largely unsupervised manner [9]. From the time that word learning begins around the age of one, children’s word learning accelerates from a rate of about word every 3 days for the first 4 months [14], to 3.6 words a day between the ages of 2 12 and 6 [1,4,14], to 12 words a day between the ages of 8 and 10 [1]. This learning would impose quite a demand on the time of parents and educators if children had to be taught all these words explicitly, but instruction does not appear to be necessary for learning language [4,27], and in fact there are communities in which children learn to speak with hardly any adult instruction at all [21]. Thus, in designing a robotic system built to learn the meanings of words quickly and without supervision, it makes sense to study the heuristics that children appear to use in learning the meanings of words.

One strategy that children use is to infer what object or person an new word is referring to from its grammatical context—realizing that “sibbing” must refer to a verb, but “a sib” must refer to a noun [6]. Another strategy children use is to contrast the new word with other words they know in order to narrow down its meaning without explicit feedback [7,9,26]. For instance, a child may believe at first that “dog” refers to all four-legged animals, but on learning the word “cat,” the child realizes that implicitly, the meaning of “dog” does not include cats, and so the definition of “dog” is narrowed.


This paper describes a word learning system called TWIG that employs both of these strategies, using sentence context and implicit contrast to learn the meanings of words from sensory information in the absence of feedback. Until now, research into learning the semantics of words in an unsupervised fashion has fallen primarily into two camps. On the one hand, there have been attempts to learn word semantics in simulation, with the world represented as collections of atomic symbols [12] or statements in predicate logic [41]. These simulations generally assume that the robot already possesses concepts for each of the words to be learned, and thus word learning becomes a simple problem of matching words to concepts. Such systems can deal with arbitrarily abstract concepts, because they never have to identify them “in
the wild.”

On the other hand, systems that have dealt with real sensor data have generally assumed that concepts and even word boundaries in speech must be learned at the same time as words. Since learning abstract concepts directly from sensor
data is difficult, these systems have focused on learning words for physical objects [37] and sometimes physical actions [48].

The TWIG word-learning system is an attempt to bridge this divide. “TWIG” stands for “Transportable Word Intension Generator.” It is “transportable” in the sense that it does not rely too heavily on the particular sensory setup of our robot (Fig. 1), but could be moved with ease to any robot that can express its sensory input in Prolog-like predicates that can include numeric values. (Note that it is the learning system, and not the definitions learned, that is transportable; each robot using TWIG will learn definitions specific to its own sensory capabilities.) A “word intension” is a function that, when applied to an object or relation, returns true if the word applies. In other words, an intension is a word meaning [8,13]. Though TWIG was originally designed to solve the conundrums posed by pronouns, which themselves straddle the boundary between the abstract and the concrete, its techniques are more generally applicable to other word categories, including verbs, prepositions, and nouns.

The first technique that TWIG introduces is extension inference, in which TWIG infers from sentence context what actual object the new word refers to. For example, if the system hears “Foo got the ball,” and it sees Bob holding the ball, it will assume that “Foo” refers to Bob; the entity in the world that is Bob is the extension of “Foo” in this example. Doing this allows the system to be picky about what facts it associates with a word, greatly reducing the impact of irrelevant information on learning. Grammatical context also informs the system as to whether it should be looking for facts about a single object or a relation between objects, as with prepositions and transitive verbs.

Second, to build word definitions, the system creates definition trees (see Section 4.3). Definition trees can be thought of as reconstructing the speaker’s decision process in choosing a word. The interior nodes are simple binary predicates, and the words are stored at the leaves. The meaning of a word can be reconstructed by following a path from the word back to the root; its definition is the conjunction of the true or negated predicates on this path. Alternately, the system can choose a word for a particular object or relation by starting at the root and following the branches that the object satisfies until arriving at a word.

Definition trees are a type of decision tree, and are created in a manner closely following the ID3 algorithm [32]—but treating word learning as a problem of word decision, with words defined in terms of the choices that lead to them, is an approach novel to TWIG. Previous robotic systems have typically assumed that word concepts can overlap with each other arbitrarily [37,48]. In assuming that words do not overlap, TWIG actually allows greater generalization from few examples, because the boundaries of each concept can extend all the way up to the “borders” of the other words. Though we described an earlier version of the TWIG system in a previous conference paper [17], that version of TWIG did not make use of the definition tree method, which we developed later [15]. (Definition trees were originally simply called “word trees,” but this led to some confusion with parse trees.)

The latest version of TWIG includes several enhancements that were not present in any previously reported results, including part-of-speech inference, online vocabulary updates that allow new words to be used immediately, and better handling of negated predicates. This paper also presents a new quantitative evaluation of the two halves of the system, and some new experiments that demonstrate the system’s flexibility in building on its previous vocabulary and learning multiple definitions for the same word.

By : Kevin Gold, Marek Doniec, Christopher Crick, Brian Scassellati

Source : DOWNLOAD Read More ..

A 2D numerical simulation of sub-cooled flow boiling at low-pressure and low-flow rates

The main purpose of this study is to apply a two-fluid mathematical model to numerical simulation of two phase flow at low-pressure condition. Although models of sub-cooled boiling flow at one-dimension and high-pressure have been studied extensively, there are few equivalent studies for numerical simulation at two-dimension and low-pressure (1–2 bar) conditions. Recent literature studies on sub-cooled boiling flow at low-pressure have shown that empirical models developed for high-pressure situations are not valid at low-pressures. Since the mathematical model used in this study is accomplished at low-pressure, the transport equations for the variables of each phase are substituted in low-pressure. The governing equations of two-phase flow with an allowance to inter-phase transfer of mass, momentum and heat, are solved using a two-fluid; non-equilibrium model. The finite volume discretization scheme is used to create a linearized system of equations that are solved by SIMPLE staggered grid solution technique for a rectangular channel.

Improvement of the void fraction prediction of our model for the case of low-pressure sub-cooled flow boiling conditions was achieved. It is found that the heat transfer due to evaporation and surface quenching is higher than that by convection. Good agreement is achieved with the predicted results against the experimental data’s available in the literatures for a number of test cases.

Introduction
Boiling occurs when the temperature of the heater surface exceeds the saturation temperature, thus causing bubble formation. If the bulk temperature of the liquid is below saturation, the process is known as sub-cooled boiling. Sub-cooled flow boiling
is an important heat transfer regime in nuclear reactors. The proper prediction of the void fraction profile and other important parameters under sub-cooled flow boiling conditions is of primary importance in thermal-hydraulic safety analysis of nuclear reactors. In the nuclear area, ensuring the safe operation of a power or research reactor is of paramount importance especially where nuclear safety analyses concern the ability to predict the void fraction distributions and other two-phase flowparameters in sub-cooled boiling conditions. In the present state-of-the-art, the
two-fluid model can be considered as the most detailed and accurate macroscopic formulation of the thermal-hydraulic dynamics of two-phase flowsystems(Konˇcar andMavko, 2003). Within the flow field equations, expressed by conservation of mass,momentumand energy for each phase, interfacial transfer terms appear in each of
the equations to couple the different phasic effects. These terms determine the rate of phase changes and the degree of mechanical and thermal non-equilibrium between phases. Much success of the two-fluid model depends on these essential closure relations, which should be modelled accurately.



Most available sub-cooled boiling flow models are developed and tested at high-pressure (above 10 bar), and one-dimension conditions typical of power reactors, but there are few equivalent studies for numerical simulation at two-dimension and
low-pressure conditions. For a number of years, an internationally coordinated effort has been carried out, focusing on continuous development and validation of best-estimate thermalhydraulic computer codes, e.g. RELAP5, TRAC, CATHARE and ATHLET. These codes are used to perform predictive safety analyses of system transients, e.g. loss-of-coolant accidents (Petelin et al., 1994; Parzer et al., 1995; Wang and Mayinger, 1995; Tuunanen et al., 2000). According to the analyses of Woodruff et al. (1996), Hari et al. (1998), Hari and Hassan (2002), Tu and Yeoh
(2002,2006), Ustinenko et al. (2007), the codes, which have been developed and validated for high-pressure conditions, cannot be applied to low-pressure conditions without adequate modifications.

Extensive studies have addressed stability at low-pressure startup of natural circulation BWRs. Aritomi et al. (1992) and Chang et al. (1992,1993) conducted basic studies on instabilities under lowpressure condition in a parallel channel natural circulation loop using a two-fluid model. Tu and Yeoh (2002) improved the CFX1
code for the sub-cooled boiling flow at low-pressure condition and applied it to numerical simulation of vertical boiling channel. They found a reasonable agreement with the experimental results of Zeitoun and Shoukri (1997). Anglart (1993) studied the sub-cooled boiling at low-pressure condition in a vertical channel. He found
that the rate of vapor generated in low-pressure condition is higher than that in high-pressure condition. Recently, Mi et al. (1998) further developed the CFX sub-cooled boiling model and validated the model against Bartolomei et al.’s (1982) high-pressure boiling experiments.

The main objective of this study is to apply a two-fluid mathematical model to numerical simulation of two-phase flow in a vertical boiling channel at low-pressure condition, which is appropriate for research reactors. Our simulation is accomplished at low-pressure, therefore the interfacial area phenomena are substituted in low-pressure. Since the sensitivity of boiling to various parameters is much higher at low-pressure, in order to show all of the effects are properly captured, the predicted results are compared with the experimental data’s of Zeitoun and Shoukri (1997) for several test cases.

Conclusion
A two-dimensional; non-equilibrium two-fluid mathematical model was applied to predictions of low-pressure; low-flow subcooled boiling, using the phasic correlations being suitable for low-pressure condition. Improvement of the void fraction prediction of our model for the case of low-pressure sub-cooled flow
boiling conditionswas achieved. Applied numerical method in this work is in the good agreement with existing experimental data’s in low-pressure condition. It is found that the buoyancy force plays an important role on the void fraction evolvement. Density of bubbles is maximum near the walls and due to diffusion phenomena its effectiveness is developed to center of channel. With increasing in length of channel void fraction is increased, due to convection of bubble. Also because the effects of buoyancy and drag forces, the vapor velocity is increasing rapidly along the channel. It is found that the heat transfer due to evaporation and surface quenching is higher than that by convection.

Finally, it should be noted that, in high flow rates, some kind of additional phenomena such as bubbles coalescence and break-up appears, therefore in order to consider these effects, it is necessary to equip the formulation with a turbulence model.

By : Said Talebi, Farshad Abbasib, Hadi Davilu

Source : DOWNLOAD Read More ..

Static analytical and experimental research of shock absorber to safeguard the nuclear fuel assemblies

The Ignalina Nuclear Power Plant (NPP) has two RBMK-1500 graphite-moderated boiling water multichannel reactors. The Ignalina NPP Unit 1 was shutdown at the end of 2004, while Unit 2 is foreseen to be shutdownat the end of 2009.At the Ignalina NPPUnit 1 remains approximately 1000 spent fuel assemblies with lowburn-up depth. A special set of equipmentwas developed to reuse these assemblies in the reactor of Unit 2. One of most important items of this set is a container, which is used for the transportation of spent fuel assemblies between the reactors of Unit 1 and Unit 2. A special shock absorber was designed to avoid failure of fuel assemblies in case of hypothetical spent fuel assemblies drop accident during uploading/unloading of spent fuel assemblies to/from container. This shock absorber was examined by using scaled experiments.

The objective of this article is the estimation whether the proposed design of shock absorber fulfils the function of the absorber and the optimization of its geometrical parameters using the results of the performed investigations.Static analytical and experimental investigations are presented in the article. The finite element code BRIGADE/Plus was used for the analytical analysis. The calculation model was verified by comparing the experimental investigation and simulation results for further employment of this finite element model in the development of an optimum design of shock absorber. Static simulation was used to perform primary optimization of design and dimension of the shock absorber.

Introduction
Ignalina Nuclear Power Plant (NPP) comprises two units with RBMK-1500 water-cooled graphite-moderated channel-type power reactors (Almenas et al., 1998). One of the specific features of RBMK type reactor is that refuelling is performed during power operation, the average burn-up depth of fuel is kept constant during all operational life of the reactor. After the reactor shutdown for decommissioning there is a substantial amount of fuel assemblies in the reactor with low burn-up depth considerably lower than design burn-up depth). Such fuel assemblies have high energetic potential and can be reused. After the shutdown of Ignalina NPP Unit 1 in 2004, approximately 1000 fuel assemblies from Unit 1 are applicable for further reuse in the reactor of Unit 2. The projectwas initiated to use fuel of Unit 1 in Unit 2, and special equipment was designed for this purpose. The schematic sketch of the equipment for loading/unloading of the fuel assemblies is presented in Fig. 1.

Transportation of spent fuel assemblies (SFA) fromUnit 1 to Unit 2 is carried out in the container designed for the maintenance of radiation and nuclear safety during transportation of SFA between power plant units. The container (see Fig. 1) is placed on the transport vehicle and represents the steel thick-walled cylindrical vessel, providing the arrangement of basket with six SFA, protection from ionizing radiation, retention of radionuclides within the container and SFA integrity during transportation.

The transport container is one of the most important components of the spent fuel transportation equipment set. Spent nuclear fuel multi-canister overpack (MCO) drop analysis (Rains, 1999) has revealed that overpack drop into the cask can produce very large impact reactions on the MCO and internals. Therefore spent fuel storage and transport cask must withstand various accident conditions such as free drop and puncture (Lee et al., 2004) in accordance with the requirement of the IAEA and national regulations. To assess potential damage to the various components that comprise the cask deformation tests of the structural material were performed along with the theoretical simulations for the shock absorber design (Sappok et al., 1994 and Pfeiffer and Kennedy, 1989). Despite the fact that container is used for on-site transportation of RBMK fuel,the requirements for its design in some aspects are even stronger than in the case of off-site transportation, because the fuel after the transportation shall be applicable for the reuse in the reactor of Unit 2. The equipment shall assure the required safety level even in case of most dangerous hypothetical accident.During loading/unloading of the basket with six SFA into a container, theoretically, the accident with drop of a basket from height of 17.5mto a container chamber is possible. The basket is supplied with brake arrangement, which in case of accidental drop ensures slowdown of a basket due to frictional force at the shaft walls. The full stop of the basket inside the container is ensured by the shock absorber (see Fig. 1).
Main requirements to the construction of the shock absorber:
- Keepweight of the loaded basket inside the container at a vertical position (normal operating conditions) without plastic deformation of the shock absorber.
- During the loading of fuel assemblies absorber must ensure damping due to elastic deformation.
- Absorb all kinetic energy of the dropping basket in case of the accident.
- Provide smooth slowdownof the loaded basket up to the full stopping on a condition that acceleration force (load) affecting the fuel loaded basket will not cause the breakdown of a SFA.
- Design of the shock absorber must prevent buckling of shock absorber and reduction of the inner diameter.
- Easy decontamination of radionuclides from the surface of shock absorber during the service.

In thiswork the static characteristics of the shock absorber models under the axial compression were determined. The comparison of the experimental and simulated results was carried out and the optimization of the construction of the shock absorber was performed.

Conclusion
Shock absorber design for use in transport container for spent nuclear fuel assemblies was proposed which represents a steel hollow cylinder with the cuts. Static analytical and experimental investigations of the shock absorber models were carried out in order to optimize geometrical parameters and damping characteristics
of the full-scale absorber. The finite element method was used for the analytical investigation of the shock absorber using the state-of-the-art BRIGADE/Plus code. Tests revealed that axial compression of the models of the given construction occurs with the retention of a uniform and smooth increase in the magnitude of force and displacement. Obtained, almost horizontal characteristic of breaking force shows that constant braking force will be maintained.

It was found that the decrease of diameter of the models during compression is relatively small. Testing of the model and simulation results shows that its symmetrical construction retains stability during compression. During the loading of fuel assemblies damping due to elastic deformation also will be ensured. The deformation curves of the shock absorber were defined by both investigations. Good agreement of simulation and experimental datawas received. With respect to the separately tested samples, the deviation of calculation data fromthose established experimentally does not exceed 5% for both model I and II. The obtained results enable to make a statement that the static FE method can be used for optimization of construction of the given design of shock absorber.

References
Abaqus/Standard, 2003. User’s Manual, volume IV, version 6.4.
Alghamdi, A.A.A., 2001. Collapsible impact energy absorbers: an overview. Thin-Walled Struct 39, 189–213.
Aljawi, A.A.N., 2002. Numerical simulation of axial crushing of circular tubes. JKAU:
Eng. Sci. 14 (2), 3–17.
Almenas, K., Kaliatka, A., Uspuras, E., 1998. Ignalina RBMK-1500. A Source Book, extended and updated version, Ignalina Safety Analysis Group, Lithuanian Energy Institute.
BRIGADE/Plus, 2003. User’s Manual, version 1.2.
Dundulis, G., Karalevicius, R., Rimkevicius, S., Kulak, F.R., Marchertas, H.A., 2006.
Strength evaluation of a steam distribution device in the Ignalina NPP accident localisation system. Nucl. Eng. Des. 236, 201–210.
Kim, S.-K., Im, K.-H., Hwang, C.-S., Yang, I.-Y., 2002. A Study on experimental characteristics of energy absorption control in thin-walled tubes for the use of vehicular-structure members. Int. J. Automot. Technol. 3 (4), 137–145.
Lee, Y.S., Kima, H.S., Kang, Y.H., Chung, S.H., Choi, Y.J., 2004. Effect of irradiation on the impact and seismic response of a spent fuel storage and transport cask. Nucl. Eng. Des. 232, 123–129.
Messmer, G., Goller, B., Hoffmann, G., Stratmanns E., Muller St., 1995. Cylindrical
tubes under dynamic axial loading causing concertina type of deformations. Transactions of the 13th International Conference on Structural Mechanics in Reactor Technology (SMIRT 13). 407–418.
Nagel, G.M., Thambiratnam, D.P., 2005. Computer simulation and energy absorption of tapered thin-walled rectangular tubes. Thin-Walled Struct 43, 1225–1242.
Olabi, A.G., Morris E., Hashmi, M.S.J., 2007. Metallic tube type energy absorbers: a
synopsis. Thin-Walled Struct. 45, 706–726.
Olabi, A.G., Morris, E., Hashmi, M.S.J., Gilchrist, M.D., 2008. Optimised design of
nested oblong tube energy absorbers under lateral impact loading. Int. J. Automot Technol 35, 10–26.
Pfeiffer, P.A., Kennedy, J.M., 1989. Free drop impact analysis of shipping cask. Nucl.Eng. Des. 114, 33–52.
Rains, D.J., 1999. Analysis for spent nuclear fuel multi-canister overpack drop into the cask from the multi-canister overpack-handling machine with air cushion.
SNF-5276 Rev 0. Site-Wide Nuclear Safety Project. Engineering Report. Sappok, M., Beine, B., Rittscher, D., Jais, M., 1994. Design and testing of a shock absorber for a type I container. Nucl. Eng. Des. 150, 459–463.
Wei, Z.G., Yua, J.L., Batrab, R.C., 2005. Dynamic buckling of thin cylindrical shells under axial impact. Int. J. Impact Eng. 32, 575–592.
Yamashita, M., Gotoh, M., 2005. Impact behavior of honeycomb structures with various cell specifications – numerical simulation and experiment. Int. J. Impact Eng. 32, 618–630.
Zhang, X., Cheng, G., Zhang, H., 2006. Theoretical prediction and numerical simulation of multi-cell square thin-walled structures. Thin-Walled Struct. 44,
1185–1191.


By :Gintautas Dundulisa, Albertas Grybenasb, Renatas Karalevicius, Vidas Makarevicius, Sigitas Rimkevicius, Eugenijus Uspurasa

Source : DOWNLOAD


Read More ..

A study on the evolution of crack networks under thermal fatigue loading

The crack network is a typical cracking morphology caused by thermal fatigue loading. It was pointed out that the crack network appeared under relatively small temperature fluctuations and did not grow deeply. In this study, the mechanism of evolution of crack network and its influence on crack growth was examined by numerical calculation. First, the stress field near two interacting cracks was investigated. It was shown that there are stress-concentration and stress-shielding zones around interacting cracks, and that cracks can form a network under the bi-axial stress condition. Secondly, a Monte Carlo simulationwas developed in order to simulate the initiation and growth of cracks under thermal fatigue loading and the evolution of the crack network.

The local stress field formed by pre-existing cracks was evaluated by the body forcemethod and its role in the initiation and growth of crackswas considered. The simulation could simulate the evolution of the crack network and change in number of cracks observed in the experiments.
It was revealed that reduction in the stress intensity factor due to stress feature in the depth direction under high cycle thermal fatigue loading plays an important role in the evolution of the crack network and that mechanical interaction between cracks in the network affects initiation rather than growth of cracks. The crack network appears only when the crack growth in the depth direction is interrupted. It was concluded that the emergence of the crack network is preferable for the structural integrity of cracked components.

Introduction
Thermal fatigue is a critical issue in nuclear power plant components. The crack network is a typical cracking morphology caused by thermal fatigue loading and has been observed in plant components (Taheri, 2007) and experiments (Maillot et al., 2005). It was pointed out that the crack network appeared under relatively small temperature fluctuations and did not grow deeply inside components (Taheri, 2007), where it was shown through operational feed back that cracks were arrested at less than 2mm depth in components of pressurized water reactors (PWRs). Nearly the same resultswere obtained under experiments (Maillot et al., 2005). Not only experiments but also numerical studies were conducted in order to evaluate the crack growth in the network (Haddar and Fissolo, 2005; Haddar et al., 2005). The two main questions concerning the crack network are: why the network develops under thermal fatigue loading and how the crack network influences the growth of cracks. Neither of these two problems is fully understood yet. In particular, the latter problem is important for the structural integrity of components of nuclear power plants.

There are two characteristics thatwe should consider for evolution of the crack network under thermal fatigue loading. The first is peculiar distribution of stress in the depth direction. The magnitude of stress amplitude caused by temperature fluctuations is large near the surface and decreases with distance fromthe surface. This reduces the stress intensity factor (SIF) in the depth direction, and the degree of reduction is dependent on the magnitude and frequency of thermal fluctuations and thermal transfer coefficient (Kasahara et al., 2002; Hayashi and Hirano, 2003; Taheri, 2007). The other characteristic is the bi-axial stress field brought about by thermal loading. The bi-axial stress causes complex mechanical interaction between cracks in the network and produces a local stress field. The local stress affects the initiation of new cracks as well as growth of pre-existing cracks (Kamaya and Haruna,2007).

By taking into account these two characteristics, we conduct a numerical study of the evolution of the crack network. First, in order to evaluate the influence of interaction on the growth and initiation of cracks, the stress field near the interacting cracks is investigated under the bi-axial stress condition. It is discussed how the interaction and bi-axial stress contribute to the evolution of the crack network. Secondly, a Monte Carlo simulation is developed in order to simulate the initiation and growth of cracks under thermal fatigue loading and the evolution of the crack network. The target of the simulation is the experiment performed using the SPLASH experimental facility, in which thermal fatigue stress was caused by applying cool water periodically to the surface of the specimen (Maillot et al., 2005). Two-dimensional cracks are assumed to be initiated on the surface according to accumulated damage calculated from local stress, which is formed by pre-existing cracks and is calculated by using the body force method (BFM). The initiated cracks growfollowing the power lawof SIF. The crack growth in the depth direction is controlled by assuming a limitation of the crack length on the surface. Finally, based on the results of analyses, the influence of the crack network on the structural integrity of plant components is discussed.



Conclusion
The mechanism of evolution of the crack network and its influence on crack growth was examined based on numerical calculations. Firstly, the stress field near two interacting cracks was investigated under the bi-axial stress condition. Secondly, a Monte Carlo simulation was developed in order to simulate the initiation and growth of cracks under thermal fatigue loading and the evolution of the crack network. Then, the influence of the crack network on the structural integrity of plant components was discussed. The following conclusions were obtained:
(1) There are stress-concentration and stress-shielding zones aroundinteracting cracks, and cracks can cross eachother under the bi-axial stress condition.
(2) The Monte Carlo simulation developed can simulate the evolution of the crack network and change in number of cracks which were observed in the experiments.
(3) The reduction in crack growth rate due to small SIF in the depth direction plays an important role in the evolution of the crack network.
(4) Mechanical interaction between cracks in the network affects the initiation rather than growth of cracks.
(5) Cracks cannot penetrate thewall thickness when the crack network appears. Namely, the emergence of a crack network is preferable for the structural integrity of cracked components.

References
Erdogan, F., Sih, G.C., 1963. On the crack extension in plates under plane loading and transverse shear. J. Basic Eng., 519–527.
Haddar, N., Fissolo, A., 2005. 2D simulation of the initiation and propagation of crack array under thermal fatigue. Nucl. Eng. Des. 235, 945–964.
Haddar, N., Fissolo, A.,Maillot, V., 2005. Thermal fatigue crack networks: an computational study. Int. J. Solid Struct. 42, 771–788.
Hayashi, M., Hirano, A., 2003. High cycle thermal fatigue crack initiation and growth behavior in simulated BWR environment. Trans. Jpn. Soc. Mech. Eng. A 69, 1353–1359.
Jaske, C.E., O’Donnell, W.J., 1977. Fatigue design criteria for pressure vessel alloys. Trans. ASME, J. Press. Vess. Technol. 99, 584–592.
Kamaya, M., 2008. Growth evaluation of multiple interacting surface cracks. Part I. Experiments and simulation of coalesced crack. Eng. Fracture Mech. 75, 1336–1349.
Kamaya, M., 2005. Influence of the interaction on stress intensity factor of semielliptical surface cracks. In: Proceedings of the 2005 ASME Pressure Vessels and Piping Division Conference, Denver, PVP, p. 71352.
Kamaya, M., Haruna, T., 2006. Crack initiation model for sensitized 304 stainless steel in high temperature water. Corros. Sci. 48, 2442–
2456.
Kamaya, M., Haruna, T., 2007. Influence of local stress on initiation behavior of stress corrosion cracking for sensitized 304 stainless steel. Corros. Sci. 49, 3303–3324.
Kamaya, M., Totsuka, N., 2002. Influence of interaction between multiple cracks on stress corrosion crack propagation. Corros. Sci. 44, 2333–2352.
Kasahara, N., Nagata, T., Iwata, K., Negishi, H., 1995. Advanced creep-fatigue evaluation rule for fast breeder reactor components: generalization of elastic follow-up model. Nucl. Eng. Des. 155, 499–518.
Kasahara, N., Takasho, H., Yscumpai, A., 2002. Structural response function approach for evaluation of thermal striping phenomena. Nucl. Eng. Des. 212, 281–292.
Maillot, V., Fissolo, A., Degallaix, G., Degallaix, S., 2005. Thermal fatigue crack networks parameters and stability: an experimental study. Int. J. Solids Struct. 42, 759–769.
Molinie, E., Monteil, N., Delatouche, S., Roux, S., Robert, N., Pages, C., 2002. Caracterisation des troncons RRA 900–1300MWe deposes: synthese des enseignements acquis. Porc. Fontevroud 5, 883–895.
Murakami, Y., Nishitani, H., 1981. Trans. Jpn. Soc. Mech. Eng. A 47, 295–303.
Murakami, Y., Nemat-Nasser, S., 1982. Interacting dissimilar semi-elliptical surface flaws under tension and bending. Eng. Fracture Mech. 16, 373–386.
Nishitani, H., Murakami, Y., 1974. Stress intensity factors of an elliptical crack or a semi-elliptical crack subject to tension. Int. J. Fracture 10, 353–368.
Taheri, T., 2007. Some advances on understanding of high cycle thermal fatigue crazing. ASME J. Press. Technol. 129, 400–410.

By : Masayuki Kamaya, Said Taheri

Source : LINK

Read More ..

Nuclear powered heat pumps for near-term process heat applications

There is a substantial market for nuclear energy in non-electric applications such as hydrogen production or water desalination. Among the Generation IV reactor concepts, the very high temperature reactor (VHTR) with a reactor outlet temperature close to 1000 oC and a power conversion efficiency of approximately 50% is believed to be the most suitable concept for cogeneration of process heat. Its high coolant exergy would enable centralize d hydrogen production and other process heat applications. In this paper it is shown that a reactor with lower coolant outlet temperature or another nearterm heat source can also meet the VHTR objectives which are high power conversion efficiency and capability to deliver high temperature process heat in the narrow temperature window required by thermochemical hydrogen production cycles.

The approach was to separate the requirement for high temperature process heat production from the nuclear part of the plant, in other words the nuclear part of the power plant would run at acceptably low te mperature while the high te mperature heat production via a heat pump system would be limited to a conventional external circuit, thus avoiding nuclear constraints. The separation of these high temperature constraints from the reactor would avoid massive R&D requirements on materials, components and fuel with uncertain outcome thus unnecessarily delaying introduction of this other wise very attractive reactor concept. We then show that the proposed technology is equally suitable for the generation of cold (e.g. for air conditioning) and for desalination of seawater.

Introduction
The sustainability of nuclear power increases linearly with power conversion efficiency and can be further raised by cogeneration of electricity and heat for various process heat applications. As an example, the Generation IV very high temperature reactor (VHTR) concept aims at approximately 50% power conversion efficiency and, according to the prevailing opinion, a temperature close to 1000 ◦C would be required to enable centralized hydrogen production. These high temperatures put severe constraints on materials, components and fuel and implymassive R&D requirements with uncertain outcome thus unnecessarily delaying introduction of this otherwise very attractive reactor concept.

The first objective of this paper is to develop an alternative option to both reach the VHTR power conversion efficiency target and the capability to deliver high temperature process heat with a reactor or other heat source with lower temperature output that would be feasible in the near-term. The approach was to separate the requirementfor high temperatureprocess heat production from the nuclear part of the plant, in other words the nuclear part of the power plant would run at acceptably low temperature while the high temperature heat production would be limited to a conventional external circuit, thus avoiding nuclear constraints. Two such methods would be feasible: electric superheating to the desired temperature level or the use of heat pump (HP) technology via compression to the desired temperature level. While the heat thus generated is then delivered to a heat exchanger, some of the compression work can be recovered in a turbine, which makes this option energetically more efficient than electric heating.

The second objective of this paper is to show how the same heat pump technology can be used for combined desalination and district cooling which corresponds to market needs in arid regions. While classical gas-cooled reactors are proven technology, the GIF VHTR still requires a strong R&D effort. Based on already available technology, the power conversion options proposed here would introduce a near-term solution to provide nuclear produced high temperature process heat.

Reverse Brayton cycle
The principal objective of a heat pump (Angelino and Invernizzi, 1994) is to remove heat from a low temperature environment and to release it to a high temperature environment. This transformation requires mechanical work, which is exactly the opposite of the working process. To achieve that goal, a fluid is used as energy carrier: it undergoes transformations to be colder than the lowtemperature environment (to remove heat there) and then hotter than the high temperature environment (to release heat there).
If the energy gained fromthe cold source is considered as a loss that would otherwise be simply rejected, energy input into a heat pump is the compression work only. Both heat andcompression
work increase the energy in the fluid: The efficiency of this system is then higher than unity. In this study, the cold energy source cannot be considered as a loss such that efficiency is <100%,
This method can be applied in a wide temperature range and reduced losses compared to a reverse Rankine heat pump or even electric heating. Although the compression work is much higher than for a liquid, a large part of it (around 60%) is earned back in the turbine. This enhances efficiency. Shortcomings are:
• Poor heat transfer to and from the gaseous fluid implying high mass flows. As the sensible heats are smaller (per unit masson a unit mass), latent heats, mass and volume flow are larger.
• The gas state, which implies non-isothermal heat exchanges.

In a reverse Brayton cycle, both mechanical power and lowtemperature power are transformed into high temperature power. As the goal is to maximize the contribution of the low temperature power compared to the valuable mechanical work, a reverse Brayton cycle with single compression and no recuperation (RB1CNR) is needed for the heat pump system. This implies production of process heat over a large temperature span. A reverse Brayton cycle with multiple recompression more would lead to an efficiency decrease but would provide process heat in a narrow temperature window


Conclusions
This paper discussed the use of the reverse Brayton cycle as a heat pump to reach VHTR process heat and efficiency objectives. This opens opportunities to design innovative, flexible and efficient power conversion cycles that can be adapted to virtually all GIF reactor concepts as well as to existing or near-term reactor designs such as the advanced gas-cooled reactor (AGR), HTR or other heat sources. While so far only HTR technology was investigated as the primary heat source, it is planned to pursue efforts towards other concepts and to enable feasible configurations with significantly less stringent technology requirements, in particular for the VHTR.

Based on this enabling technology and the expectation that switching to a hydrogen ormethanol economywould requiremuch more high temperature process heat than produced by current VHTRdesigns, innovative layoutswere developed enabling delivery of high temperature process heat in the required narrow temperature window.

This study showed the feasibility of nearer term hydrogen production and other high temperature process heat applications while avoiding costly and time consuming R&D of VHTR. The induced penalty for replacing VHTR by AGR is approximately 17% points of overall efficiency with the resulting efficiency still exceeding 50%.

Finally, a short feasibility study for combined cooling and desalination based on the technology developed in the previous objectives was performed. For these combined applications, a coefficient of performance significantly larger than unity was calculated. This figure can be further raised but complicates the cycle layout. In all three objectives, an economical study is required to assess whether these technical achievements make economically sense

References
Angelino, G., Invernizzi, C., 1994. Supercritical heat pump cycles. Int. J. Refrigeration 17 (8), 543–554, 1994.
Dostal, V., Hejzlar, P., Driscoll, M.J., 2006. High performance supercritical carbondioxide cycle for next-generation nuclear reactors. Nucl. Technol. 154 (265), 2006.
GIF, 2002, A technology roadmap for the generation IV nuclear energy systems, GIF-002-00, December 2002.
IAEA-TECDOC-1444, 2005, Optimization of the coupling of nuclear reactors and desalination systems, Final report of a coordinated research project, 1999–2003, June 2005.
Jeong, Y.H., Kazimi, M.S., Hohnholt, K.J., Yildiz, B. 2005, Optimization of the hybrid sulfur cycle for hydrogen production, Nuclear Energy and Sustainability Program, Massachusetts Institute of Technology Technical Report MIT-NES-TR-004, May 2005.
Li, X., Le Pierres, R., Dewson, S.D., 2006. Heat exchangers for the next generation of nuclear reactors. In: Proc. ICAPP’06, Reno, NV, USA, June 4–8, 2006.
Matzner, D., Kriel, W., Correia, M., Greyvenstein, R., 2006. Cycle configuration for a PBMR steam and electricity production plant. In: Proc. ICAPP 2006, Reno, NV, USA, June 4–8, 2006.
Shropshire, D.E., 2004. Lessons learned from Gen I carbon-dioxide cooled reactors. In: Proc. ICONE-12, Arlington, VA, USA, April 25–29, 2004.
Vitart, X., Le Duigou, A., Carles, P., 2006. Hydrogen production using the sulfur-iodine cycle coupled to a VHTR: an overview. Energy Conversion Manage. 47 (October (17)), 2740–2747.
Woudstra, N, 2006, Cycle-Tempo Release 5.0, Delft University of Technology, http://www.cycle-tempo.nl/.
Yildiz, B., Kazimi, M.S., 2006. Efficiency of hydrogen production systems using alternative nuclear energy technologies. Int. J. Hydrogen Energy 31 (January (1)).
Yildiz, B., Hohnholt, K.J., Kazimi, M.S., July, 2005. Hydrogen production using high-temperature steam electrolysis supported by advanced gas reactor with supercritical CO2 cycles. Nucl. Technol. 155.

By : A. Marmiera, M.A. Futterer

Source : LINK

Read More ..

Large Area Silicon Carbide Vertical JFETs for 1200 V Cascode Switch Operation

SiC VJFETs are excellent candidates for reliable high-power/temperature switching as they only use pn junctions in the active device area where the high-electric fields occur. VJFETs do not suffer from forward voltage degradation, exhibit excellent short-circuit performance, and operate at 300◦C. 0.19 cm2 1200 V normally-on and 0.15 cm2 low-voltage normally-off VJFETs were fabricated.
The 1200-V VJFET outputs 53 A with a forward drain voltage drop of 2V and a specific onstate resistance of 5.4mΩcm2. The low-voltage VJFET outputs 28 A with a forward drain voltage drop of 3.3 V and a specific onstate resistance of 15mΩcm2.

The 1200-V SiC VJFET was connected in the cascode configuration with two Si MOSFETs and with a low-voltage SiC VJFET to form normally-off power switches. At a forward drain voltage drop of 2.2V, the SiC/MOSFETs cascode switch outputs 33 A. The all-SiC cascode switch outputs 24 A at a voltage drop of 4.7 V.

INTRODUCTION
Wideband gap semiconductors like silicon carbide (SiC) and the III-IV nitrides are currently being developed for high-power/temperature applications. Silicon carbide (SiC)is ideally suited for power-conditioning applications due to its high saturated drift velocity, its mechanical strength, its excellent thermal conductivity, and its high critical field strength. For power devices, the tenfold increase in critical field strength of SiC relative to Si allows high-voltage blocking layers to be fabricated significantly thinner than those of comparable Si devices. This reduces device onstate resistance, and the associated conduction and switching losses, while maintaining the same high-voltage blocking capability. Figure 1 shows the theoretical specific onstate resistance of blocking regions designed for certain breakdown voltages in Si and 4H-SiC, under optimum punchthrough conditions [1].

The specific onstate resistance of 4H-SiC is approximately 400 times lower than that of Si at a given breakdown voltage. This allows for high current operation at relatively low-forward voltage drop. In addition, the wide band gap of SiC allows operation at high temperatures where conventional Si devices fail. Forward voltage drop versus current density of Northrop Grumman’s all-SiC vertical junction field effect transistor- (VJFET-) based cascode switch, and those of commercial Si MOSFET, Si IGBT, and Si CoolMOS switches are shown in Figure 2. The SiC switch has a lower voltage drop at a given current density, even at the elevated temperature of 150◦C. The low-loss and the high-temperature operational capabilities of SiC devices can potentially eliminate the costly cooling systems present in today’s Si based power electronics. Presently, several SiC devices are being developed for 600 V (1200V rating) power switching applications.

SiC MOS-based devices show promise as normally-off power switches but suffer from low-MOS mobility and native oxide issues that limit reliable operation to below 175◦C [2]. Furthermore, several temperature-dependant factors result in a decrease of the SiC MOSFET threshold voltage with temperature. This may lead to unwanted MOSFET turnon at temperatures over 200◦C. The SiC bipolar junction transistor is another normallyoff power switching candidate. However, as with all SiC bipolar devices, its long term performance deteriorates due to forward bias voltage degradation [3]. Also, the BJT is a current controlled device that can require substantial base drive current [4].

The SiC VJFET is a very promising candidate for highpower/temperature switching as it only uses pn junctions in the active device area, where the high-electric fields
occur, and can therefore fully exploit the high-temperature properties of SiC in a gate voltage controlled switching device. VJFETs for high voltage applications are typically normally-on devices, and an all-SiC normally-off power switch can be implemented by combining a high-voltage normally-on VJFET with a low-voltage normally-off VJFET in the cascode configuration.

In this paper, we review the reliability and high temperature characteristics of 1.25 × 10−3 cm2 area unipolar ionimplanted SiC VJFETs. Subsequently, we present the forward current and locking voltage characteristics of 0.19 cm2 area 1200 V normally-on and 0.15 cm2 area low-voltage normally-off SiC VJFETs. The 0.19 cm2 1200-V VJFETs have been connected in the cascode configuration with SiMOSFETs and 0.15 cm2 low-voltage SiC VJFETs to form
normally-off power switches.

CONCLUSION
The SiC VJFET is a very promising candidate for reliable high-power/temperature switching as it only uses pn junctions in the active device area where the high-electric fields occur. VJFETs do not suffer from forward voltagedegradation, and exhibit holdoff times higher than those
of their Si counterparts in short circuit testing. The VJFET based all-SiC normally-off cascode switch’s internal diode has exhibited a very fast 100 nanoseconds reverse recovery time, eliminating the need for antiparallel diodes in power switching circuits. VJFETs were successfully operated at 300◦C junction temperature. The measured reduction in onstate current is in good agreement with the theoretical reduction in SiC electron mobility.

To meet the current handling requirements of modern power conditioning systems, 1200 V normally-on VJFETs of 0.19 cm2 and low-voltage normally-off VJFETs of 0.15 cm2 areas were fabricated. At a gate bias of 2.5V, the 1200-V VJFET outputs 53 A with a forward drain voltage drop of 2V and a specific onstate resistance of 5.4mΩcm2. The lowvoltage VJFET’s drain current is 28 A, at a gate bias of 2.5V, with a forward drain voltage drop of 3.3V and a specific onstate resistance of 15mΩcm2.

A 1200-V SiC VJFET was connected in the cascode configuration with two commercial Si MOSFETs to form a normally-off power switch. At aMOSFET gate-to-source bias of 15V, the cascode switch outputs 33 A at a forward drain voltage drop of 2.2V. To fully exploit the high-temperature capability of SiC in a normally-off power switch, a 0.15 cm2 low-voltage normally-off SiC VJFET was connected with a 0.19cm2 1200-V normally-on VJFET in the cascode configuration. At a forward drain voltage drop of 4.7V, the all-SiC cascode switch outputs 24 A at 2.5V cascode gate bias. Operating the 1200 V normally-on SiC VJFET as a switch in an inherently safe gate-drive circuit eliminates the need for a low-voltage normally-off SiC VJFET cascode component, and enables high-current/high-gain operation with low voltage drop and low onstate resistance.

REFERENCES
[1] J. A. Cooper Jr., M. R. Melloch, R. Singh, A. Agarwal, and J. W. Palmour, “Status and prospects for SiC power MOSFETs,” IEEE Transactions on Electron Devices, vol. 49, no. 4, pp. 658– 664, 2002.
[2] S. Krishnaswami, M. Das, B. Hull, et al., “Gate oxide reliability of 4H-SiC MOS devices,” in Proceedings of the 43rd Annual International Reliability Physics Symposium (IRPS ’05), pp.
592–593, San Jose, Calif, USA, April 2005.
[3] A. Agarwal, S. Krishnaswami, J. Richmond, et al., “Influence of basal plane dislocation induced stacking faults on the current gain in SiC BJTs,” Materials Science Forum, vol. 527–529, part 2, pp. 1409–1412, 2006.
[4] S. Krishnaswami, A. Agarwal, S.-H. Ryu, et al., “1000-V, 30- A 4H-SiC BJTs with high current gain,” IEEE Electron Device Letters, vol. 26, no. 3, pp. 175–177, 2005.
[5] V. Veliadis, M. McCoy, T. McNutt, et al., “Fabrication of a robust high-performance floating guard ring edge termination for power silicon carbide vertical junction field effecttransistors,” in Proceedings of the International Conference on Compound Semiconductor Manufacturing Technology (CS MANTECH ’07), pp. 217–220, Hilton Austin, Tex, USA, May 2007.
[6] V. Veliadis, L.-S. Chen, E. Stewart, et al., “2.1 mΩcm2, 1.6 kV 4H-silicon carbide VJFET for power applications,” in Proceedings of the International Semiconductor Device Research Symposium (ISDRS ’005), pp. 166–167, Bethesda, Md, USA, December 2005.
[7] V. Veliadis, L. S. Chen, M. McCoy, et al., “High-yield silicon carbide vertical junction field effect transistor manufacturing for RF and power applications,” in Proceedings of the International Conference on Compound Semiconductor Manufacturing Technology (CS MANTECH ’06), pp. 219–222, Vancouver, Canada, April 2006.
[8] V. Veliadis, M. McCoy, L. S. Chen, et al., “Silicon carbide vertical junction field effect transistors for RF applications: processing, DC testing, and yields,” in Proceedings of the IEEE
Lester Eastman Conference on High Performance Devices, p. 77, Ithaca, NY, USA, August 2006.
[9] V. Veliadis, T. McNutt, E. Stewart, M. McCoy, H. Hearne, and C. Clarke, “Recent progress in 300◦C silicon carbide JFET technology for power conditioning in electric vehicles,” in Proceedings of the 7th International All Electric Combat Vehicle Conference (AECV ’07), Stockholm, Sweden, June 2007, paper FCXST-07060711-545141-1.
[10] T. McNutt, V. Veliadis, E. Stewart, et al., “Silicon carbide JFET cascode switch for power conditioning applications,” in Proceedings of the IEEE Vehicle Power and Propulsion Conference (VPPC ’05), pp. 574–581, Chicago, Ill, USA, September 2005.
[11] T. McNutt, J. Reichl, H. Hearne, et al., “Demonstration of high-voltage SiC VJFET cascode in a half-bridge inverter,” Materials Science Forum, vol. 556-557, pp. 979–982, 2007.
[12] P. Friedrichs, “Charge controlled silicon carbide switching devices,” in Materials Research Society Symposium Proceedings (MRS ’04), vol. 815, pp. 255–266, San Francisco, Calif, USA,
May 2004, paper J3.1.
[13] H. F. Hamann, A. Weger, J. A. Lacey, et al., “Hotspot-limited microprocessors: direct temperature and power distribution measurements,” IEEE Journal of Solid-State Circuits, vol. 42, no. 1, pp. 56–64, 2007.
[14] R. Kelley and M. S. Mazzola, “SiC JFET gate driver design for use in DC/DC converters,” in Proceedings of the 21st Annual IEEE Applied Power Electronics Conference and Exposition
(APEC ’06), vol. 2006, pp. 179–182, Dallas, Tex, USA, March 2006.
[15] A. B. Lostetter, Arkansas Power Electronics International Inc., private communication, 2007.
[16] B. Weis, M. Braun, and P. Friedrichs, “Turn-off and short circuit behaviour of 4H SiC JFETs,” in Proceedings of the 36th IAS Annual Meeting on Industry Applications Conference, vol. 1, pp. 365–369, Chicago, Ill, USA, September-October 2001.

By : Victor Veliadis, Ty McNutt, Megan Snook, Harold Hearne, Paul Potyraj,
Jeremy Junghans, and Charles Scozzie2

Source : DOWNLOAD

Read More ..

Analysis of Cavitation Phenomena in Water and its Application to Prediction of Cavitation Erosion in Hydraulic Machinery

The cavitating flow behaviour is very sensitive to nuclei content, which is undoubtedly dependent on the physical properties of the liquid. For water, it is believed that heterogenous nucleation initiators prevail over homogeneous nucleation and this is understood to be the reason why the classical nucleation theories are regarded as unreliable for the treatment of cavitation.
As a result, the problem of nuclei content is typically treated either empirically or experimentally. In this paper we show how the empirical approach can be used to obtain a useful picture of the cavitation flow aggressiveness (erosion potential) using numerical modelling of the turbulent cavitating flow. In addition, we present the latest advances in the understanding of the bubble nucleation process under cavitating conditions based on the modified binary nucleation theory.

In this article we also shortly describe the experimental research of the cavitating flow aimed at the validation of the erosion potential model, development of the nuclei-content measurement and the validation of the bubble nucleation model. As far as the practical application of our work is concerned, the paper concentrates on an evaluation of the cavitation erosion potential in the hydraulic machinery, mainly water pumps and turbines.

Introduction
In general, there are three major reasons for numerical modelling of cavitation in industrial applications. The first reason is to predict changes in the flow field caused by the cavitation phenomena, which result mainly in a degradation of machine performance. The second reason is to determine cavitation instabilities, which can generate unwanted noise and vibrations. The third reason is to assess the potential of material erosion due to cavitation and to determine the areas on the blade surface (for example in pumps), which are most endangered with erosion. In all the above cases cavitation depends on many factors, which can be divided into two categories: hydrodynamic factors (such as flow parameters or turbulence level)and factors associated with the liquid properties (such as surface tension, bubble content and liquid composition).

Nevertheless cavitation is mainly treated as a solely hydrodynamic problem and the dependence on the physical properties of the liquid is typically neglected. Prediction of cavitation phenomena in hydraulic devices is usually based on the assumption that a given spectrum of cavitation
nuclei flow through the regions with rapidly changing static pressure, which results in a very complicated dynamic behaviour of the cavitation bubbles. From the point of view of an engineer one of the important challenges of the cavitation research is the determination of the number and size of the cavitation nuclei. These quantities naturally depend on the liquid properties; however, they are usually predicted empirically or by rather expensive and complicated measurements on the case-to-case basis. The consequences are most obvious in the case of water when the experimental measurements of the cavitation events under exactly the same hydrodynamic conditions can give very different results.


The main problem in the theoretical estimation of the cavitation nuclei spectrum in water is that the classical nucleation theory predicts only one (critical) bubble size and that the bubble nucleation rate is much higher than the experimentally observed rate. This is true mainly in the case of pure water. The situation is simplified when we consider that the water running trough the hydraulic machinery parts contains a known number of air-filled or vapour-filled microbubbles of known size distribution.

Summary and Conclusion
In this paper we have shortly described the numerical modelling as well as the experimental research of cavitation in hydraulic machines. The main interest was focused on the flow aggressiveness associated with cavitation. The presented numerical model predicts regions highly endangered by the bubble collapses as well as the energy of these collapses denoted as “erosion potential”. The first stage of experimental research in the cavitation tunnel was also presented.

This stage is aimed at validation of the erosion potential model, visualisation of the cavitation bubbles, development of the nuclei content measurement and the validation of the bubble nucleation model. Though the nuclei content in the numerical model was still determined empirically, the agreement of the theoretical results and the observations is encouraging. The measurements of the cavitation nuclei in water by light scattering and the acoustic spectrometry are being completed to provide accurate nuclei distribution. Finally, we have shown that the Classical Nucleation Theory can be improved to reliably predict the process of bubble nucleation under cavitation conditions. As far as the practical application of our work is concerned, the paper demonstrates the analysis of the cavitation erosion in the impeller of the mixedflow water pump.

by : Milan Sedlář, Patrik Zima, Tomáš Němec and František Maršík

Source : LINK

Read More ..

Improved Switching Characteristics of Fast Power MOSFETs Applying Solder Bump Technology

The impact of a reduced package stray inductance on the switching performance of fast power MOSFETs is discussed applying advanced 3D packaging technologies. Starting from an overview over new packaging approaches, a solder bump technology using a flexible PI substrate is exemplarily chosen for the evaluation. Measurement techniques to determine the stray inductance are discussed and compared with a numerical solution based on the PEEC method. Experimental results show the improvement of the voltage utilization while there is only a slight impact on total switching losses.

Introduction
Fast power MOSFETs in the voltage class up to 100V realize very high current and power densities while at the same time switching transients achieve times below 50 nanoseconds. Due to the fast switching, parasitic inductances in the circuit have significant impact on converter power since overvoltages and oscillations reduce the voltage margin and hence, the maximum power rating. Optimizing the packaging of the devices is an important step to improve the utilization of the semiconductors and make use of their full current and voltage ratings.Usually, state-of-the-art devices are assembled in discrete housing, or in power modules.

The semiconductor dice are soldered to a leadframe or DBC, and thick wire bond technology is applied to contact the top source and gate pads of the dice as well as the external terminals. The wire bonds are a bottleneck with regard to stray inductance and the ohmic losses, that is, the current utilization of the chips.Further drawbacks include reliability issues and limitations with regard to 3D integration.
Several new packaging technologies have been proposed to overcome the structural limits of the wire bond technology and the related reliability problems as well as the reliability and thermal constraints associated with the die solder contact. Only a few ideas have made the transition to market yet, for example, the discrete International Rectifier “DirectFET”. The (“low-temperature joining technique”) LTJT, which is a promising alternative for the die solder bond, is close to realization for automotive applications.

Apart from the LTJT, most of the proposed packaging solutions concentrate on a substitution of the wire bonds including alternatives like ribbon bonding or spring contacts as well as planar interconnect technologies in 3-dimensional stacked assemblies. The latter include solutions with a prestructured circuit board, for example, a copper frame, a flexible substrate, or a second DBC that is usually soldered to the top side of the dice by solder bumping.Examples are the “flip-chip-on-flex” technology and the “power ball grid array” technology applying two DBCs. In derivatives of these solutions, copper posts or cylinders are soldered between the two substrates, which might be necessary for high-voltage applications. Other feasible alternatives cover solutions where the electrical circuit layout is structured during a planar integration process with metallurgically-formed interconnects leading to a multilayer assembly. Typical examples are the planar integration with an “embedded power stage” or the “Planar Power Polymer Packaging Technology” . Table 1 summarizes the basic material and geometrical data of the described 3D approaches.

There are several advantages anticipated from 3D sandwich assemblies including higher power density due to a compact layout, reduced interconnect resistances, and higher current rating compared to wire bonds as well as less parasitic stray inductance giving improved switching behavior and reduced overvoltage. Furthermore, improved heat paths and the possibility to apply double-side cooling concepts due to a second planar surface are expected. The compact integration of control electronic components into the power assembly might be a further benefit.

The reliability of 3D assemblies, on the other hand, has to be investigated thoroughly. Difficulties are to be expected, since several materials with different CTEs (coefficient of thermal expansion) are joined with large contact areas. The focus of this paper is the evaluation of the switching behavior of fast MOSFETs applying sandwich-type packaging. Exemplarily, a solder bump technology is used with the top contacts of the dice soldered to a flexible PI substrate. The analysis is based on experimental results of a half-bridge topology in buck converter configuration. The prototypes are built with 100 V-MOSFETs (Infineon OptiMOS, chip area 26mm2) with high current rating and low on-state resistance in the range below 5mΩ, and Schottky diodes (IXYS DWS 36-80 A). The wire bonds are limiting the chip utilization for this kind of MOSFET due to their current rating given by the maximum wire temperature.

SUMMARY
In this paper, the impact of a reduced package stray inductance on the switching performance of fast power MOSFETs is discussed applying advanced 3D packaging technologies. Starting from an overview over new packaging technologies, a solder bump technology using a flexible PI substrate is exemplarily chosen for the evaluation. Measurement techniques to determine the stray inductance of the package are discussed, showing that accurate results may be achieved applying an externally switched current approach while double-pulse measurements and the use of an impedance analyzer are not feasible for the package characterization.

Good agreement with a numerical solution based on the PEEC method is found. The relevance of a low stray inductance for fast switching MOSFETs is obvious from the measurement of the voltage overshoot allowing operating the new device at significantly higher DC voltages—up to 25%—than standard packages. The impact on the switching losses, however, was found to be less pronounced due to the reduced losses at turnon.

As an additional benefit, also the package resistance is reduced compared to standard housing while at the same time larger copper areas substituting the wire bonds are able to carry higher amounts of current. This thermal aspect is discussed further in [13]S. Dieckerhoff, T. Wernicke, T. Kirfe, et al., “Electric characteristics of planar interconnect technologies for power MOSFETs,” in Proceedings of the IEEE Power Electronics Specialists Conference (PESC ’07), pp. 1036–1042, Orlando, Fla, USA, June 2007.

by : Sibylle Dieckerhoff, ThiesWernicke, Christine Kallmayer, Stephan Guttowski, and Herbert Reichl

Source : DOWNLOAD

Read More ..

Performance Analysis of Trench Power MOSFETs in High-Frequency Synchronous Buck Converter Applications

This paper investigates the performance perspectives and theoretical limitations of trench powerMOSFETs in synchronous rectifier buck converters operating in the MHz frequency range. Several trench MOSFET technologies are studied using a mixed-mode device/circuit modeling approach. Individual power loss contributions from the control and synchronous MOSFETs, and their dependence on switching frequency between 500 kHz and 5MHz are discussed in detail.

Introduction

It is observed that the conduction loss contribution decreases from 40% to 4% while the switching loss contribution increases from 60% to 96% as the switching frequency increases from 500 KHz to 5 MHz. Beyond 1MHz frequency there is no obvious benefit to increase the die size of either SyncFET or CtrlFET. The RDS(ON) × QG figure of merit (FOM) still correlates well to the overall converter efficiency in the MHz frequency range. The efficiency of the hard switching buck topology is limited to 80% at 2MHz and 65% at 5MHz even with the most advanced trench MOSFET technologies.Trench powerMOSFETsare widely used as both control and synchronous rectifier switches (CtrlFET and SyncFET) in buck converters for computer, telecommunication, and consumer applications. Power MOSFETs usually account for most of the power losses, and often determine the overall efficiency of today’s DC/DC converters. Over the past decade, the power semiconductor industry has significantly improved MOSFET performance, especially in terms of the figure of merit (FOM) of RDS(ON) × QG.

The analysis, modeling, and optimization of powerMOSFET performance in synchronous buck converters have also become the focus of a significant amount of research work in the past few years. The objective is to identify the optimum design of the CtrlFETs and SyncFETs that offer the highest converter efficiency. The previous work addressed this goal with varied levels of success, but several issues still remain open especially in light of ever-evolving DC/DC converter design requirements.


The RDS(ON) × QG FOM is generally considered as the single most important indicator of MOSFET performance in DC/DC converters in the medium switching frequency range of 100 kHz to 1 MHz. As the switching frequency of buck converters increases to the MHz range to facilitate better converter transient response and smaller passive components, it is, however, not clear how closely the RDS(ON) × QG FOM correlates to the overall converter efficiency, or whether or not a different FOM needs to be defined. Furthermore, the analysis on individual MOSFET power loss contributions, namely, conduction loss of the CtrlFET, conduction loss of the SyncFET, switching loss of the CtrlFET, diode loss of the SyncFET, and gate-drive losses of both the CtrlFET and SyncFET, was previously limited to the use of simple analytical equations based on approximations and assumptions. These simple device models have only very limited ccuracy. More importantly, they are not capable of revealing or predicting the influence of variations in device structures or circuit operating conditions on each of the individual power loss terms. This is the essential knowledge required for developing future generation power MOSFETs for high-efficiency and high-density buck converters.

Lastly, the scope of the previous work on power MOSFET performance analysis was limited to the study of either one particular power MOSFET technology or just a limited number of commercial parts .While offering useful information on how to select commercially available power MOSFETs for today’s practical converter design, the previous work does not sufficiently address the perspectives and theoretical limitations of power MOSFET technology for future generation DC/DC converters operating with ever-increasing switching frequency, slew rate, and output current.

The purpose of this paper is to comprehensively investigate the performance perspectives and theoretical limitations of trench power MOSFET technology in synchronous rectifier buck converters over a wide range of operating conditions. The MOSFET device structures under investigation include but are not limited to those manufacturable with today’s semiconductor fabrication technology. The investigation was carried out with a mixed-mode device/circuit simulation approach. Device measurement data was also used to validate the physical device models. Various power loss contributions from the CtrlFETs and SyncFETs at different operating conditions were studied in detail. Several important observations weremade which may shed some light on the development of future generation power MOSFETs, as well as the optimal utilization of today’s power MOSFETs in buck converter applications.

CONCLUSIONS

In this paper, we have comprehensively investigated the performance perspectives and heoretical limitations of trench power MOSFETs in synchronous rectifier buck converters over a wide range of operating conditions. Several trench MOSFET technologies are investigated using a mixed-mode device/circuit modeling approach. Individual power loss contributions from the CtrlFETs and SyncFETs and their dependence on switching frequency between 500 kHz and 5MHz are discussed in detail.

It is observed that going from 0.5MHz to 5MHz, the conduction loss contribution decreases from 40% to 4% while the switching loss contribution increases from 60% to 96%. Under hard switching operation condition, the buck converter efficiency is limited to 80% at 2MHz and 65% at 5MHz even with the most advanced trench MOSFET technology. For the base technology we studied, beyond 1MHz frequency, there is no obvious benefit to increase the die size of either SyncFET or CtrlFET. For 5 MHz, given a constant die size of SyncFETs, the die size increase of CtrlFETs actually introduces obvious efficiency degradation. For different trench technologies, the technology,which has the lowest QG, provides smallest gate-driver losses and the smallest CtrlFET turning-off losses.

On the other hand, smaller QG gives faster switching transition, which may introduce larger reverse recovery loss due to large di/dt. Basically, there is a tradeoff between faster switching transition and smaller reverse recovery loss. The technology,which has lowest Qrr, theoretically provides the best reverse recovery loss under the same CtrlFET current slew rate. In order to give good indication on trench MOSFET design for MHz frequency operating range, we obtain the RDS(ON) × QG FOM. The simulation results show that this FOM still correlates well to the overall converter efficiency.

by : Yali Xiong, Xu Cheng, XiangchengWang, Pavan Kumar, Lina Guo, and Z. John Shen

Source : DOWNLOAD

Read More ..