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