Patent Number: 040240185
Section: summary

BACKGROUND OF THE INVENTION This invention relates to liquid metal cooled fast breeder nuclear reactors of the pool kind. In a nuclear reactor of the pool kind the reactor core and ancillary components, such as heat exchangers and coolant circulating pumps, are immersed in a pool of coolant. One known construction of liquid metal cooled fast breeder nuclear reactor of the pool kind comprises a primary vessel suspended from the cover of a concrete vault and containing a pool of liquid sodium. The reactor core, coolant pumps and heat exchangers are also suspended from the cover of the concrete vault and are immersed in the pool of sodium. Ducting is provided whereby coolant from the pool can be pumped successively through the core and the heat exchangers thence back to the pool. Replacement of spent fuel is carried out whilst the reactor is shut down and the irradiated fuel assemblies are withdrawn from the core by a charge machine located above the cover of the concrete vault. Whilst still submerged in liquid sodium the irradiated fuel is transferred sideways and lowered into a transfer rotor. Subsequently the irradiated fuel is removed from the vault to storage racks. The transfer rotor has provision for the temporary storage of twenty irradiated fuel assemblies, each within a stainless steel bucket. For insertion and withdrawal of a fuel assembly the rotor is rotated to bring an assembly to a loading or discharging station. An irradiated fuel assembly is withdrawn from the rotor within its bucket and the combination of fuel assembly, bucket and sodium (weighing approximately 100 kilograms) is lifted approximately 27 meters. Thus in the event of malfunction the combination falls back into the rotor with considerable energy. To prevent damage to the rotor an arrangement of fracture pins limits the transfer of energy to the rotor and deformable energy absorbing devices prevent damage to the reactor vessel and other parts of the reactor structure. After such a malfunction it is necessary to replace the fractured pins and the deformed energy absorbing device by remote means operated over a considerable distance from outside the vault and the operation is, therefore, very complex. It is an object of the present invention to provide a liquid metal cooled fast breeder reactor of the pool kind having improved energy absorbing means for fuel assemblies falling into a fuel transfer rotor. SUMMARY OF THE INVENTION According to the invention in a liquid metal cooled fast breeder nuclear reactor of the pool kind having a transfer rotor for the temporary storage of irradiated fuel assemblies in the pool of reactor coolant whilst the fuel assemblies are in transit from the reactor core to storage means, the fuel assemblies are supported in the transfer rotor by elastic means and there is provided a hydraulic dash pot at least in one position below the transfer rotor for absorbing the energy of a falling fuel assembly. By hydraulic dash pot is meant a construction including a piston movable within a cylinder to displace fluid within the cylinder through a restricted passage whereby the kinetic energy of the falling load is absorbed by the displacement of fluid. In a preferred construction of liquid metal cooled fast breeder reactor dash pots are provided immediately below a receiving station for irradiated fuel assemblies in the transfer rotor and immediately below a discharge station for fuel assemblies, and the transfer rotor has buckets each for housing a fuel assembly, each bucket being carried in a container which is elastically supported in the transfer rotor on a helical coil compression spring so that, in the event of a malfunction of the hereinbefore described kind, the container and bucket are returned to their normal operating position after the kinetic energy of the falling load has been absorbed by the dash pot. The transfer rotor may also be provided with recoil springs for absorbing the recoil energy of the helical coil springs. The invention embodied in the preferred construction provides that the energy of a falling fuel assembly, bucket and coolant combination is absorbed by displacement of coolant in the dash pot and for the subsequent return of the combination to its normal storage position in the transfer rotor without causing damage to the reactor construction. Thus, when the cause of the malfunction has been ascertained and corrective action has been taken, normal discharge operations can be resumed without the need for complex repairs and replacements to be made to ancillary equipment of the transfer rotor. The lower end of each container may be adapted to form a piston for the dash pot or dash pots, the cylinder being located in a static base supporting the rotor but, preferably, the complete dash pot is located in the base and presents a striker platform to the lower end of a descending container. This latter preferred feature avoids the need for very accurate positioning of the rotor whereby the lower end of a descending container forming a piston is enabled to enter the cylinder.