The interim fuel storage facility of the PBMR

The PBMR’s spent fuel and partially burnt fuel are stored in the sphere storage system (SSS), which acts as the interim fuel storage facility of the plant. It is unique in the world since the fuel is stored in bulk containers (called storage tanks), each capable of holding more than 500,000 spheres for a period of about 80 years. The SSS has the ability to transfer the contents of one tank to another tank, and to return partially burnt fuel back to the reactor core for re-fuelling. The storage tanks are individually sealed carbon steel pressure vessels. They form the final barrier of any fission products that have diffused from the fuel spheres. Sub-criticality is achieved through the geometric cross-section of the tank, and by taking credit for fuel burn-up. Protection from the corrosive environment where the PBMR Demonstration plant will be built is achieved by actively cooling the tank with clean dry air. In the event of an active cooling failure, louvres open automatically and cooling is done passively via natural convection making use of the chimney-effect. Sufficient radiation protection is provided around each tank to facilitate maintenance and inspection operations where needed. The design of the SSS is nearing the end of its basic design phase, and for some components, detail design work has already commenced. The design complies with all spent fuel storage requirements and is seen as a cost-effective solution for the interim storage of PBMR spent fuel.