Development of a novel interim bulk fuel storage facility for the PBMR
The PBMR is the first High Temperature Reactor being designed for commercial power generation in South Africa. It makes use of spherical fuel elements, containing coated uranium oxide particles encapsulated in a graphite matrix. The spent fuel generated from the reactor is stored in a storage system before final disposal. Such storage systems are called interim storage facilities, and normally make use of small transportable containers. The PBMR design makes use of bulk storage containers, capable of holding more than half a million spent fuel spheres. This is a unique concept for nuclear spent fuel storage. Also, most nuclear reactors make use of an intermediate cooling pool before the fuel is transferred to the storage facility. For the PBMR, the spent fuel is discharged directly into the interim storage facility, thus eliminating the intermediate cooling pool. All interim storage facilities have to comply with five basic requirements, namely: fuel sub-criticality; decay heat removal; radioactive material containment; fuel integrity protection; and radiation protection of the workers and the public. The solution for each requirement depends upon the type of fuel, as well as the philosophical criteria of the reactor design. For the PBMR, it involves a storage life of 80 years, passive cooling and bulk storage tanks. In addition to the basic requirements, the PBMR storage facility should also be able to store used fuel during reactor maintenance, and to transfer it back to the reactor or to another storage tank when required. During the four years of the development of the storage system, the design has undergone several changes. These changes were brought on by changes of the reactor design, and also due to developments and improvements on immature areas. The result is an integrated solution, retaining virtually none of the original concept, but still complying with all requirements. The containment design solution is a vertically suspended ASME VIII pressure vessel (or storage tank) with a loading point and an unloading device. All radioactive material is captured inside the pressure boundary, and the tank is completely sealed off when not in use. New devices were developed to systematically load the tank, and to remove the spheres from the tank. Scale tests were done to verify the performance of the new devices and to ensure proper sphere flow inside the tank. Sub-criticality of the fuel volume is achieved by adding hollow tubes to the inside of the storage tank, thereby creating a sub-critical geometry. Bum-up credit is also taken for the fuel at 20% below the average core bum-up. The fuel is therefore passively safe even if the full contents of the reactor is transferred into a storage tank. In order to ensure that the tank lasts for 80 years in a cost-effective manner, the tanks are cooled in a closed loop system. The closed loop air is continuously dried to a very low relative humidity, which minimises corrosion on even normal carbon steel. Corrosion tests have been performed to investigate the effect of radiolysis products that may build up in the closed loop. These tests are still under way. The decay heat is removed from the fuel spheres by means of air convection around the tank surface. The tubes inside the tank also allow air to pass through, creating a very strong chimney effect. A new method was developed to calculate the fuel temperatures for a given cooling flow. The technique makes use of FEA and analytical equations. Solutions are obtained at a fraction of the time it takes to perform a full CFD analysis, and within 5% compared to CFD results. Full-scale tests are planned to measure and verify the heat transfer properties of the cooling tubes in order to boost the credibility of the FEA and CFD analyses. The storage tank design is integrated into a storage unit, which performs all the nuclear functions. The storage unit can operate in four different cooling modes, namely closed loop active cooling; open loop passive cooling; open loop active cooling and closed loop conditioning. There is an automatic fallback from the active cooling mode to a passive cooling mode. The active cooling is thus only needed to prevent excessive corrosion of the tanks. A scale model has been built to demonstrate the passive cooling ability of a storage unit, and the results agree well with CFD analyses. Also, a new method was developed to calculate the passive cooling characteristics using pipe network simulation software. This method is significantly faster than CFD analyses, and allows one to easily incorporate fan characteristics and to perform sensitivity studies. Twelve storage units make up the Sphere Storage System of the PBMR. An intricate sphere pipe system allows one to transfer fuel spheres from the reactor to any tank, from any tank back to the reactor or to another tank, or to a decommissioning cask. All maintenance intensive components are placed at accessible areas, thus protecting the workers from the radiation coming from the tanks. Measures are incorporated to detect any contamination leakage, and also to enable the IAEA to verify the nuclear inventory of the storage system. The Sphere Storage System is a fully integrated, yet modular design that complies with all nuclear and process requirements. It presents a unique solution to the interim fuel storage of the PBMR, and is believed to be a cost-effective solution for 80 years of storage. Some future tests and developments are required to finalise immature areas, but overall, the system is sufficiently engineered such that detail design can continue.
- Engineering