Thorium Blended and Regular Mox Burn-up Studies for Fast Reactor Fuel Cycle Safeguards

Fast reactor fuel cycle (FRFC) is regaining importance because of its vital role in the long term development of nuclear power. The safety issues associated with the fast reactors are now well understood and newer designs can address those from the lessons learned in the past. The same cannot be ascertained about their safeguards issues. Primary reason being the presence of special nuclear material (SNM) in very high concentrations inside fresh and spent fuel of fast reactors, making it more attractive to the Proliferators and leads to less proliferation resistance (PR). Advantageously changing the fuel material without adversely affecting the normal reactor operations could improve the PR and in turn enhance the safeguards of the FRFC. The intrinsic PR characteristics estimated through computational fuel burn-up analysis of a regular and a thorium blended MOX for a typical fast reactor system is presented here. Detailed core physics computations showed that, employing either of these fuel types has insignificant effect on the reactor operations. However, build-up of denaturing material U (1300 to 1500 PPM by weight of U) along with the fissile material U during the burn-up of thorium blended MOX makes the spent fuel relatively less attractive to the Proliferators. The daughter products in the decay chain of U are significant gamma emitters and in particular, Tl emits gamma rays with energy of 2.6MeV, which not only makes the fissile material low grade but also pose handling difficulties. It is observed that the photon emission rate in the range of 2.25MeV to 2.75MeV from the actinides of a thorium blended spent fuel is higher by a factor of ~4000 compared to the regular MOX spent fuel. These gamma rays provide a distinctive signature that can be used to detect and track SNM. Also, thorium blended fresh fuel will have 2.6MeV signature because Tl is a daughter product in the natural decay chain of Th. It is concluded from the study that deployment of thorium blended MOX fuel in place of regular MOX fuel in a fast reactor should help in improving the PR characteristics and enhancing the safeguards of a FRFC. INTRODUCTION Varieties of nuclear reactors with fast neutron spectrum are in the research and development stages for future deployment and can play a vital role in the long term development of nuclear power [1] [2] [3]. Nuclear proliferation resistance (PR) characterization of spent fuel discharged from one such fast reactor (FR) is the subject matter of this paper. The PR characterization is necessary to develop safeguards approaches for the fast reactor fuel cycle (FRFC). Fuel assembly design selected for the study is that of the Indian FR, based on the information available in open literature [2] [3]. Indian FR core utilizes two kinds of fuel assemblies, namely inner and outer core assemblies. The difference is in the amount of reactor grade plutonium contained in them. Inner core fuel assemblies have less amount of plutonium compared to the outer core fuel assemblies to achieve power flattening. Both, inner and outer core fuel assemblies are made from mixed oxides (MOX) of PuO2 and depleted UO2. The ratios of PuO2:UO2 are 21:79 and 28:72 for the inner and outer core assemblies respectively. Abundance of thorium in India and the better neutronic