S.S. Chirayath, J. Ragusa, and P. Nelson, “Thorium blended and regular MOX burn-up studies for fast reactor fuel cycle safeguards”, 50th annual meeting of INMM, July 12-16, 2009, Tucson, AZ, USA.
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 fueltypes has insignificant effect on the reactor operations. However, build-up of denaturing material 232U (1300 to 1500 PPM by weight of 233U) along with the fissile material 233U 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 232U are significant gamma emitters and in particular, 208Tl 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 athorium 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 208Tl is a daughter product in the natural decay chain of 232Th. It is concluded from the study that deployment of thoriumblended 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.