Criticality investigations for the fixed bed nuclear reactor using thorium fuel mixed with plutonium or minor actinides

Prospective fuels for a new reactor type, the so called fixed bed nuclear reactor (FBNR) are investigated with respect to reactor criticality. These are ① low enriched uranium (LEU); ② weapon grade plutonium + ThO 2; ③ reactor grade plutonium + ThO 2; and ④ minor actinides in the spent fuel of light water reactors (LWRs) + ThO 2. Reactor grade plutonium and minor actinides are considered as highly radio-active and radio-toxic nuclear waste products so that one can expect that they will have negative fuel costs. The criticality calculations are conducted with SCALE5.1 using S 8–P 3 approximation in 238 neutron energy groups with 90 groups in thermal energy region. The study has shown that the reactor criticality has lower values with uranium fuel and increases passing to minor actinides, reactor grade plutonium and weapon grade plutonium. Using LEU, an enrichment grade of 9% has resulted with k eff = 1.2744. Mixed fuel with weapon grade plutonium made of 20% PuO 2 + 80% ThO 2 yields k eff = 1.2864. Whereas a mixed fuel with reactor grade plutonium made of 35% PuO 2 + 65% ThO 2 brings it to k eff = 1.267. Even the very hazardous nuclear waste of LWRs, namely minor actinides turn out to be high quality nuclear fuel due to the excellent neutron economy of FBNR. A relatively high reactor criticality of k eff = 1.2673 is achieved by 50% MAO 2 + 50% ThO 2. The hazardous actinide nuclear waste products can be transmuted and utilized as fuel in situ. A further output of the study is the possibility of using thorium as breeding material in combination with these new alternative fuels.