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

摘要

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 radioactive and radio-toxic nuclear waste products so that one can expect that they will have negative fuel costs.rnThe criticality calculations are conducted with SCALE5.1 using S_8-P3 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.rnUsing 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.rnThe 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.