Criticality and burn up evolutions of the Fixed Bed Nuclear Reactor with alternative fuels

摘要

Time evolution of criticality and burn-up grades of the Fixed Bed Nuclear Reactor (FBNR) are investigated for alternative fuels. These are: (1) low enriched uranium, (2) weapon grade plutonium, (3) reactor grade Plutonium, and (4) minor actinides in the spent fuel of light water reactors (LWRs). The criticality calculations are conducted with SCALE 5.1 using S_8-P_3 approximation in 238 neutron energy groups with 90 groups in thermal energy region. The main results of the study can be summarized as follows:rn(1) Low enriched uranium (UO_2): FBNR with an enrichment grade of 9% and 19% will start with k_(eff) =1.2744 and k_(eff) = 1.36 and can operate ～8 and >15 years with the same fuel charge, where criticality drops to k_(eff) - 1.06 and a burn-up grade of 54 000 and > 110 000 MW.D/t can be attained.rn(2) Weapon grade plutonium: Such a high quality nuclear fuel suggests to be mixed with thorium. Second series of criticality calculations are conducted with fuel compositions made of thoria (ThO_2) and weapon grade PuO_2, where PuO_2 component has been varied from 1% to 100%. Criticality with k_(eff)> 1.0 is achieved by ～2.5% PuO_2. At 4% PuO_2, the reactor criticality will become satisfactory (k_(eff)= 1.1121), rapidly increasing with more PuO_2. A reasonable mixture will by around 20% PuO_2 and 80% ThO_2 with a k_(eff) = 1.2864. This mixed fuel would allow full power reactor operation for >20 years and burn-up grade can reach 136 000 MW.D/t.rn(3) Reactor grade plutonium: Third series of criticality calculations are conducted with fuel compositions made of thoria and reactor grade PuO_2, where PuO_2 is varied from 1% to 100%. Reactor becomes critical by ～8% PuO_2 content. One can achieve k_(eff)- 1.2670 by 35% PuO_2 and would allow full power reactor operation also for >20 years and burn-up grade can reach 123 000 MW.D/t.rn(4) Minor actinides in the spent fuel of LWRs: Fourth series of criticality calculations are conducted with fuel compositions made of thoria and MAO_2, where MAO_2 is varied from 1% to 100%. Reactor becomes critical by ～17% MAO_2 content. Reasonably high reactor criticality (k_(eff)- 1.2673) is achieved by 50% MAO_2 for a reactor operation time of 15 years with a burn up of 86 000 MW.D/ t without fuel change. On that way, the hazardous nuclear waste product can be transmuted as well as utilized as fuel.