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

Abstract

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: (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. (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. (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. (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.