Low cost, proliferation resistant, uranium-thorium dioxide fuels for lightwater reactors

Our objective is to develop a fuel for the existing light water reactors (L WRs) that, (a) is less expensive to fabricate than the current uranium-diox ide (UO2) fuel: (b) allows longer refueling cycles and higher sustainable p lant capacity factors: (c) is very resistant to nuclear weapon-material pro liferation: (d) results in a more stable and insoluble waste form; and (e) generates less high level waste. This paper presents the results of our ini tial investigation of a LWR fuel consisting of mixed thorium dioxide and ur anium dioxide (ThO2-UO2). Our calculations using the SCALE 4.4 and MOCUP co de systems indicate that the mixed ThO2-UO2 fuel, with about 6 wt.% of the total heavy metal U-235, could be burned to 72 MW day kg(-1) (megawatt ther mal days per kilogram) using 30 wt.%, UO2 and the balance ThO2. The ThO2-UO 2 cores can also be burned to about 87 MW day kg(-1) using 35 wt.% UO2 and 65% ThO2 with an initial enrichment of about 7 wt.% of the total heavy meta l fissile material. Economic analyses indicate that the ThO2-UO2 fuel will require less separative work and less total heavy metal (thorium and uraniu m) feedstock. At reasonable future costs for raw materials and separative w ork. the cost of the ThO2-UO2 fuel is about 9%, less than uranium fuel burn ed to 72 MW day kg(-1). Because ThO2-UO2 fuel will operate somewhat cooler, and retain within the fuel more of the fission products, especially the ga sses. ThO2-UO2 fuel can probably be operated successfully to higher burnups than UO2 fuel. This will allow for longer refueling cycles and better plan t capacity factors. The uranium in our calculations remained below 20 wt.% total fissile fraction throughout the cycle, making it unusable for weapons . Total plutonium production per MW day was a factor of 3.2 less in the ThO 2-UO2 fuel than in the conventional UO2 fuel burned to 45 MW day kg(-1). Pu -239 production per MW day was a factor of about 4 less in the ThO2-UO2 fue l than in the conventional fuel. The plutonium produced was high in Pu-238, leading to a decay heat about three times greater than that from plutonium derived from conventional fuel burned to 45 MW day kg(-1) and 20 times gre ater than weapons grade plutonium. This will make fabrication of a weapon m ore difficult. Spontaneous neutron production from the plutonium in the ThO 2-UO2 fuel was about 50% greater than that from conventional fuel and ten t imes greater than that from weapons grade plutonium. High spontaneous neutr on production drastically limits the probable yield of a crude weapon. Beca use ThO2 is the highest oxide of thorium while UO2 can be oxidized further to U3O8 or UO3, ThO2-UO2 fuel appears to be a superior waste form if the sp ent fuel is to be exposed ever to air or oxygenated water. And, finally, us e of higher burnup fuel will result in proportionally fewer spent fuel bund les to handle, store, ship, and permanently dispose of. (C) 2001 Elsevier S cience B.V. All rights reserved.