Low cost, proliferation resistant, uranium–thorium dioxide fuels for light water reactors

Abstract Our objective is to develop a fuel for the existing light water reactors (LWRs) that, (a) is less expensive to fabricate than the current uranium-dioxide (UO2) fuel; (b) allows longer refueling cycles and higher sustainable plant capacity factors; (c) is very resistant to nuclear weapon-material proliferation; (d) results in a more stable and insoluble waste form; and (e) generates less high level waste. This paper presents the results of our initial investigation of a LWR fuel consisting of mixed thorium dioxide and uranium dioxide (ThO2–UO2). Our calculations using the SCALE 4.4 and MOCUP code 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 thermal days per kilogram) using 30 wt.% UO2 and the balance ThO2. The ThO2–UO2 cores can also be burned to about 87 MW day kg−1 using 35 wt.% UO2 and 65% ThO2with an initial enrichment of about 7 wt.% of the total heavy metal fissile material. Economic analyses indicate that the ThO2–UO2 fuel will require less separative work and less total heavy metal (thorium and uranium) feedstock. At reasonable future costs for raw materials and separative work, the cost of the ThO2–UO2 fuel is about 9% less than uranium fuel burned 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 gasses, ThO2–UO2 fuel can probably be operated successfully to higher burnups than UO2 fuel. This will allow for longer refueling cycles and better plant 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 ThO2–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 fuel 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 greater than weapons grade plutonium. This will make fabrication of a weapon more difficult. Spontaneous neutron production from the plutonium in the ThO2–UO2 fuel was about 50% greater than that from conventional fuel and ten times greater than that from weapons grade plutonium. High spontaneous neutron production drastically limits the probable yield of a crude weapon. Because 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 spent fuel is to be exposed ever to air or oxygenated water. And, finally, use of higher burnup fuel will result in proportionally fewer spent fuel bundles to handle, store, ship, and permanently dispose of.