Neutronic analysis of U-free inert matrix and thoria fuels for plutonium disposition in pressurised water reactors

Inserting reactor-grade (RG) or weapons-grade (WG) plutonium in uranium-fre e matrices and burning it in light water reactors (LWRs) is an option gaini ng a wider consensus in the nuclear community. The main results of our neut ronic studies performed in the last few years on this subject are reported. Our attention was mainly concentrated on two kinds of matrices: inert matr ix in the form of calcia-stabilised zirconia, and thoria. Both materials ar e likely to exhibit excellent behaviour under irradiation (already demonstr ated for thoria fuels) and high chemical stability. Direct disposal of spen t fuel should be made feasible and attractive. A preliminary neutronic anal ysis was performed on these U-free fuels, imposing the constraint of mainta ining the same assembly design and cycle length of a standard enriched-uran ium fuel. In particular inert matrix fuel (IMF) showed a high plutonium bur ning capability, but associated with unacceptable feedback coefficients. Th erefore, a whole IMF core results unfeasible, and only a partial core loadi ng is possible. The solution then studied consists in replacing approximate to 21% of the pins of a standard enriched-U subassembly with IMF pins. Det ailed assembly and core calculations were performed. A crucial aspect is th e choice of a suitable burnable poison, which has to dampen the power peaks in the different fuel pin types without life penalisation. Among the consi dered poisons, a thin boron coating on the IMF pellets resulted the only ef fective one. Preliminary IMF pin cell calculations and the detailed ones ga ve similar results in terms of burnt plutonium fractions: 90% of fissile an d 73% of total plutonium is burnt when RG plutonium is used. The main drawb acks of this fuel are the limited core loading capability and the lack of i n-pile technological validation. In the case of Pu-Th fuels, pin cell calcu lations showed that increasing the plutonia content, decreasing the thoria content, and decreasing the pellet diameter are all possible ways to reach a longer fuel cycle and a higher percentage of burnt plutonium. Attained va lues for RG-Pu are greater than or equal to 80% and >60% for the fissile an d total plutonium, respectively. The use of IMF is an effective solution to proliferation concerns, while some concerns remain for thoria fuels becaus e of the production of U-233. This, however, can be eliminated by a small a ddition of U-238. Long time radiotoxicity is scarcely affected by these fue ls with respect to conventional MOX, (C) 1999 Elsevier Science B.V. All rig hts reserved.