Compensation for 236U in the fuel of the VVER-440

Closing the fuel cycle is one important task that faces the nuclear power industry. The use of regenerated raw material to make new fuel for nuclear power plants will lead to a change in the radiation and neutronics characteristic of the fuel. In particular, 236U, which represents an additional neutron absorber when compared with fuel made from natural uranium, will be present in the fuel of thermal reactors made completely or in part by using regenerated uranium, even at the start of the run. The initial 236U content depends on the recycling scheme, the number of cycles, and other parameters. For example, in the versions of multiple fuel recycling considered for the VVER-440, after a few cycles the mass fraction of 236U reaches an equilibrium value of 0.6-1%. The presence of 236U in fresh fuel requires supplemental enrichment with 235U in order to preserve the life of the run (the specific energy output of the fuel). To make a quantitative estimate of this effect, the authors introduce the compensation factor K = ΔC5/C6, where ΔC5 is the increase in the 235U content corresponding to the 236U content C6 at the start of the run. They have examined the dependence of the compensation factor on the 236U content in fuel enriched by 3.6% with respect to 235U in the VVER-440