Time evolution of criticality and burnup grades of the PBMR were investigated for reactor grade plutonium and minor actinides in the spent fuel of light water reactors (LWRs) mixed with thoria. The calculations were performed by employing the computer codes MCNP and MONTEBURNS 2.0 and using the ENDF/B-V nuclear data library. Firstly, the plutonium-thorium and minor actinides-thorium ratio was determined by using the initial k(eff) value of the original uranium fuel design. After the selection of the plutonium/minor actinides-thorium mixture ratio, the time-dependent neutronic behavior of the reactor grade plutonium and minor actinides and original fuels in a PBMR-400 reactor was calculated by using the MCNP code. Finally, k(eff), burnup and operation time values of the fuels were compared. The core effective multiplication factor (k(eff)) for the original fuel which has 9.6 wt.% enriched uranium was computed as 1.2395. Corresponding to this k(eff) value the reactor grade plutonium/thorium and minor actinide/thorium oxide mixtures were found to be 30%/70% and 50%/50%, respectively. The core lives for the original, the reactor grade plutonium/thorium and the minor actinide/thorium fuels were calculated as similar to 3.2, similar to 6.5 and similar to 5.5 years, whereas, the corresponding burnups came out to be 99,000, similar to 190,000 and similar to 166,000 MWD/T, respectively, for an end of life k(eff) set equal to 1.02. (C) 2012 Elsevier Ltd. All rights reserved.