**4. Conclusions**

Parallel computing of fuel burnup in nuclear reactors give important insight about the chain reaction, the fuel consumption and buildup of radionuclides. The comprehensive calculations clearly present the characterization of the neutron flux, which is essential in the reactor design. Fuel burnup is one of the most important reactor core parameters in the operation of nuclear reactors as well as any fuel research program. In this research, Monte Carlo burnup simulations of a critical pressurized water reactor system were carried out using three different fuel grades: MOX, UOX and CEU. The effect of different clad materials on *K*eff was also analyzed.

The study of reactor neutronic parameters showed UOX fuel performs better relative to MOX and CEU. The peak reactivity of UOX was found to be 0.270 compared to 0.142 and 0.155 for MOX and CEU fuel, respectively. Also, the neutron flux for UOX exceeds that of CEU and MOX by 17.87 and 22.68%, respectively.

Zircaloy, with low thermal neutron absorption cross-section and high thermal conductivity produced better results for *K*eff and hence proves to be a much more effective clad material.

The atom densities and radioactivities in Bequerel were calculated for each of the three fuel grades at the end of the 220-day burnup. The total atom densities and radioactivities of both actinides and non-actinides were calculated for each fuel grade. This was found to be 4.251e19Bq for UOX fuel and 3.639e19Bq and 4.567e19Bq for MOX and CEU fuel, respectively. In the event of a severe reactor accident, the radiological hazard posed is proportional to the concentrations and radioactivities of these radionuclides. The total atom densities and radioactivities are also very important parameters in estimating which fuel grade performs better in the sense of posing relatively few hazards at the end of the burnup time or core lifetime of the reactor. For the same burnup time, MOX fuel was found to have a relatively less total radioactivity of the reactor core, almost 15% reduction as compared to the other fuel grades.
