Neutronic investigation of a VVER-1200 (Th-233U)O2 fuel assembly with protactinium oxide as a burnable absorber coated on the outer surface of the fuel rods

Due to the obvious excess reactivity associated with using U-233 as a fissile material, fresh fuel must include a burnable absorber (BA) to reduce excess reactivity. As a result, the objective of this study is to look into the use of protactinium oxide (PaO2) as a thin BA layer coating on the outer surface of the fuel rod for the VVER-1200 assembly, which is fueled by (Th-U-233)O-2. Through neutronic calculation, the thicknesses of PaO2 as a BA were determined using the Monte Carlo code MCNP6. For comparison, UO2 and (Th-U-233)O-2 assemblies without absorbers were used as references. The study suggests that using PaO2 with a thickness ranging from 0.11 to 0.13 mm can reduce the early excess reactivity of the (Th-U-233)O-2 assembly while also increasing the discharge burnup by an average of 2.1 GWd/ton when compared to the UO2 assembly. Furthermore, it was found that the selected (Th-U-233)O-2 + PaO2 assemblies exhibit negative fuel temperature and boron worth coefficients. However, these assemblies exhibit positive moderator temperature coefficients. Neutron spectra, radial power distribution, and fission product poisons were also evaluated to understand the viability of using PaO2 as a BA for reactivity control in the VVER-1200 assembly under consideration.