Impact of weight percent gadolinium and the number of its fuel rods on the neutronic and safety parameters

Highlights: • The reactivity swing-peak decreases with gadolinium weight percent. • Reactivity swing curve increases with increasing number of gadolinium fuel rods. • Large gadolinium weight percent impacts negatively on FTC. • Large gadolinium weight percent impacts positively on MTC. • Gadolinium is not economically viable at and above 12 w/o. Control of neutron multiplication in a nuclear reactor is fundamental in achieving stable reactor power. The present study has sought to determine the impact of gadolinium weight percent (w/o) in respect of the number of fuel rods on the neutronic and safety parameters of (Th, U)O₂ fuel in a Westinghouse small modular reactor. The MCNPX 2.7 integrated with CINDER90 fuel depletion code was used. The results show that the k-infinite and reactivity swing peak decrease with increasing gadolinium weight percent, with the highest and lowest k-infinite recorded in the fuel assembly containing zero (0) and 12 w/o, respectively. The largest and lowest reactivity swing curve occurs in the fuel assembly with 3.6 and 12 w/o at ~15 and ~35 GWd/THM, respectively. It shows that the impact of gadolinium w/o and the number of fuel rods on k-infinite follow similar trend. Conversely, the reactivity swing curve is observed to flatten with increasing gadolinium w/o but increases with increase in the number of gadolinium burnable absorber fuel rods. These phenomenological variations suggest that flat reactivity swing and power control can be achieved within 9.2–12 w/o gadolinium although not without economic penalty on fuel utilisation.