Screening-level pin-cell neutronic sensitivity of a NuScale Power Module-like SMR fuel lattice to coolant-state perturbations from primary-system degradation

Abstract

Small modular reactors such as the NuScale Power Module use an integral primary system and natural-circulation cooling, so degradation of internal components reaches the core through the coolant state rather than through a break. This study asks whether three such mechanisms (helical-coil steam generator fouling, core barrel bypass leakage and riser oxide growth) move lattice reactivity far enough to be seen. Coolant density and temperature are obtained from a coupled loop energy and momentum balance anchored to the NuScale final safety analysis report, passed through IAPWS-IF97 to an OpenMC pin-cell model, and swept over 21 degradation levels per mechanism. All three act on the lattice only through the bulk coolant state; the two mechanisms that measurably move that state, steam generator fouling and core bypass leakage, collapse onto a common coefficient of reactivity with respect to coolant density, 15,010 ± 492 pcm/(g/cm ³ ), agreeing within half a standard deviation, while the riser case provides a null control because its coolant density is effectively invariant over the swept range. Against a differencing standard deviation of 9.00 pcm the reactivity change is 2.5  σ at the design fouling allowance, 1.9  σ at the analytical bypass fraction and 0.4  σ for 3 mm of riser oxide. Using 3  σ only as a conservative computational-resolvability benchmark rather than a plant detection criterion, the corresponding fouling and bypass conditions lie roughly a quarter above their design values. At the beginning-of-cycle boron concentration the lattice coefficient is reduced by a factor of 2.34.