The Large Assembly with Small Pins (LASP) concept is an evolutionary fuel design proposed to enable a higher power density in boiling water reactors while maintaining the same operating conditions, such as power-to-flow ratio, core inlet conditions, and fuel-to-moderator ratio. It is based on replacing four traditional assemblies and the large water gap regions between them with a single large assembly having a 22 × 22 square fuel pin lattice. Twenty-five water rods within the assembly help maintain neutron moderation and accommodate as many finger-type control rods. It was previously shown that the LASP core allows operation with 20% higher power density than the core with traditional 9 × 9 fuel assemblies. However, the void reactivity coefficient of the LASP core is 25% more negative. In this study, the stability performance of the LASP core has been evaluated.

The characteristics of density wave oscillations in the LASP core and their sensitivity to the operating parameters have been investigated. Although the perturbation decay ratios for the LASP core were found to be greater than those of the reference core, the stability criteria are sufficiently satisfied. Sensitivity studies were performed on the effects of design and operating parameters. It can be concluded that the LASP and the reference core have similar sensitivity to operating parameters. Furthermore, the calculated decay ratios were much smaller than the stability criterion for all the considered parameter ranges.