Nuclear Technology / Volume 171 / Number 1 / July 2010 / Pages 27-37
Technical Paper / Fuel Cycle and Management
An advanced design of a Large Assembly with Small Pins (LASP) has been proposed at the Massachusetts Institute of Technology to increase the power density of boiling water reactors (BWRs) while keeping most of the operating conditions of current BWRs. LASP is based on replacing four traditional assemblies and the large water gap regions with a single large assembly having a 22 × 22 square lattice. In-assembly water rods accommodate control rods as well as provide help to the moderation of neutrons. Previous steady-state analysis showed that the LASP core allows for 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 and the steam flow rate is 20% higher than that of the reference core. In this study, the performances of the LASP core and reference core are compared for selected design-basis accidents and transients. Generally, the LASP design is found to behave in a manner similar to the traditional assemblies. First, the clad peak temperature during a large-break loss-of-coolant accident analysis satisfies regulatory criterion, and it is possible to preserve peak cladding temperature margin of the reference design if the capacity of the low-pressure core injection system is increased by 20%. Second, the generator load rejection with bypass failure and feedwater controller failure analyses show a decrease in dryout margin for the LASP core because of the combination of more negative void coefficient and increased steam load. However, this problem could be remedied by increasing the steam line flow area or allowing an additional flow restrictor in the steam line to attenuate the back propagating pressure wave in the main steam pipe following the turbine stop valve closure. Finally, the LASP core preserved the same level of margin to dryout as the reference core in the cases of four other evaluated events.