Nuclear Technology / Volume 171 / Number 1 / July 2010 / Pages 38-52
Technical Paper / Thermal Hydraulics
The large assembly with small pins (LASP) concept is an evolutionary boiling water reactor (BWR) fuel assembly design aimed at increasing the power density of BWR cores while keeping the same power-to-flow ratio, core inlet conditions, and fuel-to-moderator ratio. It is based on replacing four traditional assemblies and their large interassembly water gap regions with a single large assembly surrounded by a narrower gap region. The traditional BWR cylindrical UO2-fueled Zr-clad fuel pin design is retained, but the pins are arranged on a 22 × 22 square lattice. Twenty-five water rods within the assembly maintain the moderating power and accommodate as many finger-type control rods. The technical characteristics of LASP were evaluated and are systematically compared with a traditional 9 × 9 fuel assembly. This design study includes analyses of the steady-state thermal hydraulics, two-dimensional and three-dimensional burnup-dependent neutronics, flow-induced vibrations, and fuel pin thermomechanical behavior. Furthermore, the conceptual mechanical design of the LASP assembly is discussed. The analyses show that LASP can operate at a power density that is 20% higher than the traditional BWR assemblies while maintaining the same safety margin.