The power density, which is defined as the power produced per unit volume of the reactor core, is an important indicator to determine core size. It is usually provided in the unit of kW/l and the average power density of BWR is given by

Average power density = Q/Vcore = qaw x Nrod/ (LB x LB) (3-14)

The average power density can be raised by increasing the average linear heat generation rate of the fuel rods or by increasing the number of fuel rods per unit cross section. The number of fuel rods should be set as the most optimal value with a consideration on the nuclear design such as the volume ratio of coolant and fuel which is mentioned later, and on the thermal-hydraulic design such as the heat removal capability related to the heat transfer areas of fuel rods and coolant flow path. The average power density of BWRs operated until the present is in the range of about 40-60 kW/l.