BMB-LWR core moderator coefficient and its WDR reactivity worth

Control of the excess reactivity for burnup in pressurized water reactors (PWRs) is accomplished using soluble poison and/or burnable poison. The boron concentration must be limited during operating conditions to ensure that the moderator temperature coefficient (MTC) is negative. Sufficient burnable poison is installed at beginning of cycle (BOC) to give the desired cycle lifetime without exceeding the boron concentration limit. If the cyclelength is increased, the burnable poison loading has to be increased, which (for heterogeneous burnable poison, such as most boron-based schemes) displaces fuel rods and increases intraassembly power peaking. Note that even at end-of-cycle (EOC) conditions some residual poison remains, resulting in a net decrease in cycle lifetime. Accordingly, the next step is to search for concept variations that are more practical from a thermal-hydraulic view-point even at some sacrifice in fuel savings. The approach selected for subsequent study involved fuel reconstruction combined with the use if water displacement rods (WDRs) in a design concept designated as the breeder/moderator controlled/burner light water reactor (BMB-LWR). The reference reactor to which the new design was compared is a standard Westinghouse PWR, operating on an extended burnup fuel cycle, since the BMB-LWR core was also designed to have a longer cycle than has been past practice