The system 80+TM response to the small break LOCA boron dilution issue

The small break loss of coolant accident (LOCA) boron dilution issue surfaced in late 1992 as a result of concerns raised by analyses performed by the Finnish Centre for Radiation and Nuclear Safety (STUK). The concern is the potential effect on criticality and fuel temperature if the un-borated condensate that is formed during the event is transported to the core. ABB-Combustion Engineering addressed this new issue during the U.S. Nuclear Regulatory Commission (US NRC) review of the System 80+TM Advanced Light Water Reactor (ALWR) design. The US NRC approved the ABB-CE response to this postulated event and issued a Final Design Approval (FDA) for the System 80+ design in July 1994. The small break LOCA (SBLOCA) boron dilution issue addresses the potential large accumulation of un-borated condensate in the reactor coolant system (RCS) piping following a SBLOCA. This condensate is postulated to be suddenly introduced into the core by restarting a reactor coolant pump (RCP) or by regaining natural circulation. The un-borated condensate is assumed to accumulate as the result of boiling in the core and condensing in the steam generator (SG) tubes. This condensate may collect in the RCP suction piping and then later may be pushed into the reactor vessel where it could lead to an increase in reactivity. The small break LOCA boron dilution event has been studied extensively for the System 80+ design. This paper summarizes how a combination of design features, improved operating guidance, and analysis have been used to resolve this issue. The design features minimize the amount of condensate which can be formed and minimize the boron concentration required to prevent criticality. The operating guidance significantly reduces the probability of an incorrect RCP restart. Finally, analyses show that adequate core cooling is maintained even if conservative assumptions are applied