During normal operation, because the BWR uses a direct cycle, the heat rejection route is via the turbine and condenser. Under fault conditions, to preserve the barriers to fission product release the main steam and main feed isolation valves are closed by the protection system. These are shown on Fig. 10.12. Core cooling must be continued and this, for non-LOCA conditions, is achieved by either the reactor core isolation cooling (RCIC) system or the isolation condenser system. The latter is used in older GE BWR3 plants, but because it is a passive system it is also used in the advanced passive plants (see 10.11).

The RCIC is shown schematically in Fig. 10.15. The RCIC consists of a steam turbine-driven pump and its associated pipework and valves. Using steam from the steam line to power the turbine pump, it draws water from the condensate storage tank and injects it into the main feed line. The system starts automatically on low water levels being detected in the vessel.

10.15 Schematic of a BWR reactor core isolation cooling system (Source: USNRC).

©

BWRs also have ECCSs to provide core cooling in the case of LOCAs. As is the case for PWRs this must cover a range of break sizes and so there are both high and low pressure systems. The high pressure coolant injection (HPCI) system is similar in configuration to the RCIC and is independent of ac power or external cooling systems. A turbine-driven pump injects make-up water from the condensate storage tank into the main feed line. The pump can supply water at pressures above the rated reactor pressure and will continue to be effective down to pressures at which the low pressure systems can operate. Excess steam will be discharged to the containment suppression pool, which acts as the ultimate heat sink in the short term.

In addition a high pressure core spray (HPCS) system is provided. This uses electric pumps supplied by the diesel-backed essential electrical system, which draws water from either the condensate storage tank or the suppression pool and sprays it onto the top of the core using spray rings mounted in the upper part of the core barrel above the top of the core.

To provide an alternative, should the high pressure systems be unavailable or unable to recover the water level, an automatic depressurisation system (ADS) is provided. This opens selected safety relief valves to depressurise the RCS by discharging to the suppression pool. This then allows the use of the low pressure ECCSs, which are illustrated in Fig. 10.16.

The low pressure ECCS system consists of two independent subsystems: the core spray system and the low pressure coolant injection (LPCI) system. As is the case for PWRs the LPCI system is also used for residual heat removal. The core spray system consists of a number of redundant loops consisting of a diesel — backed electrically driven pump, which draws water from the suppression pool and sprays it via the low pressure core spray sparge ring, which is mounted just below the HPCS sparge ring.

The RHR pumps in their LPCI mode perform three main safety-related functions: LPCI (restore and maintain RPV water level when the RPV is depressurised); containment spray (condenses steam and reduces airborne activity) and suppression pool cooling (provides an external ultimate heat sink).