Passive Safety Injection

Various water sources are available inside the containment to provide decay heat removal and to protect the plant against LOCAs. These have been introduced earlier, e. g. the AP1000/600 and EPR have CMTs, high pressure accumulators, lower pressure CRTs and IRWSTs.

The systems need to provide protection against a spectrum of breaks. Since these systems are gravity driven or driven by overhead gas pressure, it is necessary that the reactor pressure be at a sufficiently low level for injection to take place. One way that this is achieved is by intentional automatic depressurisation via the reactor ADS. These systems are fitted in AP1000/600 and EPP reactors. Such systems were also fitted to present generation BWRs. Once the plant has been depressurised, low-pressure safety injection systems are then actuated. The AP1000/600 uses the IRWST inventory to provide reflood by gravity. Low-pressure gravity core make-up systems are available on ESBWR and SWR 1000.

The other approach is to increase the high-pressure coolant injection system capacity. The ABWR has an increased high-pressure coolant injection system.

With regard to water inventory, several of the new designs have a provision to flood the reactor to a level above the fuel. One such design is BWR 90.

The propensity of LOCAs is actually reduced in many new designs because of the elimination of certain components of primary system piping. For example, in the AP1000/600 and EPP, the recirculation pumps are situated in the steam generators. Similarly, internal pumps are used on the ABWR to eliminate recirculation piping.

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