Nuclear reactor, accidents emergency core cooling

Apart from differences of approach between world regions, in this period of time and in all the countries with nuclear reactors, the systems installed in the plants according to the requirements of the safety bodies and having the sole purpose of accident mitigation, were frequently the subject of heated debates in particular, the emergency core cooling systems and the containment systems were often discussed. 

In the event of a LOCA, the nuclear reaction in the core would be automatically cut off by the loss of the moderation associated with the coolant, while in the transient case the probability of failure of the shutdown systems is sufficiently low that continued operation at power is not a significant contributor to a core melt situation. For both classes of accident, therefore, the important requirement is the maintenance of a cooling capability sufficient to remove decay heat (see Table 12.7) from the reactor core. The emergency core cooling systems (ECCS) for the PWR and BWR are described below. 

As a direct cycle system, a BWR relies on the main feedwater subsystem to supply cooling water to the reactor, and the main steam subsystem to remove heat from the reactor core during normal operation, In the event of an accident or a serious natural disaster, both of these subsystems may be unavailable. To cope with these accident and disaster scenarios, GE and other BWR designers developed alternate heat removal systems (US Nuclear Regulatory Commission, n.d.b). A major goal of a nuclear reactor s emergency procedures is to keep the fuel elements cool, despite their continued production of decay heat energy,... 

In the event of an accident, the appropriate design of systems and installations has to ensure that radioactive substances will not be released to the environment to an extent beyond tolerable limits. To this end, a series of so-called design basis accidents (see Chapter 6) have been defined, covering conceivable failures of different installations and their possible consequences. In the licensing procedures for a nuclear power plant it has to be demonstrated that in the event of an accident the safety and emergency installations of the plant will be able to keep the reactor core in a safely cooled state and to prevent significant release of radionuclides to the environment. 

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