Passive Nuclear Safety

 

Passive nuclear safety describes a safety feature of a nuclear reactor that does not require operator action or electronic feedback in order to shut down safely in the event of a particular type of emergency (usually overheating resulting from a loss of coolant or loss of coolant flow). Such reactors tend to rely more on the engineering of components such that their predicted behaviour according to known laws of physics would slow, rather than accelerate, the nuclear reaction in such circumstances. This is in contrast to some older reactor designs, where the natural tendency for the reaction was to accelerate rapidly from increased temperatures, such that either electronic feedback or operator triggered intervention was necessary to prevent damage to the reactor.

Terming a reactor 'passively safe' is more a description of the strategy used in maintaining a degree of safety, than it is a description of the level of safety. Whether a reactor employing passive safety systems is to be considered safe or dangerous will depend on the criteria used to evaluate the safety level. This said, modern reactor designs have focused on increasing the amount of passive safety, and thus most passively-safe designs incorporate both active and passive safety systems, making them substantially safer than older installations. They can be said to be "relatively safe" compared to previous designs.

The temperature coefficient of reactivity is a measure of how the reactor responds to increased temperature. A positive number denotes a trend of increasing power production as temperatures rise, whereas a negative number denotes a trend of decreased power production as temperature rises. For liquid cooled reactors (especially those that use water as coolant) the temperature coefficient is closely linked to the reactor's void coefficient.

 

If the coolant is a liquid, increasing temperatures can cause small gas bubbles to form, displacing the coolant. The void coefficient of reactivity is a number representing how the reactor responds to the formation of such bubbles. A positive number signifies a tendency for reactor activity to increase, whereas a negative number signifies a tendency for reactor activity to decrease. Ideally the void coefficient should be close to 0, such that neither a temperature increase or decrease will cause a power surge. Very large positive void coefficients are particularly undesirable since they could lead to a rapid uncontrollable growth in heat production, as happened during the Chernobyl disaster.