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.
Temperature coefficient of reactivity
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.
Void coefficient of reactivity
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
Examples of passive safety in operation
Traditional reactor safety systems are active
in the sense that they involve electrical or mechanical operation on command systems (e.g., high-pressure water pumps). But some engineered reactor systems operate entirely passively, e.g., using pressure relief valves to manage overpressure. Parallel redundant systems are still required. Inherent
or fully passive
safety depends only on physical phenomena such as pressure differentials, convection, gravity or the natural
response of materials to high temperatures to slow or shut down the reaction, not on the functioning of engineered components such as high-pressure water pumps.
The pebble bed reactor is an example of a passively-safe reactor - as the temperature of the fuel rises, Doppler broadening increases the probability that neutrons are captured by U-238 atoms. This reduces the chance that the neutrons are captured by U-235 atoms and initiate fission, thus reducing the reactor's power output and placing an inherent upper limit on the temperature of the fuel.
Current Pressurized Water Reactors and Boiling Water Reactors are systems that have been designed with one kind of passive safety feature. In the event of an excessive-power condition, as the water in the nuclear reactor core boils pockets of steam are formed. These steam voids moderate fewer neutrons, causing the power level inside the reactor to lower.
In some designs the core of a Fast breeder reactor is immersed into a pool of liquid metal. If the reactor overheats, thermal expansion of the metallic fuel and cladding causes more neutrons to escape the core, and the nuclear chain reaction can no longer be sustained. The large mass of liquid metal also acts as a heatsink capable of absorbing the decay heat from the core, even if the normal cooling systems would fail.
The greatest departure from the old 'second-generation' designs is that many incorporate passive or inherent safety features which require no active controls or (human) operational intervention to avoid accidents in the event of malfunction, and may rely on pressure differentials, gravity, natural convection, or the natural response of materials to high temperatures.
Examples of reactors using passive safety features
The General Electric Company ESBWR (Economic Simplified Boiling Water Reactor, a BWR) is a passively-safe design. In the event of coolant loss, no operator action is required for three days.
The Westinghouse Electric Company AP-1000 ("AP" standing for "Advanced Passive") is a passively-safe design. In the event of an accident, no operator action is required for 72 hours.
The Integral Fast Reactor was a Fast breeder reactor run by the Argonne National Laboratory. It was a Sodium cooled reactor capable of withstanding a loss of flow without SCRAM and loss of heatsink without SCRAM. This was demonstrated throughout a series of safety tests in which the reactor successfully shut down without operator intervention. The project was canceled due to proliferation concerns before it could be copied elsewhere.