Passively safe

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Passively safe is a form of nuclear reactor that does not require operator action in order to shut down safely in the event of a loss-of-coolant accident (LOCA) or other emergency. Some such reactors use the laws of physics to keep the nuclear reaction under control rather than engineered safety systems.

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. It is widely believed that such reactors are the future of the industry, since their modularity allows for economies of scale via mass production while the passive-safety aspects address public safety concerns.

The General Electric Company ESBWR (Economic Simplified Boiling Water Reactor, a BWR) is also a passively-safe design. In the event of a LOCA, no operator action is required for three days.

The Westinghouse Electric Company AP-1000 ("AP" standing for "Advanced Passive") is close to a passively-safe design but in fact due to the complexity of the design some operator actions are required for safe shutdown.

Example

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. Due to the void coefficient of reactivity, these steam voids moderate fewer neutrons, causing the power level inside the reactor to lower (this is the opposite of what happened at Chernobyl).

Passively safe

Example

See also