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Before the Chernobyl accident these reactors had a positive void coefficient of 4.7 beta, which after the accident was lowered to 0.7 beta so they could safely remain in service. Fast breeder reactors do not use moderators, since they run on fast neutrons, but the coolant (often lead or sodium) may serve as a neutron absorber and reflector. For ...
Certain aspects of the original RBMK reactor design had several shortcomings, [3] such as the large positive void coefficient, the 'positive scram effect' of the control rods [4] and instability at low power levels—which contributed to the 1986 Chernobyl disaster, in which an RBMK experienced an uncontrolled nuclear chain reaction, leading to ...
Lead has a positive void coefficient, or void reactivity, meaning that as voids occur in the circulating coolant an increase in fertile fission and a decrease in the capture rate of all heavy nuclides results in reactivity increases as the void content inside the reactor increases, potentially leading to a positive feedback loop unless ...
The reactor had a dangerously large positive void coefficient of reactivity. The void coefficient is a measurement of how a reactor responds to increased steam formation in the water coolant. Most other reactor designs have a negative coefficient, i.e. the nuclear reaction rate slows when steam bubbles form in the coolant, since as the steam ...
These so-called delayed neutrons increase the effective average lifetime of neutrons in the core, to nearly 0.1 seconds, so that a core with of 0.01 would increase in one second by only a factor of (1 + 0.01) 10, or about 1.1: a 10% increase. This is a controllable rate of change.
Passive nuclear safety is a design approach for safety features, implemented in a nuclear reactor, that does not require any active intervention on the part of the operator or electrical/electronic feedback in order to bring the reactor to a safe shutdown state, in the event of a particular type of emergency (usually overheating resulting from a loss of coolant or loss of coolant flow).
It must be composed of elements with low neutron capture cross sections to prevent positive void coefficient and induced radioactivity. The use of gas also removes the possibility of phase transition –induced explosions, such as when the water in a water-cooled reactor ( PWR or BWR ) flashes to steam upon overheating or depressurization.
The use of heavy water as the moderator is the key to the PHWR (pressurized heavy water reactor) system, enabling the use of natural uranium as the fuel (in the form of ceramic UO 2), which means that it can be operated without expensive uranium enrichment facilities.