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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 ...
The total water loss combined with a high positive void coefficient further increased the reactor's thermal power. [ 21 ] A second, more powerful explosion occurred about two or three seconds after the first; this explosion dispersed the damaged core and effectively terminated the nuclear chain reaction .
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.
Control rods used to be tipped with graphite, a material that slows neutrons and thus speeds up the chain reaction. Water is used as a coolant, but not a moderator. If the water boils away, cooling is lost, but moderation continues. This is termed a positive void coefficient of reactivity. The RBMK tends towards dangerous power fluctuations.
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).
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 ...