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A positive void coefficient means that the reactivity increases as the void content inside the reactor increases due to increased boiling or loss of coolant; for example, if the coolant acts predominantly as neutron absorber. This positive void coefficient causes a positive feedback loop, starting with the first occurrence of steam bubbles ...
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 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.
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 .
4 on site; 1 involved in accident: 6 on site; 4 (and spent fuel pools) involved in accident; one of the four reactors was empty of fuel at the time of the accident. Amount of nuclear fuel in affected reactors: 1 reactor—190 tonnes (t, metric tons = 210 U.S. short tons): spent fuel pools not involved in incident [4]
The graphite moderated Soviet RBMK design tried to avoid using either enriched uranium or heavy water (being cooled with ordinary water instead) which produced the positive void coefficient that was one of a series of flaws in reactor design leading to the Chernobyl disaster. Most modern reactors use enriched uranium with ordinary water as the ...
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.