Void Coefficient

Void coefficient The term void coefficient refers to a ratio that describes how a moderator in a reactor can become less dense. In an extreme case, a reactor can lose all its water coolant (recall that water also acts as a moderator in the reactors in normal use in the United States). In this case, fewer neutrons will be “thermalized,” or slowed down, and so fewer reactions will result, causing fewer neutrons—and the reactor would stop if it didn’t melt first from all the thermal energy around, causing many other problems. In less drastic cases, the voids are less common, and the reactor would “slow down” a bit if voids occurred. Usually, the voids would be due to localized boiling (bubble formation) in the pipes. Also, water expands when heated, and this also would cause the rate of neutron production to drop. To understand what this means, consider a typical American BWR reactor. These reactors are moderated by water. What happens to the reactor when the temperature rises? The water either decreases in density or boils, reducing the amount of moderator available for fast neutrons from fissions and thereby reducing the number of thermal (slow) neutrons and making the number of fissions decrease. This makes the reactor slow, lowering the temperature. There is an increase in reactions of fast neutrons with 238U nuclei also as the temperature increases, decreasing the number of neutrons sleeting around the reactor, so reducing the number of neutrons available for fission. Both these effects tend to turn the reactor down. In PWRs the water’s thermal expansion reduces the reaction rate, just as in the BWR.(64) Because of the high pressure, however, the water in the core does not boil. Energy, Ch. 20, extension 5 Void coefficient 2 As you see, the increase of temperature in either typical reactor is self-regulating. When voids (bubbles) form or the water itself expands to become less dense, the reactor decreases thermal energy production. It is said to have a negative void coefficient. The Chernobyl reactor suffered from a positive void coefficient. The CANDU reactor also has a positive void coefficient. The RBMK reactors are moderated by graphite, but cooled by water that flows over fuel rods. There is not enough water to significantly moderate the reactor. The water does act as a neutron “poison,” or neutron absorber, because neutrons form deuterium out of the hydrogen in the water molecule 1 2 n + 1H ® 1H. When the temperature rises, the water in the core boils. This reduces poisoning, so more neutrons are available to make more fissions. In addition, the graphite expands, and this effectively increases the cross section for thermalizing neutrons, also causing more fissions. The RBMK reactor has a positive void coefficient, and so is not stable with respect to fluctuations in temperature. An increase in temperature sets up conditions leading to an even greater temperature rise. The reactor must be controlled very carefully by computers to prevent a runaway even under normal operation. This makes control much more dicey than in American-style reactors, which respond to a temperature excursion by doing things that tend to undo the change (as described above). Bodansky (Ref. 64, Sec. 12.3.6) has discussed the contribution of the void coefficient in the RBMK reactor such as is involved in the Chernobyl accident in some technical detail. While the CANDU reactor also has a positive void coefficient, it is not very important given the reactor design separating the coolant from the moderator. It has no practical effect on a CANDU reactor’s operations..

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