Void coefficient
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In nuclear engineering, the void coefficient (more properly called "void coefficient of reactivity") is a number that can be used to estimate how much the thermal output of a nuclear reactor increases (or decreases, if negative) as voids (steam bubbles) form in the reactor moderator or coolant. Reactors in which either the moderator or the coolant is a liquid typically will have a void coefficient value that is either negative (if the reactor is under-moderated) or positive (if the reactor is over-moderated). Reactors in which neither the moderator nor the coolant is a liquid (e.g., a graphite-moderated, gas-cooled reactor) will have a void coefficient value equal to zero.
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[edit] Explanation
Nuclear fission reactors run on nuclear chain reactions, in which each nucleus that undergoes fission releases heat and neutrons. These neutrons may impact another nucleus and cause it to undergo fission. The velocity of this neutron affects its probability of causing additional fission, as does the presence of neutron-absorbing material. In particular, slow neutrons are more easily absorbed by fissile nuclei than fast neutrons, so a neutron moderator which slows neutrons will increase the thermal output of a nuclear reactor. A neutron absorber will decrease the thermal output of a nuclear reactor. These two mechanisms are used to control the power output of a nuclear reactor.
In order to keep a nuclear reactor intact and functioning, and to extract useful power from it, a cooling system must be used. Some reactors circulate pressurized water, some use liquid metal, such as sodium, NaK, lead, or mercury; others use gases (see advanced gas-cooled reactor). If the coolant is a liquid, it may boil if the temperature inside the reactor rises. This boiling leads to voids inside the reactor. Voids may also form if coolant is lost from the reactor in some sort of accident (called a loss of coolant accident, which has other dangers). Some reactors operate with the coolant in a constant state of boiling, using the generated vapor to turn turbines.
The coolant liquid may act as a neutron absorber or as a neutron moderator. In either case, the amount of void inside the reactor can affect the thermal power output of the reactor. The change in thermal power output caused by a change of voids inside the reactor is directly proportional to the void coefficient.
A positive void coefficient means that the thermal power output increases as the void content inside the reactor increases due to increased boiling or loss of liquid moderator or coolant. If the void coefficient is large enough and control systems do not respond quickly enough, this can form a positive feedback loop which can quickly boil all the coolant in the reactor. This happened in the Chernobyl accident.
A negative void coefficient means that the thermal power output decreases as the void content inside the reactor increases - but it also means that the thermal power output increases if the void content inside the reactor is reduced. In boiling-water reactors with large negative void coefficients, a sudden pressure rise (caused, for example, by unplanned closure of a steamline valve) will result in a sudden decrease in void content: the increased pressure will cause some of the steam bubbles to condense ("collapse"); and the thermal output will increase until it is terminated by safety systems, by increased void formation due to the higher power, or, possibly, by system or component failures that relieve pressure, causing void content to increase and power to decrease. On the other hand, if a reactor is designed to operate with no voids at all, a large negative void coefficient may serve as a safety system. A loss of coolant in such a reactor decreases the thermal output, but of course heat that is generated is no longer removed, so the temperature can rise dangerously.
Thus a large void coefficient of either sign can be dangerous and may require more careful, faster-acting control systems. Many reactors are designed to have as small a void coefficient as possible. Gas-cooled reactors do not have issues with voids forming, but can have more severe problems of other sorts.
[edit] Reactor designs
- Boiling water reactors generally have a negative void coefficients, and in normal operation the negative void coefficient allows reactor power to be adjusted by changing the rate of water flow through the core. However, the negative void coefficient can cause an unplanned reactor power increase in events (such as sudden closure of a steamline valve) where the reactor pressure is suddenly increased. In addition, the negative void coefficient can result in power oscillations in the event of a sudden reduction in core flow, such as might be cause by a recirculation pump failure. Boiling water reactors are designed to ensure that the rate of pressure rise from a sudden steamline valve closure is limited to acceptable values, and they include multiple safety systems designed to ensure that any sudden reactor power increases or unstable power oscillations are terminated before fuel or piping damage can occur.
- Pressurized water reactors operate with no voids at all, and the water serves as both moderator and coolant. Thus a large negative void coefficient ensures that if the water boils or is lost the power output will drop.
- CANDU reactors have positive void coefficients that are small enough that the control systems can easily respond to boiling coolant before the reactor reaches dangerous temperatures.
- RBMK reactors, such as the reactors at Chernobyl, have a dangerously high positive void coefficient. This was necessary for the reactor to run on unenriched uranium and to require no heavy water.
- 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.
- Magnox reactors, advanced gas-cooled reactors and pebble bed reactors are gas-cooled and so void coefficients are not an issue. In fact, some can be designed so that total loss of coolant does not cause core meltdown even in the absence of active control systems. As with any reactor design, loss of coolant is only one of many possible failures that could potentially lead to an accident.
[edit] See also
- Chernobyl accident - occurred when an RBMK reactor overheated; its large positive void coefficient is thought to have been a factor.
- Neutron moderator
[edit] References
- Chernobyl - A Canadian Perspective - A brochure describing nuclear reactors in general and the RBMK design in particular, focusing on the safety differences between them and CANDU reactors. Published by the CANDU organization.
