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Void coefficient
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== Reactor designs == * [[Boiling water reactor]]s generally have 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. The negative void coefficient can cause an unplanned reactor power increase in events (such as sudden closure of a streamline 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 caused by a recirculation pump failure. Boiling water reactors are designed to ensure that the rate of pressure rise from a sudden streamline 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 reactor]]s operate with a relatively small amount of voids, 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 (see References). Furthermore, a [[loss of coolant accident]] automatically [[scram]]s the reactor and unlike in [[light water reactor]]s, the introduction of "regular" water to the reactor core—for example as an emergency coolant—does not pose the risk of [[Criticality accident|criticality]] as a CANDU can only reach criticality in the absence of the neutron absorption that is present in significant quantities of light water. * [[RBMK]] reactors, such as the reactors at Chernobyl, had a dangerously high positive void coefficient. This allowed the reactor to run on unenriched [[uranium]] and to require no [[heavy water]], saving costs; RBMKs were also capable of producing [[weapons-grade plutonium]], unlike the other main Soviet design, the [[VVER]].<ref name="Prelas Peck 2016">{{cite book | last1=Prelas | first1=Mark A. | last2=Peck | first2=Michael | title=Nonproliferation Issues For Weapons of Mass Destruction | page=89 | date=2016-04-07 | publisher=CRC Press | isbn=9781420028652 |url=https://books.google.com/books?id=xwrOBQAAQBAJ&pg=PA89 | access-date=2016-04-20}}</ref> Before the Chernobyl accident these reactors had a positive void coefficient of 4.7 [[Delayed neutron|beta]], which after the accident was lowered to 0.7 beta so they could safely remain in service. * [[Fast breeder reactor]]s do not use moderators, since they run on [[fast neutron]]s, but the coolant (often [[lead]] or [[sodium]]) may serve as a neutron absorber and reflector. For this reason they have a positive void coefficient. * [[Magnox]] reactors, [[advanced gas-cooled reactor]]s and [[pebble bed reactor]]s 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. In case of accidental ingress of liquid water into the core of pebble bed reactors, a positive void coefficient may occur.{{citation needed|date=November 2021}} [[Magnox]] and [[UNGG reactor|UNGG]] reactors were designed with the dual purpose of producing [[electrical power]] and weapon grade plutonium. * The [[Advanced CANDU reactor]], a never built proposed reactor type based on the CANDU, promises a negative void coefficient but it must use slightly enriched uranium as a fuel and cannot operate with [[natural uranium]] as the "regular" CANDU does. * In a [[molten salt reactor]] the salt is usually neither a strong moderator nor a neutron poison. If a [[thermal neutron]] spectrum is used, external moderators such as [[nuclear graphite]] are usually employed. Volatile fission products can "bubble out" of solution and as the fuel is dissolved in the salt, this decreases reactivity at and around the site of the bubble. Furthermore, most of the fission product noble gases—chief among them [[Xenon-135]] are strong neutron poisons. As the boiling point of the involved salts is relatively high (at a point at which the structural integrity of the housing for the molten salt would be in question), there is usually little to no emphasis put on the consequences of it boiling off. Frequently molten salt reactors employ a melt plug that melts at much lower temperatures than the boiling point of the salts and allows them to solidify in a [[core catcher]].
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