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. Nuclear engineering is the practical application of the atomic nucleus gleaned from principles of nuclear physics and the interaction between radiation and matter. ... Core of a nuclear reactor A nuclear power station. ... In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction. ... A coolant, or heat transfer fluid, is a fluid which flows through a device in order to prevent its overheating, transferring the heat produced by the device to other devices that utilize or dissipate it. ...

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. An induced nuclear fission event. ... A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions. ... Properties In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939. ... Nucleus usually refers to the center of something, but can mean: In science: Atomic nucleus, the collection of protons and neutrons in the center of an atom that carries the bulk of the atoms mass and positive charge Cell nucleus, the membrane-bound subcellular organelle found in eukaryotes, visible... A thermal neutron is a free neutron with a kinetic energy level of ca. ... A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (10 TJ/kg, hence a speed of 14,000 km/s. ... In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction. ...

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. Hot metal work from a blacksmith Look up Metal in Wiktionary, the free dictionary In chemistry, a metal (Greek: Metallon) is an element that readily forms ions (cations) and has metallic bonds, and metals are sometimes described as a lattice of positive ions (cations) surrounded by a sea of delocolised... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... NaK is an alloy of sodium and potassium, and particularly one that is liquid at room temperatures. ... This article is about the chemical element. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery white Atomic mass 200. ... Schematic diagram of the Advanced Gas-cooled Reactor. ... A Loss of Coolant Accident (LOCA) is a mode of failure for a nuclear reactor; in a nuclear reactor, the results of a LOCA could be catastrophic to the reactor, the facility that houses it, and the immediate vicinity around the reactor. ...

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. Positive feedback is a type of feedback. ... The nuclear power plant at Chernobyl prior to the completion of the sarcophagus. ...

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.

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.

A boiling water reactor (BWR) is a light water reactor design used in some nuclear power stations. ... A pressurised water reactor (PWR) is a type of nuclear power reactor that uses ordinary light water for both coolant and for neutron moderation. ... The CANDU reactor is a pressurized-heavy water, natural-uranium power reactor designed in the 1960s by a partnership between Atomic Energy of Canada Limited and the Hydro-Electric Power Commission of Ontario as well as several private industry participants. ... RBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy which means reactor (of) large power (of the) channel (type), and describes a now obsolete class of nuclear power reactor which was built only in the Soviet Union. ... General Name, Symbol, Number uranium, U, 92 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Atomic mass 238. ... Heavy water is dideuterium oxide, or D2O or 2H2O. It is chemically the same as normal water, H2O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of any hydrogen atom. ... // The fast breeder or fast breeder reactor (FBR) is a fast neutron reactor designed to breed fuel by producing more fissile material than it consumes. ... A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (10 TJ/kg, hence a speed of 14,000 km/s. ... This article is about the chemical element. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... Magnox is an obsolesent type of nuclear power reactor; When operated on a short fuel cycle (which is uneconomic) they can also produce plutonium for nuclear weapons. ... Schematic diagram of the Advanced Gas-cooled Reactor. ... The pebble bed reactor (PBR) or pebble bed modular reactor (PBMR) is an advanced nuclear reactor design. ...

See also

• Chernobyl accident - occurred when an RBMK reactor overheated; its large positive void coefficient is thought to have been a factor.
• Neutron moderator

The nuclear power plant at Chernobyl prior to the completion of the sarcophagus. ... In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction. ...

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.