What is a positive void coefficient in a nuclear reactor?

What is a positive void coefficient in a nuclear reactor?

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.

What is a negative void coefficient?

A negative void coefficient means that the reactivity decreases as the void content inside the reactor increases – but it also means that the reactivity increases if the void content inside the reactor is reduced. Boiling water reactors are all designed (and required) to handle this type of transient.

Do nuclear reactors depend on negative feedback?

In practice, control of a nuclear reactor is enhanced by several negative feedback mechanisms relying on physical phenomena. A raise of the fuel temperature increases the neutron absorption rate.

What is a void in a nuclear reactor?

In a nuclear power reactor, an area of lower density in a moderating system (such as steam bubbles in water) that allows more neutron leakage than does the more dense material around it.

How does the void coefficient work?

When the void coefficient is negative, an increase in steam will lead to a decrease in reactivity. In those reactors where the same water circuit acts as both moderator and coolant, excess steam generation reduces the slowing of neutrons necessary to sustain the nuclear chain reaction.

What is negative reactor?

If the temperature of the moderator is increased, negative reactivity is added to the core. This negative reactivity causes reactor power to decrease. As the thermal power decreases, the power coefficient acts against this decrease, and the reactor returns to the critical condition.

Why does Candu have positive void coefficient?

3 Why do CANDU reactors have a “positive void coefficient”? In any type of reactor the void coefficient refers to the reactivity feedback (the increase or decrease of the fission rate, and thus the rate of heat generation) that occurs if coolant in the core turns to steam or otherwise disappears.

Is nuclear fission a positive feedback loop?

Nuclear bombs result from a positive feedback reaction. To trigger the blast, a non-nuclear explosion is used to produce intense energy next to radioactive material inside the bomb, breaking the atoms apart (atomic fission).

Which situation is an example of positive feedback in a nuclear reaction?

Fire, explosions, even nuclear reactions (both in reactors and in weapons) are all positive feedback driven chain reactions. A single match can start a forest fire. The melting of the polar icecaps is driven by an accelerating positive feedback loop.

Do Chernobyl reactors still work?

Both the zone and the former power plant are administered by the State Agency of Ukraine on Exclusion Zone Management. The three other reactors remained operational after the accident but were eventually shut down by 2000, although the plant remains in the process of decommissioning as of 2021.

Is a nuclear explosion positive or negative feedback?

How does nuclear energy work?

Nuclear Power Nuclear energy originates from the splitting of uranium atoms – a process called fission. This generates heat to produce steam, which is used by a turbine generator to generate electricity. Because nuclear power plants do not burn fuel, they do not produce greenhouse gas emissions.

What is the void coefficient in nuclear reactors?

The void coefficient measures how the reactor’s reactivity changes as voids are introduced in the moderator or coolant of a reactor. One way these voids can be introduced is by steam bubbles forming in water that is used as a coolant or moderator.

Are CANDU reactivity and void coefficient of reactivity the same?

Since CANDU reactors and the type of reactor involved in the 1986 Chernobyl incident both have a positive void coefficient of reactivity, some people mistakenly think they also share the same safety risks.

What is NETnet reactivity in a reactor?

Net reactivity in a reactor is the sum total of all these contributions, of which the void coefficient is but one. 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).

How does coolant affect the reactivity of a nuclear 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 reactivity of the reactor. The change in reactivity caused by a change of voids inside the reactor is directly proportional to the void coefficient.