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RWM
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Why is the void coefficient in enriched US reactors negative?
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Core and reload designs are intentionally made with negative reactivity, although I have seen some preliminary designs with positive reactivity such the reload fuel (lattices) had to be redesigned until negative reactivity was achieved.RWM said:Why is the void coefficient in enriched US reactors negative?
Criterion 28—Reactivity limits. The reactivity control systems shall be designed with appropriate limits on the potential amount and rate of reactivity increase to assure that the effects of postulated reactivity accidents can neither (1) result in damage to the reactor coolant pressure boundary greater than limited local yielding nor (2) sufficiently disturb the core, its support structures or other reactor pressure vessel internals to impair significantly the capability to cool the core. These postulated reactivity accidents shall include consideration of rod ejection (unless prevented by positive means), rod dropout, steam line rupture, changes in reactor coolant temperature and pressure, and cold water addition.
Criterion 29—Protection against anticipated operational occurrences. The protection and reactivity control systems shall be designed to assure an extremely high probability of accomplishing their safety functions in the event of anticipated operational occurrences.
The void coefficient is a measure of how the reactivity of a nuclear reactor changes when there is a change in the amount of steam or voids (empty space) in the reactor. It is expressed as a fraction of a percent change in reactivity per percent change in void fraction.
The void coefficient is negative in US reactors because they use a type of reactor called a pressurized water reactor (PWR). In a PWR, water is used as both the coolant and the moderator. When the water turns to steam (voids), it absorbs fewer neutrons and therefore decreases the reactivity of the reactor. This negative void coefficient is a safety feature that helps prevent the reactor from becoming too reactive and potentially causing a meltdown.
The negative void coefficient in US reactors is beneficial for safety because it makes the reactor less likely to have a rapid increase in reactivity. This means that even if there is a loss of coolant or other accident, the reactor will automatically shut down or slow down, preventing a catastrophic meltdown.
While the negative void coefficient is a safety feature, it can also make the reactor less efficient. This is because the voids in the reactor reduce the number of neutrons available for fission, which decreases the power output. Therefore, reactor operators must carefully manage the amount of voids in the reactor to balance safety and efficiency.
No, the void coefficient can vary depending on the type of reactor. As mentioned previously, pressurized water reactors have a negative void coefficient, while other types of reactors, such as boiling water reactors, may have a positive or near-zero void coefficient. It is important for each type of reactor to be designed with the appropriate void coefficient to ensure safe and efficient operation.