Void Coefficient: Why Is It Negative in US Reactors?

[RWM]

Why is the void coefficient in enriched US reactors negative?

 

[anorlunda]

Welcome to PF.

You should start here. https://en.wikipedia.org/wiki/Void_coefficient

Reactivity coefficients vary substantially in different reactor designs. Are you asking about boiling water reactors? Are you asking about both BWR and PWR as distinct from the Canadian CANDU?

 

[RWM]

Void coefficients with respect to pressurized light water reactor designs utilizing enriched uranium fuels typical to the majority of USA reactors.

 

[etudiant]

Iirc, the late Edward Teller was the driver behind the US requirement for negative void coefficients.
The Chernobyl experience suggests this is a sensible requirement.

 

[Hiddencamper]

It’s an interesting way to ask the question. The void coefficient is negative because light water reactors are undermoderated during normal operating conditions, meaning a reduction in moderator density will result in a loss of necessary moderation and a negative impact on reactivity.

 

[Astronuc]

Why is the void coefficient in enriched US reactors negative?

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.

Reactivity control is a mandated requirement. See Code of Federal Regulations, Title 10, Part 50, Appendix A, Group III, GDCs 26-29
https://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-appa.html

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.

A positive moderator void coefficient would allow for a power increase in the event that a decrease in moderator density occurred, e.g., if nuclear boiling began in a single phase system such as a PWR, or vigorous boiling or dryout began in a BWR. An increase in local heat flux/transport would increase the voiding, and thus increase the local power through positive feedback. Such an event is precluded by designing a lattice and core with negative void coefficient. So a negative void coefficient assures control of power and power distribution in a reactor core.

With respect to a core/reload design, we necessarily must assure that during any overpower transient that limits on fuel temperature, e.g., fuel centerline melt, cladding strain (e.g., 1% cladding diametral or circumferential strain) or no cladding-liftoff are assured with some statistical confidence. These requirements relate to the integrity of the fuel and primary coolant pressure boundary under various anticipated operational occurrences (AOOs) and certain transient or postulated accident events. AOOs are expected to happen, as often as once per operating cycle, and we wish to assure that an AOO does not evolve into a worse problem.

 

[anorlunda]

Even in heavy water reactors, reactivity coefficients are critically important.

https://en.wikipedia.org/wiki/R4_nuclear_reactor

That heavy-boiling-water reactor project was abandoned mid-project, because improved calculations showed that it had a positive void coefficient which adversely affect stability.