1 Basic Concepts and Terminology

Disclaimer: I am not an expert. These are notes I made while trying to figure stuff out. Work in progress.

• You can have fast neutrons and you can have prompt neutrons. But fast does not imply prompt, and prompt does not imply fast.
• The opposite of fast neutrons is thermal neutrons (not slow neutrons).
• The opposite of prompt neutrons is delayed neutrons.
• Fast reactors, including fast breeder reactors, do not breed quickly or do anything else quickly. They make use of fast neutrons.
• A moderator is how you slow down the fast neutrons and turn them into thermal neutrons. So thermal neutrons could be called moderated neutrons.

There are at least 5 cases to consider:

• The alternative to a boiling-water reactor (BWR) is a pressurized-water reactor (PWR). But a boiling-water reactor is pressurized, too.
• A weapon is intended to go supercritical using only the fast prompt neutrons.
• In “fast” reactors, which are rather unusual, 93% of the reactivity comes from fast prompt neutrons. Things are more spread out, compared to a weapon, so the neutron takes longer to find something to react with.
• In the ordinary thermal-neutron reactors, 93% of the reactivity comes from moderated prompt neutrons. The timescale is longer, sbecause the thermal neutrons move much more slowly.
• In both kinds of reactors, the remaining 7% of the reactivity comes from delayed neutrons. The fast neutrons are delayed just as much as the slow neutrons, to an excellent approximation.

Delayed neutrons are super-important, because without them the typical(*) reactor would be unmanageable; the reaction rate would be increasing exponentially or decreasing exponentially, much too quickly. Anything that happens on a “prompt” timescale is much quick to be manageable using control rods.

Therefore a reactor is designed to be operated at the point where it is just barely delayed critical. It must be constantly adjusted to keep it there.

In a reactor, increasing the reactivity by 7% makes the thing go prompt critical, which is usually(*) a Bad Thing.

(*) An exception is a TRIGA reactor. It is cleverly designed so that if it goes prompt critical, the moderator very quickly becomes hot enough to be ineffective but not hot enough to cause a meltdown. We say the moderator creates a strongly negative temperature coefficient of reactivity.

The RBMK-1000 at Chernobyl was uncleverly designed. Among its many design faults, it was overmoderated by massive blocks of graphite, so there was nothing to limit the runaway chain reaction until “disassembly” occurred. (That’s a euphemism for “explosion”.) For a list of prompt critical accidents, see reference 1.

A fast-neutron reactor is scarier than a moderated-neutron reactor. That’s because the lack of a moderator makes it harder to create a strongly negative temperature coefficient of reactivity.

2 Other

A normal BWR has a negative void coefficient of reactivity. Because the RBMK-1000 at Chernobyl was overmoderated, it to only a small extent affected by the loss of moderation from the water, but was to a greater extent affected by the loss of absorption. It had a positive void coefficient of reactivity. Terrible design.

What’s worse, it had absorber that could leave (water) and moderator that couldn’t leave (graphite).

Also, the water was the coolant. So a loss of water resulted in loss of cooling as well as increased reactivity.

You could guarantee a negative void coefficient of reactivity by using heavy water. Comparable moderation, less absorption. Loss of coolant would still be a horror show, probably resulting in a meltdown, but probably not a huge explosion.

Xenon poisoning. Before pulling out the control rods to make up for xenon poisoning, ask what will happen when the xenon goes away. If that would take you outside the 7% margin between delayed critical and prompt critical, pulling the rods creates a time bomb. When the xenon goes away, the reactivity will increase, possibly faster than you can respond by re-inserting the control rods.

The xenon burn-out will happen non-uniformly in the enormous loosely-coupled core.

Chain of causation. But so many faults that fixing one of them might not have been sufficient.

190 metric tons of fuel in the RBMK-1000.

Low-enriched uranium. About 2% enrichment.

There are currently 94 nuclear power plants licensed to operate in the United States (63 PWRs and 31 BWRs). They generate about 20nation’s electrical use.

When designing a weapon, a great deal of effort goes into keeping all the fuel together in one place long enough for the reaction to go to completion, more or less, before the thing blows itself apart.

On the other hand, if a reactor has gone prompt critical and is about to blow up, you’d rather have disassembly occur sooner rather than later. It sounds weird to say it, but it’s true: You’d prefer a meltdown rather than an explosion, and you’d prefer a small explosion sooner rather than a humongous explosion later.

Reactor loosely coupled. A small part went prompt supercritical. Neutrons are diffusing into the rest of the core at the speed of sound. A shock moving at supersonic speed is disassembling the core.

Nobody knows exactly what happened, but this scenario fits the facts as I know them, and it’s hard to come up with a better explanation.

3 References

1.

Wikipedia article, “List of accidental prompt critical excursions”
https://en.wikipedia.org/wiki/Prompt_criticality#List_of_accidental_prompt_critical_excursions