I have a question about nuclear weapons. Anyone here know much about them?

[PamW]

103. Over-moderated reactors

I have an analogy that may help you understand about over-moderated reactors.

Have you ever adjusted the fuel mixture set screw on a small gasoline engine like on a lawn mower?

If you turn the screw to lean out the mixture too much, the engine sputters and doesn't run well.

If you turn the screw to make the mixture too rich, the engine also sputters and doesn't run well.

If you get the screw positioned to get the optimal fuel / air mixture - the engine roars to life.

The over-moderated reactor is like the engine with the screw set too rich. Even if you inject more fuel into the chamber, you don't get more power. The engine is already "flooded" with fuel and you are giving it more. The engine in that condition doesn't need more fuel, it needs more oxygen.

The over-moderated reactor is like that. It already has all the moderator it needs; and shoving graphite control rod followers into the core isn't going to increase power.

In order to trigger the runaway due to the Xenon instability; we need something that increases the power, and hence increases the reactivity. Since the reactor is over-moderated, we get more reactivity by doing something that decreases moderator, like backing off on the set screw to lean the mix.

The temperature excursion caused by the cutoff of coolant pump power gives us the necessary trigger. Less cooling means temperatures go up; which means moderator expands with temperature, and hence we get a decrease in moderation because our moderator is less dense because it got hotter.

Dr. Till is correct about Chernobyl. In fact, he was the leader of the Argonne National Laboratory team that did the analysis.

PamW