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

[PamW]

102. Let's go through the reactor physics..

Let's go through the reactor physics.

You are correct that the Xenon is a poison that is inhibiting criticality. However, the Xenon can be burned away leading to increased reactivity. Therefore, the Xenon inhibits until some other mechanism increases the neutron flux to burn away some Xenon, and off we go...

How do we get something to increase the neutron flux, which means increasing power. There are control rods with absorber and graphite tips. Graphite tips would normally increase reactivity due to increased moderation. However, the Soviet RBMK is already over-moderated. It has too much moderator for the fuel it has already. This in itself leads to a source of instability. So inserting graphite doesn't really do much to increase power and certainly inserting the control poison part of the rods reduces power. So where do we get a mechanism to increase power and thus increase flux to burn away some Xenon.

The RBMK is over-moderated. There is an optimal amount of moderator for a given reactor design. If you plot reactivity vs amount of moderator, you get a hump-shaped curve. Now in the USA, we require that all reactors be "under-moderated". That is the amount of moderator is to the left of the optimal; i.e. the amount of moderator is less than optimal. This makes the reactor stable. If you increase power, you increase temperature, and your moderator material expands, and you lose some moderating capability. So if you go to the hump curve, and your initial point is on the up-slope to the left of the peak, the lowering of moderator capability moves the point to the left, and hence downward to stay on the curve, and your reactivity goes down. That's what you want to be stable; an increase in power lowers the reactivity and moves the reactor back to its original power. It's negative feedback and that is stable.

If you instead have a reactor that is over-moderated, the point is on the right of the peak on the down-slope. This means you have too much moderator to be optimal. When the reactor power goes up, then the reactor gets hotter, and moderator expands and you lose moderating capability due to a less dense moderator; the point moves to the left again. However, in order to stay on the curve, the reactivity increases. That increases the power, which leads to hotter moderator, even less dense moderator, and it moves the point further left which increase reactivity.....and you have a vicious cycle. If the reactor is over-moderated, you have a positive feedback system, and that is unstable.

Our computer modeling showed that it was the temperature feedback due to the over-moderated instability that was the trigger; and NOT the control rod scram. The graphite tip insertion just doesn't have enough reactivity effect in an over-moderated reactor to give you enough of a trigger. But the reactivity insertion due to the temperature increase sure does.

So the sequence is the following. The operators turn off steam to the coolant pump turbines. That leads to increased temperatures. Due to the over-moderated condition, we have a positive temperature feedback so that the increased temperature increases reactivity, which increases power.

That increased power is what depletes Xenon, which leads to higher reactivity, depleting more Xenon, and off we go....

The timing of the scram and movement of the control rods was incidental. The reactivity excursion was well underway when the scram happened. Scram or no scram; the reactor was running away due to the temperature excursion.

PamW