[labawi]

> The control rods slot in between the fuel rods, and absorb the decay products without inducing further nuclear decay. Usually these are graphite rods.

AFAIU graphite rods increase fission by slowing (not capturing) neutrons which in turn have a better chance of propagating further fission, because .. physics.

Quite nifty actually - without the moderator, the fuel wont burn.

[toast0]

I think you're right; I misinterpreted the term 'graphite-moderated reactor' to mean something it doesn't. Graphite will slow the neutrons so they react more. Also, the Chernobyl reactor design has graphite tips on its control rods, which I misremembered as the primary substance of the rod.

The primary substance of the control rods is (usually) a neutron absorber, and most reactors with control rods have a passive safety system, so gravity and springs will force the control rods in to significantly slow the reaction unless actively opposed by the control system.

The Chernobyl rods had graphite ends so that when fully retracted, the reactor output was higher than if there was simply no neutron absorber present; unfortunately, this also meant that going from fully retracted to fully inserted would increase the reactivity in the bottom of the reactor before it reduced it, and in the disaster, this process overheated the bottom of the reactor, damaging the structure and the control rods got stuck, and then really bad things happened.

Long story short, most control rods don't have graphite. ;)