Can a nuke power plant just blow up?

[Morbius]

The Shutoff rods were also used as Control Rods.

Homer,

That's true in an US-designed reactors as well.

Since these rods were cooled by light water in a tube, the boron rod (a good absorber) would displace light water (also a good absorber)

Light water is NOT a good absorber relative to Boron.

so the difference in reactivity would not be that great. The graphite was called a 'displacer'. It made the effect of rod movement greater, because as the boron rod moves down it displaces graphite, not light water.

Only to the extent that the graphite ADDS reactivity - that the subsequent Boron
counteracts. That is relative to an LWR, this increase in control rod effect merely
serves to cancel the positive contribution of the graphite.

In the fully inserted position, the graphite section was out of core on the bottom. When the rods were fully out of core the control rod tubes were part graphite at the top and light water at the bottom.

During the accident, when the rods began to move in the bottom of the core became very supercritical.

Regardless of the above; the RBMK control system requires that when an
emergency shutdown is needed - the rods have to ADD reactivity before they can
DECREASE reactivity.

All in all - it's a pretty dumb thing to do from a safety standpoint - as they found out
the hard way.

Dr. Gregory Greenman
Physicist

 

[Homer Simpson]

Light water is NOT a good absorber relative to Boron.

Relative to graphite it is.

Only to the extent that the graphite ADDS reactivity - that the subsequent Boron counteracts. That is relative to an LWR, this increase in control rod effect merely serves to cancel the positive contribution of the graphite.

There is not an equal amount of graphite entering the core as there is boron. The graphite exits towards the lower core, replaced by boron.

 

[Morbius]

There is not an equal amount of graphite entering the core as there is boron. The graphite exits towards the lower core, replaced by boron.

Either way - it's still a monumentally DUMB idea to have graphite followers
on the control rods.

Dr. Gregory Greenman
Physicist

 

[jdsneeder]

NUREG-1250 has detailed description of the Chernobyl design and accident if you can find a copy (didn't see it on their wed site)

When the rods were fully withdrawn, the bottom and top meter of the rod were water and the middle 5m was graphite. So when the rods were inserted, the bottom meter of water was replaced by the graphite as Homer described.