Can a nuke power plant just blow up?

[Homer Simpson]

Have you ever heard of the Graphite tipped absorbers used at Chernobyl?? that seemed a bit of design gaffaw. Upon pulling out the absorbers to far in order to remain critical, they eventually dropped the absorbers to trip the reactor. The thing is that the graphite displaced water which led to yet another positive reactivity insertion (along with the voiding)

 

[Morbius]

Have you ever heard of the Graphite tipped absorbers used at Chernobyl?? that seemed a bit of design gaffaw. Upon pulling out the absorbers to far in order to remain critical, they eventually dropped the absorbers to trip the reactor. The thing is that the graphite displaced water which led to yet another positive reactivity insertion (along with the voiding)

Homer,

Yes - I have heard that.

Remember, the Chernobyl RBMK reactor is NOT SUPPOSED to be run with the rods
completely out for exactly the reason you point out.

By pulling the rods completely out, when the operators called for a SCRAM when
the reactor power spiked, the very first part of the rod's insertion was actually
inserting moderator - not neutron absorber.

So the first several inches of rod insertion is actually a reactivity insertion not a
neutron poison insertion. So with the rods completely out - you have to ADD
MORE reactivity to the reactor before you can DECREASE reactivity.

Yes - that's yet another BAD design aspect of the Soviet RBMK reactor.

It's another thing that I point to when the anti-nukes cite Chernobyl as a reason
for foregoing nuclear power in the USA. They always say, "Look what happened
at Chernobyl - we shouldn't build any more nuclear power plants". The problem is -
nobody is talking about building any more RBMK reactors.

Dr. Gregory Greenman
Physicist

 

[Grogs]

Homer,

Yes - I have heard that.

Remember, the Chernobyl RBMK reactor is NOT SUPPOSED to be run with the rods
completely out for exactly the reason you point out.

By pulling the rods completely out, when the operators called for a SCRAM when
the reactor power spiked, the very first part of the rod's insertion was actually
inserting moderator - not neutron absorber.

So the first several inches of rod insertion is actually a reactivity insertion not a
neutron poison insertion. So with the rods completely out - you have to ADD
MORE reactivity to the reactor before you can DECREASE reactivity.

Yes - that's yet another BAD design aspect of the Soviet RBMK reactor.

Morbius:

I'm curious. It couldn't be an accident they put one of the best moderating materials known to man on the end of the control rods. Do you have any idea what the intended purpose of the tips was? Some sort of oscillation dampening perhaps?

 

[Morbius]

I'm curious. It couldn't be an accident they put one of the best moderating materials known to man on the end of the control rods. Do you have any idea what the intended purpose of the tips was? Some sort of oscillation dampening perhaps?

Grogs,

I don't really know why they did that - perhaps to add reactivity when the rods were out?

When you look at the Soviet designs - they seem to discount the effect of their design
choices on accidents. In other words, they REALLY COUNTED ON not having any
accidents. So if a design aspect made an accident scenario worse - it didn't matter
to them because there weren't going to be any accidents.

That was just the Soviet philosophy - they didn't have accidents. If something went
wrong - it was sabotage! The Soviets were always executing or imprisoning
people after accidents - because they didn't believe in accidents - they had sabotage.

As long as they had loyal operators that wouldn't sabotage the plant - then nothing
would happen - there would be no accidents - so no use planning for them.

That appears to be their philosophy - completely foreign to us.

Dr. Gregory Greenman
Physicist

 

[Homer Simpson]

Do you have any idea what the intended purpose of the tips was?

The Shutoff rods were also used as Control Rods. 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) 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. 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.

 

[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.