Would Chernobyl reactor #4 still have exploded if....

[ElliotSmith]

TL;DR

Would Chernobyl reactor unit 4 still have exploded if the control rods had not been tipped with graphite?

Would Chernobyl reactor #4 still have exploded if the control rods had not been tipped with graphite?

Or would the severity of it just have been mitigated?

 

[.Scott]

I don't think that "tipped" is a good description of what those control rods looked like. The graphite was not at the tip, it made up a considerable length of the control rod. The top portion of the rod is an absorber, the bottom is graphite. You could equally well describe the graphite moderator as being "tipped" with an absorber. But it is best not to use the word "tipped" at all.

The control rod was intended to be in one of two positions - fully extended down into the reactor where the absorber would reduce the reaction or full lifted out of the reactor where the graphite moderator would promote the reaction.

So the net effect is to provide a lot of control for each rod. And so from the point of view of the reactor design, you are asking what would have happened if the Chernobyl operators had less control range for their reactor.

What the operators were trying to do leading up to the accident was to operate the reactor at very low power for hours. So less control would have slowed them down - making the Xenon contamination problem all the worse. And they would not have been able to burn the Xenon off as quickly, so the accident may have been avoided all together.

But, of course, the reactor would have been more difficult to control under normal conditions - which would have made the reactor less useful and potentially could have resulted is some completely different accident.

 

[Drakkith]

As Scott alluded to, the design of the reactor included the graphite 'tips' of the control rods. Without them, the reactor would function differently and they would have had to alter the design to reach their desired power output levels and such. So then it's really about whether this altered design would have suffered the same problems as the original. But since we don't know what this design would be, we can't say much about it.

That being said, if we assume the design was still very similar to the original and suffered the same problems, then I think it's semi-safe to say that no, the reactor wouldn't have exploded because the control rods would have immediately started reducing power in the reactor as they descended, instead of causing a power spike in the bottom which led to the explosion.

 

[.Scott]

In my opinion, the basic cause of the accident was the operators willingness to experiment with the reactor (including bypassing safety systems) without carefully modelling what they were doing. I would describe it as "playing around". I do not believe it is possible to design a reactor that could run safely given the work habits demonstrated at Chernobyl.

The specific "last straw" that happened at Chernobyl occurred as the control rods were being dropped. As they were lowering, the control rod channels at the very bottom of the reactor transitioned from water to graphite - thus increasing the reaction at the very bottom of the reactor. Then, (perhaps because of overheating) the rods stuck in this intermediate position eliminating any hope of controlling that local reaction.

But in order for that to happen, they had to pull all the rods - a violation of their operating rules. And then lower all of them at once as the reaction began to run away, so that all of the rods were transiting this middle position at once.

If lack of graphite "tips" had saved them, they would have returned the next week and tried again.

 

[anorlunda]

It is common to describe accidents as the result of a chain of factors. Break any link in the chain and it would not occur. That makes it not useful to speak of each link in the chain as "the cause."

 

[Dozent100]

These are called "Half Rods" in the usual Nomenclature. Violating operating parameters is never a good thing.

 

[ElliotSmith]

Didn't some of the rods get stuck at some point after being inserted?

 

[.Scott]

Didn't some of the rods get stuck at some point after being inserted?

The problem was that almost all of the rods got stuck - and it was while they were being lowered, not after.

Leading up to the accident, they had run the reactor at low power and had built up a lot of Xenon. The Xenon was poisoning the reactor, so to keep the reactor going, they needed to pull more and more rods - eventually pulling all of them or almost all of them. That was against the rules - they should have just let the reactor die down and try their experiment some other day. What happened next was that the reactor power started rising and the Xenon was rapidly being exhausted. That created a crisis where they needed to get those rods back in ASAP. As it turned out, ASAP wasn't fast enough. And the rods, almost all of them, stuck in a middle position. The problem occurred at the bottom half of the reactor that still had the moderator section of the rods - and the biggest problem was at the very bottom that was not suppose to operate with a moderator at all.

 

[ElliotSmith]

The problem was that almost all of the rods got stuck - and it was while they were being lowered, not after.

Leading up to the accident, they had run the reactor at low power and had built up a lot of Xenon. The Xenon was poisoning the reactor, so to keep the reactor going, they needed to pull more and more rods - eventually pulling all of them or almost all of them. That was against the rules - they should have just let the reactor die down and try their experiment some other day. What happened next was that the reactor power started rising and the Xenon was rapidly being exhausted. That created a crisis where they needed to get those rods back in ASAP. As it turned out, ASAP wasn't fast enough. And the rods, almost all of them, stuck in a middle position. The problem occurred at the bottom half of the reactor that still had the moderator section of the rods - and the biggest problem was at the very bottom that was not suppose to operate with a moderator at all.

The channels through which the control rods were inserted were misshapen because of severe overheating/partial core melting?

 

[.Scott]

The channels through which the control rods were inserted were misshapen because of severe overheating/partial core melting?

I would guess some misshaping. But what followed was a couple of explosions and radioactive fire. So there wasn't much left to look at.