Thursday, June 16, 2011

Fuel Pools

The video above was taken by a robot camera at the Unit 4 Fuel Pool at Fukushima in late April. At Fukushima Unit 4, there was no fuel in the core, and all the fuel had been off-loaded to the fuel pool. In the video, we can see that there is rubble on top of the fuel racks, but the racks themselves are in good order. Yesterday, the NRC announced that the spent fuel pool at Unit Four probably did not go dry. Today, Rod Adams of Atomic Insights has an excellent post on this subject, including videos and links to the NRC announcement.

Now, most of the nuclear commentators will focus on the Big Issues. Why did Jaczko think the fuel pools were dry? How did Jaczko's statements affect U. S./Japanese relationships? These are big questions.

I'll look at a smaller question which is relevent to Vermont Yankee. Did any of the Fukushima fuel pools go critical?

Fuel Pool Criticality: The Gundersen Version

On Monday, April 18, Arnie Gundersen said "it could be that the fuel pool had a self-sustaining chain reaction." (Unit 4 pool) However, as you can see in the video above, everything looks pretty darn orderly in the fuel pool, with the exception of the debris from the roof lying around. The racks and water look stable. This is not how things look after an un-controlled chain reaction.

On April 26, Arnie Gunderson suggests that the Unit 3 fuel pool went critical. He says that the nature of the Unit 3 explosion proves that the cause was not a hydrogen explosion. He says that a hydrogen explosion would only be a deflagration (burning) not a detonation (explosion) He suggests that a "prompt nuclear reaction" caused the dramatic detonation at Unit 3, because a hydrogen deflagration just "can't do it."

Fuel Pool Criticality: Nope

However, the existence of an explosion is not proof that fission took place. There were plenty of explosions before nuclear fission was even considered a scientific possibility. Nobel and dynamite and all that.

Also, though hydrogen usually burns (deflagrates) hydrogen can detonate, especially in a enclosed area, such as a reactor building. Turbulence around existing piping can also cause hydrogen to detonate. Gundersen's kindly-uncle description of detonation and deflagration is fine, but the idea that hydrogen "can't do it" (detonate) is incorrect.

Since nuclear opponents often believe that Gundersen is always right, I thought I would point out that in this case (as in many others) he was wrong. There is no evidence for criticality in any of the fuel pools.

There's a whole subculture of hydrogen explosions on Youtube. Here's a hydrogen and oxygen balloons. Detonating. Not deflagrating. Detonating.

Consensus at the Meeting

I attended an American Nuclear Society local meeting earlier this week. The general consensus was that the Unit 3 and Unit 4 explosions were hydrogen explosions, and the hydrogen was generated within the overheated reactor of Unit 3. Also, no criticalities (fission reactions) occurred in either fuel pool. Unit 3 and 4 share an off-gas venting stack, and the explosion in Unit 4 was probably from hydrogen that had migrated over from Unit 3 through the shared stack.

You understand that we don't know everything yet, so nobody is claiming anything for sure. Still, there were a lot of smart people in the room at the NRC meeting, and this was the consensus after an excellent presentation and a lot of questions.

Vermont Yankee opponents are sure to promote the idea that Fukushima proves that the Vermont Yankee fuel pool is a nuclear explosion just about to happen. They get confirmation on this belief from Gundersen's videos. It's time to point out that those videos are wrong.
  • Hydrogen can, indeed, detonate.
  • Fuel pools don't go critical.
That's just the way it is. The accident sequences are what you would expect. It is important to avoid hydrogen explosions, but hydrogen explosions are just hydrogen explosions. They don't turn fuel pools into nuclear bombs.

Updates: In a post today, Cheryl Rofer debunks the bogus Internet video on Unit 4 fuel pool which shows Cerenkov radiation (in black and white, no less!) as well as other absurdities.

That big explosion at Unit 3 could have been a deflagration or a detonation. It was certainly caused by hydrogen, not fission.


Jeff Schmidt said...

2 inter-related questions:

1) If there was still water in the cooling pool, wouldn't it be boronated so as not to act as a moderator, thus precluding the possibility of criticality?

2) If the pool was dry, as Gunderson claims at the beginning of the video (on what basis does he claim to know that? Does anyone else say the pool was dry at the time? How did it dry out - a crack in the pool wall from the earthquake?), what would act as the moderator? How could you get a criticality event without moderator?

Meredith Angwin said...

Excellent questions, Jeff!

People generally agree that the fuel pools had partially dried out. The pools lost water far more quickly than expected, and the hypothesis is that they were damaged in the earthquake, or the tsunami, or the hydrogen explosions. In other words, they leaked water, it didn't boil away due to overheating. Though some of it did boil due to overheating, once the rods were partially exposed.

You could conceivably have criticality without a moderator, but I don't think you could have this in a fuel pool, in which the fuel is held in neutron-absorbing racks. Gundersen seems to speak as if nobody had thought of these things before he came along, but actually, there was plenty of planning to be sure fuel pools are safe.

Rich said...

I have worked at both Nine Mile and Oyster Creek, the same vintage as VY, and their Spent Fuel Pools defiantly will NOT go critical when dry. Has Gunderson lost any knowledge he has of a reactor? To go "prompt" critical requires a rapid increase in multiplication factor. This means a rapid change in the moderator coefficient. Air is not a good moderator (much less than water) and thus a rapid draining isn't going to do it. The pool contains almost twice as much water above the spent fuel rods as the area with the rods, So, how could anyone "rapidly" add sufficient water to "rapidly" decrease the moderator coefficient of the borated water by replacing it with fresh/sea water? Is not going to happen.
Look at the clip of the pool - that water looks fairly clean, cleaner than some pools I have seen at operating plants. That water must have been there all along, where did they get clean water to replace all of the water that was supposedly lost? I don't think very much was lost at all. That pool is much bigger than the average Olympic size swimming pool and about 50 feet deep.

Meredith Angwin said...

My husband asked me "what is that mythbusters video doing in the post?" Good question. I wasn't clear. I was trying to show that in an enclosed area, shock waves do more damage.

Maybe I should have used this video of a hydrogen explosion though the payoff is near the end.

Meredith Angwin said...

I just added a hydrogen explosion video. So now I have three-video post. Heavens. This is a first for me.

nor850 said...

BWR spent fuel pools do not use borated water and they are all designed to maintain Keff less than 0.95 percent, i.e., not critical. There was always a very, very low probability that if, for some undefined reason the fuel racks disappeared and the fuel was all brought together that a critical reaction could occur. However, it wouldn't result in an explosion.

Gundersen is usually wrong. He may have a nuclear engineering degree but has no commercial design or operating experience. To those who understand the design and operation of nuclear plants he is comical. Unfortunately, those who have little or no knowledge consider him knowledgeable.
Charlatans are always with us. Arnie is only in it for the money but unfortunately the Entergy cow has gone dry.

Meredith Angwin said...

A friend wrote me that "hydrogen does tend to deflagrate rather than detonate. But a deflagration can also blow the building apart."

So maybe what I called detonations are very strong deflagrations? For sure, they aren't fission incidents.

Robert Hargraves said...

Indeed hydrogen explodes. My PhD experiment failed, causing 40 liters of liquid hydrogen to spill into the experiment hall, instantly flashing, killing one of us.

Meredith Angwin said...

Bob. Oh my L-rd. I know you reasonably well, we are giving a class together in the fall, and I didn't know about this. Somebody killed. Oh heavens.

I almost got killed, but it all ended up okay.. The problem was fluorine, F2, starting a fire. Unfortunately, the F2 was under a cryostat filled with liquid hydrogen which began rapidly volatilizing and rushing through the pipes. An alarm went off. As soon heard the alarm and realized what was going on, I switched the system from "recapture hydrogen within the lab" to 'roof line venting'. This took maybe 30 seconds?

As I finished THAT little operation I looked around. I was the only person left in the lab. The rest were out in the hall running for it. I may have prevented a major explosion.Or not. At any rate, I consider that a narrow escape, and have had healthy respect (no, actually, fear) for fluorine and hydrogen since then.

Around the same time, there was a hydrogen explosion at Harvard (I was at Chicago) in a similar system.

Anonymous said...

Before offgas recombiners VY had offgas detionations due to H2 gas in the offgas.One blew the plant stack door off its frame.With the addition of steam dilution and H2 recombiners in the offgas stream the H2 detionations were under control.
Radiolydic decomposition causes H2 gas to be present in the BWR's steam/offgas.
If not controllled H2 gas will accumlate and explode.