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.