Prompt Criticality

(With a hat tip to Green Mountain Daily)

Arnie Gunderson of Fairewinds Associates, Vermont’s own nuclear watchdog, has been making a series of video commentaries about events in Fukushima. One of his latest covers the explosion at reactor #3. It addresses the question of when an explosion is not an explosion and when it is more than your usual explosion.

Gundersen Postulates Unit 3 Explosion May Have Been Prompt Criticality in Fuel Pool from Fairewinds Associates on Vimeo.

Long story short, the spent fuel pool at reactor #3 actually detonated. Hydrogen was the initial culprit, but Gunderson presents evidence that the initial hydrogen explosion compressed the spent fuel rods of mixed uranium and plutonium enough that they achieved critical mass. That is, enough neutrons shot around in a tight enough space to create an uncontrolled and fast chain reaction. Gunderson called it a “prompt criticality.” The fuel pool walls and open top acted like the barrel of an upturned cannon and shot pieces of fuel rods as much as two miles away.

“Prompt criticality.” Hmmm.

As I recall from studying physics and technological history, compressing uranium and plutonium in order to create a critical mass that detonates is the job description of an atomic bomb. That flash and column of smoke we saw on the news at reactor #3 was probably a small, but quite real nuclear blast.

This is a new idea for me, and probably for a lot of people. There are spent fuel pools all over the world, including one in the southeast corner of Vermont. The concept of one boiling off and spraying radiation is bad enough, but “micro-nuke” is a new level of risk. They are not just water filled boxes of radioactive materials. They are all potential atomic bombs. It’s not that we should anticipate Hiroshima-like devastation. The engineering that goes into an efficient nuclear weapon is not trivial, and compressed spent fuel rods are not going to get anywhere near that.

The risks of a prompt criticality are catastrophic loss of containment and a wide and high spread of heavier radioactive materials. I doubt that the spent fuel pool at Fukushima #3 has any real structural integrity left, and getting near enough to it to either remove the materials or repair the containment is problematic (understatement alert). The fact that solid bits of the spent rods traveled two miles is an indication of how far dust and volatile gases must have traveled. We’ll have to wait and see how much of the nuclear material is still left in the pool, but a significant amount of uranium and plutonium must have shot up into the air. The next question is, having shot into the air, where did it, or will it, all come down?

The question after that is, how much more of this stuff do we want to make and have sitting around?

Comments

[Mark Laris]

First, prompt criticality and an nuclear explosion are two different things. Second, inorder to compress U or Pu enough to cause a nuclear explosion is not something that happens by accident; it requires a perfectly symmetric explosion. In a nuclear reactor there are 2 types of neutrons, prompt neutrons and delayed neutrons. Prompt neutrons are released immediatly at the time of the fission event Delayed neutrons are released from radioactive fission products seconds to minutes following a fission event. Prompt criticality refers to criticality (a chain reaction) with prompt neutrons only.

[Minor Heretic]

Thanks for the clarification, Mark. I saw another video by Arnie Gunderson that dug into that a bit deeper and led me to the conclusion you point out. Still, it seems to be a distinction without too much difference. There was a detonation and radioactive material was projected high into the air. The fuel pool is a ruin. It's a dirty bomb rather than an atomic bomb, but that is cold comfort to the people of northern Japan.

[Peter Lux]

Mark Laris is incorrect about prompt criticality. If you bring two bits of Uranium 235 / 233 or plutonium 239 together to produce a amount greater than the critical mass then you will get a prompt nuclear explosion. However, this will usually blow the assembly apart and quickly stop the nuclear reaction - you get a fizzel i.e. you will get very low yeild. The symmetrical explosions in a Pu nuclear warhead is to keep the masses together for long enough to produce a much heigher yield.
There has been several prompt ciritcal explosions in nuclear power plants including SL-1, CHernobyl and the Russian K-431 submarine.
If the Uranium or Plutonium is not very enriched (greater than about 20%) then the critical mass is infinite - i.e. prompt criticality is impossible. However, a nuclear reactor uses a moderator so that the critical mass is much lower and enriched (for water moderated) or even unenriched (for graphite and heavy water) fuel can be used. The US as tried to use low enriched moderated Uranium to produce a weapon but the results were disappointing.