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?