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Hi Steve,
First, I need to explain what a moderated prompt criticality is and is not.
Uranium 235 atoms split (fission) when they are hit by a neutron and when a uranium 235 atom does fission, it gives off two or three neutrons. In order for a chain reaction to occur, a single Uranium 235 fission must liberate enough neutrons to cause another uranium 235 atom to fission which in turn must liberate enough neutrons to cause another uranium235 atom to fission, and so on. Not all the neutrons create a second or third fission, because some neutrons are captured by other isotopes instead of U235. For instance, if uranium 238 absorbs a neutron, it does not cause a fission. Since only 4% of the fuel is U235 and 96% is 238, not all neutrons create fissions of U235.
There are two types of neutrons released after Uranium splits. Prompt neutrons are released immediately (one millionth of a second) after a U235 atom fissions and account for about 99% of the neutrons released. Delayed neutrons leave later and account for about 1% of the neutrons.
If a chain reaction is stable, 1 neutron collides with a U235 atom which causes another individual U235 atom to split in the second generation and yet another in the third generation. This is a “neutron multiplication factor” of 1. Since over two neutrons are created with every fission, more than half of the neutrons produced in the fission get absorbed and do not create additional fissions. If a chain reaction speeds up, the neutron multiplication factor is slightly higher than 1, like 1.0001. Over a period of many seconds, the power level gradually increases.
In order to control a chain reaction, the neutron multiplication from prompt neutrons must always be less than 1. The 1% delayed neutrons push the neutron multiplication factor over 1.0. Since the prompt neutrons fly out of the uranium so quickly, it would be impossible to control a chain reaction if there were only prompt neutrons released. So controlling the chain reaction is done very slowly by making sure that there never is a time when prompt neutrons alone can cause the neutron multiplication to exceed 1.0.
So a self-sustaining chain reaction is like a hair trigger on a gun. There is only a 1% difference between it being under control and out of control. When it goes out of control it is called a PROMPT CRITICALITY, because the prompt neutrons alone cause a rapid growth in power. And the prompt neutrons are created VERY quickly.
To complicate matters further, there are two types of prompt criticalities because of what happens after the prompt neutrons leave the U235. In a bomb, these prompt neutrons do not slow down before they hit the next uranium atom and cause the next chain reaction. Bombs need highly enriched U235 so the atoms are very close to each other and there is no chance for the neutrons to slow down. This Prompt criticality doubles in power every millionth of a second and causes incredibly rapid power increase that is the destructive nature of a bomb.
The second type of prompt criticality is called a prompt MODERATED criticality, which is what I believed happened at Fukushima. Fukushima had low enriched uranium and could not make a bomb. The prompt neutrons in the fuel pool had to travel over a longer distance to find another U235 atom. This longer journey causes them to slow down and become moderated. Slowing down takes one thousandth of a second. So a moderated prompt criticality doubles in power in a thousandth of a second, while a bomb grows at a millionth of a second.
There clearly was a significant explosion at Fukushima but it could not have been a bomb because the enrichment of the fuel was too low to make a bomb.
A prompt moderated criticality does not melt or destroy the fuel the and but it does create a large amount of heat in a very short time, as little as a tenth of a second.
The BORAX experiment created a prompt moderated criticality that looks very similar to the explosion at Unit 3.
See Minute 3:15 of
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Arnie Gundersen
Fairewinds Associates, Inc
802-865-9955
''If a Secretary of Agriculture endorsed better meat inspection, you wouldn't have a debate of near religious fervor about whether that person was pro- or anti-meat, whether he had sold out to the vegetarians.You'd debate whether the stricter regulations made sense. It's somehow unique to nuclear power that, when one refuses to have nuclear power on the industry's terms, one gets chucked into a bin labeled 'anti-nuclear.' ''
-Peter A. Bradford, former Commissioner of the Nuclear Regulatory Commission. 3/9/82
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