Monday, March 28, 2011

Thinking Criticality

If you are involved in education, you probably read the title as "Thinking Critically".  But the word I want to discuss here in Criticality.   In the nuclear context, the critical mass is the minimum amount of a fissile material needed to create a chain reaction.   All nuclear reactors sustain a chain reaction while they are working.  (They can stop the chain reaction at any time using control rods, whereas a nuclear bomb, using highly enriched Uranium/Plutonium, creates a chain reaction which is out of control.)  Criticality accidents, where the chain reaction is triggered by mistake, can be fatal, and took place during the Manhattan Project (as dramatized in "Fat Man and Little Boy") and more recently at a Japanese nuclear facility in 1999.

So lets think Critically about Criticality.

When the quake struck, control rods deployed and the three active reactors #1,2,3 all stopped their nuclear chain reactions, we are told.  We wouldn't then expect any more chain reactions.  However, nuclear fuel has the inherent potential to create a chain reaction, because of the high energy of every uranium or plutonium nucleus.  So my question is: to what extent have the reactors re-started fission chain reactions?  I find the following evidence for this happening:

--The buildings housing Reactors 1&3 exploded.  The hydrogen that caused these explosions was from the outer layers of the fuel rods reaching such high temps. that they extracted the Oxygen from the water, leaving Hydrogen behind which exploded.   This was reported as a "partial meltdown"

--Early on, the US stressed the importance of the spent fuel pool at reactor #4....I now realize why.  This spent fuel pool contained not just spent fuel but active fuel removed from the reactor core for an inspection.  This is extremely dangerous because the spent fuel pools have no containment structures around them.  Building #4 is also in ruins, destroyed by a fire in the pool.  How does a pool of water catch on fire?  The answer must be that there was no water in the pool.  The 1000+ rods there produced so much heat that they boiled the water away, then without coolant, they heated up more caught fire and probably melted down also.  TEPCO said of this "we cannot rule out the possibility of re-criticality".  Any incident of additional fission has serious health implications since the fission products (eg. I-131, Cs-137, Sr-90)  are carcinogenic and because this will cause increased radiation locally which is hazardous to worker on the scene.  So TEPCO must be carefully trying to determine if any when any of these reactor go critical again.  They just haven't shared that information.

--The presence of short-lived isotopes suggests that fission reactions are ongoing.   An earlier claimed detection of I-134 (with a half life of 1 hr. ) has been  debunked.  But it has now been over two half-lives of I-131 (2x8 days) so 80% of the original I-131 is gone.   Further detections of this isotope in large quantities would indicate ongoing fission.    TEPCO and/or the Japanese govt. yesterday admitted that reactor #2's fuel rods underwent "partial temporary meltdown" last week.  I'm guessing this is in addition to the initial meltdown.  "Temporary", I take it, is their code word for saying "we think criticality has stopped here."

Part of critical thinking is to analyze what is being discussed and see if it is consistent with the facts or if it is not supported.  But another  part of critical thinking is to ask the question "what is not even being discussed?"  In this age of media distraction, I would argue that this aspect is more important.  So here is a list of my critical questions.  If any of you have seen them answered please let me know (you can post a comment below).  I'd love to see the Japanese press raise some of these questions.

1. Why are there no detailed photos of the 3 reactor cores which melted down?  This is the first thing TEPCO should/would have done even before the radiation levels got to high to prevent anyone from getting close.   I'm guessing these photos exist and are not being released.

2. Why are there no detailed photos of the state of the spent fuel pools, esp. in reactor 4?

3. Why has TEPCO not published a detailed list of when and where "re-criticality" is likely to have taken place, and where it has not?   For each reactor TEPCO, must be able to say either:
  • fission did or probably did restart at these times  OR
  • fission did not restart and is not currently ongoing   OR
  • we have no idea: fission could be going on in there and we wouldn't even know.
If the answer is the third of those, I'd be worried.


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