[Phys-l] recriticality at Fukushima

[John Denker <jsd@av8n.com>]

I quote from
  http://www.japantimes.co.jp/text/nn20111103a1.html

> Tepco said one hundred thousandth of a becquerel per cubic centimeter
> of xenon-133 and xenon-135 was detected in gas samples.
>
> Xenon-133 and xenon-135 are materials created through nuclear
> fission, and are not usually detected even when a reactor is in
> operation, as fuel rods are covered with zirconium. This means
> fission may have occurred in the melted fuel.
>
> As the half-life of these two materials is short — five days for
> xenon-133 and nine hours for xenon-135, the xenon was probably
> created recently, ....

This is bad news.  Of course 10^-5 Bq is a very small amount of
radioactivity, but there really shouldn't be any xenon at all at
this stage of the game.  Unless the observation is completely
erroneous, it means that not only did we have a meltdown, we
had a meltdown followed by recriticality ... and recriticality 
is still occurring.

According to the standard assurances given to the regulators and
the general public, this is supposed to be impossible, even in 
the worst-case scenario.

The fact that they just now dumped more boron onto the reactor 
proves that Tepco is treating it as a recriticality problem.

==========

As usual, most of the press coverage has been ridiculously bad.
I don't know whether to laugh or cry when I see an article that 
says "criticality took place" and then goes on to say there was 
not a "self-sustaining chain reaction".  It's self-contradictory.

If nothing else, if we believe there is xenon this week and no
xenon last week, it may mean that the fuel is still on the move, 
which would mean the situation is very far from being under 
control.