Re: [Phys-l] recriticality at Fukushima

[Jeff Radtke <jr@reflectionimaging.com>]

Quoting John Denker <jsd@av8n.com>:

> > 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.

I don't believe so.

Criticality is defined in terms of the effective neutron  
multiplication factor (k), which is the ratio of the rate at which  
neutrons are produced via fission to the rate at which neutrons are  
lost via absorption by, or leakage from, the assembly.  An assembly is  
supercritical if k is greater than one, and subcritical if k is less  
than one.  A population of neutrons introduced into a subcritical  
assembly will decrease, but not to zero.  This is because spontaneous  
fissions occur in a subcritical assembly.  Cf-252 has a half life of  
2.6 years, and 3% of these decay events are spontaneous fissions.   
U-238 may also decay via spontaneous fission.   Spontaneous fissions  
produce neutrons, which occasionally produce more fissions even in a  
subcritical assembly.  Fission events produce radioactive Xenon.  So,  
presence of radioactive Xenon is not a certain indicator of recent  
criticality.

The best reference I'm aware of regarding nuclear reactor engineering  
is titled just that, and was written by Samuel Glasstone and Alexander  
Sesonske.

Finally,  the October issue of Physics Today contained two excellent  
articles on global climate change.  It is becoming apparent that  
tragedies due to nuclear accidents will pale in comparison to those  
attributable to CO2 already emitted via fossil fuel combustion.  The  
energy density of nuclear ore and the compact nature of nuclear waste  
make for a sustainable energy source.  High energy density fuels must  
also be treated with care and respect.  We humans have the capacity to  
learn from our vulnerabilities and mistakes.  We have barely begun to  
explore the advanced reactor and nuclear fuel cycle options available  
to us, and new nuclear power plants are likely to be far safer on a  
fatality per Joule basis.

--
Jeff Radtke
jr@reflectionimaging.com
jr@cloudchambers.com