[ RadSafe ] Cl-38 and neutrons

Phil Simpson phils at umich.edu
Fri Apr 1 16:12:18 CDT 2011

I'm very suspect that Cl-38 was detected for reasons already cited on 
this forum (Where are its activation product cohorts?).

I also find it extremely unlikely that a criticality event has or will 
occur at the shutdown and crippled reactors.  It is quite an 
engineering feat to create a critical mass of low enriched uranium.  
See the prior discussions touching on core design, fuel to moderator 
ratio, "buckling", etc.  Any event that degrades the core geometry will 
make criticality that much less likely to occur.

However, some sub-critical neutron multiplication is probably taking 
place.  This very low level neutron flux will be generating neutron 
activation products. But, their significance is dwarfed by the existing 
fission product inventory.  The source of the neutrons is not primarily 
spontaneous fission but instead photo-neutrons. The photo-neutrons are 
generated by fission product gamma reactions with deuterium naturally 
present in water.

As an aside, photo-neutrons from fission product gammas and activated 
reactor structural materials are also the primary neutron startup 
source for any reactor that has been operating for a while.

Phil Simpson
Assistant Reactor Manager (Ret.)
University of Michigan
Ford Nuclear Reactor

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