spent fuel storage- misc points

[Slaback@micf.nist.gov]

Criticality in spent fuel storage is an extremely
unlikely event, presuming the fuel is truly 'spent'.
   [Is ther any literature on such an event?]
Regulators normally require the licensee to assume
no depletion of the fuel when designing such storage
facilities.  There is always the somewhat unlikely 
possibility that absolutely fresh fuel would be
assembled there and someone would not notice the absence
of a fission product dose rate.  [But this is a somewhat
vague qualitative point since 'spent fuel' is simply
'not fresh fuel', i.e., it is now irradiated fuel.  But
there is no threshold for how much spent.]

But if we are talking 'spent' fuel, that typically means
that criticality cannot be acheived in the designed core
configuration, which makes it somewhat less likely in some
accidental arrangement, particularly when storage racks
include neutron absorbing materials like boron.

Also note that the possibility of criticality is linked
closely to the fuel and reactor design.  A core designed
for a heavy water reactor does not have sufficient
excess reactivity to be critical in normal water (unless
of course many more elements were added).

In research reactors fuel burnup is in the 30-50% range, 
with the NIST reactor burnup of 70-75% being the extreme
end of that range.


A little risk adds spice to life.
slaback@MICF.NIST.gov