Re: spent fuel storage- misc points

[Ellen_Hochheiser@RL.gov]

     Spent fuel has sufficient Uranium and Plutonium to become critical, it 
     does not have enough Uranium and Plutonium to keep the reactor running 
     at full power.  The geometry of the spent fuel pits, frequently with 
     additional neutron poisons added to the pool, prevent any criticality 
     excursion.  
     
     Ellen Hochheiser
     Ellen_Hochheiser@RL.Gov
     Usual Disclaimer


______________________________ Reply Separator _________________________________
Subject: spent fuel storage- misc points
Author:  Slaback@micf.nist.gov at -MailLink
Date:    8/29/96 2:28 PM


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
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Subject: spent fuel storage- misc points
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