[ RadSafe ] Criticality in LEU

[Derek Putley]

Dear Ed / Bryan / George / All
 
A copy of the definitive reference on past criticality accidents, LA-13638 "the accidents book" can be accessed via this link:-
 
http://www.orau.org/ptp/Library/accidents/la-13638.pdf 
 
To date there has not been a fuel-cycle criticality accident with lower enrichment material than 6.5%. As you reduce enrichment, critical masses increase, making it harder to accumulate a critical system by accident.
 
In the context of the likelihood question, BWR fuel is obviously intended to achieve criticality under controlled conditions in a reactor. 
 
Elsewhere, in transport and storage, arrangements will be designed to prevent criticality, even under reasonably foreseeable accident conditions. 
 
For recently irradiated fuel, such arrangements will allow for the presence of water as a coolant - e.g. by the use of appropriate spacing and neutron poisons in fuel storage arrangements and/or by the use of "burn up credit", which might be done to take credit for the reduced reactivity of used fuel. 
 
Elsewhere, the contingency of water ingress is likely to be an assessed fault condition for which deterministic safety may be demonstrated.
 
Calculating accurate likelihoods for criticality events is hard to do for a number of reasons, not least because there are not many of them in practice. In general (in the UK (and worldwide?)) regulated nuclear operators are expected to provide safety cases to prove the safety of their plants and then, in spite of this, they will be required to produce emergency plans and train in emergency response.
 
In a real (or simulated) emergency, emergency controllers will need to take expert advice and make judgements about likelihood on the fly - in order to inform the decisions they need to take regarding possible intervention strategies. But the risks and benefits would need to be weighed up together.
 
 
 
 
 
 
Message: 7
Date: Fri, 18 Mar 2011 10:03:37 -0500
From: "Ed Battle" <radsafeinst at cableone.net>
Subject: [ RadSafe ] RadSafe Digest, Vol 571, Issue 1
To: <radsafe at health.phys.iit.edu>
Message-ID: <005001cbe57d$a8d75c70$fa861550$@net>
Content-Type: text/plain;charset="us-ascii"

Bryan or George; If you have time, would you please comment on the
probability of a "criticality event" in fuel that's only 3 to 5% enriched???
Thanks, Ed Battle



Bryan said:"Looks like they are about to start spraying water from police
water
cannon through a hole in the wall in an attempt to refill the pool (per
NHK report at 7am Japan time).

Bryan'

Bryan, I hope someone tells them A) hot, dry zirc4 rapidly oxidizes in the 
sudden presence of water (H2 given off in quantity and much extra heat) 

and 

B) Powerful jets of water can change the geometry (critical spacing) of fuel

rods, possibly asking for a criticality event.


George Dowell



*************************************
 
 
Best Regards

Derek Putley

Technical Area Lead (Criticality) 
 
Serco (Technical Services)
Thomson House, Risley, Warrington, Cheshire, WA3 6GA, UK



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