[ RadSafe ] BWR SCRAM sequence & criticality

Derek Putley derek.putley at serco.com
Thu Mar 17 13:53:01 CDT 2011

Dear All
I agree with Doug Huffman and Doug Aitken's colleague.
Also, a typical BWR low enriched uranium (or equivalent MOX) fuel will need to be mixed with a moderator such as water (or graphite) to go critical.
The most effective moderation arrangements for low enriched fuels involve an ordered lattice of fuel and moderator - but in a practical reactor the lattice parameters will be optimised for aspects of heat transfer and mechanical properties rather than just criticality.
Hence if fuel melts, it is likely to drive off water and revert to a non-optimal geometry. In simple terms, if the shape of a fuel mass changes so that its surface area increases, that will benefit neutron leakage, so the reactivity will go down. This is formally recognised in the geometric buckling term in buckling theory, as described in the excellent textbooks by Glasstone (and his various co-authors, e.g. Edlund and Sessonske) or (in less detail) in more general Nuclear Physics textbooks (e.g. Krane or Lilley). It is also possible to calculate a minimum critical thickness for any given material. Hence if a molten core were able to spread out and a form a thin slab geometry, that would quite likely be inherently sub-critical.
The case of spraying water onto the Pond 4 spent fuel sounds more problematical. However, most spent fuel is removed from reactors because it will not go critical as easily as fresh fuel. In the UK most power station cooling ponds employ both fixed and soluble neutron poisons to ensure sub-criticality but this provides protection for extreme cases, including placing more reactive fresh fuel into the fuel pond. So on balance, if you already have radiation leaking from overheated damaged fuel, applying cooling water is probably the right thing to do.
To close with a question - in my previous job at Harwell in the early 1990s, I was involved with nuclear robotics. Even back then, there was talk of accident intervention robots, not least by the Japanese. Broadly we consider potential applications for monitoring and surveillance and then perhaps for actual intervention, much in the manner of robots used for bomb disposal. I have not seen any mention of the deployment of any such devices as part of the response to the Japanese emergency - have I missed anything or is the technology not really ready for "real  nuclear emergencies" yet?
Kind Regards

Message: 27 Date: Thu, 17 Mar 2011 09:58:30 -0500
From: Doug Huffman <doug.huffman at wildblue.net>

It did not at Chernobyl.  Shape is an essential term in the
determination of critical mass.  I knew it as 'Buckling'.  See IIRC
Glasstone's source Book on Atomic Energy


Message: 28 Date: Thu, 17 Mar 2011 10:22:02 -0500
From: Doug Aitken jdaitken at sugar-land.oilfield.slb.com 
I received this reply from a colleague that clearly sets to rest the
question and I thought it would be good to share it.

Many thanks! Regards Doug


No. The boron and other neutron absorbers in the control rods already
inserted into the core(s) coupled with the very low enrichment of the fuel
and the burnable neutron absorbers used therein will not at all allow a
re-criticality. A similar analysis applies to the fuel in the spent fuel
pools. Things are going to be very hot for a while, and we want them to keep
water on whatever the geometry is of the core(s)/spent fuel to keep the hot
fission products from vaporizing/volatizing  and escaping into the
atmosphere. Fortunately, a re-criticality is not an issue in any of this

Doug Aitken wrote: 

As a person with only a basic understanding of the workings of these
reactors, any detail will be very welcome.
One question I keep getting is what happens if the rods melt and pool in the
base of the containment. Will the mass of material go critical? 

Best Regards

Derek Putley

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

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