Re: Japanese nuclear accident: Did the reaction oscillate?

[Enrico SARTORI <sartori@nea.fr>]

Yes, it did oscillate and then became more stable. It is due to feedback
from the different physical phenomena intervening: heating through
fission power release, variation in density of the liquid, bubble
formation, evaporation, condensation, collapsing of bubbles through
cooling, increase of density etc. It is too long to explain in detail,
but the phenomenon is well understood. The report from the IAEA does not
contain everything. In addition this was an experiment but an accident,
so not everything 'was measured' - in fact the required measurement
devices to be able to record this were not present : it was assumed it
would/could not happen.

You have "a neutron dose rate sustained at approximately the same level
for hours after a criticality accident" because criticality is not over
but continues. Negative feedback phenomena stabilise a critical system,
a nice natural feature, that's why reactors work; the natural Oklo
reactor worked for millions of years, it started 'probably' with a
natural criticality  accident.  

There are experiments in which criticality excursions are carried out in
laboratories; there the oscillations (or transients) are observed and
they depend on a number of parameters. Well known experiments include
those carried out in France (CRAC- SILENE) and elsewhere. They have been
interpreted thoroughly and the latest papers were presented at the
International Conference on Criticality Safety (ICNC'99) held at
Versailles France, just the week before the JCO accident occurred.
For more information on the conference and papers see:
http://www.ipsn.fr/icnc99/

> Date: Mon, 29 Nov 1999 12:53:40 CET
> From: "Bjorn Cedervall" <bcradsafers@hotmail.com>
> To: radsafe@romulus.ehs.uiuc.edu
> Subject: Japanese nuclear accident: Did the reaction oscillate?
> Message-ID: <19991129115340.80554.qmail@hotmail.com>
> 
> Several early information sources about the Japanese nuclear accident said 
> that the nuclear reaction oscillated. I find this puzzling when looking at 
> the neutron dose rate measurements in the surroundings (IAEA report a few 
> weeks ago). My quetion is: How can a neutron dose rate be sustained at 
> approximately the same level for hours after a criticality accident?
> 
> I realize that there may be (if so, probably a very small fraction of) 
> radionuclides that emit neutrons after a fission event - but if so - which 
> ones would they be? Or was it an accident where - after the initial Cerenkov 
> flash - the nuclear reaction was continuing in some more balanced mode? (I 
> assume that the neutron dose rate measured in the surroundings outside the 
> building essentially came from the direct source - rather than from any 
> released, minor fission product).
> 
> As the information stands I question the oscillatory mode description.
> 
> Please comment or clarify my thoughts here,
> 
> Bjorn Cedervall   bcradsafers@hotmail.com
> 

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Enrico Sartori               Tel.: +33 1 45 24 10 72 or 78
OECD NEA Data Bank           Fax : +33 1 45 24 11 10 or 28
12 Bd des Iles
92130 Issy-les-Moulineaux    e-mail: sartori@nea.fr
France                       www-->  http://www.nea.fr
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