A commenter on one of my articles about the possibility of a recriticality accident said such an accident was not possible. Well actually, a study of the type of reactor now having problems in Japan (BWR) shows that a recriticality accident is possible:
ftp://ftp.cordis.europa.eu/pub/fp5-euratom/docs/09-sara.pdf
From the summary of the document:
Recriticality in a BWR during reflooding of an overheated partly degraded core, i.e. with relocated control rods, has been studied for a total loss of electric power accident scenario. In order to assess the impact of recriticality on reactor safety, including accident management strategies, the following issues have been investigated in the SARA project: (1) the energy deposition in the fuel during super-promt power burst, (2) the quasi steady-state reactor power following the initial power burst and (3) containment response to elevated quasi steady-state reactor power. The approach was to use three computer codes and to further develop and adapt them for the task. The codes were SIMULATE-3K, APROS and RECRIT. Recriticality analyses were carried out for a number of selected reflooding transients for the Oskarshamn 3 plant in Sweden with SIMULATE-3K and for the Olkiluoto 1 plant in Finland with all three codes. The core initial and boundary conditions prior to recriticality have been studied with the severe accident codes SCDAP/RELAP5, MELCOR and MAAP4.
The results of the analyses show that all three codes predict recriticality – both super-promt power bursts and quasi steady-state power generation – for the range of parameters studied, i.e. with core uncovering and heat-up to maximum core temperatures of around 1800 K, and water flow rates of 45 kg/s to 2000 kg/s injected into the downcomer. Since recriticality takes place in a small fraction of the core, the power densities are high, which results in large energy deposition in the fuel during power burst in some accident scenarios
So does that mean such an accident has happened at Fukushima? Well we can’t be certain and we may never be certain but the evidence points in that direction.
One must also add that without the evidence (neutrons, Iodine 134) computer codes are not definitive. When the “experiment” matches the code you may actually have something.
Cross Posted at Power and Control
Comments
4 responses to “Is A Recriticality Accident Possible?”
Some years ago, geologists discovered (in Africa?) evidence of a naturally occurring sustained fission reaction in an accumulation of uranium minerals. Was it critical?
bob,
Yes. It was critical. It was made possible by the fact that U235 was in the uranium at higher concentrations than currently (it is slowly decaying).
Probably a good thing or mining and concentrating uranium would have caused criticality accidents well before we had a clue on the nuke stuff.
I was that commenter but what I said was “not plausible.” We have not had an real experience with degraded LWR cores besides TMI. I can see some local effects in undamaged portions of the core but this is expected to be rare and unlikely. A batch of molten corium from an LWR can not go critical but adjacent, relatively undamaged fuel, in the presence of water and with the absence of control rods, could conceivably do so. We thought all rods went in with the seismic scram at the time of the initial earthquake.
If the post-mortum shows that this really happened, live and learn.
Please also consider that the IAEA is not held in high status within the commercial nuclear engineering community. Remember El Baradei?
Joseph,
I agree about the IAEA. But what can you do when it is the most accurate and timely source of information?
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The evidence – neutrons, I134, the chlorine isotope, all points to criticality. Maybe it was a minor blip. Maybe a large excursion. But something happened and it is being covered up IMO.