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