Some info on the Japanese nuclear accident:

What happened in Japan:  The criticality event
(based on media reports, e-mails from experts, and IAEA updates)

The criticality accident occurred at the Conversion Test Building, JCO,
Ltd. at Tokai-mura, starting at 10:35 am Japan time on Thursday
September 30.  Some reports said that this facility makes fuel for the JOYO
experimental fast reactor.  The accident lasted about 17 hours.

The criticality involved about 16 kilograms of 18.8 percent enriched
uranium in solution in a tank.  The tank is used to precipitate uranium
from nitric acid liquid solution.  The tank contained far more uranium
than allowed for safe operation.  The tank should not have contained more
than 2.4 kilograms.

Because the uranium was in solution, the criticality event proved
difficult to stop.  Following the initiation of the accident, when nearby
workers
saw a distinctive blue glow, the chain reaction continued. What happened
next is uncertain. Available reports are unclear whether there was a
continuous nuclear reaction throughout the period of the 17-hour accident,
or
periodic critical reactions happened.  This second situation could have
occurred
if, as some reports indicate, the heat produced during the event boiled the
solution, expanding the fluid and creating air voids which stopped the
chain reaction.  But after the solution cooled, criticality resumed.
Otherreports stated that after the first criticality, the chain reaction
continued at a lower level for the entire time of the accident.

This type of criticality event is extremely dangerous because it does
not necessarily destroy the container holding the uranium.  In essence, the
container became a nuclear reactor, but without the customary materials
and equipment to keep the reaction under control and protect the workers
and public.

Plant officials moved to stop the reaction by draining water from
cooling pipes surrounding the tank.  This water jacket served to cool the
tank
(and perhaps protect workers against the lower levels of radiation emitted
routinely from the tank).  However, during the accident, the water in
the jacket reflected neutrons back into the tank, intensifying the number
of
fissions of uranium 235.

Once this water was drained, the uranium in the tank apparently could no
longer go critical. The criticality reaction is reported to have stopped
by about 5:30 pm Japan time on Friday.

Subsequently, boron was injected into the tank to absorb neutrons and
further reduce the chances of another criticality reaction.

The total number of irradiated persons under medical observation as of
early October 1 was 49 persons.  The breakdown is 3 staff irradiated
severely, 36 staff irradiated less severely, and 10 nonstaff irradiated
less severely (including 3 firemen involved in rescuing the seriously
exposed workers).

A release of radiation to the atmosphere happened, but stopped.
According to the IAEA, the noble gases have dissipated, but radioactive
cesium and
iodine have escaped the facility.  These materials remain a concern,
pending the results of more radiation monitoring in the populated area
surrounding the accident site.

On the International Nuclear Event Scale (INES), the event was given a
provisional rating of four, with seven being the most severe type of
accident.  Level 4 signifies an event without significant off-site risk;
however, this ranking may be changed as more information is obtained
about the accident, including the level of off-site radiation
contamination.