A Review of Criticality Accidents A Review of Criticality Accidents

Table 4. The Seven Vessels Related to the Reconstruction of the Criticality Accident.
At approximately 22:25 on 5 December, an operator
working alone in the room initiated a vacuum transfer
of carbonate solution from vessel R2 to the holding
vessel. Shortly after starting the transfer the operator
noticed that one of the criticality alarm system detectors
located about 4.5 m from the holding vessel was
sounding intermittently. The detector had a dynamic
range of 10 to 3,600 mR/h and was set to trigger at
110 mR/h. Without stopping the transfer, he left the
room to report this observation to the shift supervisor.
His decision to leave the room may have saved his life;
shortly after his departure the entire alarm system
began sounding continuously. All personnel immediately
evacuated to an underground tunnel per
procedure. Later, radiation safety personnel entered a
corridor located about 10 to 15 m from the room to
measure the radiation level. Using a portable PMR–
1gamma sensing device it was determined that the
radiation level exceeded the 18 R/h upper range of the
instrument. Facility managers, the head of the site
safety organization, and the site technical specialist
arrived at the building about an hour after the criticality
alarm sounded.
Following their arrival, the facility managers and
the technical specialist began interviewing personnel
and reviewing measurements and working documents
to locate the site of the accident. They concluded that
the accident had occurred in the room with
gloveboxes 9 and 10 and that the holding vessel was
the most likely location of the accident. Remote
readout of the criticality alarm detector located in the
room indicated the average radiation level to be in the
range of 1.5 to 1.8 R/h. The peak–to–minimum
radiation level was oscillating in amplitude by more
than a factor of 10. Using a long pole, personnel placed
an integrating dosimeter into the room from the outside
corridor during a radiation level minimum. Based on
20
Vessel
Designation a
Vessel
Location
Vessel
Diam.
(mm)
Vessel
Height
(mm)
Approx.
Vessel Volume
( l) Vessel Function
R0 G-9 b
R1 G-10
500 900 180 First oxalate precipitation
b
400 500 60 Second oxalate precipitation
R2 G-10 400 500 60 Second oxalate precipitation
Holding Vessel c
Outside 350 450 40 Sample extraction and solution
G-10
staging between R1 or R2 and R3
R3 G-10 300 400 27 Third oxalate precipitation
Transfer Vessel G-10 250 300 15 Transfer precipitate from R3 to
filter vessel
Filter Vessel G-10 — — 4 Collected precipitate while
passing lean solution
a
All seven had vertical axes of cylindrical symmetry.
b
G-9 = Glovebox 9, G-10 = Glovebox 10
c
Accident location
the exposure time, the gamma radiation level about
2 m from the holding vessel was estimated to be 10 R/
h. The facility was not equipped with instrumentation
that could have automatically recorded the excursion
history. In addition, because of the high stress level,
personnel did not take the time to manually record the
excursion history.
About an hour and a half following the first excursion,
the vacuum system in the holding vessel was still
switched on. Ordinarily the transfer of solution from
R2 to the holding vessel would have been completed in
much less than 1.5 hours. One speculated explanation
for the continuing excursions was that the vacuum
system would cause the transfer of enough solution to
initiate an excursion, which would build up pressure
sufficient to saturate the vacuum system momentarily
interrupting the transfer. Eventually the vacuum system
would recover, and more solution from R2 would be
transferred leading to another excursion. To test this
hypothesis the vacuum system was switched off.
Despite this action, the excursions continued.
The next attempt to terminate the excursions took
place about 00:15 on 6 December. Three operators
entered the room during a period of low radiation
levels. They closed the holding vessel’s vacuum system
and R2 transfer line valves. They then opened the
holding vessel’s compressed air supply and the transfer
valves to vessel R3. These tasks were accomplished in
about 10 to 12 seconds. During this episode, the
average radiation level in the room was about 1.8 R/h.
A hand held PMR–1 instrument measured about
14 R/h in the vicinity of the holding vessel. The
vacuum system was then switched on and 5 l of
solution were transferred from the holding vessel to
vessel R3. This action terminated the excursions. The
transfer was limited to 5 l to prevent a criticality
accident from occurring in vessel R3.