A Review of Criticality Accidents A Review of Criticality Accidents

20. Siberian Chemical Combine, 13 December 1978
Plutonium metal ingots in a storage container; single excursion; one serious exposure, seven significant
exposures.
Various operations with α-phase plutonium metal
ingots were performed in Building 901, Department 1,
of this plant. There were 16 interconnected gloveboxes
manned by a total of 7 operators. Although the
operators were trained on all of the individual operations,
each would be assigned only a particular subset
at the beginning of each shift. By written procedure, it
was not allowed for an operator to deviate from his
assigned tasks even if the deviation involved assisting
others with their tasks.
Transfer and temporary storage of the ingots within
the gloveboxes was accomplished using cylindrical
storage containers designed specifically for this
purpose. The containers were lined with a 0.5 mm
thick cadmium layer and had a 30 mm thick shell of
polyethylene encased in a stainless steel sleeve
(Figure 28). The design decreased the neutron interaction
for a planar array to such a degree that it was not
necessary to control the number or storage arrangement
of the containers within any of the gloveboxes.
The ingots were produced from a direct oxide
reduction process and were in the shape of a frustum of
a cone. The dimensions of the ingots are no longer
Stainless
Steel can of
unknown wall
thickness
~152 mm
Polyethylene
~30 mm side walls, floor, and lid
Figure 28. Storage container.
To Glovebox 6 (via conveyor)
~0.5 mm Cadmium liner on
all internal surfaces
~192 mm
~222 mm
1392 1391-A 1391-B
Figure 29. A simplified layout of Glovebox 13.
known. Depending on the origin of the feed material
the upper mass limit of an ingot was either 2 kg (waste
recovery, precipitation/calcination) or 4 kg (relatively
pure oxide). The general administrative limit for the
storage containers in this building allowed up to
2 ingots totaling 4 kg or less. However, the size of the
container did not preclude the possibility of loading
multiple ingots to levels in excess of a critical mass. It
was assumed that the operating personnel, because of
their proficiency and discipline, would not make gross
errors (more than twice the administrative limit) in
loading the containers or ignore the safety limits.
Glovebox 13, where the criticality accident took
place, consisted of three workstations, (1391-A,
1391-B, and 1392, see Figure 29) and was connected
to gloveboxes 12 and 6 by a pass–through port and
conveyor, respectively. All of the ingots produced at
the plant passed through this glovebox operation. For
these particular workstations, the administrative limit
for the containers was even stricter, allowing only one
ingot regardless of its mass. Workstation 1391-A was
used for extracting drill samples (up to 0.1 g) for
chemical analysis of the impurities. The ingots were
weighed in workstation 1391-B, where the samples
were also temporarily stored. Workstation 1392 was
used for measuring the dimensions of the ingots.
Within the workstations the ingots were removed and
returned to their original containers one at a time. The
workstations were equipped with pass–through ports in
the connecting walls, gloveports, and 50 mm thick
leaded glass windows. The front of the workstation
had a 30 mm thick lead shield to reduce the γ-radiation
from the plutonium.
At the beginning of the shift on 13 December 1978,
three containers were in workstation 1391-B, and four
containers were in workstation 1392, each with one
ingot. The ingots are numbered sequentially in
Figure 29 for the purpose of the discussion that
follows. Although the containers are depicted as being
linearly arranged, in actuality the containers holding
ingots 4 and 5 were behind those with ingots 6 and 7,
making visual inspection of the contents more difficult
7 6 5 4 3 2 1
Glovebox 12
47