A Review of Criticality Accidents A Review of Criticality Accidents

16. Mayak Production Association, 16 December 1965
Uranyl nitrate solution, U(90), in a dissolution vessel; multiple excursions; insignificant exposures.
This accident occurred in a residue recovery area of
a metal and fissile solution processing building. The
residues being recovered were produced from dissolution,
precipitation, and reduction processes. A schematic
of the residue recovery area is shown in Figure
21. The residues, which were difficult to dissolve,
were first calcined to convert the uranium content,
usually less than 1% by weight, to U 3 O 8 . Residues
with abnormally high uranium content, which were
occasionally generated (failed castings, cracked
crucibles, etc.), were directed by operating procedures
to other handling areas subject to special requirements.
The residue dissolution glovebox where the
accident occurred had three identical sets of process
equipment as shown in Figure 22. Each set was
equipped with a cylindrical dissolution vessel, a
holding vessel, a filter vessel, and a filtrate receiving
vessel. The dissolution vessels had elliptical bottoms,
were 100 l in volume, and 450 mm in diameter, and
were equipped with a pulsating device for mixing, a
flat cover plate with a feed hopper, and a pressure
relief valve. Heating of the dissolution vessels was
accomplished with a 25 mm thick, steam-water jacket.
Solution was moved within the system through a
transfer line by drawing vacuum.
Glovebox in which the
accident occurred.
Gloveboxes
Windows
Concrete Walls
Maintenance Area
Figure 21. Residue recovery area layout.
The residues were loaded into the dissolution vessel
via the feed hopper located on the cover plate. The feed
hopper was equipped with a lid that was sealed and
locked in place during dissolution. Dissolution was
accomplished by adding acid and heating the solution
while mixing with the pulsating mixing device. Once
dissolution was complete, the resulting solution was
vacuum transferred to the holding vessel. The solution
was then passed through the filter vessel (to remove
non–dissolved solids) to the filtrate receiving vessel.
The day before the accident, 15 December 1965, a
shift supervisor instructed an operator to calcine
residue batch 1726 (uranium content greater than 1%)
in a glovebox with furnaces intended only for the
processing of residues with less than 1% uranium
content. This was a direct violation of the criticality
safety rules. After calcination, batch 1726 was
sampled, and before the results of the analysis had
been obtained, was transferred to another glovebox
already storing multiple batches of residue scheduled
for dissolution. The analytical laboratory determined
the uranium content of the sample from batch 1726 to
be 44% by weight. This result was recorded in the
laboratory sample book but was not transmitted to the
recovery wing for recording on the batch’s accountability
card.
Operations Room
Freight
Elevator
and Stairs
Control Room
Operations
Area Exit
Glass Block Walls
Detectors
37