A Review of Criticality Accidents A Review of Criticality Accidents

1. Mayak Production Association, 15 March 1953
Plutonium nitrate solution in an interim storage vessel; single excursion; one serious exposure, one significant
exposure.
The accident occurred in a plutonium processing
building. The plutonium had been recovered from
irradiated uranium rods. After separation from the fuel
rods, the plutonium in the form of nitrate solution, was
put through several purification steps. Operations were
performed on 6 hour shifts, 4 shifts per day. The
building was not equipped with a criticality alarm
system.
Following purification, the plutonium nitrate
solution was routed through a staging area before
being sent on for further processing. Operations
performed within the staging area included mixing,
dilution, volume measurement, sampling for plutonium
concentration and purity, and interim storage. Plutonium
solution that failed purity requirements was
returned for further purification. The staging area
consisted of a concrete cell and adjacent space located
in the corridor outside of the cell. Figure 6 shows the
layout of the concrete cell and corridor.
The staging area contained 15 identical short right
circular cylindrical stainless steel vessels, each with a
unique identification number. The vessels were
400 mm in diameter and 320 mm high with the
cylindrical axis oriented vertically. The staging area
was not equipped with radiation monitoring instruments.
Criticality control was implemented by a
500 gram plutonium mass limit per vessel. Criticality
Cast Iron
1 2 3 4 5 6 7
Cell
Concrete
safety support consisted of part time effort by a staff
physicist. Operators were not trained in criticality
safety.
The 15 vessels were separated into 2 linear arrays.
The first array containing 7 evenly spaced vessels was
located near the back wall of the concrete cell. This
array was installed in May 1952 and was supplied with
solutions through permanently installed transfer lines.
The concrete cell was 3 m wide, 2 m deep, and 2.5 m
high. Top surfaces of the vessels were less than 1 m
above the cell floor. Located in front of the array was a
vertical, 200 mm thick cast–iron plate. In addition, a
125 mm thick cast–iron top plate was located horizontally
above the 7 vessels. This plate had cutout holes
above each vessel to allow access for making hose
connections. Cadmium plates were positioned vertically
between the 7 vessels. Procedures required that,
for criticality safety purposes, vessels 2, 4, and 6 never
contain solution. This constraint was expected to
reduce neutronic interaction between the vessels.
The second array containing 8 evenly spaced
vessels was located in the corridor outside the concrete
cell. This second array was installed after realization
that the first array had inadequate capacity for the
volume of plutonium solution being processed. Each
vessel in the array was individually shielded by
approximately 175 mm of cast–iron on 4 sides and
Vacuum
Trap
Cast Iron
Vacuum Pump
Figure 6. Layout of vessels and equipment in the staging area.
Corridor
17 18 19 22 15 16 11 12
7