A Review of Criticality Accidents A Review of Criticality Accidents

determined by a radiation control person called to the
scene. Measurements indicated that an intense gamma
radiation field was emanating from the filtrate receiving
vessel. These measurements were made about 15 to
20 minutes after the accident. The radiation control
person immediately ordered a prompt evacuation of the
area.
About 5.5 hours after the event, the exposure rate
was measured to be 18 R/h at a distance of 1.5 m from
the filtrate receiving vessel. It was estimated that this
exposure rate corresponded to a fission yield of
approximately 1.0 × 10 17 fissions. Seventeen hours
after the accident, measurement of the specific activity
of the 24 Na in the operator’s blood showed
245 Bq/cm 3 . Based on analysis done at that time, this
activity was consistent with an absorbed whole body
dose of about 3,000 rad. The operator died twelve days
after the accident.
Five other operators had been in the room at varying
distances from the reacting vessel at the time of the
accident. They received doses estimated to be upwards
3. Mayak Production Association, 2 January 1958
This particular accident was unique in that it
occurred during operations with a vessel used for in–
plant critical experiments. However, since the accident
occurred well after the cessation of an experiment and
during handling operations associated with transferring
the fissile solution into favorable geometry bottles, it
has been categorized as a process criticality accident.
After the 21 April 1957 accident, it was decided to
set up a small-scale experiment capability for measuring
the critical parameters of high concentration,
highly enriched uranyl nitrate solution. This was
deemed necessary in light of the widespread use of
unfavorable geometry process vessels, the uncertainties
in the critical parameters, and in recognition of two
prior criticality accidents at the same plant. Previously,
critical vessel dimensions and critical solution concentrations,
masses, and volumes were estimated based on
calculations, since directly applicable experimental
results were unavailable in Russia at that time. The
experimental capability was set up in the same building
where large volumes of this material were being
processed, giving efficient access to the fissile solution.
The small critical experiment setup, shown in
Figure 8, was located in a separate room in the main
process building. It had only been in operation for two
months at the time of the accident. During measurements,
the critical experimenters worked at a control
of 300 rad. All of them suffered from temporary
radiation sickness but recovered without apparent
long-term health effects.
After dismantling the glovebox and cleaning the
various pieces of equipment, a total of 3.06 kg of 235 U
was discovered in the filtrate receiving vessel. This
material was primarily in two forms: a thin, hard crust,
due to long-term buildup, and a flocculent precipitate,
the concentration of which decreased with height in the
vessel. The accident caused no mechanical damage to
this vessel, and the room was not contaminated. The
glovebox was taken apart, cleaned, and reassembled
with essentially the original equipment. The operation
was resumed after just a few days. During the downtime,
a radiation meter was installed on the glovebox,
operating instructions were revised, and enhanced
operator training was implemented.
This accident led to the decision to set up an in–
plant critical experiment measurement capability to
better determine critical parameters for vessels in
routine use. The next criticality accident at Mayak on
2 January 1958 involved this critical experiment set up.
Uranyl nitrate solution, U(90), in an experiment vessel; one prompt critical burst; three fatalities one serious
exposure.
panel located a few meters away from the setup and
shielded by a 500 mm thick, water–filled slab tank
located 600 mm from the experiment vessel. There
were no criticality alarms installed in the building.
This was the first day of work after the New Year’s
holiday. While the plant generally ran continuously on
four 6–hour shifts daily, there was only one team of
critical experimenters. They were working on their first
shift of the new year, 13:00 to 19:00. Other plant
operators were involved in preparing the fissile
solution and in assembling the experimental apparatus,
but the critical experiments were performed solely by
this dedicated, knowledgeable team.
The prior series of experiments had focused on
determining critical parameters for smaller vessels and
had been concluded before the end of the year. During
the last working days of December the equipment for
the next experimental series had been assembled. This
was the first experiment conducted with this larger
vessel. It was a cylindrical stainless steel vessel,
750 mm inside diameter, with a wall thickness of 2 to
4 mm, representative of the vessels in common use at
the facility. The fissile solution, of known concentrations
and volumes, was added from an overhead,
graduated cylinder 3 l in capacity.
The experiment vessel was bolted to a stand and sat
atop an 8 mm thick steel support plate approximately
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