A Review of Criticality Accidents A Review of Criticality Accidents

established that in addition to the error in judgment,
the specialist violated several operational procedures.
For the evening assembly, the lower half of the
uranium reflector was not positioned 20 mm above its
lower stop as required to ensure an adequate margin of
criticality safety for the assembly process. Instead, the
lower reflector was positioned 90 mm above its lower
stop which is 30 mm below its upper stop. The upper
stop corresponds to the lower reflector making contact
with the lower core. The failure to reposition the lower
reflector following the daytime assembly was identified
as the primary cause of the accident.
The investigation revealed several additional
violations of procedures:
• The instrument system designed to be sensitive
enough to alert the specialist that system multiplication
was rapidly increasing as they lowered the
upper reflector half was not operating. This system
had been switched off following the daytime assembly
and not switched back on for the evening assembly.
• A third specialist was required to be present in the
control room, but the control room was unmanned.
• The assembly required the presence of a health
physicist. The health physicist was not present
because he was not notified of the evening assembly.
Following the excursion, the two specialists
remained conscious and maintained their self-control.
They were able to inform administrative officials that
the accident had occurred and to place a phone call
requesting an ambulance. The senior specialist carried
out dose estimates for himself and the junior specialist.
The senior specialist made the following entry into his
experimental log.
“Eighty-mm diameter plug * was removed.
The gap was 30 mm. Polyethylene was
inserted. When moving the shell down a
pulse was produced. Safety system was
actuated. The operator was at the distance
of 0.5 meters away from the shell. The
responsible person - at 1.7 meters away
from the overhead-track hoist pendant.
There was a flash, a shock, a stream of
heat in our faces. The polyethylene contribution
exceeded the expected magnitude.”
The two criticality safety specialists working with
this assembly at the time of the excursion were
knowledgeable in neutron physics and the experimen-
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tal procedures required for criticality measurements.
The senior specialist was born in 1929, joined the
facility in 1955, and became qualified to handle fissile
material in 1958. The junior specialist was born in
1938, joined the facility in 1961, and became qualified
to handle fissile material in 1962. Both were qualified
to carry out the experimental procedures taking place
at the time of the accident. At the time of the excursion,
the junior specialist was standing approximately
0.5 m from the assembly axis. The senior specialist
was located approximately 1.7 m away. The junior
specialist received an accumulated neutron plus
gamma dose in the 20–40-Sv range. The senior
specialist received an accumulated neutron plus
gamma dose in the 5–10-Sv range. Following the
accident, both specialists were taken to the local
hospital and then immediately flown to the
Bio-Physics Institute in Moscow. The junior specialist
died three days after the excursion. The senior specialist
survived for 54 days after the excursion.
The estimated yield of the excursion was 6 × 10 16
fissions. At the beginning of the excursion, the upper
half of the natural uranium reflector was descending on
the core at approximately 100 mm/s, driving the
assembly above prompt critical. This closure speed
corresponds to a reactivity insertion rate of about
40 β/sec. A 5.2 × 10 6 (neutrons per second) 238 Pu-Be
source was located off-center outside of the uranium
core of the assembly.
The investigation concluded that although the
specialists were highly experienced in working with
critical assemblies, it was their overconfidence and
haste that resulted in their loss of life. Both specialists
had theater tickets for the same evening as the accident.
The senior specialist prepared the procedure for
the evening assembly disregarding a principle rule for
criticality safety which stated: Every unmeasured
system is assumed to be critical. The investigation
concluded that the accident was caused by “severe
violations of safety rules and regulations which
occurred due to insufficient control by facility managers
and health physics personnel.”
* This plug was located at the north pole of the upper reflector and passed through the full thickness of the reflector.