A Review of Criticality Accidents A Review of Criticality Accidents

At the equatorial plane, the two halves were
separated by a 5 mm thick disk of duralumin that had a
radial groove into which twenty-four, 80 mg pellets of
U 3 O 8 (90% enriched in 235 U) were placed. The pellets
were used to determine an integrated fission yield for
the assembly.
Initially, the distance between the lower and upper
parts was 200 mm. The minimal distance the pieces
could be brought together depended on the thickness of
the steel stops manually installed by the operator. The
stops were placed in a horizontal cut in the lower part
of the assembly prior to beginning the closure (Figure
46). As the steel stops contacted the upper part of
the assembly, the upward movement of the table would
automatically cease. The operator would then measure
the neutron flux and determine the corresponding state
of subcriticality.
The accident on 9 April 1953 was due to an error
made by an operator working alone in violation of
regulations. The operator mistakenly installed 5 mm
steel stops instead of the 10 mm required by the
experimental plan. This caused an excursion as the
assembly was brought together and resulted in a
significant release of heat that melted a portion (~70 g
mass) of the plutonium core. The plutonium flowed out
into the horizontal channel in the natural uranium
reflector.
The excursion activated an emergency alarm, at
which time the operator pushed a button on the control
panel and hydraulically lowered the table back down to
its original position terminating the excursion. The
accident occurred during lunch, when most of the
Neutron Detectors
FKBN
Control Room
FKBN
Control
Panel
Corridor
Figure 47. Building B floor plan.
experimental team (~10 people) was out. About 2
hours afterwards, the experiment supervisor and the
operator entered the FKBN experimental cell and
performed a visual inspection, during which they
received doses of 1.6 rad and 1 rad, respectively.
Subsequent inspection of the assembly parts
determined that three (of the four) plutonium
hemishells had gotten so hot that they fused with the
duraluminum disk. Therefore, that portion of the
system was shipped to the Chelyabinsk-40 (Mayak)
facility for further disassembly.
Analysis of the U 3 O 8 pellets within the assembly
gave an integrated energy release equivalent to about
~1 × 10 16 fissions. There was no significant contamination
within the facility allowing it to be reused for
later experiments.
The accident investigation report stated,
“...The design of FKBN and its
automatic safety features are such that it
provides safeguards for only slow changes
in reactivity. FKBN is not safeguarded
against negligent operation, and therefore
per safety regulations operations should
be performed only in a manner that would
not require any safety system response.”
As a result, it was concluded that FKBN did not
comply with safe operation requirements for critical
assemblies, and it was dismantled. As a replacement,
the design was modified several times resulting in the
construction by VNIIEF of FKBN-1 (1955), FKBN-2
(1963), FKBN-2M, and others equipped with fast
response safety systems. 49
Office
Entrance
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