A Review of Criticality Accidents A Review of Criticality Accidents

had been conjectured, was solidified (~2 g/cm 3 ) crust
containing uranium. This crust was dissolved directly
in the vessels again using nitric acid. The uranyl nitrate
solutions that were produced were transferred to
favorable geometry vessels and placed in storage. A
second γ-scanning of the vessels on 29 May confirmed
that as much of the deposits as practical had been
removed. The total mass of uranium recovered was
~24.4 kg at an average enrichment of 70% by weight.
To determine the actual thickness of the receiver
vessels, a large device capable of measuring the
distance between two opposite, external surface points
with a margin of error of ~3% was constructed. A
complete characterization of the thickness of both
receiver vessels was then generated. At several points,
the internal thickness was determined to be 132 mm,
i.e., ~32% greater than the design thickness. On the
average, the internal thickness in the region of the
deposits was ~17% greater than the design thickness,
or about 117 mm, a factor that clearly reduced the
criticality safety margin. The location of the solid crust
was closely correlated with the deformed regions.
Before the first excursion, uranium was present in
both receiver vessels in three forms, the solid crust, the
unknown density UO 2 particle slurry, and the low
concentration solution. Transfer of etching solutions
into and out of the receiver vessels occurred up to
300 times a year. The crust and slurry that consisted of
a mixture of uranium dioxide, aluminum hydroxide,
and other particulates had accumulated gradually over
many years. This, coupled with the deformation of the
vessels, ultimately resulted in the criticality accident.
The dynamics of the excursions were estimated by
analyzing the response of the various alarm system
detectors and by analysis of the 140 La and 235 U in
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samples of the UO 2 particles taken on 20 May. The
detectors were located at 12 monitoring stations.
However, during the excursions, at most only three of
the monitoring stations alarmed, indicating very low
peak power for each excursion. These three stations
were located on the ground, first, and second floors
directly above one another. From the central control
room, health physicists documented the sequence of
excursions, the response of the detectors, and the
length of time that the detector alarm threshold
(36 mR/h) was exceeded (Figure 33).
Using known distances, the thickness of the floors
and ceilings, and prompt gamma–ray attenuation
coefficients for concrete, power doubling times (T 1/2 )
and uncompensated excessive reactivity estimates (¢)
for the first three excursions were made. The ratio (f)
of the total number of fissions to those that occurred in
the first excursion was also estimated. These results are
shown in Table 8.
Positive reactivity was introduced into the system as
the concentration of the UO 2 slurry increased under
gravity. After each excursion, the system was driven
subcritical as radiolytic gas generation decreased the
density of the system. The absolute value of these
Table 8. Characteristics of the First Three
Excursions
Excursion
Parameter 1 2 3
T1/2 (sec) 1.5 77 147
¢ 67 9 5
f 1.000 0.130 0.075
Figure 33. Sequence of alarms and duration that the radiation levels exceeded the alarm level (36 mR/h).
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