A Review of Criticality Accidents A Review of Criticality Accidents

The accident investigation determined that the
excursions had occurred in the vacuum pump oil
reservoir (Figure 16). The gamma exposure rate on
contact was measured daily at the midpoint of the oil
reservoir for five days using a PMR-1 instrument.
Results of these measurements are shown in Table 7. A
total of 42.95 l of oil were drained into 5 l bottles on
18 and 19 July. Analysis of the oil using the luminescent
method indicated a uranium concentration of
173 g/ l with an uncertainty of ±30% to 40% (the
luminescent method involved introducing a liquid
luminescing agent into the solution and estimating the
concentration from the intensity of the emitted light).
The uranium enrichment was uniformly 22.6% by
weight.
The time evolution of the excursions had several
unusual features. The UF 6 had been accumulating at a
normal rate until four days before the accident when a
large increase occurred. There was no delay in the
initiation of the chain reaction due to the very slow
Figure 16. Oil reservoir (Dimensions are in mm).
reactivity insertion rate and the relatively large neutron
source (~1.3 × 10 4 n/s from α,n reactions). Consequently,
the first excursion on 14 July (04:45) was very
small and probably did not exceed 2 × 10 14 fissions.
After this excursion terminated, the oil in the reservoir
probably remained only slightly subcritical. It should
be emphasized that the concentration, solution volume,
and material configuration at the time of this burst are
not well known. Furthermore, it is not known if the
vacuum pump was operating during this first excursion.
When the pump was turned on at 07:30, the oil in
the pump’s cavities was forced out into the central pipe
of the oil reservoir. This sudden injection of oil
containing uranium led to the second and much larger
excursion. The shutdown of this excursion was due to a
combination of temperature rise and radiolytic gas
generation, which caused ejection of the oil from this
central pipe back into the pump’s cavities and the gas
purification equipment. As with the first excursion, the
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