A Review of Criticality Accidents A Review of Criticality Accidents

9. Siberian Chemical Combine, 14 July 1961
Uranium hexafluoride, U(22.6), accumulation in a vacuum pump oil reservoir; two excursions; one significant
exposure.
This accident occurred at a gaseous diffusion
uranium enrichment facility. The enrichment process
was a continuous, 24 hour operation. The end product
was uranium hexafluoride (UF6 ) enriched to various
levels depending on the final reactor fuel to be produced.
The initial UF6 sublimation, introduction into the
cascade, intermediate purification, and its final
desublimation to the solid phase (in cylinders) was
conducted in desublimation/sublimation stages (DSS).
DSS–6 (Figure 14), occupied a large room (7.2 by
18 m) and was an intermediate purification stage (the
enrichment was 22.6% by weight at this point in the
cascade). This stage was used to remove contaminants
such as air and excess hydrogen fluoride (HF) from
the UF6 . This process was accomplished by
1. continuously diverting a portion of the UF6 cascade
stream to DSS–6,
2. desublimating the UF6 at temperatures between
–60 to –80°C in the 5 main cylinders and in the 3
intermediate cylinders,
3. condensing excess HF at a temperature of about
–180°C in the 2 sedimentation vessels,
4. drawing off the impurities, followed by
5. sublimation and reintroduction of the UF6 into the
diffusion cascade.
Enrichment Cascade
Main Cylinders (5)
45 , 35 cm O.D.
-60 °C to -80°C
Contaminated UF 6
Purified UF 6
To Gas Purification
Figure 14. Layout of DSS–6.
During normal operations, most of the UF 6
desublimated in the main and intermediate cylinders.
The HF, containing only trace amounts of UF 6 , passed
on and collected in the 2 HF sedimentation vessels.
The air and any other non–condensable impurities
were collected downstream in a bank of 5 holding
vessels, about 4,500 l in capacity each. The contents
of these holding vessels were then transferred to the
gas purification equipment by the action of a vacuum
pump (Figure 15) that used oil as its working fluid.
About once every 15 days, it was necessary to replace
the oil because accumulation of UF 6 caused an
increase in density (from 0.9 to ~0.92, with a concentration
of ~20 g U/ l) reducing the pump’s efficiency.
While clearly having some criticality safety significance,
this replacement schedule was motivated
primarily by operational considerations.
Both the main and intermediate cylinders were
cooled by liquid nitrogen flowing through a coil
embedded in iron-shot filled jackets surrounding the
cylinders. When there was an adequate supply of liquid
nitrogen, its flow was actuated automatically by a
signal from thermal transducers (resistance thermometers).
However, when in short supply, liquid nitrogen
was fed manually from portable dewars. The sublimation
(heating) cycle was activated automatically by a
pressure sensor in the UF 6 accumulation vessel.
Bypass
Intermediate Cylinders (3)
24 , 26.8 cm O.D.
-60°C to -80°C
Vacuum Pump with Oil Reservoir
UF 6
Accumulation
Vessel
HF Sedimentation Vessels (2)
24 , 26.8 cm O.D.
-180°C
Holding Vessels (5)
4500
2 mm Steel Partition
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