A Review of Criticality Accidents A Review of Criticality Accidents

12. Siberian Chemical Combine, 30 January 1963
Uranyl nitrate solution, U(90), in a collection vessel; multiple excursions; insignificant exposures.
This accident occurred in a waste recovery line of a
uranium metal production building. The waste feed
was a dry precipitate. The first step in recovering the
uranium was a time–intensive, concentrated nitric acid
dissolution process. Two sets of identical equipment
were used to increase the throughput and to optimize
the process. Figure 18 is a material flow diagram for
this process.
The solution was vacuum transferred from the
dissolution vessels to the intermediate feed vessels,
through a filter vessel, and then finally to the collection
vessels. The dissolution and intermediate vessels were
320 mm in diameter and 520 mm in height. The
collection vessels were 390 mm in diameter and
500 mm in height. All of the vessels had curved ends
and were about 50 l in volume. Since the process
vessels were not favorable geometry, limiting the
fissile material mass was the only control used to
prevent criticality accidents. Compliance with the mass
limit was solely dependent on the reliability of the
chemical analysis of the dry precipitate.
Despite the importance of chemical analysis results
for implementing the criticality safety mass control,
there were two allowed reporting formats:
Intermediate
Vessels
Figure 18. Process vessels and material flow diagram.
30
61-A
62-A
Filter
Vessel
63
Collection
Vessels
62-B
61-B
64-A 64-B
Dissolution
Vessels
1. as grams of uranium per kilogram of precipitate,
g U(90)/kg, or
2. as a mass fraction, the uranium-to-precipitate mass
ratio.
The bulk amount of precipitate to be processed was
calculated using the results of the chemical analysis.
The precipitate was then weighed before loading into
the dissolution vessels.
When dissolving consecutive batches of precipitate,
the operational procedure allowed for the recycling of
solution with low uranium concentration from the
collection vessels, as long as the total mass of uranium
in the dissolution vessels did not exceed 400 g. The
uranium concentration in the recycled solution was
determined by analyzing samples from the collection
vessels. This concentration was always reported as
grams of uranium per liter (g U/ l) and was also used
for compliance with the criticality safety mass limit.
On 30 January 1963, two waste containers were
received at the waste recovery line with the chemical
analysis results attached. These results were reported
as a mass fraction, approximately 0.18 in this case. The
supervisor for the shift erroneously recorded this result
in the work orders as 18 g U/kg of dry precipitate
(instead of 180). The actual uranium content in this
batch of dry precipitate was thus in error and underestimated
by a factor of 10.
Using the erroneous work orders, an operator
loaded 2 kg of precipitate from one container and 5 kg
from another into dissolution vessel 61-A, believing
the total uranium content to be 126 g when it was
actually about 1,260 g. Because of a shift change, a
different operator completed the dissolution process.
The solution was then filtered and transferred to
collection vessel 64-A, where a sample was taken to
determine the concentration. Later during that same
shift, the operator asked for the sample results by
telephone. While this normally would have led to the
discovery of the original recording error, the laboratory
erroneously reported the results for a vessel other than
64-A. By an unfortunate coincidence, this other vessel
contained solution with a uranium concentration
10 times lower than that in 64-A, thus reinforcing the
operator’s expectations.
The supervisor for the second shift, relying on this
erroneous information, decided to recycle this supposedly
low concentration solution to dissolve the next
batch of dry precipitate. This next batch, still subject to
the original recording error, contained ~1,255 g of
uranium, about the same as that of the preceding batch.
The dry precipitate was loaded into dissolution
vessel 61-A and then the solution from collection