A Review of Criticality Accidents A Review of Criticality Accidents

heater and stirrer were turned off again, leading to a
second excursion at 01:10. This was apparently no
larger than the first, as it did not trip detectors that had
not tripped during the first excursion.
After this second excursion the plant and building
managers arrived at the accident site along with the
manager of the safety organization and supporting
physicists. They then directed further recovery operations.
Attempts to remotely drain vessel 2 by various
means continued until a third and final excursion
occurred at 01:55. Detectors which had not previously
tripped (some as far away as 150 m) were activated,
indicating that this third excursion was the largest. At
this time the stirrer and heater were once again turned
on (to keep the system subcritical) and left on until the
solution could finally be transferred from vessel 2.
The final draining of the vessel was a two–part
process. First, about one–half of the solution in
vessel 2 was transferred to an aqueous collection vessel
and then from there partitioned into several bottles.
This procedure was then repeated with the remaining
solution in vessel 2. All the bottles were stored in an
isolated room, and the contents were reprocessed only
after the radiation levels decreased to acceptable levels.
The trip level of the criticality alarm detectors was
110 mR/h. They were spaced a maximum of 30 m
apart but were commonly much closer. Fifteen minutes
after the first excursion the exposure rate in the vicinity
of the glovebox was about 2.2 R/h. Thirty minutes after
the third excursion the exposure rate in the vicinity of
the glovebox was about 1.8 R/h.
Ultimately it was determined that there had been a
total of 1,324 g of plutonium in the dissolution vessel
Stirrer Motor
Water
Nitric Feed
Reagent Addition Line
Product Discharge Line
Inspection Light
Stirrer
Steam/Water Jacket
Dissolution Vessel #1 Dissolution Vessel #2
Figure 17. Layout of glovebox equipment.
at the time of the accident. This was about three times
the criticality safety limit. The recovered nitric solution
contained 933 g of plutonium and a very rich precipitate
was found in the bottom of vessel 2. It contained
391 g of plutonium in 660 g of solids; the remainder
was mostly graphite pulp from molds. Using approximate
techniques, the energy release for all three
excursions was estimated at 2 × 1017 fissions. The
excursion history was not recorded. There was minor
ejection of solution from the vessel onto the glovebox
floor, likely from the third excursion.
Several factors contributed to the accident:
• Unfavorable geometry equipment.
• The charging of high grade residues into the dissolution
vessel when the criticality controls were
based on residues with an average 1% plutonium
content.
• Inadequate isolation of high grade residues from the
more common low grade residues in the staging
glovebox.
• Unclear and difficult to read labels on the residue
cans.
• Procedural violations on the sequence of reagent
additions.
• Inadequate supervisory monitoring of operations;
inadequate attention to the completion of accountancy
documents.
• Lack of real time fissile material accounting instrumentation.
No equipment damage occurred and only a short
downtime was needed to clean up the spilled solution.
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