A Review of Criticality Accidents A Review of Criticality Accidents

On Friday, 24 July, at approximately 18:00 the
operator assigned to work the solvent extraction
columns asked his supervisor if it was necessary to
wash some of the contaminated TCE. Since the
contaminated TCE was to be used for rinsing a process
column, he was told that washing the TCE was not
necessary. Nevertheless, the operator proceeded to
locate a bottle of TCE with the intention of washing it,
perhaps only to obtain an empty bottle. Unfortunately,
the operator mistook one of the bottles containing the
high concentration solution for one containing TCE.
The bottle was transported to the stairwell leading to
the third floor location of the carbonate makeup vessel
by cart, and then hand carried the rest of the way. The
bottle’s label, which correctly characterized the
contents as high concentration solution, was found
after the accident on the floor near the cart.
After arriving at the third floor, the operator poured
the contents of the bottle into the makeup vessel
already containing 41 l of sodium carbonate solution
that was being agitated by a stirrer. The critical state
was reached when nearly all of the uranium had been
transferred. The excursion (1.0 to 1.1 × 10 17 fissions)
created a flash of light, splashed about 20% of the
solution out of the vessel and onto the ceiling, walls
and operator. The operator who fell to the floor,
regained his footing and ran from the area to an
emergency building ~180 m away.
An hour and a half after the excursion, the plant
superintendent and shift supervisor entered the
building with the intent of draining the vessel. When
they reached the third floor, the plant superintendent
entered the room and approached the carbonate reagent
vessel while the supervisor remained behind in the
doorway. The superintendent removed the 11 l bottle
(still end up in the vessel) and turned off the stirrer. He
then exited the room, passing the supervisor and
preceding him down the stairs. Unknown to anyone at
the time (the alarm was still sounding from the first
excursion), the change in geometry, created as the
stirrer induced vortex relaxed, apparently added
enough reactivity to create a second excursion, or
possibly a series of small excursions. The estimated
yield of the second excursion was 2 to 3 × 10 16 fissions
and no additional solution was ejected from the vessel.
The two men proceeded down to the second and first
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floors and began to drain the tank through remote
valves. When the drain line became clogged with
precipitate, the superintendent returned to the vessel,
restarted the stirrer, and then rejoined the supervisor
who was draining the solution into ~4 l bottles on the
first floor.
That the second excursion had occurred was not
realized until dose estimates for the superintendent and
supervisor were available. The supervisor received
~100 rad, while the superintendent received ~60 rad.
Both doses were much higher than expected and were
inconsistent with their reported actions. Only after
significant analysis was it realized that the two had
been exposed to a second excursion, which most likely
occurred just as the superintendent passed the supervisor
on the way down the stairs.
The radiation dose to the operator as a result of the
initial excursion was estimated to be about 10,000 rad.
He died 49 hours later. Other persons in the plant
received very minor doses. The investigation determined
that there had been 2,820 g of uranium in 51 l
of solution in the makeup vessel at the time of the first
excursion. No physical damage was done to the
system, although cleanup of the ejected solution was
necessary. The total energy release was equivalent to
1.30 ± 0.25 × 10 17 fissions.