A Review of Criticality Accidents A Review of Criticality Accidents

The investigation identified several contributing
factors to the accident:
• The shift supervisor’s decision to take actions that
were improvised, unauthorized, and against all
regulations, to recover from the plutonium mass
limit excess in tank 2.
• Changes to the original piping system that had
precluded organic solution transfers to the aqueous
solution tanks. As a result of these changes, organic
solution could be sent to these tanks in three different
ways. These included, (1) the operation of cer-
18. Windscale Works, 24 August 1970 25,26,27
Plutonium organic solution in a transfer vessel; one excursion; insignificant exposures.
This criticality accident is one of the more interesting
and complex because of the intricate configurations
involved. The plant was used to recover plutonium
from miscellaneous scrap, and the processes used were
thought to be subject to very effective controls.
Recovery operations started with a dissolver charge of
about 300 g of plutonium. Following dissolution, the
supernatant was transferred through a filter to a
conditioner vessel, where the concentration was
adjusted to between 6 and 7 g Pu/ l, less than the
minimum critical concentration.
The solution was vacuum lifted from the conditioner
to a transfer vessel (Figure 25). When the transfer was
Dissolver
Vacuum Line
Conditioner
~32 ft
tain valves out of the proper sequence, (2) through
the vacuum and vent lines in the event of stop–valve
failures, and (3) through the purposeful transfer of
aqueous solution containing held up organic solution
from the extraction facility
• A transfer from tank 1 to tank 2 of about 10 l solution
with an unknown plutonium content
on 10 December 1968 between 07:00 and 13:00.
The small scale organic solution research and
development operation was discontinued in this
building as a result of the accident.
completed, the vacuum was broken and the transfer
vessel contents were allowed to drain into a constant
volume feeder that supplied a favorable geometry,
pulsed, solvent extraction column. The connection
from the transfer vessel to the constant volume feeder
was through a trap 25 feet (7.6 m) in depth, that
prevented any potential backflow and thus controlled
contamination.
The excursion occurred on completion of the
transfer of a 50 l batch of solution from the conditioner
to the transfer vessel. The small size
(10 15 fissions) and brief duration (less than 10 s) of the
excursion precluded the termination of the excursion
Transfer
Vessel
25 ft
Recycle
Constant
Volume
Feeder
Solvent
Figure 25. Process equipment related to the criticality accident.
Product
Pulsed Column
Raffinate
Waste
43