A Review of Criticality Accidents A Review of Criticality Accidents

vessel 64-A was added. The dissolution vessel thus had
over 2,500 g of uranium, an amount close to the
unreflected critical mass for this diameter vessel. The
solution was then filtered in 10 l batches and transferred
to the larger diameter collection vessel 64-B. In
the course of this filtering/transfer operation, the
solution exceeded the critical height in 64-B, and the
first excursion occurred at 18:10 on 30 January 1963.
The criticality alarm system sounded and all personnel
evacuated. The γ-ray detectors were set to activate at
110 mR/h. Several of the personnel were directed to
undergo medical assessment.
During approximately the next 10 hours, eight
additional excursions occurred, decreasing in power
each time. The total number of fissions was estimated
to be 7.9 × 1017 . This was based on a sample analysis
that determined the total 140La fission product content
after the accident. The shutdown mechanisms for the
excursions were
• expansion caused by the formation of radiolytic gas
bubbles,
• ejection of some of the solution into the service
piping,
13. Siberian Chemical Combine, 2 December 1963
This accident occurred in an enriched uranium,
U(90), reprocessing and purification facility. Operations
were being conducted on four 6–hour shifts per
day. The combination of an unfavorable geometry
holding vessel and the unplanned accumulation of
much larger than expected quantities of organic
solutions led to the accident.
The normal use of the vacuum system routinely
resulted in the accumulation of small amounts of
solution within the vacuum system as drops and
condensate. In addition, occasional operator mistakes
resulted in the overflow of process vessels, again
resulting in solution entering the vacuum system. In
order to protect the vacuum system from the corrosive
effects of this solution, and to prevent loss of solution
from the process stream, two traps and a holding vessel
were installed in the vacuum line. The traps, 696 and
697 in Figure 19, were intended for the collection of
solutions that entered the vacuum lines. Holding vessel
694 served as a backup in the event that either of the
two traps filled to preset levels. This would result in the
automatic draining of some of their solution. All three
vessels had straight cylindrical sidewalls of 500 mm
diameter with hemispherical bottoms and a volume of
about 100 l. The vessels were spaced about 1.5
meters, surface–to–surface.
• thermal expansion of the solution (decreased density),
and
• an increase in the solution temperature, which
tended to harden the neutron spectrum.
Each time the solution drained back into the vessel
from the service pipes and cooled down another
excursion took place. The excursions were terminated
on 31 January 1963 at 04:30, when part of the solution
was drained from the collection vessel to portable 5 l
containers.
The accident investigation determined that a total of
2,520 g of uranium in a volume of about 35.5 l
(~71 g/ l) was in the collection vessel at the time of the
accident. All of the solution (35.5 l) was stored in a
concrete shielded room for one year, then reprocessed.
Four people standing at a distance of 10 m from the
collection vessel received radiation doses of 6 to
17 rad. No damage occurred to the vessel nor was there
any contamination of the surroundings. The process
was inoperative for no more than 12 hours.
Uranium organic solution, U(90), in a vacuum system holding vessel; multiple excursions; insignificant
exposures.
Each vessel had a level indicator that was actuated
when an electrically conductive solution reached a
preset height. For traps 696 and 697, the preset height
was only half as high as for vessel 694. When the level
indicator in either 696 or 697 was actuated, the transfer
of solution occurred automatically until the level was
drawn down (decanted) to the bottom of the dip tube.
However, when the level indicator in holding vessel
694 was actuated, its entire contents were drained via
the outlet line.
When these vessels were installed, it was believed
that only high conductivity aqueous solutions would
accumulate in them. However, because organics such
as tributylphosphate were transferred in this facility
using this same vacuum system, considerable amounts
of organic solution were in fact also ending up in these
vessels. The very low electrical conductivity of these
organic solutions was insufficient to actuate the level
indicators.
During typical operations, sufficient quantities of
aqueous solutions would accumulate in vessels 696
and 697, causing the automatic decanting to occur up
to four times per day. The elevation difference between
the level indicator and the dip tube was slight, thus
causing only 1 or 2 l to be transferred at a time.
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