A Review of Criticality Accidents A Review of Criticality Accidents

ased on a radiation survey, that the central control
room (Figure 21) could be safely occupied by personnel
even during the excursions. Consequently, surveillance
and recovery management operations were
moved to this location.
Based on interviews with operating personnel,
examination of the accountability records and system
schematics, and the results of γ-ray surveys conducted
with portable collimated detectors, dissolution vessel 1
or its associated holding vessel was identified as the
most likely location of the accident. The surveys
indicated that the average exposure rate, 1.5 minutes
after an excursion, was ~8 R/h at a distance of 2 m
from the dissolution glovebox.
After the ninth excursion, cadmium poisoned
solution was remotely added to the holding vessel.
Despite this action, approximately 20 minutes later a
tenth excursion occurred, establishing dissolution
vessel 1 as the site of the accident with a high degree
of probability. The removal of the vessel’s contents or
addition of cadmium poisoned solution were considered
to be too dangerous to personnel because either
action would have required time intensive manual
manipulation of valves located within the glovebox.
Instead the emergency response personnel decided to
• remove two gloves from the glovebox ports to gain
access,
• unlock and open the feed hopper lid, and
• insert a ball consisting of crumpled up strips of
0.5 mm thick cadmium foil into the vessel through
the feed hopper.
The first two tasks were successfully accomplished
by two, specially briefed, experienced operators taking
no more than 30 and 60 seconds, respectively, including
the time necessary to enter and exit the operations
area. The recovery actions were then halted as the
radiation level began to rise again indicating that an
eleventh excursion was under way. After waiting for a
significant decrease in the radiation level, a senior
engineer/physicist entered the operations area and
loaded the cadmium foil ball into the vessel, taking
care to disturb the surface of the solution as little as
possible so as not to initiate another excursion. This
took about 20 seconds total. As evidenced by the
release of a characteristic orange smoke, the cadmium
foil began dissolving immediately in the nitric acid.
The introduction of the foil terminated the excursions
as shown by a steady decrease in the γ-ray exposure
rate measurements. All three of the individuals
involved in the recovery actions carried personal
dosimeters. No one received doses greater than
0.3 rem.
One day after the termination of the accident, the
solution was transferred from the dissolution vessel,
using temporary piping, to favorable geometry
containers. These containers were then sent to a special
storage facility and later reprocessed by conventional
techniques after the cadmium had been removed.
The total number of fissions for the 11 excursions
was estimated from activity measurements to be
~5.5 × 10 17 . The total volume of material in the tank
was 28.6 l, and sample analysis indicated that the
uranium concentration was 77 g/ l. Of the personnel in
the area at the time of the accident, 17 received doses
of 0.1 rem or less, 7 between 0.1 and 0.2 rem, and 3
between 0.2 and 0.27 rem. The process equipment was
not damaged and no contamination occurred. Normal
operations were resumed within several days. During
the following 2 to 3 years most of the process equipment
(~94%) was replaced with favorable geometry
vessels.
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