A Review of Criticality Accidents A Review of Criticality Accidents

During the next shift, radiation safety supervisors
decided that the room could be reentered since the
remote readout of the criticality alarm system detector
measured a radiation level of less than 0.15 R/h.
Working from special instructions, three operators
entered the room. Consciously limiting their time in
the room and using three 20 l bottles, they were able
to remove the 5 l of solution in R3 and make two
transfers of 6 and 8 l from the holding vessel. The
three 20 l bottles were placed in a specially designated
storage location with the contents eventually
being returned to the recovery process.
An investigation was carried out to reconstruct the
events leading up to the accident. The investigation
determined that the accident occurred because the mass
limits per batch in vessel R0 had been exceeded.
Table 5 presents the chronology leading up to the
overloaded batch in vessel R0.
Nitric waste solutions deposited in vessel R0
typically had a plutonium concentration of a few grams
per liter. Criticality safety for the entire operation
relied entirely on the mass limit for vessel R0. This
mass limit was 400 g of plutonium. As can be seen in
Table 5, the mass limit was exceeded on 2 December
when the third batch of waste solution arrived at R0.
Following the fourth transfer of solution on 3 December,
vessel R0 contained a total of 682 g of plutonium.
However, the shift production engineer deliberately
changed the R0 vessel log to show only 400 g in the
vessel. On 4 December the oxalate precipitate in
vessel R0 was dissolved and transferred in the form of
plutonium carbonate solution to an empty vessel, R2,
in glovebox 10. The same shift production engineer
then violated procedures again by transferring an
additional 30 l of carbonate solution to R2. This latter
transfer contained 115 g of plutonium. Consequently
vessel R2 contained a total of 798 g of plutonium in a
little less than 50 l of solution in the early evening of
5 December. The plutonium in R2 was then
precipitated with approximately 15 g of plutonium
discharged with the supernatant. The precipitate was
again dissolved, and the carbonate solution was
transferred to the holding vessel, leading to the
criticality accident.
Following the accident, the vacuum system trap, the
holding vessel, and vessels R2 and R3 were carefully
flushed. The flushing of the holding vessel resulted in
40 l of discharge which contained 180 g of plutonium.
The contents of the three 20 l bottles which had
collected the contents of the holding vessel and
vessel R3 were analyzed. Table 6 presents the results
of these analyses.
Table 6. Analysis of Accident Solution Recovered
from Holding Vessel and R3.
Bottle
Number
Volume
( l)
Plutonium
Conc. (g/ l)
Plutonium
Mass (g)
1 5 39.4 197.0
2 6 37.7 226.0
3 8 36.4 291.0
Total 19 714.0
To reconstruct the volume and plutonium mass that
were in the holding vessel at the time of the accident,
the 714 g from Table 6 were combined with the 180 g
from the holding vessel flush to obtain 894 g of
plutonium in 19 l of solution and precipitate. The
flushing of vessel R3 resulted in 10 l of flush containing
43 g of plutonium recovered from non–dissolved
precipitate. The total mass of plutonium recovered
from all the vessels plus the three 20 l bottles was
1003 g, suggesting that 66 g was held up in vessel R2.
During the accident and the subsequent cleanup
phase, five individuals received doses in the range
0.24 rem to about 2.0 rem. There was no contamination
or damage to any of the equipment. Following the
accident, the holding vessel was immediately replaced
with a favorable geometry vessel. The total fission
yield was estimated to be about 2.5 × 10 17 fissions.
Table 5. Chronology of Batch Composition Leading to Overloading of Vessel R0 in Glovebox 9.
Date
Volume
( l)
Concentration
(g Pu/ l)
Pu Transferred
(g)
Pu Discharged
(g)
Running total of
Plutonium in R0 (g)
1 Dec 160 1.67 267 51.0 267 – 51 = 216
2 Dec 80 2.20 176 21.0 216 + 176 – 21 = 371
2 Dec 100 3.28 328 39.0 371 + 328 – 39 = 660
3 Dec 80 0.58 46 24.0 660 + 46 – 24 = 682
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