A Review of Criticality Accidents A Review of Criticality Accidents

additional reflection afforded by the added poisoned
aqueous solution was apparently insufficient to
compensate for this effect.
After addition of the cadmium nitrate solution, a
hose was inserted into the holding vessel through a top
port to siphon out the solution. Surprisingly, at the very
beginning of the siphoning, a sixteenth, and final,
excursion occurred. Later, upon consideration of the
facts, it was recognized that this last excursion was a
result of siphoning from the bottom of the vessel. Even
though the draining of the cadmium nitrate decreased
reflection, it allowed the organic to assume a more
reactive geometry as it reentered the hemispherical
bottom of the vessel.
The siphoning action was continued (in spite of the
last excursion) until the vessel was drained. Altogether
about 65 l of organic with a uranium concentration of
33 g U(90)/ l, corresponding to about 2.14 kg of
U(90), were removed from holding vessel 694.
14. United Nuclear Fuels Recovery Plant, 24 July 1964 23,24
This accident occurred in a chemical processing
plant at Wood River Junction, Rhode Island, which was
designed to recover highly enriched uranium from
scrap material left over from the production of fuel
elements. The plant operated three, 8–hour shifts, 5
days a week. The scrap material was shipped to the
plant as uranyl nitrate solution in 55 gallon (208 l)
drums, at concentrations ranging from 1 to 5 g/ l of
uranium. The uranyl nitrate was then purified and
concentrated by solvent extraction using tributyl
phosphate mixed with kerosene as the organic wash.
After the final acid strip, the purified uranyl nitrate
solution was bubbled through a column containing a
fixed charge of 4 to 6 l of trichloroethane (TCE). The
TCE removed any organic that remained in the
solution. The original plant design was based on a
predicted life of 6 months to 1 year for a single charge
of the TCE.
Typical of the difficulties that should be expected
with a new operation (the plant had begun operations
on 16 March 1964) a larger amount of organic
carryover and a higher retention of uranium reduced
the TCE’s useful life to approximately 1 week. In early
April, therefore, a procedure was developed to remove
the uranium from the TCE before discarding it. The
very low concentration of uranium in the solution
(400 to 800 ppm) was recovered by washing the TCE
with sodium carbonate solution. Originally, the
During the nearly 16 hours over which the accident
took place, the plant operators and supervisors on shift
at the beginning of the excursions remained at the
facility to assist with recovery operations and to
document the events.
The total number of fissions, based on 140 La fission
product analysis, was estimated to be about 6 × 10 16 . A
prompt and efficient evacuation was initiated by the
sounding of the criticality alarm system. The largest
individual dose received was less than five rem. There
was no damage to the equipment or radioactive
contamination.
The accident occurred (despite the fact that the
vessels were steam cleaned monthly) presumably
because of a rather quick and large accumulation of
both organic and aqueous solutions. Subsequently,
changes were made to the process design and between
January and September 1964, large numbers of process
vessels were replaced. At the end of this renovation,
nearly all of the vessels were of favorable geometry.
Uranyl nitrate solution, U(93), in a carbonate reagent makeup vessel; two excursions; one fatality, two significant
exposures.
operation was performed by manually shaking or
rocking the solution within 5 inch (127 mm) diameter,
11 l bottles. This somewhat laborious process was
used until 16 July. On that date, because an unusually
large amount of contaminated TCE had accumulated,
an operator was given permission by his supervisor to
use a sodium carbonate makeup vessel to perform this
process as long as the uranium content did not exceed
800 ppm. The vessel was 18 inches (457 mm) in
diameter and 26.375 inches (670 mm) deep and was
located on the third level of a tower housing solvent
extraction columns. This new procedure was communicated
to one other operator on a different shift.
Between 16 and 24 July, these two operators washed
10 to 12 bottles each using the carbonate makeup
vessel. It should be noted that the treatment of the TCE
by any method was not part of the facility license, and
therefore was not approved by the regulating authority.
On the day before the accident, a plant evaporator
had failed to operate properly, making it necessary to
disassemble it for cleaning. During the cleaning, a plug
of uranium nitrate crystals was found in a connecting
line. The crystals were dissolved with steam, and the
resulting concentrated solution (256 g U/ l) was
drained into polyethylene bottles identical to those that
normally held the very low concentration TCE
solution. All of the bottles containing the high concentration
solution were labeled as such.
33