A Review of Criticality Accidents A Review of Criticality Accidents

In the final experiment, the critical number of fuel
rods with the smallest pitch (7.2 mm) was measured.
This number was 1,790 and exceeded the minimum
critical number of rods for an optimum pitch by
approximately 7 times.
After the experiment was finished, the supervisor of
the experimental team ordered the insertion of all
control and emergency protection rods and the neutron
source was removed from the core. Four staff members
entered the critical assembly compartment for examination.
The supervisor then ordered the water be
removed through the fast dumping (emergency) valve.
The water from the critical assembly tank could
have been drained through the slow dumping valve in
15 to 20 minutes or through the fast dumping valve in
20 to 30 seconds. After the previous experiments, the
water had been drained through the slow dumping
valve.
The plexiglas support plate almost completely
covered the tank section, and the size of the gap
between the plate edge and tank wall was less than the
size of the fast dumping valve outlet. For this reason,
upon initiating the fast dumping operation, the
Plexiglas base plate sagged and the fuel rods fell out of
the upper lattice plate (Figure 61). The fuel rods
separated into a fan shaped array in which the pitch
between the rods came close to optimum and the lattice
went highly supercritical. The rate of reactivity
insertion was calculated to have reached ~2 β eff /s.
13. The RA-2 Facility, 23 September 1983 79
The RA–2 Facility was located in Buenos Aires,
Argentina. Control rods for this essentially zero power
experimental reactor assembly were MTR elements in
which 4 of the 19 fuel plates were removed and
replaced by 2 cadmium plates. Just outside the fueled
region (approximately 305 mm × 380 mm) was a
graphite reflector approximately 75 mm thick. The
large reactor vessel was filled with de-mineralized
water during operations and was supposed to be
drained during changes in fuel configurations when
people were required to be present.
The technician, a qualified operator with 14 years
experience, was alone in the reactor room making a
change in the fuel configuration. The moderator had
not been drained from the tank, though required by
During the criticality accident, two peripheral rows
of fuel rods were destroyed by the energy release.
Fragments of these fuel rods resembled welding rod
fragments. Water splashed out of the tank. The energy
release in the burst, estimated from the core radioactivity,
was ~5 × 10 18 fissions. This value is, in all probability,
universal for uranium–water systems in an open
tank under conditions of rapid reactivity insertion.
Excursions of this type are terminated due to fuel
destruction or loss of water.
Radioactive contamination of the critical assembly
room was minimal and there was no contamination of
the outer premises; however, the accident consequences
were tragic for the personnel involved. A
technician, who was close to the tank when the pulse
occurred received a radiation dose of ~6,000 rem and
died 5 days after the accident. The supervisor received
2,000 rem and died in 15 days. Two other staff
members inside the critical assembly room received
doses of 700 –800 rem. Physicians managed to save
their lives but both suffered long term health effects.
The construction of the critical assembly was the
main cause of the accident. No calculations were
performed for the components of the system and the
construction as a whole. Improper and hasty actions by
personnel during the final stage of the experiment also
contributed to the cause of the accident.
MTR–type fuel element, water reflected, assembly; single excursion; one fatality, two significant exposures.
procedures. Two fuel elements had been placed just
outside the graphite, instead of being removed completely
from the tank. Two of the control elements,
without the cadmium plates installed, were being
placed in the fuel configuration. Apparently, criticality
occurred as the second of these was being installed,
since it was found only partially inserted.
The excursion consisted of 3 to 4.5 × 10 17 fissions;
the operator received an absorbed dose of about
2000 rad from gamma-rays and 1700 rad from neutron.
This exposure was highly non–uniform, with the upper
right side of the body receiving the larger exposure.
The operator survived for 2 days. Two people in the
control room received doses of about 15 rad from
neutrons and 20 rad gamma–rays. Six others received
lesser doses, down to 1 rad, and 9 received less than
1 rad.
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