A Review of Criticality Accidents A Review of Criticality Accidents

A. FISSILE SOLUTION SYSTEMS
1. Los Alamos Scientific Laboratory, December 194937,38 Water boiler reactor; control rods removed by hand; single excursion; insignificant exposure.
This accident occurred when two new control rods
(poisons) were being tested in the water boiler reactor.
The water boiler was a 12 inch diameter stainless steel
spherecontaining 13.6 l of uranyl nitrate. In 1949 it
was reflected by thick graphite.
The rods had been installed, and the operator was
manually checking their drop times. After several tests
of each individual rod (a safe procedure since one rod
was sufficient to maintain subcriticality) both rods
were pulled, held for about 5 seconds, and then
dropped simultaneously. A short time later the rods
were again pulled and dropped together.
2. Hanford Works, 16 November 1951 39
Plutonium solution assembly; cadmium rod removed too rapidly; single excursion; insignificant exposures.
The critical assembly in which the excursion
occurred was an aqueous solution of 1.15 kg of
plutonium in the form of plutonium nitrate contained
in an unreflected 20 inch diameter aluminum sphere.
The purpose of the experimental program was to
determine the critical mass of plutonium for various
container geometries and solution concentrations. The
excursion occurred during the approach to criticality,
when the sphere was 93% full, as a result of withdrawing
a remotely controlled, hollow cadmium safety rod
in a series of steps with insufficient time between
steps. The excursion yield was 8 × 10 16 fissions, and a
small amount of fuel was forced through gaskets at the
top of the reactor assembly. Because the gaskets sealed
about 18 l of air above the fuel level before the
accident, pressures considerably in excess of atmospheric
must have existed in the assembly during the
accident.
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The removal of the two rods increased the reactivity
to about 3 cents over prompt criticality, corresponding
to a period of 0.16 seconds. The power probably rose
with this period to a very broad peak of 2 or 3 × 10 16
fissions/s and remained close to this value for about 1.5
seconds. The excursion was not immediately detected
because all the instruments were turned off except for a
direct reading thermometer that showed a temperature
rise of 25°C, equivalent to a yield of 3 or 4 × 10 16
fissions.
The operator received a 2.5 rad dose. The reactor
was not damaged.
The published data suggest that the reactivity
insertion rate resulting from the safety rod withdrawal
must have been about 4.7 $/s, which would lead to a
fission yield of about twice the observed value if
known temperatures and void coefficients of reactivity
are used. In this case, however, the action of the scram
circuit was sufficiently fast that the cadmium rod most
probably contributed to the shutdown of the excursion.
A slight reduction in the assumed reactivity insertion
rate would lengthen the time, making it even more
certain that the excursion was stopped by the falling
poison rod.
No personnel were injured in this excursion,
although plutonium nitrate solution contaminated the
experimental area. The building was successfully
decontaminated in a few days, but before cleanup of
the test area was completed, a fire occurred and the
building was abandoned.