A Review of Criticality Accidents A Review of Criticality Accidents

3. Los Alamos Scientific Laboratory, 1 February 1951 42,43,45
Critical separation experiment, two large 235 U metal masses in water; multiple excursions; insignificant
exposures.
A water reflected system was set up in 1949 to
obtain the neutron multiplication of a single unit of
fissile metal in water. The system had two scram
devices. The first, with a quick response, consisted of a
pneumatic cylinder that raised the unit out of the water;
the second, a slower device, was the draining of the
tank. Later, a traveling support was added so that
critical separation distances between two units could
be determined; a dropping cadmium screen provided
an additional scram (Figure 45).
The excursion was precipitated by an experiment
that measured the critical separation distance of two
enriched uranium masses (each of 93.5% 235 U) in
water: one, a solid cylinder of 24.4 kg and the other, a
hollow cylinder of 38.5 kg. Sheet cadmium 10 mils
thick was fastened to the outer surface of the solid
cylinder and to the inside surface of the hollow
cylinder. A paraffin slug filled the cavity in the hollow
cylinder.
At the completion of the critical separation experiment
(at a multiplication of 65.5), the assembly was
scrammed. The water started draining, the cadmium
screen dropped, the solid cylinder (left-hand body in
Figure 45) was lifting, and an excursion (later deter-
mined to be 10 17 fissions) was made evident by the
jamming of neutron counters and the appearance on
television of a vapor cloud above the water.
Later reconstruction of the accident showed that the
pneumatic tangential scram was the first to be effective
and led directly to two types of difficulty. First, the
center of reactivity of the left-hand cylinder (Figure
45) proved to be below that of the stationary
cylinder; second, the rapid lift through the water
created hydrodynamic forces that swung the cylinders
closer together. The combination of the two effects was
enough to drive the assembly prompt critical and to
have maintained at least this much reactivity for
0.2 seconds if the power excursion had not occurred.
The first power spike is estimated to have contained
6 × 10 15 fissions. It is possible that one or more
excursions into the prompt region followed because
boiling was the primary quenching mechanism.
In this excursion of 10 17 fissions, no radiation doses
were received, and no contamination was found in the
experimental area. Damage to the uranium consisted of
a small amount of oxide flaking and blistering. The
experimental area was in use two days later.
Figure 45. The Los Alamos Scientific Laboratory assembly machine employed for measurements of critical
separation distances.
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