A Review of Criticality Accidents A Review of Criticality Accidents

Figure 50. Burst rod and several sections of Lady Godiva showing oxidation and warpage that accompanied the
second accident, 12 February 1957.
8. Los Alamos Scientific Laboratory, 17 June 1960
235 U metal, graphite reflected, assembly; single excursion; insignificant exposures.
The critical parameters of highly enriched
(93% 235 U) uranium metal cylinders in thick graphite
(about 9 inches) and near infinite water reflectors were
being investigated. In the experiment of interest, an
approximate 48 kg uranium annulus was built up on a
cylinder of graphite that, in turn, rested on a hydraulic
lift device. This annulus was raised by remote control
into a reflector of graphite resting on a stationary steel
platform. The system became critical before complete
assembly and was scrammed both manually and
automatically at about 1 inch from closure. Following
the scram signal, the lift dropped rapidly and the
system became subcritical, but about one–third of the
metal mass stuck in the graphite reflector for a few
seconds before falling to the floor. The yield was
6 × 10 16 fissions; there was no contamination or
damage to the fissile metal. Personnel radiation doses
were immeasurably small.
This accident was, in many respects, similar to that
of Jemima (II-B.4). The reactivity sensitivity of this
particular experiment was not measured after the
power transient but, when investigators examined
similar systems, the reactivity insertion rate probably
did not exceed a few dollars per second and the initial
spike could have included 10 15 fissions.
The fission yield was very close to that of the first
Godiva accident (3 February 1954, 5.6 × 10 16 fissions),
and the two masses are quite comparable. In the earlier
case, all of the energy release took place during the
power spike and some warping of pieces and damage
to supports was seen. In this transient, the metal was
undamaged, thus supporting the assertion that the
initial power spike was small compared to the total
yield.
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