A Review of Criticality Accidents A Review of Criticality Accidents

4. Los Alamos Scientific Laboratory, 18 April 1952 44,45
Jemima, cylindrical, unreflected 235 U metal assembly; excursion history unknown; insignificant exposures.
The system in which the excursion took place was a
cylindrical, unreflected, enriched 93% 235 U metal
assembly made up of a number of plates, each 267 mm
in diameter and 8 mm thick.
Complete assembly of the two components had
been made previously with six plates in the lower
component, but with first three and then four plates in
the upper component.
A plot of the reciprocal multiplication versus
number of plates, or total uranium, shows clearly that
the system should not have been assembled with
11 plates. Nevertheless, such an assembly was attempted
following a computational error made
independently by two people. Contrary to operating
regulations, a graph of the data had not been plotted.
The burst yield was 1.5 × 10 16 fissions.
There is no way to determine the power history
experienced by the 92.4 kg mass without reproducing
5. Sarov (Arzamas-16), 9 April 1953 49
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the experiment. At the time the system was near
prompt criticality, the lower component was coasting
upward and probably inserting no more than 2 or 3 $/s,
a rate that could cause a power spike of about
10 15 fissions. The power would then stabilize at about
10 17 fissions/s, just enough to compensate for the
reactivity insertion rate. Most of the 1.5 × 10 16 fissions
must have occurred in this plateau. The power dropped
essentially to zero when the automatic scram system
separated the two masses of metal.
During the remotely controlled operation no
damage was done to the system, even to the fissile
material. None of the personnel received any radiation,
and the experimental area was not contaminated. The
apparent self terminating property of this excursion
stimulated study with Lady Godiva, 46,47,48 which
became a facility for generating large bursts of fission
spectrum neutrons in less than 100 µs.
Plutonium, natural uranium reflected, assembly; single excursion; insignificant exposures.
This accident occurred on 9 April 1953 as an
experiment was being conducted on a critical assembly
constructed on a vertical split table, FKBN (Figure 46).
FKBN, designed and built in 1950, had a hydraulically
driven vertical table as its main feature. It was not,
however, equipped with a fast acting, gravity driven
scram mechanism. FKBN located in Building B
(Figure 47), was operated remotely from an adjacent
control room.
The critical assembly involved in the accident had
an ~100 mm outside diameter plutonium core (about 8
kg mass) surrounded by a 300 mm outside diameter
natural uranium reflector. The core was composed of
four hemispherical shells (hemishells) of δ-phase
plutonium with a thin nickel coating. The core had a
28 mm diameter central cavity in which a neutron
source (~107 n/s) had been placed. The reflector was
composed of six nesting hemishells that had a 26 mm
diameter channel at their plane of separation.
The assembly being constructed was separated into
two pieces (Figure 46):
1. the upper part on a fixed support consisted of a
single natural uranium hemishell, with inner and
outer diameters of 120 mm and 300 mm,
respectively; and
2. the lower part, constructed on the table, consisted of
three lower natural uranium hemishells, the
plutonium core, and two upper natural uranium
hemishells.
Figure 46. FKBN and assembly involved in the
9 April 1953 accident.
Reflector
(Natural U)
Core (Pu)
Steel Stops
Duralumin
Plate
Neutron
Source
Reflector
(Natural U)
Hydraulic Lift