A Review of Criticality Accidents A Review of Criticality Accidents

completed the construction of the lower half of the
assembly on the table (Figure 56). This lower half
consisted of the copper reflector (with an outside
diameter of 258 mm since that was all that was
available) and a uranium core (9 spherical layers, with
an inside diameter of 28 mm and an outside diameter
of 167 mm) with a neutron source (~10 5 n/s) placed in
its center. As he went to position the first upper copper
hemishell (with an inside and outside diameters of
167 mm and 183 mm, respectively) into place it
dropped onto the bottom assembly. This led to the
initial prompt critical spike and the ~6.5 day excursion.
The automatic scram system activated and dropped the
table to its down position 60 but otherwise had no effect
on the supercritical assembly. After the initial fission
burst, the assembly eventually reached steady state
power conditions.
Seeing a flash of light, the experimenter left the
experimental cell, shutting the safety door behind him.
He reported the accident to the engineer and attending
health physicist who were in the control room at the
time. The head of the laboratory and the facility chief
were then notified by phone. The facility chief arrived
shortly thereafter, at which time the experimenter
related the composition and dimensions of the assembly
as documented in his logbook. 60
The accident burst conditions were reproduced from
measurement of the assembly’s neutron flux (19 June
1997) and from computations shown in Figure 57
Support
Core
(HEU)
Reflector
(copper)
Neutron
Source
Figure 56. Accident configuration.
(these computations were performed by
Dr. V. F. Kolesov and Dr. V. Kh. Khoruzhy). The
asymptotic power was estimated to be 480 watts,
corresponding to an event with an initial excess
reactivity 6.5 ¢ above prompt critical, i.e.,
∆k = 0.00745. The initial superprompt critical spike of
~4 × 10 15 fissions was followed by a subprompt critical
excursion lasting 3 to 5 minutes and generating
~2 × 10 17 fissions. This second excursion was followed
by highly damped power oscillations separated by
about 40 minute intervals. Several hours afterwards,
the assembly power, and the core and reflector temperatures
had reached equilibrium levels. The temperature
of the uranium core was estimated to have been
865°C.
The excursion was terminated around 00:45 on
24 June 1997 when a vacuum gripper was used to
remove most of the assembly from the stand, leaving
only the outermost lower copper hemishell in place.
The removed portion of the assembly was placed onto
a support stand in the room (Figure 58). All the
recovery operations were done remotely.
Later, another two hemishells from the lower
reflector were removed, and the system was then
separated into two parts and packaged into containers.
After the γ-radiation decreased to allowable levels, it
was planned to further disassemble the system (into
single components) to restore the damaged coppernickel
protective coating on fissile material parts.
Ring
Assembly
Support
Support
Pipe
Lower Support
Hydraulic Lift
Carriage
Cart
91