A Review of Criticality Accidents A Review of Criticality Accidents

14. Aberdeen Proving Ground, 6 September 1968 59
Uranium–molybdenum metal fast burst reactor; single excursion; insignificant exposures.
The Army Pulse Radiation Facility Reactor
(APRFR), located in Maryland in the United States,
was another of the series of Godiva-like reactors. The
APRFR design evolved from the Health Physics
Research Reactor of the Oak Ridge National Laboratory
and was intended to provide large values of
neutron flux and fluence.
During pre-operational testing, several minor
variations in the reactor configuration were studied in a
program to optimize performance. During this testing,
an unexpectedly large burst (6.09 × 10 17 fissions)
occurred. It exceeded, by about a factor of three, the
maximum burst size the reactor was expected to
withstand without damage. Internally the core reached
the melting point of the fuel, 1150°C. The initial period
was measured as 9.1 µs, and the reactivity was estimated
to have been about 18 ¢ above prompt criticality.
The planned excess reactivity for this burst was 8.05 ¢,
which was expected to result in a burst of 1.68 × 10 17
fissions.
15. Sarov (Arzamas-16), 17 June 1997 49,60,61,62
U(90) metal, copper reflected, assembly; multiple excursions; one fatality.
This accident occurred at 10:40 on 17 June 1997 as
an experimental assembly was being hand constructed
on a vertical split machine, FKBN-2M. FKBN-2M is
located in an experimental cell (12 m × 10 m × 8 m) of
a dedicated building (Figure 54) at a reactor site. 60 A
schematic of FKBN-2M is shown in Figure 55. The
lower part of a research assembly is constructed on a
table that can be moved vertically up and down using
hydraulics, although the table had to be in the up
position during construction of an assembly. The upper
part is placed on a stand consisting of a ring, upper and
lower support, and a carriage that can be moved
horizontally and into position over the lower part of the
system. Closing of the assembly (by moving the
carriage over and the table up) is performed remotely
from a control room behind an ~3 meter thick concrete
shielding wall (Figure 54). FKBN-2M is equipped with
a fast acting gravity driven scram that automatically
causes the table to drop to its down position if the
neutron flux exceeds a preset value.
Post accident analysis indicates that the extra
reactivity resulted from a reactor configuration such
that the burst rod passed through a reactivity maximum
before seating. This condition had not been recognized;
apparently on previous operations the burst rod had
reached its seated position before the arrival of an
initiating neutron. In the absence of a strong neutron
source, wait times before an excursion occurs can be
long. 97
Damage was limited to the fuel components of the
reactor and included some warping and spalling as
well as elongation of bolts. The four central rings fused
together at the inner surface and experienced some
cracking.
There were no detectable external or airborne
radiation hazards and no personnel overexposures.
Various sets of nesting fissile (uranium, plutonium)
and nonfissile hemishells (steel, copper, polyethylene,
etc.), standardized in size, exist to allow for a variety of
experimental configurations. Assemblies are constructed
by consecutively stacking the hemishells
together into the desired configuration.
On 17 June 1997, an experimenter working alone
and without having completed the proper paper work
(both violations of safety requirements), was constructing
an experimental assembly consisting of a core of
uranium, U(90) reflected by copper. 61,62,63 The
experimenter was an expert who was confident that he
was recreating an assembly that had been the focus of
an experiment performed successfully in 1972. He had
taken the dimensions for all of the system components
from the original 1972 logbook. However, when he
copied down the inside and outside dimensions of the
copper reflector (167 and 205 mm, respectively) he
incorrectly recorded the outside dimension as 265 mm.
Using this incorrect information, the experimenter had
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