A Review of Criticality Accidents A Review of Criticality Accidents

a technician prescribing the loading of a manual rod
followed by the unloading of another one. The technician
did not wait until the moving control rod reached
its bottom position and started the manipulation in the
wrong order. He first extracted a manual rod instead of
first inserting the other.
During the extraction of the manual rod the reactor
became critical. The technician had his left foot
projecting over the edge of the tank and resting on a
grating that was about 50 mm above the reflector; his
right foot and leg were somewhat behind him and
partly shielded. He noticed a glow in the bottom of the
reactor, immediately dropped the control rod, and left
the room.
11. Kurchatov Institute, 15 February 1971 77
Experiments to evaluate the relative effectiveness of
iron and metallic beryllium as a reflector on a power
reactor core were in progress at the critical experiment
facility, SF-7. The core measured 1,200 mm high and
1,000 mm in diameter and held 349 fuel rods. Criticality
was obtained by adding water to the core and
immersing the fuel rods. The safety rods consisted of a
lattice of boron carbide rods that could be inserted
throughout the core to compensate for the operative
reactivity margin. The boron carbide lattice did not
cover the three outer rows of fuel rods. The fuel rods
were enriched to about 20% 235 U (typical of icebreakers).
The first stage of the experiment consisted of a core
configuration in which the neutron flux was nonuniformly
distributed along the core radius. Measurements
showed that the completely water flooded core
with the boron carbide safety rod lattice inserted was
deeply subcritical (~10%), and the reactivity increased
only slightly (+0.8%) after replacing the radial iron
reflector with one of beryllium.
The second stage of the experiment consisted of a
core configuration in which the neutron flux was
uniformly distributed along the core radius. One
hundred forty–seven fuel rods with the maximum
loading of burnable neutron absorber were inserted
into the central part of the core that was heavily
poisoned by a safety rod lattice. Two surrounding rows
of rods (118 rods) containing less absorber, in this case
a burnable poison, were then added. The outermost
row (84 rods) did not contain burnable neutron
absorber material.
The energy release was 4.3 × 10 17 fissions and,
apparently, the excursion was stopped by the falling
manual rod, although the scram may have been
speeded up by a combination of the Doppler effect and
emptying of the vessel, which was automatically
“provoked.” This is uncertain.
No steam was created, no damage was done to the
fuel, and there was no contamination. The technician
received a severe radiation dose, primarily gamma–
rays. Eight days later and after 300 measurements in a
phantom, rough estimates were that the dose to his
head was 300 to 400 rem, to his chest 500 rem, and to
his left ankle 1,750 rem. At the end of his foot the dose
approached 4,000 rem. Medical treatment of the
patient was successful, except that the left foot had to
be amputated.
U(20)O 2 fuel rod, iron and beryllium reflected, assembly; multiple excursions; two serious exposures.
The second stage of the experiment, according to
the plan, began with the beryllium reflector in place
because it was in place at the end of the first stage.
However, criticality calculations for this core configuration
were performed only for an iron reflector. Based
on results of the comparison between beryllium and
iron reflectors for the first configuration, the supervisor
of the experimental team determined that substituting
iron for beryllium would not result in any considerable
increase in reactivity. Therefore, additional calculations
were not performed.
The core configuration with the beryllium reflector
was assembled in the dry critical facility tank and left
for the night. The next morning, the supervisor entered
the facility control room (Figure 60) and without
waiting for the arrival of the control console operator
and the supervising physicist, switched on the pump,
and began adding water to the critical assembly tank.
The supervisor considered the system to be far from
critical. The control equipment was switched on, but
the neutron source had not been placed in the critical
assembly, and the control rods were not actuated.
A scientist arrived from Gorky who was training at
the SF-7 experimental facility and was standing near
the critical assembly tank discussing the experiment
with the supervisor. Suddenly they saw a blue luminescence
reflecting from the ceiling and heard a rapidly
increasing signal from an audible neutron flux indicator.
They thought that something had happened in
another facility and ran from the critical assembly
room. Other workers who were in the room also left.
The manager of the facility was informed of the event.
The manager and a dosimetry technician tried to enter
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