A Review of Criticality Accidents A Review of Criticality Accidents

13. Chelyabinsk-70, 5 April 1968 57,58
U(90) metal, natural uranium reflected, assembly; single excursion, two fatalities.
The accident occurred on 5 April 1968 at the
Russian Federal Nuclear Center (VNIITF) located in
the southern Ural mountains between the cities of
Ekaterinberg and Chelyabinsk. Criticality experiments
began at VNIITF in 1957 using the FKBN vertical lift
assembly machine. * FKBN is a Russian acronym for
“a physics neutron pile.” At that time, intensive work
was under way in the development of powerful reactors
for studying radiation tolerance. Assembled on the
FKBN in particular were a number of critical configurations
with a thick reflector and a large internal cavity.
This enabled operation in the static and pulsed mode
up to several kilowatts. In the case under consideration,
research was being conducted on the effect that a
spherical polyethylene sample would have on the
kinetic characteristics of the reactor system by means
of the boiler noise method.
The FKBN assembly machine and system components,
as configured at the time of the accident, are
shown in Figure 53. The core of the assembly consists
of a U(90) spherical metal shell with an internal cavity.
80 mm Plug
Upper Reflector
Core
30 mm
Polyethylene
Sphere
Lower Reflector
Figure 53. Approximate accident configuration of the
FKBN vertical lift assembly machine and core.
The core could be surrounded by a thick, spherical,
natural uranium metal reflector. Figure 53 illustrates
that the external reflector is split into upper and lower
halves. The accidental excursion resulted in the death
of two knowledgeable nuclear criticality specialists
standing near the assembly at the time of the excursion.
A criticality alarm system was not installed at the time
of the accident. The upper reflector was natural
uranium metal having a total uranium mass of 308 kg.
The inside radius was 91.5 mm and outside radius was
200 mm. The core was a 90% enriched uranium metal
spherical shell having an inside radius of 55 mm and
an outside radius of 91.5 mm. The core uranium mass
was 47.7 kg of uranium or 43.0 kg of 235 U. The
uranium core was constructed of nested, close fitting,
hemispherical components with radii of 55 mm,
67.5 mm, 75.5 mm, 83.5 mm, and 91.5 mm. A polyethylene
sphere had an outside radius of 55 mm. The
lower reflector was natural uranium with an inside
radius of 91.5 mm and an outside radius of 200 mm.
The accident occurred on a Friday evening after
normal working hours. Earlier that same day an
assembly had been constructed on the FKBN machine.
Two specialists present for the daytime assembly
decided to continue working into the evening in order
to complete a second assembly. The evening assembly
was to be a repeat of the daytime assembly with one
variation—a solid polyethylene sphere was to be
inserted into and fill the cavity of the core which was
void for the daytime assembly. Using a hand-held
controller panel, the senior specialist operated an
overhead tackle to lower the upper half of the reflector
to make contact with the core (the operation of
lowering the upper-half of the reflector could not be
carried out under remote control). The junior specialist
stood next to the FKBN with both hands on the upper
reflector to guide it into place. The accident occurred
as the upper half of the reflector was being lowered
onto the core and was about to make contact with it.
The emergency instrument system was operating and
responded after the power level of the excursion
reached the kilowatt level. The system dropped the
lower half of the reflector, which was sufficient to
drive the system deeply subcritical and terminate the
excursion.
The senior specialist made an error of judgment
when he expected the polyethylene sphere to have a
small effect on system reactivity. The investigation also
* The prototype of the FKBN was developed and used earlier at the Arzamas-16 research center (VNIIEF). It is mentioned in the
reminiscences of A. D. Sakharov. See also proceedings of ICNC’95, Vol. 1, paper 4-44, “Criticality Measurements at VNIITF
Review,” V. A. Teryokhin, V. D. Pereshogin, Yu. A. Sokolov.
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