A Review of Criticality Accidents A Review of Criticality Accidents

9. Oak Ridge National Laboratory, 10 November 1961 53
235 U metal, paraffin reflected, assembly; single excursion; insignificant exposures.
This power transient in about 75 kg of highly
enriched (about 93% 235 U) uranium metal reflected
with paraffin took place when one portion on a vertical
lift machine was approaching the other stationary
portion. The experiment was the last of a series during
which uranium or paraffin had been added by increments
to change the reactivity of the complete system;
all previous experiments had been subcritical when
fully assembled. In this case, the system became
supercritical while the lift was in motion, leading to a
yield of between 10 15 and 10 16 fissions.
The closure speed of the lifting device was
16 inches per minute; delayed criticality was later
determined to be at a separation distance of 2.7 inches.
The sensitivity of the system at this point was
10. Sarov (Arzamas-16), 11 March 1963 54
84
8.6 $/inch. Thus, the reactivity insertion rate was
2.3 $/s and a lift slowdown, which became effective at
1.94 inches, did not affect the course of the transient.
The reactivity and power histories must have been
similar to those of the Jemima (II-B.4) accident, except
that the pertinent scram delay time was only 50 ms in
this case. The initial spike could not have exceeded
10 15 fissions, and the remaining energy must have been
created during the subsequent plateau. The appearance
of the metal (smooth, no oxide) and the fact that the
paraffin did not melt qualitatively confirmed the yield
estimate of 10 15 to 10 16 fissions. Personnel radiation
doses were trivial, and the laboratory was ready for
normal use within 1.5 hours.
Plutonium, lithium deuteride reflected assembly; inadvertent closure; single excursion; two serious exposures.
This accident occurred on 11 March 1963 as
modifications and maintenance were being performed
on a vertical split machine, MSKS. MSKS (Figure 51)
was located within Building B at a reactor site (Figure
52) and was used to perform experiments other
than critical approaches. It was limited to assemblies
whose shape, composition, maximum subcritical
multiplication (≤1,000), safe assembly–disassembly
procedures, and the contribution of fixtures and
equipment in use had already been verified by experiments
on the vertical split machine, FKBN-1, located
in the same Building B (Figure 52). MSKS was
equipped with an automatic gravity driven scram
system and closure of an assembly was performed
remotely from a control panel behind concrete shielding
(Figure 52).
The assembly involved in the accident had a
135 mm outside diameter plutonium core surrounded
by a ~350 mm outside diameter lithium deuteride
reflector (Figure 51). The core was composed of a set
of δ–phase plutonium metal hemishells in a thin
(0.1 mm) nickel coating. The core had a 63 mm central
cavity in which a neutron source (~10 6 n/s) had been
placed. The neutron reflector was a set of LiD
hemishells, ~107 mm thick.
On 9 March 1963, the MSKS chief and operations
engineer constructed the assembly on MSKS and
conducted approach to critical experiments without
first performing the required experiments on FKBN-1.
Pu Core
Neutron
Source
Liquid
Damper
Electromechanical
Drive
Figure 51. MSKS and assembly involved in the
11 March 1963 accident.
LiD Reflector
Steel Ring
LiD Reflector
Steel Cup
Emergency
Block
Support Pipe
Bedplate
Electrical
Drive Rod