A Review of Criticality Accidents A Review of Criticality Accidents

11. Lawrence Livermore Laboratory, 26 March 1963 55
235 U metal, beryllium reflected, assembly; single excursion; insignificant exposures.
The critical assembly consisted of concentric
cylinders of highly enriched uranium metal surrounded
by a beryllium reflector. The total enriched uranium
mass of 47 kg was divided into two parts with the
central core on a lift device and the larger diameter
rings with the reflector on a fixed platform. The
approach to criticality was to be achieved by lifting the
core in a series of steps into the reflected annulus. The
experiments were performed in a heavily shielded vault
adjacent to the area where the Kukla reactor produced
prompt bursts of neutrons.
This step wise assembly procedure was successfully
followed for seven multiplication measurements. After
the eighth apparently normal assembly, the system
suddenly became highly supercritical. An explosive
sound was heard, scrams and alarms were actuated,
and after a few seconds, the uranium could be seen
melting and burning. The yield was later measured at
12. White Sands Missile Range, 28 May 1965 56
The success of the Godiva reactor in creating very
sharp, intense bursts of near fission spectrum neutrons
stimulated the development of several similar reactors
for production of pulsed irradiations. One of these was
the White Sands Missile Range (located in southern
New Mexico in the United States) fast burst reactor,
which was composed of 96 kg of an alloy of highly
enriched uranium and 10 wt% molybdenum. This
reactor design was somewhat similar to the Godiva II
reactor 42 —seven rings and a top plate all of which
partially enclosed a large central volume that at
criticality was filled with a safety block. Two control
rods and a burst rod penetrated the rings. The assembly
was held together by three metal bolts. Initially, the
bolts were stainless steel, but just prior to the accident
they were replaced by bolts composed of the uranium–
molybdenum alloy, and re–calibration of the reactivity
worth of various components was underway. The new
worth of the control rods, burst rod, minor components,
and the first inch withdrawal of the safety block
had been measured.
Further calibration of the safety block seemed to
require higher neutron flux than that given by a
polonium–beryllium neutron source. To obtain a power
of about 1 watt, an interlock was bypassed and the
86
3.76 × 10 17 fissions, but little or no explosive energy
was generated. About 15 kg of uranium burned and
about 10 kg melted and spread over the floor. Dose to
personnel in or near the building was low and in no
case exceeded 0.12 rem. The reactor room was highly
contaminated.
The accident is believed to have been caused by the
central cylinder of metal on the lift being very slightly
off center. When it was lifted into the fixed half, one or
more of the metal rings were carried upward. Following
the eighth assembly, the system adjusted itself and
the rings settled properly around the central core,
abruptly increasing the reactivity. The rate is not
known nor is the maximum reactivity. The initial spike
probably did not exceed 10 16 fissions; most of the
energy was generated during a high plateau. Quenching
came through thermal expansion and melting.
Unreflected uranium–molybdenum metal fast burst reactor; single excursion; insignificant exposures.
safety block was set into motion inward, approaching a
state thought to be known. The excursion took place as
the safety block neared the one–half inch. position.
All scrams functioned as designed, but the short
period allowed a very high power to be attained, and
the excursion was actually terminated by thermal
expansion of the metal. The new uranium-molybdenum
assembly bolts failed (the heads snapped); the two top
rings and minor parts were tossed 5 to 15 feet.
This accident was well instrumented. The minimum
period was 9.2 µs, the maximum reactivity 15 ¢ above
prompt criticality, the reactivity insertion rate 2.2 $/s,
and the burst width 28 µs. The internal temperature rise
of 290°C suggested a fission yield of 1.5 × 10 17 , which
is only 1.4 times the maximum expected from normal
operations.
During this unexpected burst, damage was limited
to the failure of the assembly bolts and very slight
chipping of the nickel coating of the rings. Personnel
radiation doses were immeasurably small. One hour
after the excursion the cell was entered and radiation
levels were determined to be higher than normal
background, but not appreciably higher than those
measured after a routine burst.