A Review of Criticality Accidents A Review of Criticality Accidents

2. Chalk River Laboratory, late 1940s or early 1950s 63
ZEEP assembly; single excursion; three significant exposures.
The ZEEP assembly consisted of aluminum clad
uranium metal rods in a heavy water moderator. The
cylindrical aluminum reactor tank was reflected by
graphite on the sides and bottom, shielded on the sides
by 3 foot thick water tanks, and unshielded on the top.
Reactivity was controlled by the level of the heavy
water, which was supplied from a storage tank by an
electrically driven pump. As a safety feature, the pump
was controlled by a timer that required the pump to be
restarted by a pushbutton switch every 10 seconds or
so.
Cadmium coated plates suspended on cables
between the reactor tank and the graphite reflector
served as safety rods. The scram circuit was set to trip
at a power of about 3 watts.
At the time of the accident, two physicists were
working on the top of the reactor, inserting foils into
reentrant tubes. A technician raising the water level in
the reactor with the pump control had instructions to
stop at a water level predetermined to be many minutes
of pumping below predicted criticality.
3. Argonne National Laboratory, 2 June 1952 64
UO 2 particles in plastic; water moderated; single excursion; 4 significant exposures.
This accident occurred in a light water moderated
core in which 6.8 kg of 235 U oxide were embedded in
strips of polystyrene plastic. All but 0.5% of the oxide
particles were 10 µm in diameter or less, the remainder,
up to 40 µm in diameter. Seven strips of the plastic
fastened to 6 zirconium strips (0.91 inches ×
0.110 inches × 43 inches) formed 1 standard fuel
element. The core was roughly cylindrical and contained
324 fuel elements. The zirconium, fuel strips,
and water occupied 60%, 7.71%, and 32.2%, respectively,
of the core volume.
The experiment in progress at the time of the
accident consisted of making comparisons of the worth
of central control rods of different design. The system
became superprompt critical following an attempt
(contrary to operating procedures) to replace the
central control rod when the normal amount of water
was in the core. Peripheral poison rods were in
position but were inadequate to prevent criticality.
*See Appendix A.
94
One of the physicists asked the technician to bring a
tool to the top of the reactor. Not wanting to lose time
and in direct contravention of instructions, the technician
inserted a chip of wood into the pump control
button so the timer would reset each time it ran out. He
then went to the reactor top and became involved in the
work being done there, while the heavy water level
continued to rise.
The reactor became critical and scrammed as
designed. The pump was stopped automatically by an
interlock in the scram circuit. The NRX reactor in the
adjacent building was scrammed by skyshine radiation.
Subsequent evaluations indicated that the ZEEP power
level had coasted above the preset scram power level,
perhaps by several factors of two.
The three people on the reactor top each received
doses in excess of the quarterly permissible limit and,
perhaps, above the annual limit.
The quenching mechanism for the excursion of
1.22 × 10 17 fissions was the near uniform expansion of
the plastic as the 10 µm particles became hot and the
bubble formation in the neighborhood of the 40 µm
particles. This forced most of the water out of the core,
and the entire excursion was over about 0.6 seconds
after the operator started raising the control rod. The
maximum reciprocal period was nearly 100 s –1 , the
maximum power was 1.7 × 10 8 watts, and the halfwidth
of the power spike was 18.5 milliseconds.
In this excursion, the core fuel elements were
ruined, but no significant amount of fissile material
was lost. The activity in the reactor room was above
tolerance for about a day. The core elements were
removed after 5 days, and decontamination was
completed by a single application of detergent and
warm water. Four persons received radiation doses of
136, 127, 60, and 9, rep.*