process to assure the reviewer that nuclear criticality safety concerns are
being properly addressed at the facility.
CRITICALITY ACCIDENT EXPERIENCE
Criticality accidents, sometimes called criticality excursions, can either be short-duration
pulse-type excursions or continuous excursions. In the history of plutonium handling and
processing, there have been five criticality accidents involving plutonium materials. Three
of the accidents occurred during research activities and the other two accidents during
plutonium-processing operations. The two processing accidents are reviewed in this
Types of Criticality Accidents
In a pulse-type criticality accident, there is an initial pulse of typically 1015-1017
fissions over a short time-period (less than 1 sec), sometimes following by
additional lower-intensity pulses. In a fissionable material solution, the pulse or
spike is terminated by the heating and consequent thermal expansion of the
solution and by bubble formation that serves to reconfigure the fissile mass into a
noncritical configuration (Paxton, 1966). If the initial pulse results in a loss of
solution from the container (e.g., by splashing) or redistribution of material, the
criticality event may conclude without further pulses. However, if there is no loss
of material as the solution cools, it may form a criticality mass once again and
pulse with slightly lower fission yield (Paxton, 1966).
considerable distances from the accident site (on the order of tens of meters). There
can also be high beta-gamma residual radiation levels from fission products after
the excursion is concluded. The heat generated during the excursion can melt parts
of the system that held the fissionable material (Moe, 1988).
Moe (1988) reviewed estimated prompt radiation doses from excursions in a
moderated system and a metallic system, as well as dose rates from residual
contamination left by a criticality excursion. Assuming a burst of 1018 fissions in an
unshielded, water-moderated system, the total absorbed dose is estimated to be
>600 rad up to 6 m and >100 rad up to about 15 m. The gamma/neutron ratio of the
total absorbed dose was 2.8. The gamma/neutron absorbed dose ratio was 0.1. In
general, for a moderated system, the gamma dose would be expected to be higher
than the neutron dose and, for a metal system, the neutron dose would be expected
to be higher than the gamma dose.