Criticality Accident Experience - doe-std-1128-98

DOE-STD-1128-98

process to assure the reviewer that nuclear criticality safety concerns are

being properly addressed at the facility.

7.3

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

section.

7.3.1

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).

Criticality accidents can result in lethal doses of neutron and gamma radiation at

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.

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