Criticality Accident Experience - STD-1128-98master0212

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

criticality concerns, the development of failure modes and the potential effects of the

accident, and the consequences of the accident. This safety analysis report, and the

associated technical safety requirements, should document both the entire nuclear

criticality safety program and the analysis 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.

7-8