Summary of Past Criticality Accidents

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

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 fissionable 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 critical mass once again and

pulse with slightly lower fission yield.

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 level 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 contain the fissionable material (Moe

1988).

Moe 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.

An excursion of 3 x 1015 fissions in a metallic, partially reflected 239Pu assembly, assuming no shielding,

yielded total absorbed doses of >600 rad up to approximately 10 m and >100 rad up to approximately 25 m.

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.

Moe (Moe 1988) noted that for an excursion of >1018 fissions, dispersion of the fissionable material

and fission products would occur, resulting in heavy local contamination and subsequent high residual

dose rates. This dose rate was estimated at >1000 rad/h at 100 ft shortly after the burst and >10 rad/h at 30

ft an hour after the burst. This is the basis for instructing workers to immediately run from the work area

when the criticality alarm is sounded. Seconds can save significant dose, if not from the excursion itself,

then from any residual radiation that is in the area.

Additional guidance for estimating dose following a criticality accident may be found in NUREG/CR-

5504 An Updated Nuclear Criticality Slide Rule (NRC 1994).

7.3.2 Summary of Past Criticality Accidents

Current criticality safety practice has been influenced both by the overall experience of the nuclear

industry and by the analysis of the accidental criticality excursions that have occurred. Los Alamos

National Laboratory has published LA-13638 Review of Criticality Accidents (McLaughlin et al. 2000)

which provides a description of 60 criticality accidents. According to LA-13638, there have been 22

criticality accidents in chemical process facilities. Twenty-one of the 22 occurred with fissile material in

solutions or slurries, one occurred with metal ingots. No accidents occurred with powders.

Overall, the consequences from the 22 accidents have been 9 deaths, 3 survivors with limbs

amputated, minimal equipment damage, and negligible loss of fissionable material. One of these

incidents resulted in measurable exposure to the general public (well below allowable worker annual

exposures). All accidents have been dominated by design, managerial, and operational failures. The

focus for accident prevention should be on these issues.

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