On November 22, 1994, personnel at Los Alamos National Laboratory reported a fire in a glovebox in the Plutonium Processing Facility. They placed a can that had exterior surface corrosion in a glovebox, and cotton rags in the can were removed and laid out to dry. The rags began to smolder, filling the glovebox with smoke and prompting an employee to actuate a fire alarm. The rags caught fire and burned to completion without damaging any other material or equipment.

There was no spread of radioactivity as a result of the event. (ORPS Report ALO-LA-LANL-TA55-1994-0037, OEWS 94-48) The Office of Operating Experience Analysis and Feedback (OEAF) is developing simple but credible analytical approaches to review operating events and families of events to determine their safety and risk significance.

OEAF will evaluate features such as risk of injury or fatality, comparison to benchmarks (such as the risk of accidental fatality at U.S. industrial sites), and the cost-benefit aspects of corrective actions. A method for such analyses has been developed based on the degree of residual protection or the number of barriers remaining against an undesirable consequence and the risk posed by the event. The method has been applied to the glovebox fire event as described in this report. Analysis of selected operating events will be presented in future issues of the Operating Experience Weekly Summary. Any constructive comments readers may have on improving the methodology and its use will be appreciated.

The first step in the analysis was a semi-quantitative ranking of the operating event based on two factors. One factor was the estimated maximum consequence of a similar event. The other was the conditional probability of that consequence. The conditional probability was based on the degree of residual protection measured by the number of barriers remaining against injury, fatality, or other undesirable result. In this case, there were three barriers:

(1) detection of the fire and intervention to contain it,

(2) glovebox containment, and

(3) the negative pressure in the glovebox provided by its ventilation system.

Based on the barriers, the analysts characterized the conditional probability of a significant release as medium. The fire was caused by enough Pu-238 to generate about 10 watts of heat, which ignited a rag weighing 113 grams and generated about 2 megajoules of heat. The quantity of Pu-238 required to generate 10 watts is radiologically significant; however, the fraction that could be inhaled by a worker was small, so the analysts judged the consequence to be medium also. According to the classification scheme, events classified as "medium/medium" are subject to further analysis. The classification matrix is shown in Figure 1. (Please refer to hard copy of weekly summary for figure) Consequences associated with the significant release were based on the following assumptions. Variable names are in parentheses.

1. A Pu-238 inventory of 18 g was estimated to be in the rags (I).

2. The airborne release fraction of the Pu-238 was 6¥10-3 (ARF).

3. Ten percent of the Pu-238 particles were released from the glovebox (GRF).

4. Particles were instantaneously and uniformly distributed over the free volume of the room, which was 311 m3 (V).

5. The respirable fraction of the particles was 1% (RF).

6. The worker was able to leave the room within 10 seconds (T).

7. The breathing rate was 3.5¥10-4 m3/s (BR).

8. A value of 460 rem/mCi was used for the committed

effective dose equivalent of Pu-238 (CEDE).

9. A value of 17.2 Ci/g was used for the specific activity of Pu-238 (SA). The estimated inhalation dose was I ARF GRF RF SA BR T CEDE/V, or 9.6 rem.

Variations in the distribution of the particles might double this dose for some workers to 19.2 rem. Combining the conditional probability of 2.5¥10-3 per event with the worst case dose of 19.2 rem gives an average dose of 2.5¥10-3 19.2 rem or 0.045 rem/event.

The fatal cancer risk coefficient for the worker was taken to be 4¥10-4 fatality/rem (ICRP Publications Nos. 15 and 21 Combined: Protection Against Ionizing Radiation from External Sources). The analysts computed the fatality risk as 2¥10-5 fatality/event.

In Figure 3, the fatality risk from the event is compared to the OSHA threshold for significant risk to workers for exposure to benzene (1¥10-3 fatality) and the average lifetime accident fatality risk in other U.S. industries (4¥10-3 fatality).

As is evident from Figure 3, the fatality risk of this event is about two orders of magnitude less than the other two benchmarks, and therefore, no further analysis is warranted.(Please refer to hard copy of weekly summary for figure) However, the event could have been prevented easily by placing the rags in an inerted glovebox, and future glovebox fires can be avoided by a procedure change.

Inerted gloveboxes are already available at the facility: thus, the cost of avoiding similar events is low. The benefits of using inerted gloveboxes is larger than the cost, so they should be used to store rags that may be contaminated with Pu-238.

The cost-benefit matrix is shown in Figure 4. (Please refer to hard copy of weekly summary for figure) Risk-based analyses such as this may be a valuable decision analysis method when used with other information and techniques available to managers.

Users of the method should include all available information to establish the events in the event tree and the numerical values on the branches of the tree (split fractions). Plant personnel with knowledge of the equipment and procedures are particularly important sources for this information. Users should also examine common-cause relationships between events. For instance, inadequate maintenance could affect both the glovebox containment and the ventilation function.

Finally, users should be aware of the uncertainties associated with the resulting values from these analyses. If uncertainties are too large, uncertainty analyses should be performed to quantify their magnitudes.
For further information about this methodology, please contact Andy Marchese, Manager, Office of Operating Experience Anaylsis and Feedback, at (301) 903-2712.