WSRC-MS-99-00540
Accident Analysis for a Postulated Large Research
and Development Laboratory Fire
A. Blanchard
Westinghouse Savannah River Company
Aiken, SC 29808
C. C. Fields, E. P. Hope, and S. P. Tinnes
Westinghouse Safety Management Solutions LLC
Aiken, SC 29804-5388
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An accident analysis has been updated for a large radiological laboratory, the Savannah River Technology Center (SRTC), at the Savannah River Site (SRS). The dominant event is a postulated fire affecting a significant portion of the multi-winged facility. The deterministic accident analysis for this nuclear installation was performed in support of a Safety Analysis Report (SAR) meeting requirements of Department of Energy (DOE) Order 5480.23, "Nuclear Safety Analysis Reports", and the DOE standards supporting that Order. The significant features of this analysis including analysis of the material-at-risk, the governing phenomena and an alternative study evaluating the thermal plume portion of the scenario are discussed.
SRTC management and the SRS DOE Operations Office specified that innovative analysis methods be employed to ensure calculated accident doses were not excessively conservative. Excessive conservatism could result in controls and equipment classification that could lead to increases in operating costs (with no significant resultant increase in safety). Such increases could affect the attractiveness of the facility to obtain new missions.
The source term portion of the analysis employed the "five-factor" methodology described in the DOE Airborne Release Fraction and Respirable Fraction Handbook.1 The analysis treats aerosolization from fires by accounting for thermal stresses (overheating), falling objects (objects, such as ceiling tiles that might fall onto the radioactive inventory in the facility during the fire), falling inventory (stresses from the inventory itself falling during the fire, such as if glovebox supports collapsed due to the fire), and resuspension during recovery operations. Generally, thermal stresses are the most important aerosolization mechanism for radioactive solutions, while falling objects are the most important mechanism for radioactive powders.
The "large fire" event is a key element of the overall analysis because (1) this event is explicitly assessed in the SAR (a full-building fire) and (2) postulated large fires also appear in the SAR as consequential events in other scenarios (e. g., an explosion which starts a large fire). Because of this, efforts to remove excessive conservatism from the large fire event are particularly cost effective; they remove excess conservatism from several event sequences.
A special factor in this accident analysis is that SRTC is closer to the site boundary than most other SRS facilities (670 meters). Thus, plume dilution and deposition during transport to the site boundary are less significant for SRTC than for most other SRS facilities.
A key analysis approach adopted for the 1998 SAR was to use reduced Airborne Release Fractions (ARFs) for much of the material-at-risk (MAR) in the building (i. e., ARF values lower than the bounding values listed in Reference 1). The justification for use of these values was based in part on the diversity of inventories, equipment, and activities carried on within the building. This diversity is consistent with the facility's research and development mission. The form and arrangement of the MAR in this facility, with its current set of diverse missions, makes it impossible that a fire could affect the entire inventory so as to result in bounding ARF values for the entire MAR. To ensure the results of the analysis were conservative (and physically reasonable), bounding ARF values were used for the highest-dose-contributing portion of the facility.
An alternative analysis approach that was considered was to credit the thermal plume expected from a large fire. Such a thermal plume can behave roughly similar to a stack, enabling additional dispersion before aerosolized material is deposited on the ground and on vegetation (the heat from the fire can be viewed as "lofting" the aerosols). Calculations indicated that this approach could reduce calculated offsite doses by as much as a factor of 100 (this large reduction factor is due, in part, to SRTC's closeness to the site boundary). Calculations of this type have to be done very carefully, because the highest calculated doses are not necessarily at the site boundary, and because oversimplified analyses can give unreasonable results. In the latter case, for example, a small fire (without appreciable thermal plume) has larger calculated doses than a large fire (if a plume does develop).
Issues associated with the thermal plume analysis approach include: (1) in certain scenarios a slow-burning fire might not develop a thermal plume and (2) a significant portion of the release could take place before a thermal plume develops. Factoring in these considerations, and because the other approaches met the objectives of the analysis, a management decision was made to not employ the thermal plume approach for the 1998 SAR. However future revisions may review these decisions particularly if new missions for the laboratory are considered.