Fire in a Process SCB

Section 3.3.2.3.5, Accident Selection, identified one beyond DBA as, Multiple simultaneous errors or events that affect multiple SCBs, resulting in release of the contents of multiple targets. Release of the fission products from multiple targets (six) in a single SCB has already been analyzed in two DBAs Spill of Process Materials in an SCB and Fire in a Process SCB. The source term and consequences of the Multiple simultaneous errors or events... beyond DBA are discussed in Section 3.4.3.1 and S.4.3.2 below. Two additional hazards are discussed in Sections 3.4.3.3 and 3.4.3.4 as beyond DBAs to provide a perspective of the residual risk associated with the operation of the facility. These beyond DBAs are 1) seismic events resulting in a release of radiological material and 2) dispersal of radiological material by an explosion in the HCF. 

It is unlikely that a fire will occur in any of the SCBs. However, if a fire were to occur, it would not extend beyond the involved SCB due to the solid stainless steel construction and lack of combustibles both inside the SCBs and in Zone 2A. The worst-case, unmitigated fire in a process SCB is conservatively evaluated in Chapter 3 and would result in an off-site dose of approximately 1.8 Rem at the exclusion area boundary (3000 m). 

Figure 3E.3-1 Event Tree Analysis for a Fire in a Process SCB.

Of the various scenarios evaluated, the most serious consequences resulted from the spill of volatile process materials or a fire in an SCB. Based on these evaluations, structures, systems and components with the potential to mitigate this hazard were identified. While many features in the HCF control or mitigate the hazards for both workers and the public, the most significant mitigative feature for the protection of the public is the filtered HCF exhaust ventilation system. 

While fire protection systems are typically considered important to safety, fire protection In the HCF does not provide a major contribution to defense in depth due to the limited quantity of flammable materials permitted in the process extraction SCB s and the form and containerization of the hazardous process materials. A fire in an SCB does not threaten the ventilation system filters and would normally not result in any significant release of hazardous materials. The fire scenario has been evaluated and included in the accident analyses described in Section 3.4, which indicates that the release is bounded by the consequences of a process spill in the SCB. 

In all of the SCB fire scenarios, the fire was assumed to envelop the liquid dissolution cocktail in the process containers during or following UO2 dissolution and release of radioactive material to the SCB and Zone 1 ventilation systems. While dilution of combustion products is expected to preclude damage to the ventilation system, an unmitigated release bounds the scenario where the filters or the ventilation system itself have been degraded or compromised as a consequence of a fire. The frequencies per year for such an accident developed in the event tree analysis shown in Appendix 3E.3 agree with the frequency for an extraction SCB fire as assessed in event CP-7 in the hazard evaluation (Appendix 3C). 

Potential doses at the exclusion area boundary (3000 m) have been evaluated using the methodology described in Section 3.4.1. These techniques have been used to calculate the potential dose consequences resulting from the released source term for this DBA, which are 110% of the process spill scenario, or a maximum of 2 rem. The probability of a fire in an SCB, however, has been assessed in Appendix 3E.3 to be three or more orders of magnitude lower than spill events (Appendix 3E.2), and fires which could result in release of radioactive materials are even less likely. 

The total radioactive material inventory simultaneously in-process within the Zone 2A canyon is limited to the equivalent of six maximally irradiated targets to ensure that a dose of 2.0 rem at 3000 meters is not exceeded in the most bounding radiological accident scenario (unmitigated fire). The amounts of radioactive material permitted in operable glove boxes and fume hoods are negligible compared to the SCBs, and are administratively controlled to be consistent with prudent Health Physics practices. Significant amounts shall be addressed in detailed procedures that are reviewed and approved by the RCSC. 

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