Unmitigated consequence

Some HCF safety-significant SSCs serve primarily a defense in depth function. That is. they reduce either the likelihood of occurrence or the consequence of an accidental release of radioactive material (unmitigated consequences cannot exceed the off-site Evaluation Guideline of 25 rem). Other safety-significant SSCs serve primarily a worker safety function for the HCF. The evaluation of these potential consequences and the identification of the SSCs that mitigate them are described in Chapter 3 and are summarized in Table 4.4-1. 

The unmitigated consequences at the exclusion area boundary for credible HCF accidents are well within the off-site Evaluation Guideline of 25 Rem. Since the HCF had no Technical Safety Requirement (TSR) related safety limits, there have been no past safety limit wolations. Furthermore, there have been no significant violations of OSR-related surveillances or administrative controls In the HCF historical record. 

Event No. Event Description Causes Prevention Features Freq. Method of Detect Mitigative Features Unmitigated Consequences Risk Bin Safety Enhancement Consideration... 

Activities, exposure rates in air, and dose rates (effective dose equivalent (EDE), anterior/posterior) were calculated for the unshielded case for the °°Mo product extraction samples in the SGB as weli as 20 A7100 ml Dilution samples in the Hood. The results (shown below) were used to assess unmitigated consequences. 

The severity of unmitigated consequences range from low to high and include both safety and environmental considerations. Unmitigated consequences are estimated by considering the harm that specific hazardous events can cause. 

The consequence of a false rejection decision error will be a more severe one because of the unmitigated threat to the environment. The consequence of a false acceptance decision error will be a more severe one because of the violation of the NPDES permit. 

First we will look at a release scenario that is unmitigated, and then at the modification of a scenario to include a postrelease mitigation technique. The effect of the postrelease mitigation technique will be evaluated by applying the consequence modeling techniques described above. It is important to note... 

For each of the postrelease mitigation techniques examined a description of the consequences of the unmitigated release will be presented first, followed by the mitigated release. The results have been tabulated and shown in figures. 

Frequency of the unmitigated accident consequences Frequency of the initiating events (1.2 a ) probability that staff in the surroundings is affected (0.3) = 0.36 a ... 

These DBA s include those postulated events that pose the greatest risk and therefore bound the HCF operational risk. Failure of the ventilation system filters will be considered as a subset of each DBA by quantifying dose consequences with filtration as well as for a totally unmitigated release. Additionally, an accident from each of the major types of events identified in the hazard tables was selected even if the risk was evaluated to be low. 

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). 

Dose consequence calculations for mixed fission products that are typical of the isotope production target show the dose at 3000 m is approximately 0.015 mrem for each curie released (Naegeli 1999, Mitchell and Naegeli 1999), or a maximum of 0.08 mrem for an unmitigated release of 5 curies of respirable material released during a fire. These calculations were done using the methodology described in Section 3.4.1. 

The maximum potential dose consequences for the unmitigated release are approximately 0.08 mrem and the estimated maximum potential dose consequences for a mitigated release would be less than 0.008 mrem, both well below the evaluation guideline of 25 rem. 

Based on DOE-STD-1027 criteria and methodology, a dose consequence at 3000 m. of 44 mrem is calculated for unmitigated release of the entire maximum inventory associated with each radioactive material storage area (RMSA). The likelihood of a release of the RMSA inventory was not assessed in the DBE analysis. If the inventories of all RMSAs were released... 

Since dose to the pubiic from the reiease of the fission products from six in-process tai ets was calcuiated for two DBAs, the consequences for the resuits of multiple simultaneous events that affect multiple SCBs have already been assessed. They are bounded by the maximum potential dose consequences of 1.8 Rem for an unmitigated release. These dose values are lower than the evaluation guidelines (25 rem) so no additional safety-class SSCs or TSRs are identified. 

As indicated in Section 4.3 of Chapter 4, because of the nature of the HCF and its hazardous material inventory, no unmitigated accident scenario will result in radiological exposures at the exclusion area boundary that will approach the off-site EG of 25 rem. Therefore, there are no SSCs required to maintain the consequences of facility operations below the EG (safety-class SSCs). As a result, no TSR Safety Limits (SL) or Limiting Control Settings (LCS) are required for HCF SSCs. Section 4.4 of the chapter addresses those SSCs that perform a significant defense in depth or worker safety function. These safety-significant SSCs are summarized in Table 4.4-1. 

The NRC and others performed a variety of studies of the consequences of an inadvertent boron dilution event. The conclusions of the NRC assessment along with other studies were (1) that the consequences of an unmitigated boron dilution event, although undesirable, are not severe enough to warrant backfit of additional protective features at operating plants and (2) Standard Review Plan (SRP) Section 15.4.6 (Reference 2) is adequate for plants presently undergoing license review. 

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