Exclusion area boundary

Population data are site specific and will be presented at the time of application for a specific site. However, for purposes of estimating population doses, a population density is assumed to average 500 persons per square mile over a radial distance out to 30 miles. A bounding low population zone of 425 meters (i.e. coincident with the exclusion area boundary) has been selected for purposes of safety analyses. 

A program of onsite data collection, supplemented by National Weather Service (NOAA) summaries from locations near a specific site, will be conducted. Information from the onsite program will be used to confirm that the limiting offsite dose analyses and annual average X/Q values are adequate relative to exclusion area boundary radius and low population zone requirements. 

The limiting exclusion area boundary short term atmospheric dispersion factors... 

The annual average atmospheric dispersion factor at the exclusion area boundary is asstuned to be 2 x 10 sec/m. ... 

Two-hour exclusion area boundary and 30-day low population zone accident doses less than 25 rem whole body and 300 rem thyroid. 

While the maximum potential consequences are similar for the process spill and the SCB fire DBAs, the process spill scenario dominates the overall risk to the public by several orders of magnitude as compared to all other DBAs. The maximum potential consequence at the exclusion area boundary (3000 m.) is calculated to be 2 Rem, and the sequence of events that results in this dose is assessed to be extremely unlikely. Accidents that are expected to occur within the lifetime of the facility are (conservatively) calculated to have dose consequences of up to several miliirem at the exclusion area boundary. These DBAs bound ail other potential accidents that have been postulated to occur in the facility. 

These process spill scenarios range from anticipated events which result in dose consequences of 2 to 13 mrem at the exclusion area boundary to extremely unlikely events which result in dose consequences of 1.8 Rem. 

The maximum potential dose consequence at the 3000 meter exclusion area boundary of 1.8 Rem for the process spill scenario is well below the evaluation guideline of 25 rem. 

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. 

Mitigative features for this DBA are inherent in the distance to the exclusion area boundary. In addition, low or zero ventilation rates from the HVAC system exhausts may reduce the amount of radioactive material escaping the building for airborne release. 

In the more likely event of failure of the ventilation system, the inventory contained in the SCBs is released to the Zone 2A canyon, and subsequently diffuses into Zone 2. The rate of this diffusion Is relatively slow, and at most, approximates 1 curie of 1-131 after about 4 hours. The resulting dose consequences at the 3000 m exclusion area boundary in this scenario are less than 0.1 mrem. 

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. 

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). 

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. 

The source term potentially available for release consists of the cold trap noble gas inventory, the residual contamination in each SCB, the halogen inventory built up on the in-box filters, the residual process liquid source term, and the inventory in a maximally irradiated target. The specific mix of each of these inventories in each DBE scenario varies, and is dependent on the specific release scenario under consideration. The dose consequence at the 3000-meter exclusion area boundary for each of these potential Inventories is summarized in Table 3E.7-1. 

The SFP building is also equipped with high-efficiency particulate air (HEPA) and charcoal filters to collect and filter radioactive gases that might be released from a spent fuel assembly in the event of an accident. An accident of a dropped assembly is postulated to occur that causes a breach of all fuel rods in a single assembly and release of the radioactive gases trapped in the fuel rod. Under such conditions, the dose to an individual at the exclusion area boundary (EAB) would be much less than the acceptance criteria specified in Section 15.7.4 of the Standard Review Plan (NUREG 1981). 

The potential doses at the exclusion area boundary and the low population zone are calculated assuming that the accident occurs when the meteorological conditions are worse (from the standpoint of the calculated doses) than those that would be expected to prevail at the site approximately 95% of the time [Regulatory Guides 1.3 and 1.4]. Table 2.1-4 presents the results from typical calculations of potential offsite doses due to several kinds of design basis accidents. Even with the considerable number of pessimistic assumption employed, the calculated doses that a person out-of-doors in the vicinity of the plant might receive for the entire course of the accident are usually well below the 10 CFR Part 100 guidelines. 

Doses are calculated for a hypothetical person standing outside in the radioactive plume, for 2 hours at the exclusion area boundary and during the entire period of plume passage at the low population zone outer boundary. [10 CFR 100 (d)]... 

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