STP Nuclear Operating Company; South Texas Project, Units 1 and 2
- Jul 19, 2017 8:59 pm GMT
DATES: The exemption was issued on
ADDRESSES: Please refer to Docket ID NRC-2016-0092 when contacting the NRC about the availability of information regarding this document. You may obtain publicly-available information related to this document using any of the following methods:
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SUPPLEMENTARY INFORMATION: The licensee is the holder of Facility Operating License Nos. NPF-76 and NPF-80, which authorize operation of the STP Units 1 and 2, respectively. The licenses provide, among other things, that the facility is subject to all rules, regulations, and orders of the NRC now or hereafter in effect. The facility consists of two PWRs located in
In 1996, the NRC identified Generic Safety Issue (GSI)-191 associated with the effects of debris accumulation on PWR sump performance during design-basis accidents. As part of the actions to resolve GSI-191, the NRC issued Generic Letter (GL) 2004-02, "Potential Impact of Debris Blockage on Emergency Recirculation during Design Basis Accidents at Pressurized-Water Reactors," dated
By letter dated
Specifically, the licensee requested an exemption from 10 CFR 50.46(a)(1)(i), which, in part, requires ECCS cooling performance to be calculated in accordance with an acceptable evaluation model, as described in 10 CFR 50.46(a)(1), for postulated LOCAs of different sizes, locations and other properties sufficient to provide assurance that the most severe LOCAs are evaluated in order to demonstrate that acceptance criteria in 10 CFR 50.46(b) are met. The NRC staff interprets 10 CFR 50.46(a)(1) requirement to calculate ECCS performance for "other properties" as requiring licensees to consider the impacts of debris generation and transport in containment. The most significant form of debris in nuclear power reactor containments is piping and component insulation that becomes debris during LOCAs, is transported and accumulates in the sumps, and clogs the sumps strainers, thus creating resistance to coolant flow. Fibrous debris from this insulation can also enter the reactor core and directly impede heat transfer from the fuel to the coolant. The licensee also requested exemptions from GDC 35, which contain ECCS performance requirements, and GDCs 38 and 41, which respectively set performance requirements for reactor containment heat removal following a LOCA and for containment atmosphere cleanup following postulated accidents.
The approval of a risk-informed methodology would require exemptions from 10 CFR 50.46(a)(1)(i) and GDCs 35, 38, and 41 because the NRC has interpreted these regulations as requiring a deterministic approach and bounding calculation to show compliance with ECCS and CSS performance criteria in 10 CFR 50.46(b) and GDCs 35, 38 and 41. Issuance of exemptions is an appropriate means to grant relief from the use of a deterministic approach to show compliance with these requirements.
The licensee's 10 CFR 50.46 deterministic analysis considered the debris in containment and demonstrated that the debris loading could prevent acceptable ECCS and CSS operation and core cooling for certain pipe ruptures. Based on its analysis, the licensee concluded that the amount of debris in the STP containment would need to be reduced to demonstrate compliance with 10 CFR 50.46 criteria using a deterministic analysis for certain large-break LOCA sizes because, for those breaks, the plant-specific testing threshold for generation and transport of debris was exceeded.
Additionally, the licensee's deterministic thermal-hydraulic (TH) analysis could not show that hot-leg LOCAs greater than 16 inches could maintain adequate cooling. While not all large-break hot-leg LOCAs resulted in a loss of in-core cooling due to strainer blockage, the licensee categorized all hot-leg breaks greater than 16 inches as assumed to fail in order to simplify the TH analysis.
The licensee requested exemptions from the requirement to use a deterministic analysis for specific scenarios of LOCA breaks producing and transporting debris in excess of the plant-specific tested debris limits and for large hot-leg breaks. Since it determined that the probability of consequences from debris effects is very low, the licensee requested an exemption to use a risk-informed analysis to show adequate assurance of ECCS and CSS functionality, in accordance with the criteria in Regulatory Guide (RG) 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis." The RG 1.174 was developed in consideration of the Commission's Policy Statements on safety goals and the use of probabilistic risk assessment methods in nuclear regulatory activities ("Safety Goals for the Operations of Nuclear Power Plants; Policy Statement,"
The GDC 35, in part, requires that the ECCS safety system functions adequately to transfer heat from the reactor core following a LOCA and in the presence of a worst single failure, at a rate such that (a) fuel and clad damage that could interfere with continued effective core cooling is prevented and (b) clad metal-water reactor is limited to negligible amounts. The licensee stated in its submittal that the function of the ECCS emergency sump is assumed to fail for debris that exceeds the amount determined in acceptable plant-specific testing. Failure of the sump and strainers result in loss of cooling to the core. The licensee requested an exemption from the deterministic requirements of GDC 35 to use a risk-informed approach to show ECCS function for those LOCA breaks that exceed the plant-specific testing debris threshold, and for large hot-leg breaks. The use of a risk-informed analysis, in accordance with the criteria in RG 1.174, would allow the licensee to show that the risk from debris effects is very low.
--This is a summary of a
Citation: "82 FR 33161"
Document Number: "
Federal Register Page Number: "33161"
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