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STP Nuclear Operating Company; South Texas Project, Units 1 and 2

  • Jul 19, 2017 10:44 pm GMT
Nuclear Regulatory Commission Documents & Publications

The U.S. Nuclear Regulatory Commission (NRC) is granting exemptions from certain portions of the acceptance criteria for emergency core cooling, and the general design criteria for emergency core cooling, containment heat removal, and atmosphere cleanup for the use of a risk-informed analysis to evaluate the effects of debris in containment following a loss-of-coolant accident (LOCA) for the South Texas Project (STP), Units 1 and 2, located in Matagorda County, Texas, Docket Nos. 50-498 and 50-499, respectively. The exemptions are in response to a request dated June 19, 2013, from the STP Nuclear Operating Company (STPNOC, the licensee) related to STPNOC's proposed approach to resolve a generic safety concern for pressurized water reactors (PWRs) associated with potential clogging of emergency core cooling and containment spray system strainers during certain design basis events.

DATES: The exemption was issued on July 11, 2017.

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:

* Federal Rulemaking Web site: Go to and search for Docket ID NRC-2016-0092. Address questions about NRC dockets to Carol Gallagher; telephone: 301-415-3463; email: For technical questions, contact the individual listed in the FOR FURTHER INFORMATION CONTACT section of this document.

* NRC's Agencywide Documents Access and Management System (ADAMS): You may obtain publicly-available documents online in the ADAMS Public Documents collection at To begin the search, select " ADAMS Public Documents" and then select " Begin Web-based ADAMS Search." For problems with ADAMS, please contact the NRC's Public Document Room (PDR) reference staff at 1-800-397-4209, 301-415-4737, or by email to For the convenience of the reader, the ADAMS accession numbers are provided in a table in the "Availability of Documents" section of this document.

* NRC's PDR: You may examine and purchase copies of public documents at the NRC's PDR, Room O1-F21, One White Flint North, 11555 Rockville Pike, Rockville, Maryland 20852.

FOR FURTHER INFORMATION CONTACT: Lisa Regner, Office of Nuclear Reactor Regulation, U.S. Nuclear Regulatory Commission, Washington DC 20555-0001; telephone: 301-415-1906, email:

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 Matagorda County, Texas.

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 September 13, 2004, to holders of operating licenses for PWRs. In GL 2004-02, the NRC staff requested that licensees perform an evaluation of their emergency core cooling systems (ECCS) and containment spray system (CSS) recirculation functions considering the potential for debris-laden coolant to be circulated by the ECCS and the CSS after a LOCA or high energy line break inside containment and, if appropriate, take additional actions to ensure system function. The GL required that licensees provide a written response to the NRC, pursuant to section 50.54(f) of title 10 of the Code of Federal Regulations (10 CFR), describing the results of their evaluation and any modifications made, or planned, to ensure the ECCS and CSS remain functional.

II. Request/Action

By letter dated June 19, 2013, as supplemented by letters dated August 20, 2015, and April 13, 2016, STPNOC submitted requests for exemptions pursuant to 10 CFR 50.12, "Specific exemptions," from the requirements of 10 CFR 50.46, "Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors," and 10 CFR part 50, appendix A, General Design Criterion (GDC) 35, "Emergency core cooling," GDC 38, "Containment heat removal," and GDC 41, "Containment atmosphere cleanup," to use a risk-informed methodology instead of the traditional deterministic methodology, to resolve the concerns associated with GSI-191 and respond to GL 2004-02.

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," August 4, 1986, 51 FR 30028; and "Use of Probabilistic Risk Assessment Methods in Nuclear Activities; Final Policy Statement," August 16, 1995, 60 FR 42622, respectively). Therefore, RG 1.174 provides an acceptable method for licensees and NRC staff to use in assessing the impact of licensing basis changes when the licensee chooses to use risk information.

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 Federal Register article originally published on the page number listed below--

Exemption; issuance.

Citation: "82 FR 33161"

Document Number: "Docket Nos. 50-498 and 50-499; NRC-2016-0092"

Federal Register Page Number: "33161"


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