Resolution of Generic Safety Issues: Issue 191: Assessment of Debris Accumulation on PWR Sump Performance (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–34 )
Results of research on BWR ECCS suction strainer blockage identified new phenomena and failure modes that were not considered in the resolution of Issue A-43. In addition, operating experience identified new contributors to debris and possible blockage of PWR sumps, such as degraded or failed containment paint coatings. Thus, this issue was identified1691 by NRR and called for an expanded research effort to address these new safety concerns and to examine whether the events and new research being conducted for the BWR strainers warranted similar evaluation and/or changes for ensuring the adequacy of PWR recirculation performance.
A study was deemed to be required to determine whether PWR ECCS sumps are adequate to ensure proper ECCS operation. Based on the existence of an action plan1692 to address the safety concerns, the issue was considered nearly-resolved in September 1996. It was later given a HIGH priority ranking in SECY-98-166.1718
Preliminary parametric calculations were completed in July 2001 indicating the potential for debris accumulation for 69 cases. These 69 cases were representative of, but not identical to, the operating PWR population. The staff’s Technical Assessment concluded that GSI-191 was a credible concern for the population of domestic PWRs, and that detailed plant-specific evaluations were needed to determine the susceptibility of each U.S.-licensed PWR to ECCS sump blockage. Following the ACRS agreement with the staff’s Technical Assessment of the issue in 09/2001, the issue was forwarded to NRR.1910 NRR has evaluated the technical assessment, and prepared a Task Action Plan1911 for developing appropriate regulatory guidance and resolution of GSI-191.
Following meetings with stakeholders, the NRC issued Bulletin 2003-011912 to PWR licensees to: (1) confirm their compliance with 10 CFR 50.46 (b)(5) and other existing applicable regulatory requirements, or (2) describe any compensatory measures that have been implemented to reduce the potential risk due to post-accident debris blockage, as evaluations to determine compliance proceed. All PWR licensees provided a response to the Bulletin, indicating interim compensatory measures and candidate operator actions that would be implemented. Closure letters were issued to the PWR licensees as these reviews were completed and generic close-out of Bulletin 2003-011912 was completed in December 2005. Responses to the Bulletin from licensees and Bulletin closure letters are available at: http://www.nrc.gov/reactors/operating/ops-experience/pwr-sump-performance/bulletin03-01-correspondence.html
The NRC staff concluded that plant-specific analyses should be conducted to determine whether debris accumulation in PWR containments could impede or prevent ECCS operation during recirculation, and that appropriate corrective actions should be taken if necessary. This expectation was communicated to licensee via Generic Letter 2004-02.1913
GL 2004-021913 was issued in September 2004 requesting licensees to perform plant-specific mechanistic evaluations of sump performance following LOCA and HELB events, and to implement corrective actions as required to ensure compliance with regulatory requirements. NEI provided a guidance report (GR) to the staff in May 2004 containing the industry's proposed evaluation methodology for performing the plant specific evaluations. The staff reviewed the GR and issued a draft Safety Evaluation (SE), which supplemented the GR. The staff presented the SE to CRGR and to the ACRS Subcommittee and Full Committee in September and October 2004, respectively. The final SE was issued in December 2004,1914 resulting in an NRC-approved evaluation methodology.
GL 2004-021913 required licensees to respond within 90 days to document the actions planned by the licensee to perform the sump evaluation, and the proposed schedule for completion. All PWR licensees responded to the GL on schedule in September 2005. All PWR licensees committed to modify their containment sump strainer, except for three plants who had modified their containment sump strainers within the last five years. The staff evaluated all 90-day responses to GL 2004-02 1913 and in January 2006 issued comments to licensees to be addressed in their final response submittals. (Licensees responses and NRC’s comments are available at: http://www.nrc.gov/reactors/operating/ops-experience/pwr-sump-performance/generic04-02-correspondence.html)
To address concerns regarding the potential for chemical precipitates and corrosion products to significantly block a fiber bed and increase the head loss across an ECCS sump screen, a joint NRC/Industry Integrated Chemical Effects Testing program was started in 2004 and completed in August 2005. Chemical precipitation products were identified during the test program, and follow-up testing and analyses will be needed to address the effect on head loss. IN 2005-26,1915 "Results of Chemical Effects Head Loss Tests in a Simulated PWR Sump Pool Environment," was issued on September 16, 2005.
The NRC conducted additional research in certain areas to support evaluation efforts and provide confirmatory information. These areas include research on chemical effects to determine if the pressurized-water reactor sump pool environment generates byproducts which contribute to sump clogging, research on pump head losses caused by accumulation of containment materials and chemical byproducts, and research to predict the chemical species that may form in these environments. The staff completed reports on the chemical effects on ice condenser containments,1916 and on PWR containments.1917 Supplement 1 to IN 2005-261918 was issued on January 26, 2006 to specifically provide additional information regarding test results related to chemical effects in environments containing dissolved phosphate (e.g., from trisodium phosphate) and dissolved calcium.
Between July and September 2006, the staff completed research on: (1) the thermodynamic simulation of containment sump pool chemical constituents, to predict the chemical reactions/byproducts in the pools; (2) the pressure loss across containment sump screens due to fiber insulation, chemical precipitates, and coating debris; and (3) a literature survey to summarize the knowledge base to date on the potential contribution of material leached from containment coatings to the chemical products formed in the containment sump pool, after a loss-of-coolant accident.
As part of the plan to confirm adequate implementation and resolution of GSI-191, the NRC has conducted detailed plant audits examining the analyses and design changes used to address the technical issues. Visits to strainer vendor test facilities have often been included as part of this audit process. The NRC staff is also systematically evaluating remaining technical questions related to GSI-191 to support a decision on whether additional confirmatory research is needed and if so, on what time frame. In early 2008 the NRC issued additional guidance1919 on several important subjects related to GSI-191 for the use of licensees and NRC staff reviewers of licensee submittals. This guidance is intended to help ensure an adequate technical basis is provided for conclusions that corrective actions are complete and sufficient. In December 2007 the NRC approved topical reports intended to address chemical and ex-vessel downstream effects1920, 1921. An additional topical report on in-vessel downstream effects is under NRC review.
In addition to the plant audits identified above, the staff will use inputs from review of licensee responses to GL 2004-021913 and items identified from Regional inspections using Temporary Instruction TI-2515/1661922 to support closure of GSI-191. Inspections by regional staff will verify proper implementation of planned modifications. All licensees submitted supplemental responses to GL 2004-02 in early 2008. However, the NRC had authorized many plants some additional time (generally on the order of a few months) to complete one or more specified corrective actions (e.g., a particular plant modification or a test of strainer function). Therefore, some of the supplemental responses received in early 2008 did not support a final conclusion that the licensee had fully addressed GL 2004-02. In such cases an additional response will be due within 90 days of completion of the last corrective action for a given plant. The NRC expects that only a very few corrective actions will occur later than the end of 2008.