The current status of activities to resolve Generic Safety Issue 191 (GSI-191) is documented in the Generic Issues Status Report (2011, 3rd Quarter, pp. 8-13).
For boiling-water reactors (BWRs), the suction strainer performance issue has been addressed by licensee responses to Bulletin 96-03, "Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling-Water Reactors." The NRC issued this bulletin asking licensees to implement procedures and modifications to minimize potential clogging of suction strainers. The staff confirmed the adequacy of the licensee solutions implemented in response to the bulletin.
Based on some further study of the issue, the NRC was concerned about licensees' calculations for the amount of water available to the sump pumps in the case of a loss-of-coolant accident. The NRC issued Generic Letter 97-04, "Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps," asking licensees to provide these calculations for review. No generic concerns were identified during the NRC's review.
Events at a number of plants raised concerns regarding the potential for coatings in containment to form debris and clog sump screens and suction strainers. The NRC subsequently issued Generic Letter 98-04, "Potential for Degradation of the Emergency Core Cooling System and the Containment Spray System After a Loss-of-Coolant Accident Because of Construction and Protective Coating Deficiencies and Foreign Material in Containment," to obtain information in order to evaluate how the licensees maintain and monitor these coatings. Also, the NRC revised Regulatory Guide 1.54, "Service Level I, II, and III Protective Coatings Applied to Nuclear Power Plants," to employ current American Society for Testing and Materials standards for the selection, qualification, application, and maintenance of protective coatings.
The NRC issued Bulletin 2003-01, "Potential Impact of Debris Blockage on Emergency Sump Recirculation at Pressurized-Water Reactors," in which PWR licensees were asked to either confirm their compliance with existing regulatory requirements or describe interim compensatory measures they would put in place to reduce potential risks associated with sump performance. The NRC reviewed the adequacy of the licensee's responses to the bulletin. Also, a temporary instruction (TI 2515/153) was issued as guidance for NRC inspectors to use during the inspections of licensee's implementation of compensatory measures. The NRC also issued revision 3 to Regulatory Guide 1.82, "Water Sources for Long-Term Recirculation Cooling following a Loss of Coolant Accident." This Regulatory Guide provides the industry with the most current guidance on sump performance and long-term cooling.
The NRC issued GL 2004-02 in September 2004, asking the PWR licensees to perform a mechanistic evaluation of the recirculation functions and, as appropriate, to take additional actions (e.g., plant modifications) to ensure system functionality. This generic letter asked licensees to perform plant-specific evaluations to determine whether sump performance is an issue. The industry submitted evaluation guidelines, which were accepted and modified by an NRC Safety Evaluation in December 2004, are mentioned in the generic letter as an acceptable method for demonstrating compliance with the regulations.
All licensees have made significant modifications to address sump performance issues. These modifications have significantly enhanced safety with regard to sump performance.
As licensees pursued development of responses to the GL, further testing indicated that licensees needed to address additional issues (e.g., chemical effects and in-vessel downstream effects) whose scope was not fully known at the time GL 2004-02 was issued. These issues have required additional testing and plant modifications for many licensees. As a result, the licensees’ completion of corrective actions for GL 2004-02 has been extended beyond the date specified in the GL (December 31, 2007).
In addition, the NRC audited selected plants in order to evaluate the compensatory actions (e.g. plant modifications) performed to support resolution of GSI-191. The Regions inspected each PWR to verify that licensees have made changes to which they have committed in correspondence with the NRC.