ML12066A232
| ML12066A232 | |
| Person / Time | |
|---|---|
| Issue date: | 03/16/2012 |
| From: | Leeds E Office of Nuclear Reactor Regulation |
| To: | Marion A Nuclear Energy Institute |
| Mathew R, NRR/DE/EEEB X2965 | |
| Shared Package | |
| ML12068A211 | List: |
| References | |
| TAC ME8144, Y020120059, RIS-11-012, Rev 1 | |
| Download: ML12066A232 (2) | |
Text
March 16, 2012 Mr. Alexander Marion Vice President, Nuclear Operations Nuclear Energy Institute 1776 I Street, N.W., Suite 400 Washington, DC 20006 3708
Dear Mr. Marion:
On behalf of the U.S. Nuclear Regulatory Commission (NRC), I am responding to your February 28, 2012, letter (Agencywide Documents Access and Management System (ADAMS)
Accession Number ML12059A461) regarding concerns associated with the NRCs release of Regulatory Issue Summary (RIS) 2011-12, Revision 1, Adequacy of Station Electric Distribution System Voltages, dated December 29, 2011.
Consistent with the NRCs regulatory requirements and processes, the staff did review and consider the concerns raised in draft versions of the RIS through the public comment process and during public meetings. In addition, the NRC Committee to Review Generic Requirements and the Office of the General Counsel staff have reviewed this RIS and the comment/resolution document. With respect to the specific issues raised in your letter, the staff offers the following clarifications on key issues.
Although, the Nuclear Energy Institute letter did not discuss the purpose and role of the degraded voltage relays (DVR) in protecting the safety-related systems, the staff wants to emphasize its safety function. Specifically, the DVR monitors sustained degraded voltage conditions at the plant safety bus and automatically takes protective actions when a plant reaches unacceptable voltage conditions. This ensures that all safety-related equipment is protected from a potentially damaging degraded voltage condition. Since degraded voltages could potentially damage both trains of emergency core cooling systems, the DVR must automatically initiate isolation of the degraded offsite power source and transfer the safety buses to the emergency power source within the time period assumed in the accident analysis.
The staff does not agree that implementation of the RIS is contrary to operational safety and plant system reliability. Licensees established analytical limits and plant technical specifications (TS) requirements long before the RIS was issued to ensure that safety-related equipment can meet their intended functions. This means that all equipment can operate (start and run) in accordance with the design and licensing basis of the plant at any voltage at or above the minimum allowable values of the plants TS. The licensees must maintain the design-basis analyses throughout the life of a plant to demonstrate that the trip setpoint adequately protects the Class 1E equipment from a potentially damaging degraded voltage condition. They must also show that the time delay to transfer from a degraded offsite source to the standby power source to support the emergency core cooling equipment operation is consistent with accident analysis time assumptions. The NRC issued the RIS to clarify the staffs position because
A. Marion recent inspections have identified that some licensees have not implemented the degraded voltage protection for their plants in accordance with regulatory requirements and staff positions.
The potential for double sequencing is inherent in existing plant designs, regardless of the DVR setpoint, primarily because of the assumption that loss of offsite power and loss-of-coolant accident event occur simultaneously. Any stagger in the two events would result in double sequencing. However, it is important to note that the analytical limits and setpoints chosen for the degraded voltage protection scheme must be supported by the capacity and capability of the offsite power system and grid. If not, there is a potential for the separation of offsite power to occur prematurely, which is contrary to the requirements specified in General Design Criterion 17, Electric Power Systems, of Appendix A, General Design Criteria for Nuclear Power Plants, to Title 10 of the Code of Federal Regulations, Part 50, Domestic Licensing of Production and Utilization Facilities, or applicable principal plant design criteria.
Your letter stated that an industry technical working group performed analyses for six different nuclear power stations using the methodology described in the RIS, and that results of the analyses indicated that setting the DVR on the basis of providing motor starting protection would likely result in increased DVR setpoints and higher grid voltages. As stated before, the offsite power system must support the DVR analytical limits and setpoints that the licensee established. If these are existing plant conditions, they are not in compliance with NRC regulations. The staff also noted that the reset values (3 percent to 4 percent of dropout values) specified for six different nuclear power stations mentioned in the attachment of the letter may be incorrect because the reset values typically are a very small percentage change from the dropout values.
Your letter also stated that a regulatory guide would be the appropriate vehicle for guidance contained in the RIS; therefore, the RIS should be withdrawn and a regulatory guide developed.
The staff does not plan to withdraw the RIS. As stated in the RIS, the staff technical positions outlined are consistent with the aforementioned regulations and generic communications, and provide more information on calculations needed. The NRC staff is working with the Institute of Electrical and Electronics Engineers (IEEE) Standard 741 working group (Working Group 4.7 of IEEE Subcommittee SC-4, Auxiliary Power) to revise the existing guidance. If acceptable, the staff may endorse this guidance through a future regulatory guide.
Should you or your staff have any questions, please contact James Andersen at (301) 415-3565 or Roy Mathew at (301) 415-8324.
Sincerely,
/RA/
Eric J. Leeds, Director Office of Nuclear Reactor Regulation cc: Anthony Pietrangelo, NEI Martin Virgilio, NRC
ML12066A232 OFFICE TL: NRR/DE/EEEB BC:NRR/DE/EEEB DD:NRR/DE OD: NRR NAME RMathew JAndersen PHiland ELeeds DATE 03/07/12 03/08/12 03/08/12 03/16/12