05000440/FIN-2016008-01
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Finding | |
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| Title | Coincidence Logic to Preclude Spurious Trips of the Offsite Power Source |
| Description | The inspectors identified an unresolved item (URI) concerning the installed designed of the safety-related 4-kilovolt under voltage protection scheme. On February 11, 2016, while the plant was shut down, a fuse failure in a bus voltage detection scheme resulted in the actuation of associated under voltage relays and a trip of the safety related EH11 bus. With the under voltage condition locked in, the Division 1 safety-related equipment remained unavailable when the Division 1 EDG started and powered up the EH11 bus. As a consequence of the under voltage scheme, the ESW pump for the EDG was not available to provide cooling water and the EDG was manually shut down by operators to prevent damage to the diesel engine. By letter dated June 3, 1977, Statement of Staff Positions Relative to Emergency Power Systems for Operating Reactors (Agencywide Document Access and Management System (ADAMS) Accession No. 8111230342), the NRC requested all licensees, including Perry Nuclear Power Station, to assess the susceptibility of Class 1E electrical equipment to sustained degraded voltage conditions from offsite power sources and to the interaction between the offsite and onsite emergency power systems. In this same letter, the NRC requested that licensees compare the current design of the emergency power systems at plant facilities with the NRC staff positions and that licensees propose plant modifications, as necessary, to meet the NRC staff positions, or provide a detailed analysis which shows that the facility design has equivalent capabilities and protective features. The NRC staff subsequently issued BTP PSB1, Adequacy of Station Electric Distribution System Voltages, Revision 0, dated July 1981 (Appendix A to Standard Review Plan Chapter 8), which provided additional guidance on technical requirements of the degraded voltage scheme. The NRC staff in addressing Perrys design for conformance with BTP PSB1, stated in Section 8.2.4.1 of the Safety Evaluation Report (SER) for Perry (ADAMS Accession No. 8211120305 dated May 31, 1982) that the applicant states in the FSAR that degraded voltage conditions for the Class 1E power system are detected by an under voltage protection system on each division... The system includes specific coincident logic for each bus. The staff also stated that the applicant has committed to provide the final design of the first and second level under voltage protection of the safety equipment in conformance with the staff position, before plant startup. Therefore, the staff finds this to be acceptable pending confirmation of the set point values and analysis. The staff identified this item as Confirmatory Issue (34) in the Section 1.0 of the SER. In Section 8.2.5 of this SER the staff also stated that on the basis of this review..., the staff concludes that the offsite power system for Perry meets the requirements of GDC 17 and 18 and is, therefore, acceptable. By letters dated June 8, 1982, and August 26, 1982, (ADAMS Accession No. 8206170298 and 820310416) the licensee provided details of the degraded voltage relay scheme to be utilized at Perry. In the August 26, 1982, letter, the licensee provided details associated with set point for the first level of under voltage protection. Specifically the licensee stated that the first level of voltage protection had been changed to trip at 75 percent of motor rated voltage instead of 86 percent and the three second fixed time delay would still remain in effect. The licensee also stated the diesel generator start signal after the 15 second time delay for the second level of under voltage had been eliminated. The licensee attached sketches to clarify the logic for a typical bus under voltage protection scheme. However, the licensee did not provide a detailed description of how the attached sketch satisfied the requirements of BTP PSB1. The August 1982 letter again states that the final design and set points will be established after a review of onsite preoperational test results. In a Supplemental Safety Evaluation Report (SSER 2) (ADAMS Accession No. 8301280169 dated January 13, 1983), the NRC staff acknowledged that based on information provided in the August 26, 1982 letter, the relays in the loss of power protection relays were arranged on a two-out-of-two coincident logic to initiate a timer with a 3-sec time delay. The SSER also acknowledged that in the degraded grid voltage protection, the relays were arranged in a two-out-of-two coincident logic to initiate two separate time-delay relays. The staff stated that the applicants final design of the first and second level under voltage protection of safety equipment was acceptable; thus Confirmatory Issue (34) listed in Section 1.10 of the SER, was satisfactorily resolved. The licensee revised the USAR to document conformance with BTP PSB1 and the requirements of Institute of Electrical and Electronics Engineers (IEEE) Std. 2791971, Criteria for Protection Systems for Nuclear Power Generating Stations. Specifically, USAR Section 8.3.1.1.2.9.2 states that under voltage protection shall include coincidence logic on a per bus basis to preclude spurious trips of the offsite power source. This is in conformance with staff guidance stating that improper voltage protection logic can itself cause adverse effects on the Class 1E systems and equipment such as spurious load shedding of Class 1E loads from the standby diesel generators and spurious separation of Class 1E systems from offsite power. The inspection team noted that the licensees installed design utilizes a single voltage sensor that is shared between two relays in the under voltage trip logic. As a consequence of a shared voltage sensor between both under voltage relays, the overall under voltage protection did not constitute a coincidence logic as stated in BTP PSB1. A single malfunction in the voltage sensing circuit resulted in a trip of the offsite power source and precluded the onsite power source from performing its safety related functions. As evidence by the February 11, 2016, event, a secondary potential transformer fuse failure caused the EH11 bus to separate from offsite power (even though there were no deficiencies in offsite power voltage) and resulted in shutdown of the division 1 EDG bus due to ESW being unavailable. The inspectors believe that this design is deficient in that there is no coincident logic to ensure that spurious trips are precluded as delineated in BTP PSB1 Section B.1.c. The licensee agrees that the issue relative to the failure of a single fuse resulting in actuation of both relays, satisfying the under voltage protection scheme logic, represented a design vulnerability. However, the licensee contended that this original design had been maintained, and was approved by the NRC during the initial licensing. Specifically, the licensee concluded, in a white paper addressing the SIT concerns, that The current design of the potential transformers fusing and the under voltage relays for the division 1 4160 V ESF bus is reflective of the original design and licensing basis. The NRC Staff reviewers clearly used PSB1 in their review of the PNPP licensing and design bases. They were aided by the schematics contained in the following letter [First Energy Nuclear Operating Company (FENOC) letter docket Nos. (50440; 50441), dated August 26, 1982]. They recognized and approved of the relays arrangement in a two-out-of-two coincident logic. (This is identified on the second schematic - reference Sheet 2 of 2 on lines 19 and 21, which address the time delay relays.) This design is reflective of PNPP's current design and is consistent with the original PNPP licensing basis. Additionally, the NRC stated in the SER that Perry meets the requirements of GDC 17 and 18. This issue is considered an unresolved item (URI 05000440/20160801) pending receipt of clarification of the design basis with respect to 4 Kilovolt safety bus under voltage protection scheme. |
| Site: | Perry  |
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| Report | IR 05000440/2016008 Section 4OA5 |
| Date counted | Mar 31, 2016 (2016Q1) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 93812 |
| Inspectors (proximate) | D Szwarc I Hafeez J Havertape P Louden R Baker W Schaup |
| INPO aspect | |
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Finding - Perry - IR 05000440/2016008 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Perry) @ 2016Q1
Self-Identified List (Perry)
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