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ACCESSION NBR:9810190291 DOC.DATE: 98/10/15 NOTARIZED: NO DOCKET # FACIL:50-261 H.B. Robinson Plant, Unit 2, Carolina Power & Light C 05000261 AUTH.NAME AUTHOR AFFILIATION 'WILK9RSON,T.M. Carolina Power & Light Co. RECIP.NAME RECIPIENT AFFILIATION Records Management Branch (Document Control Desk)

SUBJECT: Summarizes considerations & discussions presented in 980817 meeting with NRC to discuss backfeeding of plant loads from C unit auxiliary transformer.

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Carolina Power & Light Company Robinson Nuclear Plant 3581 West Entrance Road Hartsville SC 29550

Robinson File No: 13510 Serial: RNP-RA/98-0178 0CT 15 1995 United States Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555

H. B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO. 2 DOCKET NO. 50-261/LICENSE NO. DPR-23

ADDITIONAL INFORMATION REGARDING UNIT AUXILIARY TRANSFORMER BACKFEEDING AVAILABILITY ENHANCEMENTS

Sir or Madam:

On August 17, 1998, Carolina Power & Light Company met with the NRC to discuss the backfeeding of plant loads from the unit auxiliary transformer at the H. B. Robinson Steam Electric Plant, Unit No. 2. This letter summarizes the considerations and discussions presented in that meeting.

If you have any questions concerning this matter, please contact me or Mr. H. K. Chernoff of my staff.

Very truly yours,

" AA. Wlkl T. M. Wilkerson 9810190291 981015 Manager - Regulatory Affairs PDR ADOCK 05000261 P PDR MSL/msl : J

Attachments: Attachment I: Backfeeding Loads from the Unit Auxiliary Transformer Attachment II: Timelines c: NRC Resident Inspector, HBRSEP Mr. R. Subbaratnam, NRC, NRR Mr. L.A. Reyes, NRC, Region II

Highway 151 and SC 23 Hartsville SC United States Nuclear Reg 'ory Commission Attachment I to Serial: A/98-0178 Page 2 of 4

"HBR 2 was constructed prior to the issuance of General Design Criteria 17. Therefore, a single startup transformer connects the multiple sources of offsite power to the onsite electric distribution system. Should a failure of the startup transformer occur, a spare startup transformer located onsite could be jumpered into service. During the time that the startup transformer was out of service, the unit auxiliary transformer could supply power to the onsite distribution system by back-feeding the main transformer from the 230KV switchyard. Prior to back-feeding the main transformer from the 230KV switchyard, the generator must be disconnected from the main transformer by removing the connecting straps. The main transformer backfeeding will only be done during cold shutdown unless nuclear safety considerations require it to be done during hot shutdown when no other power sources are available... .A minimum of 4 hours is estimated to disconnect the generator straps to enable backfeeding through the unit auxiliary transformer."

It should be noted that the backfeeding capability is available, but not required, to mitigate an accident. Backfeeding is routinely performed during refueling outages to allow maintenance of the startup transformer. The need to backfeed the electrical distribution system via the unit. auxiliary transformer is limited to a small set of specific events which result in the inability to provide power to the safety-related electrical buses via the Startup Transformer.

Additionally, the coping analysis for 10 CFR 50.63, "Loss of All Alternating Current Power," concluded that HBRSEP, Unit No. 2 can be classified as an Alternate AC (AAC) plant, which is required to cope for eight (8) hours after a Station Blackout (SBO). HBRSEP, Unit No. 2 is classified as a one hour ACC plant, i.e., the Dedicated Shutdown Diesel Generator can be started and loaded within one hour after SBO, and the Dedicated Shutdown Bus will supply power for the next seven hours to equipment necessary for safe shutdown. Therefore, provided backfeeding is established within this coping duration or within approximately 4 hours as stated in the UFSAR, fuel design limits and design conditions of the reactor coolant system pressure boundary will be maintained.

On July 31, 1997, CP&L met with the NRC to discuss details of the electrical distribution system at HBRSEP, Unit No. 2 and described certain actions, both in place and planned, to enhance the ability to establish backfeed. CP&L committed to develop a modification, tentatively planned for implementation during Refueling Outage (RO 19), to install a switch to enable prompt disconnecting of the main generator from the isophase bus to allow backfeed of electrical buses through the unit auxiliary transformer. This commitment was contingent on the technical feasibility of such a design and final management concurrence that the design change was warranted.

Discussion

The enhancements described at the July 31, 1997, meeting regarding backfeeding capability at HBRSEP, Unit No. 2 have been completed. During RO 18, in the Spring of 1998, HBRSEP, Unit No. 2 demonstrated that backfeeding the plant electrical distribution system can be accomplished within the four (4) hour time period stipulated in the UFSAR. In United States Nuclear Reg ory Commission Attachment I to Serial: -RA/98-0178 Page 3 of 4 addition, conceptual work was begun on the proposed disconnect switch modification. The proposed modification was anticipated to cost approximately $500,000.

Based on the successful results of the enhancements demonstrated during RO 18 and the anticipated significant cost of the proposed modification, a probabilistic safety assessment (PSA) was initiated to determine if there was a risk informed, cost-justifiable basis for proceeding with the proposed modification.

The PSA of the proposed modification was performed to assess the effect,on Core Damage Frequency (CDF), of the time saved in establishing backfeed through the unit auxiliary transformer during a SBO . In this assessment, it was assumed that four (4) hours was required to establish backfeed with the current plant configuration and that the proposed modification would reduce the required time to two (2) hours. These assumptions were conservative, based on demonstrated performance, of the steps required to establish backfeed, during RO 18. The assessment determined that there was no appreciable improvement in CDF as the result of the modified plant configuration. The CDF for the current configuration is 4.907E-05 per reactor year and with the modified plant configuration the CDF would be 4.902E-05 per reactor year which equates to a change in CDF of 5.OE-8 per reactor year or less than 0.1%.

The results of this PSA, in conjunction with the demonstrated ability to conform with the discussion of backfeed in the UFSAR and the availability of the Dedicated Shutdown Diesel Generator have led CP&L to conclude that the proposed modification does not appreciably change (1) the risk to the public from the accidental offsite release of radioactive material; (2) the potential impact on radiological exposure to facility employees; or (3) the Core Damage Frequency. Therefore, the modification was not considered cost-justifiable from a risk perspective. A meeting with the NRC Staff to propose deletion of the disconnect switch modification was scheduled for August 17, 1998.

During the August 17, 1998 meeting, CP&L described the results of the RO 18 demonstration activities, provided activity timelines, and summarized the PSA results. Assuming a SBO, the time required to establish backfeed with the current as-built configuration of the electrical distribution system has been estimated to be 221 minutes after the event. This timeline includes the time dedicated to SBO coping activities directed by the emergency procedures, the time to hang and remove clearances on the equipment used for backfeed, the time required to remove the generator links, and the time to perform the required switching to establish the backfeed. The time required to establish backfeed under the same circumstances with the proposed disconnect switch modification in place was estimated to be 176 minutes after the event. The NRC Staff had several questions regarding the timelines presented in the meeting, including questions regarding the time required for hanging clearances. Subsequent to the meeting additional review of the timelines was conducted.

As a result of CP&Ls review of the timelines presented in the August 17, 1998, meeting, it was determined that the timelines could be modified to delete the time needed for hanging clearances for the case that assumed the disconnect switch modification was installed. In an United States Nuclear Re ory Commission Attachment I to Serial: - A/98-0178 Page 4 of 4

effort to further optimize the timeline for this case, the scope of the conceptual modification for the disconnect switch was changed to also include permanently installed switches to replace temporary jumpers, and permanently installed switchyard breaker control power connections. The cost of this optimized modification has been estimated to be $889,000.

The optimized modification was reviewed to determine the potential improvement in the time required to establish backfeed from the unit auxiliary transformer. The time required to establish backfeed with the current as-built configuration, as previously stated, has been estimated to be 221 minutes after the event. The time required to establish backfeed with the optimized modification described above is estimated to be 138 minutes after the event. This timeline includes the time dedicated to SBO coping activities directed by the emergency procedures, time to prepare for backfeed as directed by the emergency procedures, an estimated time to open the proposed disconnect switch, and the time to perform the required switching to establish the backfeed. A graphical depiction of these timelines is provided in Attachment II.

It should be noted that the anticipated required backfeed time for the optimized modification is still conservative with respect to the two (2) hours originally assumed in the PSA. Therefore the determinations from the PSA remain bounding for the proposed optimized modification.

Conclusions

As described above, CP&L concludes that the existing plant configuration need not be changed since: (1) CP&L has demonstrated the capability of providing adequate assurance of the ability to establish backfeed through the unit auxiliary transformer within approximately four (4) hours as stated in the UFSAR; (2) the Dedicated Shutdown Diesel Generator provides power to equipment necessary for safe shutdown within one (1) hour of a loss of onsite power and for the duration of the eight (8) hour SBO coping period; (3) proposed optimized modifications to the plant do not result in an appreciable improvement to the CDF; and (4) there is no appreciable increase in the risk to the public from the accidental offsite release of radioactive material or potential impact on radiological exposure to facility employees. Ufited States Nuclear Re 'fory Commission Attachment II to Serial: R -RA/98-0178 Page 1 of 1

Backfeed Timelines

Time to Establish Backfeed with Station Blackout - Current Configuration

47 min 70 min 55 min 38 min 11 min (1) (2) (3) (4) (5)

Total time to backfeed - 221 min (3 hours 41 min)

(1) SBO activities (2) Hang clearances, initial backfeed setup, circuit breaker control power installed (3) Remove generator links (4) Install jumpers, remove clearances (5) Switching manipulations to establish backfeed

Time to Establish Backfeed with Station Blackout - Optimized Modification

47 min 60 min 10 10 11 S mm mlH mm (1) (2) (3) (4) (5)

Total time to backfeed - 138 min (2 hours 18 min)

(1) SBO activities (2) Initial backfeed setup (3) Open disconnect switch (4) Manipulate switches that replace jumpers (5) Switching manipulations to establish backfeed Urnited States Nuclear Re ory Commission Attachment I to Serial: - A/98-0178 Page 1 of 4

H. B. Robinson Steam Electric Pant, Unit No. 2, Backfeeding Loads from the Unit Auxiliary Transformer

Purpose

The purpose of this letter is to summarize the considerations and discussions presented during a meeting conducted on August 17, 1998, between Carolina Power & Light (CP&L) Company and the NRC, regarding the establishment of backfeeding of plant loads from the unit auxiliary transformer at the H. B. Robinson Steam Electric Plant (HBRSEP), Unit No. 2 in the event of a loss of onsite AC power.

Additional information concerning the backfeeding issue is contained in correspondence between from NRC to CP&L dated February 14, 1997, from CP&L to NRC dated March 27, 1997, from NRC to CP&L dated June 10, 1997, from CP&L to NRC dated September 2, 1997, and from CP&L to NRC dated October 30, 1997 as well as other correspondence.

Background

HBRSEP, Unit No. 2 was constructed prior to the issuance of General Design Criterion (GDC) 17, and therefore, the requirements of GDC 17 are not applicable to HBRSEP, Unit No. 2. The Final Safety Analysis Report (FSAR) contained a discussion of how HBRSEP, Unit No. 2 would comply with a plant specific adaptation of the proposed Appendix A to 10 CFR 50, "General Design Criteria for Nuclear Power Plants," published in the Federal Register on July 11, 1967. This was presented in the FSAR as GDC 39.

The HBRSEP, Unit No. 2 GDC 39 states: "An emergency power source shall be provided and designed with adequate independency, redundance, capacity, and testability to permit the functioning of the engineered safety features and the protection systems required to avoid undue risk to the health and safety of the public. The power source shall provide this capacity assuming a failure of a single active component (GDC 39)." It should be noted that the plant specific adaptation of proposed GDC 39 to which HBRSEP, Unit No. 2 is committed does not require the assumption of a single failure in the onsite power system and the offsite power system.

The capability for backfeeding was recognized in the Safety Evaluation Report (SER) issued by the NRC by letter dated May 18, 1970, in support of the issuance of the HBRSEP, Unit No. 2 Operating License. The SER states in Section 3.8.1: "Our review indicated that the only portion of the offsite system vulnerable to the random failure of a single component (as opposed to a cascading blackout), is the single startup transformer. A failure of this transformer during normal reactor operation would not cause an accident. However, should a loss-of-coolant accident occur following a transformer failure, the engineered safety features would be dependent on the redundant diesel generators for power until disconnect links could be removed from the station generator to permit the backfeeding of offsite power to the auxiliary electrical system through the station main transformer. This would require about 8 hours to accomplish." This position was further modified in the Updated FSAR (UFSAR) Section 8.2.2, "Analysis" which states: