ML071770043

6/21/07 E-Mail from M. Thadani, Nrr/Dorl/Lpliv, to F. Madden and T. Hope, Txu Power - Request for Additional Information on License Amendment Request on Extension of Completion Times (MD4066, MD4067)

ML071770043
Person / Time
Site:	Comanche Peak
Issue date:	06/21/2007
From:	Thadani M NRC/NRR/ADRO/DORL/LPLIV
To:	Hope T, Madden F TXU Power
Thadani, M C, NRR/DORL/LP4, 415-1476
References
TAC MD4066, TAC MD4067
Download: ML071770043 (5)

Text

From: Mohan Thadani To: Internet:fred.madden@txu.com; Internet:Timothy.Hope@txu.com Date: 6/21/2007 11:16:35 AM

Subject:

Fwd: CPSES_EXTENSION EDG CTs (TAC Nos. MD4066 and MD4067) - RAI Fred/Tim:

The NRC staff has reviewed your application for extension of completion times for diesel generators (TAC Nos. MD4066 and MD4067), and has identified a need for additional information. Early response to the following request for additional information is needed to complete our review in a timely manner.

Please advise your schedule for response to this request, and indicate if you require that this request be formally sent to you by a letter.

Thanks.

Mohan REQUEST FOR ADDITIONAL INFORMATION

1. The calculations of the change in core damage frequency (DCDF) and change in large early release frequency (DLERF) effectively assume a single entry into the extended 14-day completion time (CT) each year, but no such restrictions have been identified and the licensee specifically states they will use the 14-day CT for corrective maintenance if needed. The licensee has identified the recent corrective maintenance history, but has not identified frequencies and durations of any proposed planned preventive maintenance which would be implemented using the extended CT. The licensee is requested to justify the assumption of one 14-day CT per year, or provide appropriate risk analyses for more realistic assumptions, or proposed appropriate restrictions on the applicability of the extended CT.

2. The licensee stated with regards to the potential for common cause failure (CCF) of the remaining operable diesel generator (DG), that if a common mode failure exists, Technical Specification (TS) 3.0.3 would require the plant to be shutdown. It is not clear to the staff why TS 3.0.3 would always apply under these circumstances, since the existing TS 3.8.1 provides action requirements. The licensee is requested to clarify the TS applicability in the event of a discovery of a CCF mode affecting the DGs, including any cross-unit considerations.

3. The licensee has identified the modeling assumptions with regards to the alternate AC power source (AACPS). The staff has additional questions regarding how the AACPS is credited in the risk analyses supporting this proposed change:

a. It is not clear whether the model assumes an automatic start and load capability of the AACPS with manual operator backup, or whether the manual operator action is always required. Section 1.0 of Attachment 1 of the licensee submittal states that the AACPS would be started manually or automatically, and connected to the bus when it has achieved rated voltage and speed. Section 4.2.4.2 however states that no credit is taken for scenarios due to insufficient time for manual starting and loading. The licensee is requested to clarify the specific assumptions for the risk analyses with regards to starting the AACPS, connecting it to the emergency bus, and starting the required equipment, including whether actions are accomplished locally or from the control room. If local actions are required, the licensee should further discuss how human reliability for the AACPS was evaluated, including dependencies with other potential actions required by the probabilistic risk assessment (PRA) model and by Comanche Peak Steam Electrical Station (CPSES) emergency procedures for station blackout.

b. The licensee stated an assumption that the AACPS would be connected to the emergency bus within 15 minutes of detection of a loss of offsite power (LOOP), and therefore the AACPS would have the capacity required for safe shutdown. No basis was provided for the 15-minute time, and additional time to manually start required loads was not addressed. The licensee further identified that the reactor coolant pump seal loss of coolant accident (LOCA) model used in the PRA model uses Westinghouse Commercial Atomic Power (WCAP)-15603 Revision 1-A. This model assumes that a 13-minute interruption of pump seal cooling may result in the development of excessive leakage. The licensee is requested to clarify the basis for the assumption of 15 minutes to connect the AACPS and its potential impact on the assumptions of the PRA model with regards to seal LOCAs.

c. It is not clear from the licensees submittal if a specific AACPS has been identified, either permanent or temporary. Therefore, specific assumptions in the risk analyses regarding the reliability of the AACPS, its fuel supply, its output breaker, procedures for operation, human reliability and the associated procedural bases, are not defined. The licensee is requested to provide a basis for the assumptions regarding reliability of the AACPS as a system (including the above specific items) being equivalent to the existing DGs.

d. The licensee stated that the AACPS would require a 24-hour fuel oil supply. This is less than the 7-day supply required by the CPSES TS 3.8.4. Typically, the PRA mission time is 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, but this assumes a safe stable end state has been achieved, which would not be the case if the AACPS did not have fuel oil beyond 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. The licensee is requested to discuss the availability of additional fuel oil beyond 24-hours to support the continued use of the AACPS under emergency conditions.

4. The licensee is requested to identify the specific version and date of the probabilistic risk assessment (PRA) model applied for the risk evaluations supporting the proposed change, and identify any plant changes (i.e., modifications, procedure revisions, or other items) not yet incorporated into the PRA model, including justification that such unincorporated changes do not adversely impact the stated risk impact.

5. The licensee stated that the computation of incremental conditional core damage probability (ICCDP) and incremental conditional large early release probability (ICLERP) were per the definitions in Regulatory Guide (RG) 1.177, and identified specific equations used to perform the calculations. However, RG 1.177 uses the increase above the nominal baseline risk, including contributions from nominal expected equipment unavailability, while the licensee calculations specify the use of the baseline CDF without test or maintenance contributions included. The licensee is requested to clarify its calculation basis, which appears to be different than the specific RG 1.177 guidance.

6. The licensee stated that for emergent repair-type use of the extended CT, the AACPS would be in place prior to exceeding the 72-hour CT, consistent with the TS Required Actions as proposed; however, the analyses of risk assume the availability of the AACPS throughout the 14-day CT. The licensee is requested to provide the applicable risk analyses without crediting the AACPS for the first 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> consistent with its stated intent of AACPS availability to support emergent repairs.

7. The licensees qualitative evaluation of external events including internal fires and floods considers only events which may cause a LOOP but which specifically do not impact the DGs or their support systems. The staff does not agree with this approach, since there may be internal flooding or fire scenarios which can cause a LOOP and also affect one of the two safety trains, which would be potentially significant for this application. The licensee is requested to identify whether there are such scenarios possible given the physical layout and separation of the offsite power circuits with regards to internal fires and floods, and if necessary provide additional analyses to disposition these scenarios.

8. A seismically-induced LOOP frequency of 5E-5 per year was demonstrated and compared to the nominal LOOP frequency of 3.49E-2 per year to conclude that the additional

risk from seismic events was small. However, for this application, the non-seismic LOOP events can be mitigated by the AACPS which would not be available after a seismic event.

Further, recovery of offsite power following a seismic event is not likely until some significant time after the event. The licensee is requested to provide additional evaluation of the conditional core damage probability given a seismically -induced LOOP, with no credit for offsite power recovery, no credit for AACPS, and one DG unavailable over the 14-day extended CT, to provide a more conclusive argument for low risk of seismic events.

9. The licensee identified insights from the CPSES Fire PRA with regards to scenarios which cause a LOOP. The staff has additional questions regarding these events:

a. The licensee had previously stated that it only had an internal events PRA model; it is not clear to what the CPSES Fire PRA refers.

b. Three control room cabinet fires were identified as causing a LOOP but no further details were provided. The licensee should discuss: 1) the frequency of occurrence of these events (identified only as very low), 2) mitigation of these events, and 3) the capability to restore offsite power given the expected fire damage.

10. The licensees analysis of high wind events stated that such events which cause a LOOP are already included in the internal event PRA model LOOP initiating event frequency.

However, the licensee stated that the AACPS is not protected from natural phenomena or abnormal environmental or dynamic effects. Therefore, the availability of the AACPS is not assured by the licensee for these types of LOOP events. The licensee is requested to provide additional justification that the risk of such events is not significant for this application.

11. The licensees analysis of tornado events identifies a strike frequency of 5E-4 per year, and identifies that the risk exposure over the 11 additional days of the extended CT is very small. The licensee then states that offsite power non-recovery probability is very small beyond 3 days. The staff does not understand the significance of the 3-day period identified.

As previously noted by the staff for these types of events, offsite power recovery may be significantly delayed due to the damage incurred, and a simple comparison with the nominal LOOP frequency is an inadequate justification that the risk impact is small. Further, as noted in RAI 10, the AACPS is not protected from natural phenomena and therefore may be unavailable to mitigate the impact of a tornado. The licensee is requested to provide additional justification that this external event is not a significant risk contributor.

12. The licensees analysis of internal floods stated that these events contribute less than 1% of the internal events risk. The licensee had previously stated that it only had an internal events PRA model which did not include contributions from internal floods. The licensee is requested to discuss the basis for this quantitative assessment of risk.

13. The licensees submittal did not identify if the risk analyses provided point estimates of the mean or actual means, nor was there any discussion of uncertainty analyses to support the calculations. The licensee is requested to address PRA model and parametric uncertainty using the guidance of RG 1.174 Section 2.2.5.

14. Section 4.1 of the licensees submittal identifies administrative controls which would be applicable during the extended CT. In addition, Section 4.2.3 identifies plant equipment and activities which, if unavailable simultaneous with the DG, would likely result in a high risk configuration. The staff has additional questions regarding these portions of the submittal:

a. The licensees submittal does not specifically identify whether these statements represent commitments. The staff notes that the licensees risk analysis assumes no other testing or maintenance activities on other plant equipment. The licensee is requested to clarify their intent with regards to the RG 1.177 tier two portion of their request.

b. The staff notes that the section 4.1 administrative controls items 2 and 3 are worded subtly different; specifically, weather conditions must be conducive to perform planned maintenance, and offsite power supply and switchyard conditions must be conducive to perform maintenance. The licensee is requested to clarify the intent, if any, of the use and omission of the word planned.

c. Section 4.1 states switchyard access will be monitored and controlled, and discusses the procedure STA-629. It is not clear that this represents any unique administrative control, since switchyard access should normally be so monitored and controlled using approved plant procedures. The licensee is requested to clarify the intent of this administrative control as regards its representing a unique additional restriction on switchyard activities.

d. Section 4.2.3 does not explicitly identify that the potential high risk configurations would be prohibited, consistent with the assumptions of the risk analysis, during the extended CT. In fact, the submittal states that Tier 3 risk management actions will address the availability of these systems. The licensee is requested to clarify the intent of identifying these configurations in the tier 2 section of their submittal, and identify any associated commitments consistent with RG 1.177 for tier 2.

15. RG 1.177 Section 2.3.7 describes various attributes of contemporaneous configuration control and the CRMP which can support risk-informed decisionmaking. Certain aspects of the licensees program have not been adequately described to assure that the guidance of RG 1.177 is met. Specifically, the licensee only states that added or emergent activities, or activities which have slipped from the scheduled completion time, are addressed. RG 1.77 Section 2.3.7.1 requires specific descriptions to be provided, as to their capability to perform contemporaneous assessment of overall plant safety impact of proposed plant configurations, how the tools or other processes are used to ensure risk-significant configurations are not entered, and that appropriate actions will be taken when unforseen events put the plant in a risk-significant configuration. Further, it identifies four key components of the CRMP, which have not been addressed by the licensee. The licensee is requested to confirm and describe how their CRMP conforms to the RG 1.177 Section 2.3.7 guidance.

16. The licensee has submitted a proposed change to extend the CT for LCO 3.8.1 with regards to one inoperable offsite circuit from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 30 days. The staff requests clarification of certain aspects of the proposed change which may impact the proposed changes for the DGs.

a. The second CT of LCO 3.8.1 applicable to contiguous application of the actions of the TS 3.8.1 is proposed to be increased from 6 days to 17days in this amendment request, and from 6 days to 33 days for the offsite circuit request. The licensee is requested to identify the proposed final CT. The staff also notes that TSTF-439-A eliminated this second CT, and the licensee may want to consider implementation of this TSTF along with these amendment requests.

b. Because these two requests are directly related to AC power sources, the staff considers them to be a combined change request as defined by RG 1.174 Sections 2.1.1 and 2.1.2. The licensee is requested to submit the additional information identified in RG 1.174 with regards to the synergistic impacts of the proposed changes.

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Subject:

Fwd: CPSES_EXTENSION EDG CTs (TAC Nos. MD4066 and MD4067) - RAI

Creation Date 6/21/2007 11:16:35 AM From: Mohan Thadani Created By: MCT@nrc.gov Recipients fred madden (Internet:fred.madden@txu.com)

Timothy Hope (Internet:Timothy.Hope@txu.com)

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