IR 05000461/2007006

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February 7, 2007

EA-06-291Mr. Christopher M. CranePresident and Chief Nuclear Officer Exelon Nuclear Exelon Generation Company, LLC 4300 Winfield Road Warrenville, IL 60555

SUBJECT: FINAL SIGNIFICANCE DETERMINATION FOR A WHITE FINDING AND NOTICE OF VIOLATION; NRC INSPECTION REPORT NO. 05000461/2007006(DRS) FOR CLINTON POWER STATION

Dear Mr. Crane:

The purpose of this letter is to provide you the final results of our significance determination of the preliminary Greater Than Green finding identified in Inspection Report No. 05000461/

2006011(DRS). The inspection finding was assessed using the Significance Determination Process and was preliminarily characterized as Greater Than Green, a finding of greater than very low safety significance, resulting in the need for further evaluation to determine significance; and therefore, the need for additional NRC action. This Greater Than Green finding involved the failure to select an appropriate method for calculating the minimum elevation (i.e., the analytical level) of water above the high pressure core spray (HPCS) pump suction line to preclude vortex formation and subsequent air entrainment in the pump's suction. As a result, the analytical level would result in significant air entrainment potentially causing the HPCS to be incapable of completing its safety function. At our request, a Regulatory Conference was held on December 19, 2006, to further discussyour views on this issue. The public meeting summary, including the handouts, can be found in the Agencywide Document Access and Management System (ADAMS) ML063520445. During the meeting, your staff described the results of recent scaled model testing. Specifically, the scaled model testing showed that a localized depression briefly formed which immediately collapsed and resulted in significant (about 24 percent) air being entrained in the suction piping. You determined that this air/water mixture would result in a slowed level decrease in the reactor core isolation cooling tank resulting in a delay in transferring suction to the suppression pool. However, your calculations showed that the suction valve from the suppression pool wouldopen and suction would be primarily from the suppression pool prior to the air reaching the suction of the HPCS pump. Therefore, you concluded that the HPCS pump would be capable of performing its safety function. In assessing the test model results, you assumed that flow C. Crane-2-would not be characterized as slug flow, that is, the flow would be less than 24 percent airentrained. Your conclusion was based on: (1) visual confirmation during Alden Laboratory Testing; (2) visual comparison of test results against a "known" flow having 24 percent air entrainment; and (3) a computer model (RELAP) prediction that slug flow would exist above 24 percent. The NRC identified the following concerns with your conclusion: *During testing at Alden Laboratory, the pump was stopped immediately (about 5 seconds) upon visual observation of 'break through', i.e., air becoming entrained in the suction pipe. This was done to preserve and prevent damage to the test pump.

This quick stopping did not allow time to verify the absence of slug flow.*The visual comparison of the test results against a "known" flow having 24 percent airentrainment was short in duration. This visual comparison may not represent actual test or in-plant flow conditions. With flow and flow conditions unstable and oscillating, this short time duration did not provide definitive proof that slug flow would not exist. Actual void fraction measurements may typically have as much as 8 percent uncertainty.*The computer model (RELAP) is a generic 2-phase flow code and is not necessarilytuned or calibrated to this exact scenario. To consider results 'exact' and without any consideration of analytical error is imprudent. The assumption of 24 percent air entrainment was key in assessing the ability of HPCS toperform its function for several reasons. First, the 24 percent provides a basis for the rate of decrease in the RCIC tank. A greater void fraction would slow down the rate of change, increasing the time to the swap-over point. This increase in time, would allow the air wave front to travel further down the line and potentially reach pump suction prior to full opening of the suppression pool suction valve. Secondly, although your staff calculated a 2-phase fluid flowvelocity, an increase in void fraction will increase the transport velocity, increasing the possibility of air arrival at the pump. Lastly, should slug flow exist, there is a potential for system waterhammer affecting system piping or the HPCS pump or both.In summary, the staff does not concur with your evaluation regarding the amount of airentrainment; and therefore, does not agree with your assessment on the past operability of the HPCS pump. Your assessment is not conclusive, complete or robust, in that the basis for 24 percent was not well founded. Small changes to these assumptions may significantly impact the conclusion regarding past HPCS pump operability. In addition, during the Regulatory Conference, you also provided your assessment of thesignificance of the finding. Specifically, you provided information regarding the potential for operators to throttle HPCS flow and the estimated contribution to the risk from fire events. The NRC reviewed the information regarding throttling the HPCS injection valve and determined C. Crane-3-that it should be considered in the final significance determination. Based on the discussion atthe Regulatory Conference, operators would be directed to throttle HPCS in response to transient (i.e., non- Loss of Coolant Accidents and non- Anticipated Transient Without a Scram)

scenarios. If operators successfully throttle the HPCS injection valve, the system flow rate will be low enough that air entrainment during suction swap-over to the suppression pool would no longer be a concern. For the final significance determination, the NRC assumed that HPCS would fail in response to transient initiating events only if the operator failed to properly throttle the HPCS injection valve. For all other initiating events, HPCS was assumed to fail during the suction transfer, consistent with the assumption in the preliminary significance determination. Given the inherent uncertainty in estimating human error probabilities, the NRC used its best estimate of 2.6E-2 for the human error probability in the final significance determination.The NRC also reviewed the estimation of fire risk contribution that you provided and determinedthat it was the best available information; and therefore, it was used directly in the final significance determination.After considering the information presented at the Regulatory Conference and the additionalinformation you provided in your letter dated December 21, 2006, the NRC has concluded that the inspection finding is appropriately characterized as White, an issue with low to moderate increased importance to safety, which may require additional NRC inspections. Using the estimation of fire risk contribution and best estimate for human error probability, the NRC determined the total change in core damage frequency to be about 4.4E-6 per year. You have 30 calendar days from the date of this letter to appeal the staff's determination ofsignificance for the identified White finding. Such appeals will be considered to have merit only if they meet the criteria given in NRC Inspection Manual Chapter 0609, Attachment 2.The NRC has also determined that the failure to ensure the adequacy of design of the HPCSsystem by performance of design reviews or by use of alternate or simplified calculational methods is a violation of Title 10 Part 50, Appendix B, Criteria III, as cited in the enclosedNotice of Violation (Notice). The circumstances surrounding the violation are described in detailin Inspection Report No. 05000461/2006011(DRS). In accordance with the NRC Enforcement Policy, NUREG-1600, the Notice of Violation is considered escalated enforcement action because it is associated with a White finding.You are required to respond to this letter and should follow the instructions specified in theenclosed Notice when preparing your response.Because plant performance for this issue has been determined to be in the regulatory responseband, we will use the NRC Action Matrix, to determine the most appropriate NRC response for this event. We will notify you, by separate correspondence, of that determination.

C. Crane-4-In accordance with 10 CFR 2.390 of the NRC's "Rules of Practice," a copy of this letter, itsenclosure and response will be made available electronically for public inspection in the NRC Public Document Room or from the Publically Available Records (PARS) component of NRC's document system (ADAMS), accessible from the NRC Web site at http://www.nrc.gov/reading-rm/adams.html (the Public Electronic Reading Room).

Sincerely,/RA/James L. CaldwellRegional AdministratorDocket No. 50-461License No. NPF-62

Enclosure:

Notice of Violationcc w/encl:Site Vice President - Clinton Power StationPlant Manager - Clinton Power Station Regulatory Assurance Manager - Clinton Power Station Chief Operating Officer Senior Vice President - Nuclear Services Vice President - Operations Support Vice President - Licensing and Regulatory Affairs Manager Licensing - Clinton Power Station Senior Counsel, Nuclear, Mid-West Regional Operating Group Document Control Desk - Licensing Assistant Attorney General Illinois Emergency Management Agency State Liaison Officer, State of Illinois Chairman, Illinois Commerce Commission 1 HQ concurrence received via e-mail from D. Starkey, OE on February 2, 2007