05000346/LER-2003-003

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LER-2003-003, 1 OF 9
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
10 CFR 50.73(a)(2)(v), Loss of Safety Function
10 CFR 50.73(a)(2)(vii), Common Cause Inoperability
10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
Initial Reporting
3462003003R01 - NRC Website
v • d • e

DESCRIPTION OF OCCURRENCE (continued)! In response to this concern, the licensee contacted the Nuclear Steam Supply System (NSSS) vendor, Framatome ANP (formerly Babcock & Wilcox (B&W)) in the fall of 2002. Break sizes between the capacity of the Makeup (MU) System and up to the 0.01 ft2 range were not covered by the vendor's existing SELOCA analyses.

Framatome ANP then reexamined the range of break sizes less than 0.01 ft2.

The application of the NRC-approved EM for very small breaks, which includes very conservative design basis assumptions, indicated a vulnerability of the HPI Pumps' minimum recirculation flow protection. It predicted that cyclic repressurization of the RCS could occur prior to depletion of the EWST and continue following realignment of the BPI pumps to the emergency sump via the LP' pump discharge (piggy-back operation), after the recirculation valves are closed. RCS pressure during this condition was analytically predicted to exceed the shutoff head of the pumps. Without minimum flow through the pumps, they could be damaged from overheating in this potential deadhead operating condition and fail to perform their intended safety function. This condition has existed since the original design of DBNPS.

Notification of this condition was provided to the NRC on April 11, 2003 (Event No. 39750).

This condition is being reported as

50.46(a)(3)(ii). This regulation requires a 30-day report be submitted to the NRC describing each significant change or error discovered in an acceptable evaluation model or application of such model that affects the peak fuel clad temperature (PCT) calculation. The application of the EM for very small break sizes determined a vulnerability of the HPI Pumps minimum recirculation flow protection. This is not considered a change or error in the EM or its application that resulted in a calculated PCT change of greater than 50 degrees F. Rather, this application of the EM model resulted in the determination that the HPI Pumps would not perform their function for certain small size breaks using very conservative design basis assumptions.

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APPARENT CAUSE OF OCCURRENCE:

When the DBNPS was originally designed and licensed, a LOCA analysis was performed by B&W to demonstrate compliance with the requirements of 10 CFR 50.46, 'Acceptance Criteria for ECCS for Light Water Nuclear Power Reactors.' This analysis was based on the application of a May 1972 Dl. The computer codes used in this early BM were not sufficiently sophisticated to identify repressurization cycles of the type now understood to occur during very SBLOCA events. The evaluation methodologies met the standards of the time.

As a result of the accident at Three Mile Island Unit 2 in March 1979, the NRC initiated an examination of slow RCS depressurization resulting from SBLOCAs.

These SBLOCAs had not previously received detailed analytical studies comparable to those devoted to large breaks. A product of this NRC re-examination was NUREG-0565, "Generic Evaluation of Small Break Loss-Of-Coolant Accident Behavior in Babcock & Wilcox Designed 177-FA Operating Plants." In NUREG-0565, the NRC concluded that "the small break analysis methods used by B&W are satisfactory for the purpose of predicting trends in plant behavior following small break LOCAs and for training of reactor operators,' but concerns regarding the small break model were identified. NRC recommendations included that analysis methods should be revised and submitted for NRC approval, and plant-specific calculations using the NRC-approved model for small breaks should be submitted to show compliance with 10 CPR 50.46.

In response, B&W re-examined SBLOCAs to a minimum break area of 0.01 ft2 for the raised loop design (elevated once-through steam generators (OTSO) relative to the reactor vessel) which DBNPS incorporates. The analysis of the 0.01 ft2 break predicted that repressurization cycles would occur while the HPI Pumps are aligned to the BWST but rapidly dampened after three cycles due to system heat removal dynamics. Recognizing the lower shutoff head of the DBNPS HPI Pumps.

the NRC was concerned with the potential for repressurization. However, based upon mitigative actions provided by B&W, the NRC found "the range of small breaks and small break accident cooling modes arc considered appropriate for demonstrating plant response to potential SBLOCAs.' This analysis supported establishment of a 0.01 ft2 break area as the lower limit of the spectrum of breaks analyzed for the DBNPS.

For the DBNPS Operating Cycle 13 and forward, the large and small break LOCA spectrum was reanalyzed using improved computer codes in the EM, Framatome Topical Report BAW-10192PA, July 1998. The DBNPS-specific analysis results are summarized in Framatome ANP 86-5006232-00, "DB-1 LOCH Summary Report.' This report was issued in March 2000. This analysis included a break area of 0.01 ft2 as the lower limit of the break sizes analyzed based on the conclusion that with OTSO heat transfer available, the consequences of the small break transient decrease with decreasing break size. Revision 22 to USAR 6.3.3 discusses the evaluation results of this report. This Summary Report indicates that the repressurization phenomenon would occur during the injection mode of operation when the HPI pumps have minimum flow recirculation. However, Auxiliary APPARENT CAUSE OF OCCURRENCE (continued):

Feedwater (AFW) cooling in conjunction with intermittent HPI would result in adequate long-term depressurization of the RCS long before the MST would be depleted and, therefore, before the recirculation valves would be closed. A revised LOCA Summary Report, Framatome ANP 86-5006232-01 was issued in August 2002. It continued to maintain the 0.01 ft2 break as the lower limit of break sizes analyzed. It indicated that for this break size, initial repressurization would occur that would be well above the shutoff head of the HPI pumps.

Following that, smaller repressurization cycles would occur, but RCS pressure would then drop off continuously. The results of the 0.01 ft2 break analysis were determined to meet the acceptance criteria of 10 CFR 50.46, and Framatome concluded that with OTSO beat transfer available, the consequences of the small break transient decrease with decreasing break size. Therefore, break sizes smaller than 0.01 ft2 were not specifically analyzed.

Following the questioning during the fall 2002 NRC inspection of a potential deadhead condition of the pumps and the adequacy of thermal protection for the pumps, Framatome performed a study in November 2002 for the DBNPS, Framatome ANP 06-5022260-00, to determine whether BPI pump operability during post-LOCA sump recirculation could be assured for all break sizes and transient scenarios.

This study identified a range of break sizes from 0.00206 ft2 (leak-to-LOCA transition area) to 0.0045 fewhich would result in RCS repressurization cycles that may continue following HPI Pump realignment to the containment emergency aump and closure of the minimum flow recirculation valves. The study concluded that for this new range of break sizes analyzed, past operability of the HPI pumps was a concern from an analytical perspective.

The condition described in this LER has existed since original design of DBNPS, and is a latent issue.

ANALYSIS OF OCCURRENCEs The following scenario describes an analytical event in accordance with NRC requirements of 10 CFR 50.46 and Appendix K. The Framatome LOCA Summary Report currently contained in the DBNPS USAR includes analysis of SBLocAs that may allow RCS repressurization while HPI pumps are injecting from the BWST to the RCS. During this operation, thermal protection of the pumps is provided by a minimum flow recirculation line. With continuous heat transfer to AFW in the OTSO, RCS depressurization would be accomplished, and the pressure limit of the HPI pumps would not be challenged. The small break areas analyzed were over the range of 0.01 to 0.75 ft'. However, based upon the November 2002 reanalysis of possible transient scenarios for break sires smaller than those typically included in the SELOCA analysis spectrum range (i.e., less than 0.0045 ft2), it may be possible for interruption in natural circulation to occur that leads to RCS repressurization above the minimum KPI flow pressure criterion (1750 psia).

This repressurization phenomenon can become cyclic as predicted by the analysis, ANALYSIS OF OCCURRENCE (continued):

lasting from 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to several days when the classical 10 CFR 50, Appendix K assumptions are applied. If the minimum flow is not preserved, the pump is vulnerable to damage such that continued operability is no longer assured.

Although operator actions are procedurally directed for actual events, the Appendix K LOCH analyses have substantial prescribed conservatism's, use only safety-related equipment, consider the worst single failure, and make very limited use of operator actions. In the case of DBNPS, no credit is taken in the analysis for the mitigative benefits of highly reliable but non-safety grade equipment such as the Pilot-Operated Relief Valve (PORV) and Makeup (MU) pumps.

The MU pumps are capable of providing continuous flow to the RCS from the Blinn.

Continuous operation of the MU pumps would protect the HPI pumps by assuring that subcooled natural circulation of the RCS could be restored long before the BWST inventory is depleted. Opening of the PORV and its Block Valve increases the RCS equivalent break area by at least 0.01 ft2 . This ensures that the RCS would depressurize below the shutoff head of the HPI pump.

This type of accident is a slowly evolving transient which is included in the plant's emergency operating procedure. Framatome calculated the range of break sizes of interest to be 0.00206 to 0.0045 ft2 . The minimum time to switch the HPI pump suction from the BWST to the emergency sump was calculated to be approximately 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. For the smallest break area simulated, the switchover time was calculated to be approximately 92 hours0.00106 days <br />0.0256 hours <br />1.521164e-4 weeks <br />3.5006e-5 months <br />. Plant operators are trained to recognize and act on the symptoms of this type of event and the actions necessary to mitigate the accident prior to entering an RCS repressurization condition that would challenge HPI pump operability. Based upon the analytical results, substantial time would be available to execute accident mitigative actions.

Additional evaluations determined that potential mitigative actions allowed by design basis assumptions would not eliminate the need for the HPI Pumps to be provided with minimum flow protection after realignment to the emergency sump.

An additional protective recirculation flow path is being installed to assure compliance with 10 CFR 50.46 licensing basis requirements. However, Framatome AMP Summary Report 86-5022260-00 indicates that use of a MU Pump or use of the PORV to reduce RCS pressure is a successful mitigative strategy, but these components are not credited ECCS sub-systems, although they are expected to be available and are provided with essential power. The existing Emergency Operating Procedure already utilizes the MU pumps. In addition, the PORV would also be available, and its use would be indicated by the event's symptoms.

Therefore, the theoretical repressurization cycles would not occur when existing procedural and elective steps are performed.

The significance of this event was evaluated assuming the unavailability of non- safety grade equipment, consistent with 10 CFR 50.46 methodology. The Core Damage Frequency for this scenario was calculated to be less than SE-9/year.

Overall, this condition represents a very small contribution to the CDF, is mitigated by highly reliable equipment and proceduralized operator actions, and is, therefore, not risk significant.

CORRECTIVE ACTIONS 1 The re-analysis which predicts the cyclic RCS repressurization phenomenon for very SBLOCAs was further evaluated to validate the plant-specific application at the DBNPS. Based on the results of this further evaluation, several corrective actions are being implemented.

An additional minimum flow recirculation line was installed during the present outage for each HPI Pump that taps off the existing minimum flow line through two isolation valves and a non-cavitating pressure breakdown device connecting to the LPI Pump discharge upstream of its respective Decay Heat cooler for the corresponding safety train. These additional recirculation lines are designed to provide adequate minimum flow protection of the HPI Pumps, as required, when aligned to the emergency sump in "piggy back" operation with the LPI Pumps. In this lineup, the DH Coolers will provide cooling for the respective HPI Pumps without loss of sump inventory.

Operator action will be required to open the valves on these additional recirculation lines prior to pump realignment from the BLAST to the emergency sump. Because the postulated transient is very slow in developing, ample time would be available for operators to take this action. This action does not replace any existing automatic action.

Revision to DB-OP-02000, "Emergency Procedure," to provide direction to plant operatiors on establishing HPI alternate minimum recirculation flowpath was issued September 8, 2003. Operator training on this procedure was completed August 29, 2003.

Beyond these specific corrective actions being implemented to conform to the licensing basis requirements of 10 CFR 50.46, other plant improvements have been pursued to further enhance the high reliability of non-safety grade equipment that can be used to realistically mitigate the effects of BBLOCA sequences.

Previous upgrades to the PORV and MU Pumps were described in DBNPS responses (Serial 1836 dated September 18, 1990, and Serial 1884 dated December 21, 1990) to NRC Generic Letter 90-06, 'Resolution of Generic Issue 70, 'Power-Operated Relief Valve and Block Valve Reliability' and Generic Issue 94, 'Additional LTOP Protection for Light Water Reactors.'" In addition, during the current outage, NU Pump 1 motor was rewound to safety grade requirements. Both MU Pumps are supplied with Class 1E electrical power and are seismically qualified. The PORV was upgraded to install a rewound solenoid that meets safety-grade requirements.

The PORV cut-in switch was replaced with a switch upgraded from commercial grade to safety grade, and its power cable, previously qualified to safety grade and EQ standards, was replaced.

CORRECTIVE ACTIONS (continued)2 The DBNPS is unique among B&W pressurized water reactors in requiring EM application at these very small break sizes. The predicted pressure transients were not identified in previous applications of the existing EM because the very.

SBLOCA break sizes were not explicitly analyzed. A finite number of break sizes is selected to be analyzed per the EM. Because of the continuity of predicted response throughout the balance of the break spectrum, there is no indication that there is any other range of break sizes that require additional application of the EM.

As a related issue, the NRC requested on November 21, 2979, additional information regarding protection of HPI pumps should RCS repressurization occur causing the pumps to deadhead. The DIMS response was provided on December 28, 1979 (Serial Letter 568). The response to Item 42 of this letter has been reviewed in light of the newly identified analytical prediction of HPI pump operation and plant modification. The following revised response is provided.

Responses The DBNPS HPI Pumps are protected against deadheading should the RCS repressurize by its design. During post-accident injection phase operation when the pumps are aligned to the BWST, a recirculation path is provided downstream of each HPI Pump back to the BWST. No operator action is required to initiate this protection because the valves in this recirculation line are normally open.

Since the original response to this NRC request for additional information was provided in 1979, improvements in analytical methodology have occurred with respect to the 10 CFR 50.46 EM. With the current understanding of the predicted potential for repressurization, a recirculation flow path from the HPI Pumps' discharge to the discharge piping of the LAX Pumps upstream of the DH Coolers was installed during 13RFO. This recirculation flow path is designed to protect the HPI Pumps against the effects of deadheading. Operator action is required to open the valves in this new recirculation flow path prior to closing of the valves in the recirculation piping to the BWST. Since the RCS repressurization is a slowly evolving transient, ample time will be available to perform these actions. This operator action is being incorporated into the plant emergency procedure.

FAILURE DATA:

In the previous two years, two conditions of non-conservative LOCH analysis were identified. These are documented in CR 01-1538 and CR 00-1921. Neither of these CRs addressed issues regarding the EM that were similar to the issues involved with this LER. In addition, one LER (LER 2003-002) was submitted to the NRC involving potential inoperability of the HPI Pumps. LER 2003-002 addressed the potential harmful effects to the pumps of debris entrained in the pumped fluid which is a different issue than addressed in this LER. During this same time period no condition was identified and no LER was submitted with respect to the 10 CFR 50.46 accident analysis.

Energy Industry Identification System (EIIS) codes are identified in the text as UKX].

NP-33-03-003-01 CR 02-06702 CR 03-02876 CR 03-04912 CR 02-08915 CR 03-03848 CR 02-09159 CR 03-04868 4. �

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