ML17059C570
| ML17059C570 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 02/18/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML17059C567 | List: |
| References | |
| IEB-95-002, IEB-95-2, NUDOCS 9902240121 | |
| Download: ML17059C570 (10) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATIONBYTHE OFFICE OF NUCLEAR REACTOR REGULATION RELATEDTO NRC BULLETIN95-02 NIAGARAMOHAWKPOWER CORPORATION NINE MILE POINT NUCLEAR STATION UNIT2 DOCKET NO. 50-410
1.0 INTRODUCTION
The U.S. Nuclear Regulatory Commission (NRC) issued Bulletin 95-02, "Unexpected Clogging of a Residual Heat Removal (RHR) Pump Strainer While Operating in Suppression Pool Cooling Mode," on October 17, 1995.
It requested all holders of boiling-water reactor (BWR) operating licenses or construction permits for nuclear power reactors to take five actions to ensure that unacceptable buildup of debris that could clog strainers does not occur during normal operation.
By letters dated'November 16, 1995, and January 6, 1999, Niagara Mohawk Power Corporation (NMPC) responded to the bulletin for Nine Mile Point Nuclear Station, Unit 2 (NMP2). In its response, NMPC stated its intent to comply with the requested actions in the bulletin.
2.0 DISCUSSION The following describes the requested actions in NRC Bulletin 95-02 and NMPC's response to each requested action.
Action 1 Verifythe operability of all pumps which draw suction from the suppression pool when performing their safety functions (e.g., ECCS [emergency core cooling system], containment spray, etc.), based on an evaluation of suppression pool and suction strainer cleanliness conditions. This evaluation should be based on the pool and strainer conditions during the last inspection or cleaning and an assessment of the potential for the introduction of debris or other materials that could clog the strainers since the pool was last cleaned.
Summa of NMPC's Res onse NRC Bulletin 95-02 is applicable to the following NMP2 ECCS systems: high pressure core spray (HPCS), low pressure core spray (LPCS), and three loops of RHR. RHR functions include low pressure core injection, decay heat removal, containment spray, and containment cooling. The Reactor Core Isolation Cooling (RCIC) system is not an ECCS, but is considered within the scope of this requested action because it is capable of taking suction from the suppression pool during performance of a safety function.
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NMP2 suppression chamber was thoroughly cleaned and inspected prior to initial filling, in order to remove original construction debris. The ECCS and RCIC suction strainers and suppression pool at NMP 2 were also thoroughly cleaned during the spring 1995 refueling outage, which was the first such cleaning following initial plant startup. The cleaning of the pool and strainers was accomplished utilizing an underwater diver and vacuum system.
AII significant debris was removed from the pool. The ECCS and RCIC suction strainers were visually inspected following cleaning and the evolution was videotaped. No debris remained in the pool of a size or quantity that could plug the strainers. The strainers contained no significant debris nor has there been any indicated deterioration in pump net positive suction head (NPSH) after approximately 10 years of operation without thorough cleaning of the suppression pool or the pool water. Improved Foreign Material Exclusion (FME) practices were implemented for the containment during the spring 1995 outage and extensive walkdown and cleaning of the drywell was completed prior to restart from the outage.
Based on the suppression pool and strainer cleaning performed during the refueling outage, improved FME practices following the cleanup and satisfactory system performance demonstrated by normal operation and testing, NMPC concluded that the ECCS and RCIC systems are operable, and are not susceptible to common mode failure caused by suction strainer clogging.
Action 2 Confirm the operability evaluation in requested action 1 above through appropriate test(s) and strainer inspection(s) within 120 days of the date of this bulletin.
Summa of NMPC's Res onse The evaluation provided in the response to action (1) provides assurance that there willbe no operability concern during the present operating cycle. NMPC committed to a program to periodically inspect and dean the suppression pool and to trend ECCS pump suction pressure.
The RHR and LPCS systems take suction from the suppression pool during surveillance testing and for ECCS functions. Although the HPCS would take suction from the suppression pool during operation concurrent with low condensate storage tank (CST) or high suppression pool level, it is aligned to the CST during routine surveillance testing and ECCS function.
Similarly, the RCIC system is normally aligned to the CST and is capable of transferring to the suppression pool on low CST level. The HPCS and RCIC suction strainers willtherefore not become clogged during normal operation. Based on suppression pool and strainer cleaning performed during the refueling outage, there is no reason to suspect the presence of any debris on any of the suction strainers, including those for the HPCS and RCIC systems. NMPC willcontinue to monitor the performance of ECCS systems taking suction from the suppression pool during routine surveillance testing. This monitoring, as part of the pump and valve inservice testing program, willprovide indication of any significant deterioration in pump or
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suction strainer performance.
NMP2 also has low suction pressure annunciators for ECCS pumps which take suction from the suppression pool. These alarms would alert operators to significant deterioration of pump NPSH ifitwere to occur. An inspection of the suppression pool was performed during Refueling Outage 5 in 1996 and during Refueling Outage 6 in 1998.
Action 3 Schedule a suppression pool (torus) cleaning. The schedule for cleaning the suppression pool should be consistent with the operability evaluation in requested action 1 above.
In addition, a program for periodic cleaning of the suppression pool should be established, including procedures for the cleaning of the pool, criteria for determining the appropriate cleaning frequency, and criteria for evaluating the adequacy of the pool cleanliness.
Summa of NMPC's Res onse Based on previous experience at NMP2, it is not expected that any significant amount of debris willbe found in the suppression pool. The suppression pool was visually inspected during the 1996 refueling outage.
NMPC willdevelop a program for periodic cleaning and inspection of the suppression pool based on the inspection results and the BWR [ Boiling Water Reactors] Owners'roup guidance. As part of the planning for refueling outages, a
contingency plan will be developed for cleaning of the suppression pool should significant debris be identified during scheduled inspections.
These actions willmaintain suppression pool cleanliness to ensure ECCS operability during future operating cycles.
Action 4 Review foreign material exclusion (FME) procedures and their implementation to determine whether adequate control of materials in the drywell, suppression pool, and systems that interface with the suppression pool exists.
This review should determine ifcomprehensive FME controls have been established to prevent materials that could potentially impact ECCS operation from being introduced into the suppression pool, and that workers are sufficiently aware of their responsibilities regarding FME. Any identified weaknesses should be corrected.
In addition, the effectiveness of the FME controls since the last time the suppression pool was cleaned and the ECCS strainers inspected, and the impact that any weaknesses noted may have on the operability of the ECCS should be assessed.
Summa of NMPC's Res onse Administrative procedures establish the overall controls for FME. FME material accountability measures are used for activities in the suppression chamber.
The drywell is not generally subject to material accountability measures.
FME for the dryweil is accomplished via housekeeping inspections and final closure inspection.
System cleanliness inspection during work activities and walkdowns prior to system closure are performed for plant systems, including those which interface with the suppression pool. Drywell coordinators provided oversight of housekeeping activities during outages involving primary containment entries.
P Hg During the spring 1995 refueling outage, a suppression pool coordinator provided oversight of the cleanup and inspection, and reinforced material accountability requirements for outage activities in,the suppression chamber.
A training lesson plan relative to local work zones and system cleanness was prepared in December 1994. Prior to the spring 1995 refueling outage, NMPC personnel and contract supervisors with outage work responsibilities were trained per the lesson plan.
Existing FME controls provide assurance that operability of the ECCS and RCIC systems will not be adversely affected by debris generated during normal operation.
Improved FME training was provided to NMPC and contractor personnel with outage work responsibilities, prior to the fall 1996 refueling outage.
Action 5 Consider additional measures such as suppression pool water sampling and trending of pump suction pressure to detect clogging of ECCS suction strainers.
Summa of NMPC's Res onse NMP2 monitors ECCS and RCIC pump performance as part of the pump and valve inservice
.test program.
Pump NPSH will be trended for the RHR and LPCS systems, which are normally tested by taking suction from the suppression pool. This trending willprovide early indication of reduction in ECCS pump NPSH for those systems, which is indicative of potential strainer plugging.
3.0 EVALUATION The purpose of the requested actions in the bulletin is to ensure that ECCS and other pumps drawing suction from the suppression pool do not experience unacceptable buildup of debris that could clog strainers during normal operation which would prevent them from performing their safety function. Requested action 1 requested licensees to evaluate the operability of their pumps based on the cleanliness of the suppression pool and strainers.
Requested action 2 then requested a verification of the licensee's assessment through a pump test and strainer inspection.
These two actions serve to ensure that the pumps are currently operable and not experiencing unacceptable debris buildup. Requested actions 3, 4 and 5 serve to ensure that appropriate measures, such as cleaning of suppression pools and strengthening of FME practices, are taken in the long term to prevent debris accumulation in the pool.
Since NMPC cleaned its suppression pool and ECCS strainers during the spring 1995 refueling outage, the NRC staff concludes that NMPC had a reasonable basis for concluding that its pumps were operable in its response to NRC Bulletin 95-02 dated November 16, 1995.
Similarly, the pool/strainer cleaning seemingly provided an appropriate basis for not conducting pump tests as requested in action 2 of the bulletin. However, on October 17, 1996, NMPC reported that a visual inspection of the suppression pool conducted during the 1996 refueling outage found a significant amount of debris in the suppression pool downcomers.
Some of this debris (e.g., plastic bags) may have had the ability to partially clog the ECCS suction strainers.
While NMPC had cleaned the suppression pool floor and ECCS strainers during the
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1995 refueling outage, NMPC had not inspected for floating debris in the downcomers.
This failure, in combination with other unrelated violations, resulted in escalated enforcement action by the NRC, including the imposition of a civil penalty.
NMPC removed the debris found during the 1996 refueling outage.
As a result of these actions, the NRC staff has concluded that the suppression pool/strainer cleaning conducted during the 1995 refueling outage, and the suppression pool inspection and associated removal of debris conducted during the 1996 refueling outage, is sufficient to ensure operability of the ECCS pumps, and together meet the intent of requested actions 1 and 2 of Bulletin 95-02. The NRC staff also notes that, durin'g the spring 1998 refueling outage, NMPC replaced NMP2's old strainers with new large capacity strainers in response to related Bulletin 96-03, "Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling-Water Reactors."
The new strainers willminimize the potential for clogging of the ECCS suction strainers.
The NRC staff has also concluded that NMPC's evaluation of NMP2's FME program and establishment of a suppression pool cleaning program meet the intent of requested actions 3 and 4, and are acceptable.
NMPC's programs for trending pump NPSH data, the low suction pressure annunciators, and periodically inspecting the strainers and torus provide additional opportunity for early identification of potential strainer fouling. The NRC staff has concluded that these additional actions meet the intent of requested action 5 and are acceptable.
4.0 CONCLUSION
On the basis of the above evaluation, the NRC staff finds that NMPC has complied with the actions requested by the bulletin, and therefore, NMPC's response to NRC Bulletin 95-02 for NMP2 is acceptable.
Principal Contributors:
J. Cushing R. Elliott Date February 18, 1ggg
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