ML20206U691
| ML20206U691 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 05/19/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20206U689 | List: |
| References | |
| IEB-96-003, IEB-96-3, NUDOCS 9905250296 | |
| Download: ML20206U691 (7) | |
Text
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UNITED STATES g
j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. Sam *1
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RElATED TO AMENDMENT NO. 99 TO FACILITY OPERATING LICENSE NO. NPF-85 PECO ENERGY COMPANY LIMERICK GENERATING STATION. UNIT 2 DOCKET NO. 50-353
1.0 INTRODUCTION
By letters dated November 1,1996, and October 6,1997, PECO Energy Company (the licensee) submitted their responses to NRC Bulletin 96-03, " Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling Water Reactors" (NRCB 96-03), for Limerick Generating Station (LGS), Units 1 and 2. The October 6,1997, letter requested a license amendment under 10 CFR 50.90 to revise the licensee's basis for sizing the suction strainers on their emergency core cooling system (ECCS) low pressure pumps; specifically, the residual heat removal (RHR) and the core spray (CS) systems. The licensee's previous licensing basis did not account for debris loading on the strainers. The October 6th letter provided the methodology the licensee intends to use for estimating the debris loadings on their new ECCS suction strainers being installed in response to NRC Bulletin 96-03. The letter requested staff review and approval of their criteria. By letter dated December 30,1997, the NRC staff requested additional clarifying information. The licensee's February 2,1998, response provided the information requested for LGS, Unit 1, only. The licensee stated that the net positive suction head (NPSH) analyses for LGS, Unit 2, RHR and CS system pumps had not been updated to reflect the new strainer design values and therefore were not submitted. Therefore, the NRC evaluation of LGS, Unit 2, was deferred until the requested information was provided.
Subsequently, the requested information for LGS, Unit 2, was submitted in a letter dated August 28,1998. The August 28,1998, letter provided clarifying information and did not change the original proposed no significant hazards consideration determination. This safety evaluation (SE) provides the results of the staff's review of the licensee's criteria for sizing their new suction strainers for the RHR and CS systems.
The licensee's proposed resolution is based on installation of passive large capacity suction strainers, designed and manufactured by ABB Combustion Engineering. The submittal provided estimated debris loadings on the strainers following a postulated " worst-case" break using methodologies developed by the Boiling Water Reactors Owners Group (BWROG) described in General Electric Topical Report NEDO-32686, " Utility Resolution Guidance for ECCS Suction Strainer Blockage," (URG) dated November 1996. Estimates for quantities of fibrous debris transported to the strainer were evaluated on a plant specific basis by the licensee using Method 2 of the URG. The licensee modified their application of Method 2 to provide a 122%
operational margin in order to accommodate various calculational uncertainties. Generation and transport of Min-K and glass wool were also taken into account when estimating the total 905250296 990519 pDR p
ADOCK 05000353 PDR
, quantities of generated debris. The quantity of sludge used to size the strainers was shown to bound the actual plant operating history relative to the sludge generation rate. Generic estimates provided in the URG were used for paint chips, dust and dirt, and rust from unpainted structures. The unqualified or indeterminate coatings were accounted for by artificially increasing the quantity of particulate debris used to size the strainers by 1000 lbs, The licensee's foreign material exclusion (FME) program is assumed to prevent foreign material from significantly contributing to head loss across the strainer by minimizing the presence of such material in the containment and suppression pool. In addition, the licensee maintained 2 feet of water of net positive suction head (NPSH) margin to account for any uncertainties from other debris sources.
The licensee designed their strainers to be single failure proof. Based on the worst case single failure, the debris loadings would all be transported to the strainers for 3 RHR and 2 CS pumps.
The new strainers have been designed to accommodate the debris and resultant head losses without exceeding the designed NPSH margins of 12 feet of water for the RHR pumps and 9.2 feet of water for the CS pumps. The corresponding flow rates are 10,000 gallons per minute (GPM) and 3950 GPM for the RHR and CS, respectively. Estimates for NPSH margin are based on atmospheric pressure assumed in the wetwell and an assumed suppression pool temperature of 212 degrees Fahrenheit (*F).
The licensee's submittals request that their licensing basis be revised from the current basis which assumes no limits relative to fouling of the strainer to a new basis which accounts for debris loading on the strainer when evaluating the NPSH margin for the RHR and CS pumps.
The licensee's proposed new licensing basis would assume a strainer loading of 900 ft of insulation plus 1000 lbs. of corrosion products (sludge),150 lbs. of dust / dirt,50 lbs. of rust from unpainted surfaces, and 47 lbs. of inorganic zinc coating. In addition, to account for potential foreign material which may inadvertently be left in the suppression pool or drywell, the licensee factored in a headloss to account for these materials. This additional heaaloss was equivalent to an additional 1000 lbs. of sludge. To allow for operational margin in the strainer design basis, the licensee designed the strainers to handle debris loadings of 100% of the fibrous insulation in the containment (approximately 2000 ft ) in addition to the paint, dirt, dust, and corrosion 3
products assumed in their licensing basis. The strainers are designed to handle this debris loading while maintaining a minimum NPSH margin of 2 feet of water.
2.0 plSCUSSION 2.1 Strainer Design The staffs contractor, Science and Engineering Associates, Inc. (SEA), performed a technical evaluation of the licensee's submittals and their responses to the staffs requests for additional information. Their evaluation focused on the following:
- 1) Evaluating the estimated debris loading to be used in sizing the strainers, including selection of breaks evaluated by the licensee and the methodology estimating debris generation and transport.
.- 2) Conducting order of magnitude confirmatory calculations of the strainer designs to handle the debris loadings calculated by the licensee.
Break Selection: Using method 2 of the URG required the licensee to determine the largest potential zone of influence (ZOI) and then place that in the most congested part of the drywell, which contains the largest quantities of the insulation materials of interest. As such, this method does not require detailed break-by-break analysis and is considered by the staff to be conservative and consistent with the guidance of Regulatory Guide 1.82, Revision 2 (RG 1.82),
and is therefore acceptable. In addition, the pleated surface of the strainer being used by the licensee is not expected to have a " thin bed effect"; therefore, Regulatory Position 2.3.1.5 of RG 1.82 does not apply.
Debris Generation: As noted above, the licensee used method 2 for debris generation. This used the largest ZOI for all insulation types and located the bounding amount of debris generation by placing the ZOI in the region of the containment with the highest density of debris sources. Approximate calculations performed by SEA based on the NUREG/CR-6224 reference plant show that the licensee's estimate of debris generation of 900 ft is reasonable.
8 The licensee's estimate of debris generation appears to be an appropriate use of the URG, is consistent with RG 1.82, and is therefore acceptable.
Debris Transport: A drywell transport factor of 1.0 was applied to the calculated quantity of debris generated in the drywell to estimate the amount transported to the suppression pool. In addition, a transport factor of 1.0 was used to estimate the amount of transport within the
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suppression pool to the surface of the strainers. These factors are consistent with RG 1,82, and are, therefore, considered to be acceptable. The staff notes that the licensee did not use the URG recommended transport fractions for Mark ll containments, which the staff concluded in its draft SER on the URG were unacceptable.
Head Loss Correlation: Estimates of head loss performed using a modified form of the NUREG/CR-6224 have led the staff to conclude that the proposed strainers would most likely be cble to handle the debris loadings estimated by the licensee without exceeding the NPSH ma gins of 12 feet and 9.2 feet, respectively, for the RHR and the CS pumps. These calculations do not address uncertainties in the prediction of head loss; however, the staff notes that the margin of 2 feet of water allows for some uncertainty as well as the 122% margin used in the strainer design for fibrous material.
2.2 NPSH Analyses The NPSH available (NPSHA) for the RHR and core spray pumps at Limerick is calculated using the following equation:
NPSHA =h,-hgh,-h,,
1
. where:
h, = absolute pressure on the surface of liquid supply level
- h. = head corresponding to the vapor pressure of the liquid being pumped h, = static head of liquid above centerline of impeller eye h,, = suction line friction losses.
NPSH margin is the difference between NPSHA and NPSH required (NPSHR); NPSHR is usually specified on the vendor pump curves for the specific pumps.
NPSH Mamin NPSHA-NPSHR The current design basis analyses for ECCS NPSH accounts for a maximum suppression pool water temperature of 212 'F, a suppression pool air space pressure of 1 atmosphere, and RHR and core spray flow rates of 10,000 gpm and 3950 gpm, respectively. The NPSH required for the RHR pumps and the core spray pumps at design conditions is 5 feet and 10 feet, respectively. Based on these assumptions, the NPSH margin for the RHR and core spray pumps is 14 feet and 11.2 feet, respectively, excluding strainer losses. These margins represent the amount of head above that required by the pumps.
The staff notes that Limerick is designed to meet the guidance specified in Safety Guide 1 (Regulatory Guide 1.1, Revision 0), " Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal System Pumps." Safety Guide 1 states that ECCS and containment heat removal system should be designed so that adequate NPSH is provided to system pumps assuming maximum expected temperatures of pumped fluids and no increase in containment pressure from that present prior to postulated loss-of-coolant-accidents. The staff notes that it is the licensee's intent to continue to meet the guidance set forth in Safety Guide 1.
As such, the new large capacity strainers were designed to have a fully fouled headloss
- (AP.,.) of 12 feet for RHR at 212.5 'F and 11,000 gpm and 9.2 feet for core spray at 212.5 *F and 3950 gpm. This design allows for 2 feet of margin above that required by the pumps, including strainer losses. The staff notes that this reduction of margin (from old ECCS strainer to new ECCS strainer) is margin assuming atmospheric conditions in containment and is acceptable as discussed below. The design basis accident and limiting single failure for Limerick with respect to strainer sizing and NPSH concems is a recirculation line break coupled with a loss of offsite power and failure of Division I or 11 power. This scenario would render one RHR pump and two core spray pumps inoperable while the remaining three RHR pumps in the low pressure coolant injection mode and two core spray pumps take suction from the suppression pool to initially reflood the core. This scenario is represented as Mode B for RHR and two pump runout for core spray in the NPSH calculations for Unit 2.
The Mode B NPSH analysis for RHR adjusted the maximum allowable dirty AP., from 12 feet at 212.5 'F and 11,000 gpm to the design basis conditions of 212 'F and 10,000 gpm. The adjustment accounted for the system losses at various flow rates, which are proportional to the square of the flow rates; and the bed losses, which have a linear relationship to the flow rates.
The licensee also stated that the differences in temperature are considered negligible based on
. the insignificant change in water properties between 212 'F and 212.5 'F, which would have no impact on NPSHA. The resultant AP,,,,n., for Mode B was 10.67 feet. This was added to the other suction line friction losses from the 24-inch and 30-inch suction lines and inserted into the NPSHA equation. For Mode B, NPSHA was calculated to be 10.62 feet, which is 5.62 feet
. greater than NPSHR. The staff notes that this analysis does not rely upon or require containment overpressure, i.e., containment pressure greater than 1 atmosphere (14.696 psia),
and therefore is acceptable.
The two pump runout NPSH analysis for core spray did not adjust the maximum allowable dirty AP,.n., from 9.2 feet at 212.5 'F and 3950 gpm to the design basis conditions of 212 'F and 3950 gpm since the temperature difference is considered insignificant. The AP,,,n.,was added to the other suction line friction losses and inserted into the NPSHA equation. For the two pump runout case, NPSHA was calculated to be 12 feet, which is 2 feet greater than NPSHR for the core spray pumps with no reliance on containment overpressure.
The staff notes that the actual dirty AP,,,,n., at design basis conditions for the RHR and core spray were calculated by the licensee. The actual dirty APon.,is the sum of the strainer system losses and the strainer bed losses at design basis conditions. These values were less than the maximum allowable dirty AP,,,,n, for which the strainers were designed. Therefore, when the actual dirty AP,,,,n., is added to the other suction line friction losses and inserted into the NPSHA equation, a larger NPSHA was calculated than described above. In all cases, reliance on containment overpressure was not required and, thus, the guidelines of Safety Guide 1 are still met.
3.0
SUMMARY
Based on the staff's evaluation of the licensee's submittals, the contractor's Technical Evaluation Report, and all other relevant information, the staff concludes the following:
f 1.
The licensee's proposed new licensing basis for calculating the worst case potential debris loading on the suction strainers for their RHR and CS pumps is acceptable. The licensee used Method 2 of the BWROG's Utility Resolution Guidance (URG) document for determining the zone of influence (ZOI) for the worst case pipe break in their containment.
In addition, the licensee performed the calculation for debris generation more conservatively than recommended in the URG by using a destruction factor of 1.0 and transport factor of 1.0 versus the URG recommendations of 0.28 and 1.0 respectively for NUKON fiberglass insulation.
2.
The staff has reviewed the licenme's design basis NPSH analyses, which include the installation of the new ECCS suction strainer and associated maximum allowable dirty A P..n.,. The licensee's analyses demonstrated that NPSHA exceeds NPSHR for the worst case scenario. The staff notes that containment overpressure is not required to meet the NPSH requirement with the new strainers installed for both the RHR and core spray cases.
Since the licensee's analyses demonstrate that adequate NPSH is available to meet the required NPSH and the analyses are consistent with the guidance of Safety Guide 1, the staff concludes that the NPSH analyses for the RHR and core spray systems are l
l
6-acceptable.
3.
In the license amendment request, the staff was not asked to evaluate the adequacy of the strainer to withstand the calculated licensing basis and design basis debris loadings.
However, the staff's estimates of headioss based on the debris loadings and the strainer sizes lead to the conclusion that the strainers appear adequately sized to handle the debris loadings estimated by the licensee. The headloss across the new strainers with the calculated debris loadings, the basis for the estimated headloss across the new strainers (e.g., the headloss correlation, supporting test data, headloss calculations, and test data scaling analysis), have not been completely reviewed by the staff. As a result, the staff has not drawn specific conclusions as to the adequacy of the strainer design to perform its function with the calculated debris loadings. However, the staff believes that the new strainers will result in an improvement in NPSH margin due to the increase in strainer size and the corresponding decrease in strainer pressure drop.
4.
The staff also notes that adding additional margin for potential foreign material in the suppression pool is a conservative practice and will assist the licensee in minimizing potential operability concerns should they find foreign material in the suppression pool.
The staff also believes that this margin is a prudent measure and provides some margin for potentially degraded or improperly applied coatings that could be transported to the strainers during an accident. However, the staff wants to make it clear that increasing the margin in the strainer size does not in any way reduce the licensee's responsibility to maintain effective foreign material exclusion and coating programs, and to take all steps necessary to minimize the amount of material that can accumulate in the suppression pool, vent pipes, vent header, downcomers, drywell, and in any other system or component that communicates with the suppression pool.
5.
The issue of whether or not Technical Specification (TS) Surveillances are needed wi!! be addressed generically as part of the staff's post-implementation audits of NRC Bulletin 96-03 resolutions. By letter dated April 3,1997, related to the licensee's response to NRCB 95-02, the licensee provided their plan for inspection of the strainers and j
suppression pool every other refueling outage. The staff concurs that, given the installation of the new strainers and the margin in the ney; strainer design, the licensee's proposed inspection plan is appropriate.
6.
This amendment authorizes the licensee to incorporate in the Updated Final Safety Analysis Report (UFSAR) certain changes to the description of the facility as described in the licensee's applicat. ion dated October 6,1997, as supplemented by letter dated August 28,1998. The staff finds these changes acceptable.
4.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Pennsylvania State official was notified of the proposed issuance of the amendment. The State official had no comments.
5.0 ENVIRONMENTAL CONSIDERATION
Pursuant to 10 CFR 51.21,51.32, and 51.35, an environmental assessment and finding of no significant impact have been prepared and published in the Federal Reaister on May 18,1999 (64 FR 27014). Accordingly, based upon the environmental assessment, the staff has determined that the issuance of this amendment will not have a significant effect on the quality of the human environment.
6.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors: K. Kavanagh R. Elliott B. Buckley Date:
May 19, 1999
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