ML20202H961
| ML20202H961 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 02/16/1998 |
| From: | HOUSTON LIGHTING & POWER CO. |
| To: | |
| Shared Package | |
| ML20202H951 | List: |
| References | |
| NUDOCS 9802230053 | |
| Download: ML20202H961 (11) | |
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ATTACHMENT 2 SAFETY EVALUATION c:\\wpW\\nrc ap\\TSC-98vXT/4 doc STI30526288
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NOC-AE-0074 Page1of5 SAFETY EVALUATION in the development of the voltage-based repair criteria, draft Reg Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes," and draft Reg Guide 1.83, Rev.1, " Inservice Inspection of PWR Steam Generator Tubes," are used as the bases for determining that steam generator integrity considerations are maintained within acceptable limits. Draft Reg Guide 1.121 describes a method acceptable to the NRC staff for meeting General Design Criteria (GDC) 14,15,31, and 32. The probability and consequences of steam generator tube rupture are reduced by determining the limiting safe conditions of degradation of steam generator tubing, beyond which tubes with unacceptable degradation, as established by inservice inspection, would be remosed from service by plugging.
This regulatory guide uses safety factors on loads for tube burst that are consistent with the requirements of Section Ill of the ASME Code. For the degradation occurring in the steam generator tube support plate elevations, tube burst criteria are inherently satisfied during normal operating conditions by the presence of the tube support plate. The presence of the tube support plate enhances the integrity of the degraded tubes in that region by precluding tube deformation beyond the diameter of the drilled hole. It is not certain whether the tube support plate would function to provide a similar constraining effect during accident condition loadings. Therefore, no credit is taken in the development of the voltage-based repair criteria for the presence of the tube support plate during accident condition loadings. Conservatively, based on the existing database, burst testing shows that the safety requirements for tube burst margins during both normal and accident condition loadings can be satisfied at end-of-cycle conditions with bobbin coil signal amplitudes of approximately 5.45 volts for tube support plates and 4.47 volts for the flow distribution baffle, regardless of the depth of tube wall penetration degradation. Generic Letter 95-05 requirements for use of the latest NRC-approved database will be met and will ensure plant safety for future c ycle structural repair limits.
Reg Guide 1.83, R tv.1, describes a method acceptable for implemen'.ing GDC 14,15,31, and 32 through periodic inservice inspection for detection of significant tube wall degradation. South Texas Unit 2 is applying for voltage-based repair criteria for flow distribution baffle intersections.
For the voltage-based repair criteria developed for the steam generator tubes, no leakage is expected during normal operating conditions even with the presence of through-wall degradation. This is because the stress corrosion cracking occurring in the tubes at the support plate elevations in the steam generators are shoit, tight, axially oriented microcracks separated by ligaments of material.
Relative to the expected leakage during accident condition loadings, the limiting event with respect to primary-to-secondary leakage is a postulated steam line break event.
The following items support this proposed license amendment.
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NOC-AE-0074 Page 2 of 5 1.
Chemistry South Texas has undertaken steps to help mitigate steam generator tubing corrosion. Plant design was upgraded during construction to:
add a full flow feedwater deaerator for dissolved oxygen control, e
add cation condensate polishers in addition to the full flow mixed bed condensate e
polishers, double the capacity of the steam generator blowdown system to 1% of main steam e
- flow, remove copper components from the secondary system, and e
use all volatile treatment During the past three years, alternate amine pH control was implemented to reduce iron transport. CurTent information included in the EPRI Secondary Chemistry Guidelines is used to monitor the effectiveness of the chemistry program.
2.
Steam Generator Leakane Monitorina Steam generator leakage monitoring employs a sampling program in conjunction with radiation monitors permanently installed on the condenser air removal system (RT8027), the unit vent monitor (RT8010), the steam generator blowdown flash tank (RT8043), and employing N-16 primary-to-secondary leak monitors permanently installed on each of the four main steam lines (RT8130B, RT8131B, RT8132B, and RT8133B). The South Texas program for detection and mitigation of steam generator tube leak events was upgraded earlier in response to industry lessons leamed such as Information Notice 91-43. The program for early leak detection provides for prompt detection and response, thus minimizing the likelihood of a steam generator tube rupture event. In addition to the monitors described below, supplementary monitors which are less sensitive to small leaks have been provided on each of the four main steam lines and on the four steam generator blowdown lines. These are provided primarily for detection of a steam generator tube rupture event and are not discussed in detail.
Samplinn Each steam generator is sampled for various purposes, including the detection of tube leaks and determination of secondary specific radioactivity.
Steam Generator Blowdown (SGBD) Radiation Monitor The SGBD radiation monitor continuously checks the steam generator blowdown flash tank effluent to provide indication and alarms locally and in the control room. The radiation monitor detects water activation products as well as corrosion activation oroducts and fission e.\\wp\\nlinrc-ap\\TsC 98VXU4 doc sTI 30526288 l
NOC-AE-0074 Page 3 of 5 products. It is sensitive to leakage as low as five gallons per day. An alert or high alarm would be an indication of a primary-to-secondary leak.
Condenser Air Removal System Radiation Monitor The condenser air removal system is provided with a radiation monitor which continuously monitors the effluent line from the condenser vacuum pump. This monitor is designed to detect low levels of noble gas radioactivity and is sensitive to leaks as low as five gallons per day. An alarm from this detector indicates a primary-to-secondary system leak.
Unit Vent Monitor The unit vent is provided with a radiation monitor which samples the plant vent stack prior to discharge to the environment and monitors for particulates, iodine, and noble gases. The unit vent monitor provides for sampling of plant effluents in compliance with NUREG-0737, item II.F.1.
N-16 Radiation Monitors The N-16 gamma detectors provide continuous indication of individual steam generator primary-to-secondary leakage. The detectors provide real time indication in the control room of steam generator leak rate in gallons per day when reactor power is 225%. The N-16 monitors are reactor power compensated for accurate leakage trending during power level reductions and increases. A recorder monitors the N-16 detector readings and provides a trend recording of steam generator leak rate. The N-16 monitors alarm in the cold chemistry lab, from which they are controlled, while monitor readings are continuously available in the control room via the plant computer.
Station Response to a Steam Generator Tube Leak Abnormal radiation in a steam generator indicates primary-to-secondary leakage. This can be shown by trends or alarms on main steam line N-16 monitors, the condenser vacuum pump effluent monitor, the steam generator blowdown radiation monitor, or from chemistry samples. A large leak would be indicated by feedwater flow being less than steam flow, decreasing feed flow, a mismatch in charging and letdown flow, or decreasing feed regulating valve position in conjunction with a stable steam generator level. These symptoms, however, would more likely be noticed with a tube rupture event. Procedures provide actions to mitigate the entire spectrum of steam generator tube leaks from the threshold of detectability up to a steam generator tube rupture event.
Upon any confirmed indication of leakage, the frequency of monitoring and sampling is increased in a manner proportionate to the severity of the leak. Additional confirmatory or diagnostic samples would be taken from the steam generator blowdown and from the condenser air removal system effluent. Operators begin to closely monitor the N-16 monitors in the control room.
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NOC-AE-0074 Page 4 of 5 Traininc The operator training program has been upgraded previously to reflect training scenarios based on actual plant response to previous industry steam generator leak events. Plant operators and chemical analysis technicians have been trained in the use of the N-16 monitors and in the upgraded station procedures for response to steam generator leaks.
Steam Generator Leak Detection Procram Adeauacy The plant leak rate monitors and procedures provide the required indications and alarms to ensure reactor coolant system leakage is detected early, while the leakage rate is low. In addition, leakage verification is provided by chemistry procedures which provide alternate means of calculating and confirming reactor coolant system leakage. These procedures maximize assurance that leak evaluation and mitigation can occur before small leaks propagate to steam generator tube rupture events.
3.
Eddy Current Test and the Data Analysis Guidelines The data acquisition and the analysis guidelines will be in accordance with Westinghouse Steam Generator Report SG-98-01-004 for type of ca;ibration, recording, and analysis requirements.
4.
Eddy Current Data Analyst Trainine and Oualifications All analysts will be qualified per SNT-TC-1 A and a minimum of 90% will be qualified as
" Qualified Data Analyst." Supervisors are not required to be " Qualified Data Analysts." All analysts must pass the site-specific data analysis course prior to beginning work. The site-specific data analysis course will sensitize the analysts to identify indications attributable to primary water stress corrosion cracking (PWSCC) and to recognize the potential for PWSCC to occur at dented tube support plate intersections. This information will be contained in the current " Steam Generator Eddy Current Data Analysis Training Manual" or its equivalent.
5.
Tube Pulls Removal of a minimum of four tube suppon intersections containing larger distorted support indications (DSIs) is planned for refueling outage 2RE06. The results of Unit I destructive examination of eighteen tube support plate intersections from four tube pulls performed in 1993 and three tubes pulls performed in 1995 are included in Section 8 of BAW-10204-P, Rev. 3 (refer to letter ST-HL-AE-5395 dated 6/6/96). From these pull samples, one ~70%
through wall axial defect and onc ~54% axial defect were burst tested. No other degradation morphologies, such as PWSCC, circumferential degradation, or significant IGA, were found in the support plate intersections. Helium leak testing indicated no through-wall penetrations and thus hot leak rate testing was not performed. This information has been incorporated as appropriate into the industry-wide tube pull database.
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NOC-AE-0074 Page5of5 South Texas Project Unit 2 is committed to tube pulls as required by Generic Letter 95-05 or to participation in an industry tube pull program should such a program be approved by the NRC as allowed by Generic Letter 95-05.
The proposed amendment may preclude occupational radiation exposure that would otherwise be incurred by plant workers involved in tube plugging operations. Operation in accordance with the proposed am:ndment would reduce loss of margin in reactor coolant flow through the steam generator and is therefore safety enhancing, its implementation would assist in assuring that minimum flow rates are maintained in excess of that required for operation at full power. Reduction in the amount of tube plugging can reduce the length of plant outages and reduce the time the steam generator is open to the containment environment during an outage. South Texas has determined that this methodology is applicable to our steam generators and provides a safe and effective alternative to plugging.
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i ATTACHMENT 3 DETERMINATION OF NO SIGNIFICANT HAZARDS CONSIDERATION i
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NOC-AE-0074 Page 1 of 4 DETERMINATION OF NO SIGNIFICANT HAZARDS CONSIDERATION The following evaluation determines that the proposed amendment to the Technical Specifications involves no significant hazards consideration as defined in 10 CFR 50.92:
1.
Operation of the facility in accordance with the proposed amendment would not involve a significant increase in the probability or consequences of an accident previously evaluated.
Structural Considerations Industry testing of model boiler and operating plant tube specimens for free span tubing at room temperature conditions sho'vs typical burst pressures in excess of 5000 psi for indications of ODSCC with voltage measurements at or below the current structurallimit of 5.45 volts. One model boiler specimen with a voltage amplitude of 19 volts also exhibited a burst pressure greater than 5000 psi.
Burst testing performed on one intersection pulled from South Texas Unit 1 in 1993 with a 0.51 volt indication yielded a measured burst pressure of 8900 psi at room temperature, Burst testing performed on another intersection pulled from South Texas Unit 1 in 1995 with a 0.48 volt indication yielded a measured burst pressure of 9950 psi at room temperature.
The next projected end-of-cycle (EOC) voltage compares favorably with the current structural limit considering the voltage growth rate for indications at South Texas, Using the methodology of Generic Letter 95-05, the structural limit is reduced by allowances for uncertainty and growth to develop a beginning-of-cycle (BOC) repair limit which should preclude EOC indications from growing in excess of the structurallimit. The non-destructive examination (NDE) uncertainty to be applied per Generic Letter 95-05 is approximately 20%. The growih allowance will be 30%/EFPY or a South Texas Unit 2-specific growth iate, to be calculated in accordance with Generic Letter 95-05, whichever is greater. Where the generator-specific growth rate exceeds both the Unit 2-specific average growth rate and 30%/EFPY, that generator-specific growth rate will be used for that generator. Each succeeding cycle upper voltage repair limit will also be conservatively established based on Generic Letter 95-05 methodology. By adding NDE uncertainty allowances and a growth allowance to the repair limit, the structural limit can be validated.
The upper voltage repair limit could be applied to bobbin coil voltages between the lower and upper repair limits to leave such indications in service independent of RPC confirmation. However, RPC-confinned indications will be conservatively removed from service consistent with Generic Letter 95-05.
Leakace Considerations As part of the implementation of voltage-based repair criteria, the distribution of EOC degradation indications at the TSP intersections has been used to calculate the primary-to-secondary leakage which is bounded by the maximum leakage required to remain within the applicable dose limits of 10CFRI00 and GDC 19. This limit was calculated using the Technical Specification RCS Iodine-131 ehp\\rd\\nrup\\TsC48WU4 doc sTI30526288
NOC.AE-0074 -
Page 2 of 4 transient spiking values consistent with NUREG-0800. Application of the voltage based repair criteria requires the projection of postulated hiSLB leakage based on the projected EOC voltage distribution from the beginning of cycle voltage distribution. Projected EOC voltage distribution is developed using the most recent EOC eddy current results and a voltage measurement uncertainty, Draft NUREG-1477 and Generic Letter 95-05 require that all indications to which voltage-based repair criteria are applied must be included in the leakage projection.
The projected h1SLB leakage rate calculation methodology prescribed in Generic Letter 95-05 will be used to calculate the EOC leakage. A hionte Carlo approach will be used to determine the EOC leakage, accounting for all of the bobbin coil cddy current test uncertainties, voltage growth, and an assumed probability of detection of 0.6. The fitted log-logistic probability ofleakage correlation will be used to establish the h1SLB leak rate for each cycle. This leak rate will be used for comparison with a bounding allowable leak rate in the faulted loop which would result in radiological consequences which are within the dose limits of 10CFR100 for offsite doses and GDC 19 for control room doses. Due to the relatively low voltage levels of indications at South Texas to date and low voltage growth rates, it is expected that the actual calculated leakage values will be far less than this limit for each successive cycle.
Therefore, implementation of voltage-based repair criteria does not adversely affect steam generator tube integrity and the radiological consequences will remain below the limits of 10CFR100 and GDC 19. Operation of the facility in accordance with the proposed amendment would not result in any increase in the probability or consequences of an accident previously evaluated.
2.
Operation of the facility in accordance with the proposed amendment would not create the possibility of a new or different kind of accident from any accident previously evaluated.
Implementation of the proposed steam generator tube voltage-based repali criteria for ODSCC at the TSP intersections does not introduce any signifcant changes to the plant design basis. Use of the criteria does not provide a mechanism which could result in an accident outside of the region of the 4
TSP elevations because the criteria do not apply outside the thickness of the TSPs. It is therefore expected that for all plant conditions, neither a single nor multiple tube rupture event would likely occur in a steam generator where voltage-based repair criteria has been applied.
Specifically, South Texas Project Unit 2 has implemented a maximum leakage rate of 150 gpd per steam generator to help preclude the potential for excessive leakage during all plant conditions. The
- draft Reg Guide 1.121 criterion for establishing operational leakage rate limits governing plant shutdown is based upon leak-before-break (LBB) considerations to detect a free span crack before potential tube rupture as a result of faulted plant conditions. The 150 gpd limit is intended to provide for leakage detection and plant shutdown in the event of unexpcted crack propagation outside the tube suppost plate thickness resulting in excessive leakage. Draft Reg Guide 1.121 acceptance criteria for establishing operating leakage limits are based on LBB considerations such that plant shutdown is initiated if permissible degradation is exceeded.
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NOC AE-0074 Page 3 of 4 Thus, the 150 gpd limit provides for plant shutdown prior to reaching critical degradation lengths.
Additionally, the leak-before-break evaluation assumes that the entire crevice area is uncovered during the secondary side blowdown of a MSLB. Typically,it is expected for the vast majority of intersections, that only partial uncovery will occur. Therefore, the proximity of the TSP will enhance the burst capacity of the tube.
Steam generator tube integrity is continually maintained through inservice inspection and primary-to-secondary leakage monitoring. Any tubes falling outside the voltage-based repair criteria limits are removed from service. Therefore, operating the facility in accordance with the proposed amendment would not create the possibility of a new or different kind of accident from any accident previously evaluated.
3.
Operation of the facility in accordance with the proposed amendment would not involve a significant reduction in a margin of safety.
The use of the voltage based bobbin probe for dispositioning ODSCC degraded tubes within TSP intersections is demonstrated to maintain steam generator tut e integrity in accordance with the requirements of draft Reg Guide 1.121. Draft Reg Guide 1.121 describes a method acceptable to the NRC staff for meeting GDCs 14,15,31, and 32 by reducing the probability or the consequences of steam generator tube rupture. This is accomplished by determining the limiting conditions of degradation of steam generator tubing, as established by inservice inspection, for which tubes with unacceptable degradation are removed from service. Upon implementation of the criteria, even under the worst case conditions, the occurrence of ODSCC at the TSP elevation is not expected to lead to a steam generator tube rupture event during normal or faulted plant conditions. The EOC distribution of ind'eations at the TSP clevations for each successive cycle will be confirmed to result in acceptable primary-to-secondary leakage during all plant conditions.
In addressing the combined effects of loss of coolant accident (LOCA) and safe shutdown earthquake (SSE) on the steam generators, as required by GDC 2, it has been determined that tube collapse may occur in the steam generators at some plants. This is not the case at South Texas Unit 2 as the TSPs do not become sufficiently deformed as a result of lateral loads at the wedge supports at the periphery of the plate due to the combined effects of the leak-before-break-limited LOCA rarefaction wave and SSE loadings to affect tube integrity.
Because the leak before-break methodology is applicable to the South Texas reactor coolant loop -
piping, the probability of breaks in the primary loop piping is sufficiently low that they need not be considered in the structural design of the plant. Implementation practices using the bobbin probe voltage based tube plugging criteria bounds Reg Guide 1.83, Rev.1, considerations by:
1)
Using enhanced eddy current inspection guidelines consistent with those used by EPRI in developing the correlations. This provides consistency in voltage normalization.
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NOC-AE-0074 Page 4 of 4 2)
Performing a 100% bobbin coil inspection for all hot leg tube support plate intersections and all cold leg intersections down to the lowest cold leg tube support plate with known ODSCC indications at each cycle. The determination of the tube support plate intersections having ODSCC indications shall be based on the performance of at least a 20% random sampling of tubes inspected over their full length, and 3)
Incorporating rotating pancake coil inspection for all tubes with bobbin voltages greater than 1.0 volt. This further establishes the principal degradation morphology as ODSCC.
Implementation of voltage-based repair criteria at TSP intersections will decrease the number of tubes which must be repaired at each subsequent inspection. Since the installation of tube plugs to remove ODSCC degraded tubes from service reduces the RCS flow margin, voltage-based repair criteria implementation will help preserve the margin of flow.
For each cycle the projected EOC primary-to-secondary leak rate allowed is bounded by a leak rate which limits the radiological consequences of a EOC MSLB to within the dose limits of 10CFR100 for offsite doses and GDC 19 for control room doses. Therefore, this change does not involve a significant reduction in the margin to safety.
The assessment of radiological consequences of an assure.ed steam line break applicable to South Texas Project Unit I was provided in Attachment 2 tc ST-HL-AE-5359 on May 2,1996. The submittal was made in response to questions from :he Emergency Preparedness and Radiation Protection Branch and is applicable to Unit 2 as well. The staff concluded that the thyroid doses for the EAB, LPZ, and control room are within the acceptance criteria.
It is therefore concluded that the proposed license amendment request does not result in a significant reduction in the margin of safety as defined in the plant Final Safety Analysis Report or Technical Specifications.
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