ML20079S236
| ML20079S236 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 10/13/1994 |
| From: | BOSTON EDISON CO. |
| To: | |
| Shared Package | |
| ML20079S234 | List: |
| References | |
| NUDOCS 9410270069 | |
| Download: ML20079S236 (27) | |
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ATTACHMENT l
l REDUCTION IN IGSCC INSPECTION FREQUENCIES l TABLE OF CONTENTS l
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SUMMARY
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A. Introduction B. Discussion C. Hydrogen Water Chemistry Performance D. Proposed IGSCC Inspection Schedule ;
E. BWR Owner's Group Topical Report l
l F. Reduction in Radiological Exposure and Resources i G. Conclusion !
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! II. HYDROGEN WATER CHEMISTRY PROGRAM l
A. IGSCC welds inspection Program
( B. PNPS Hydrogen Water Chemistry )'
l 1. Hydrogen Water Chemistry System Description l
- 2. Crack Arrest Verification System Description j 3. System Operation
- 4. Conformance with BWROG Topical Report, NEDC-319151P l C. Reduction in Radiological Exposure and Resources D. Proposed IGSCC Inspection Program E. Figures:
- 1. ECP for 304SS, Feb.1990 to March 1991.
- 2. Crack Growth for INC66 Specimen, Feb.1990 to March 1991.
- 3. Crack Growth for SSINC Specimen, Feb.1990 to March 1991.
- 4. Crack Growth for PRPS1 Specimen, Feb.1990 to March 1991.
- 5. ECP for 304SS, Jan.1992 to April 1993.
- 6. Crack Growth for INC66 Specimen, Jan.1992 to April 1993.
- 7. Crack Growth for SSINC Specimen, Jan.1992 to April 1993.
- 8. Crack Growth for PRPS1 Specimen, Jan.1992 to April 1993.
- 9. ECP for 304SS, May 1993 to June 1994.
- 10. Crack Growth Rate for Alloy 182 Weld Metal.
- 11. ISI Drawings, ISI-l-2RA and B (Two Sheets), Rev. E1.
F. Enclosure 9410270069 941013 3 PDR ADOCK 05000293 D PDR
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SUMMARY
A. Introtiuction On January 25,1988, the Nuclear Regulatory Commission (NRC) issued Generic Letter (GL) 88-01. This GL provided the Staff's position on Intergranular Stress Corrosion Cracking (IGSCC)in BWR austenit.c stainless steel piping including weld examination frequencies. It also specified methods 'or applying for reduced examination frequencies when Hydrogen Water Chemistry (HWC)is in use. Pilgrim Nuclear Power Station (PNPS) has implemented a HWC Program and a Crack Arrest Verification System (CAVS) with successful results. This document requests reduced frequencies for eleven Category D Recirculation System IGSCC weld examinations and provides the justification for this request.
B. Discussion A total of 222 safety-related welds are included in the PNPS GL 88-01 Examination Program (Reference 1 and 2). The 68 non-safety related welds (all RWCU) currently fall under the provisions of GL88-01, Supplement 1 and are outside the scope of the relief request. The number of safety related welds and the categories as defined in GL 88-01 are as follows:
TABLE 1: SAFETY-RELATED IGSCC WELDS (GL 88-01)
(INCLUDES RHR, RCS, CS, RWCU WELDS)
Materials / Mitigation IGSCC Inspection Extent Number of Process Category & Schedule Safety-Related Welds Resistant Matenal A 25% every 10 years (at 164 least 12% in 6 years)
Non-Resistant Materials - B 50% every 10 years (at 0 Stress Improvement (SI) least 25% in 6 years) within 2 years of operation Non-Resistant Materials SI C All within the next 2 2 after 2 years of operation refueling cycles, then all every 10 years (at least 50% in 6 years).
Non-Resistant Materials D All every 2 refueling 47 with no Si cycles Cracked Reinforced by E 50% next refueling 1 Weld Overlay or Mitigated outage, then all every 2 SI refueling cycies Cracked inadequate or No F All every refueling 0 Repair outage Non-Resistant Not G All next refueling outage 8 Inspected l These safety related welds are in the Residual Heat Removal (RHR), Reactor Recirculation, Core Spray tCS), and Reactor Water Cleanup (RWCU) Systems.
l PNPS has an aggressive inspection program that fulfi!!s the requirements of GL88-01. PNPS has performed 88 IGSCC (safety and nnn-safety) weld examinations since the GL88-01 inspection schedule was implemented and 24 examinations since HWC was initiated. The only cracking discovered was in RWCU piping prior to continuous use of HWC in September 1991. The safety-related portion of the RWCU system is undergoing replacement with resistant material during the 1995 refuel outage while the non-safety-related RWCU piping will l fall under Supplement 1 of GL88-01. No pipe cracking has been observed in any safety-
l Ref. 1 BECo Response to GL 88-01, dated August 4,1988 (BECo Letter #88-119)
Ref 2. BECo Response to NRC RAI, dated June 19,1989 (BECo Letter #89-084) 2
C. .Hydroaen Water Chemistry Performance HWC has been in continuous operation at PNPS since September,1991 (beginning of Cycle l
- 9) with hydrogen injection more than 90% of the time at greater than 60% reactor power.
HWC can be placed into service at approximately 60% reactor power and injects 23 SCFM hydrogen at 100% reactor power.
A Crack Arrest Verification System (CAVS) was installed at PNPS to monitor the Reactor Recirculation piping ECP levels and has been in continuous operation since January 1992. .
Electrochemical Corrosion Potential (ECP) and crack growth data is measured by this system. 1 CAVS samples reactor recirculation water from the B Loop recirculation inlet riser which ;
circulates through external autoclaves containing crack growth specimens and ECP electrodes. A Data Acquisition System (DAS) provides on-line crack growth and ECP readings and stores the data for later use. The CAVS ECP data is used in the HWC program ,
to maintain, by hydrogen injection, the ECP levels below- 230 millvolta on the standard l hydrogen electrode (mVSHE) scale during reactor operation at greater than 60% power.
The threshold for IGSCC mitigation is ECP levels below -230mV (SHE). The CAVS ECP values in the recirculation system have been below -230 mV (SHE), and readings as low as
-500 mV (SHE) have been noted. At the current hydrogen injection rate and ECP values, full IGSCC protection of the recirculation system is accomplished. This is confirmed by the weld ;
examination results during the Refueling Outage 9 which revealed no new IGSCC indications ,
since the implementation of HWC. A comparison of crack growth rates between Cycle 8 I (under normal plant water chemistry) and Cycle 9 (under HWC) shows a decrease in the l crack growth rate for INCONEL 182 (recirculation nozzle weld butter material) and for 304 SS. !
This HWC performance is consistent with the conclusions of the BWROG Topical Report, NEDC-31951P.
D. Proposed IGSCC Inspection Schedule This request for NRC approval only applies to the eleven Category D nozzle safe-end welds of the reactor recirculation system. A factor of 2 reduction in inspection frequency is proposed based on the proven effectiveness of HWU at PNPS as shown in Table 2 below.
TABLE 2. PROPOSED GL 88-01 SCHEDULE FOR RECIRCULATION WELDS Materials / Mitigation IGSCC Inspection Extent & Number of Proposed l Process Category Schedule Welds Changes Resistant material A 25% every 10 years (at 61 N/C least 12% in 6 years)
Non-resistant materials B 25% every 10 years (at 0 N/C (stress improvement [SI) least 12% in 6 years) within 2 years of operation)
Non-resistant materials SI C 50% every 10 years (at J N/C after 2 years of operation least 25% in 6 years)
Non-resistant materials D All every 2 refueling 11 All every 4 1 with no Si cycles refueling !
cycles Cracked reinforced by E All every 4 Refueling 0 N/C weld overlay or mitigated cycles SI Cracked inadequate or no F All every refueling 0 N/C repair outage Non-resistant not G All next refueling outage O N/C inspected ,
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The specific category D Recirculation welds are listed below (Figure 11: ISI-l-2RA and B Drawings, altached):
2R-N2A-1 12" inlet nozzle 2R-N2F-1 12" inlet nozzle 2R-N18-1 28" outlet nozzle 2R-N2B-1 12" inlet nozzle 2R-N2G-1 12" inlet nozzle I 2R-N2C-1 12" inlet nozzle 2R-N2H-1 12" inlet nozzle 2 R-N2 D-1 12" inlet nozzle 2R-N2J-1 12" intet nozz'e 2 R-N2 E-1 12" inlet nozzle 2R-N2K-1 12" inlet nozzle The reduction in examination frequency is consistent with the BWROG Topical Report and will only apply to eleven Category D welds in the Recirculation System. There are ten 12 inch inlet nozzh, .3afe-end welds and one 28 inch outlet nozzle safe-end weld (Category D welds) in the recirculation system of which all are currently examined every two fuel cycles. Relief from the current schedule is requested to allow a 4 fuel cycle interval.
This request does not change the Technical Specification leakage monitoring program that meets Generic Letter 88-01. Also, requests for relief on RWCU, RHR, and CS welds are not included since there are no Electrochemical Corrosion Potential (ECP) measurements i performed on RWCU, RHR and Core Spray Systems.
E. BWROG Topical Report l
l j
i The BWROG-NEDC-31951P is hereby incorporated and endorsed as applicable to this relief l request. The technical content of this submittal concerning the PNPS experience with HWC is consistent with the BWROG Topical Report relating to revised piping inspection schedules.
The PNPS HWC program conforms with the BWROG Topical Report Criteria as follows:
! BWROG Topical Report Criteria PNPS Status i
-230 mV SHE. -230mV SHE.
- 2. Hydrogen availability should be greater Hydrogen availability has been greater than 00%. than 90% since September 1991.
- 3. ECP measurements should be made for ECP measurements are made by an the system piping. autoclave for the recirculation system.
- 4. ECP measurements shou ld be Very low ECP and O2 values were representative. maintained. Additionally, an "In-situ" measurement probe will be installed by the end of Refueling Outage #11 if relief is granted beginning RFO #10.
(Conditional Commitment)
F. Reduction in Radioloaical Exposure and Resources PNPS has estimated the ALARA benefits anticipated during future refueling outages based on reduction in inspection frequency. The revised inspection frequency would reduce exposure by 3 2/3 man-rem each fuel cycle, and will reduce BECo expenditures in the range of $300,000 per refueling outage without compromising the underlying objective of the IGSCC program.
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G. Con 61usion This Request is in conformance with the BWROG Topical Report, NEDC-31951P, does not compromise the underlying objective of the IGSCC program and is within the scope of NRC Generic Letter 88-01.
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II. HYDROGEN WATER CHEMISTRY PROGRAM A. IGSCC Weids Inspection Proaram
- 1. BECo Response to GL 88-01 Boston Edison's response to Generic Letter 88-01 actions is contained in BECo Letter 88-119 (dated August 18,1988) and supplemented by BECo Letter 90-140 (dated November 15, 1990). The IGSCC susceptible welds within the scope of GL 88-01 are included in Table I below:
TABLE 1: IGSCC Related Welds IGSCC Description Number of Safety Number of Number of Category Related Welds Safety Related Non-Safety Excluding RWCU RWCU Related Welds (RWCU)
A Resistant 121 43 19 Matenal B Non-resistant 0 0 0 material, SI within 2 years of operation C Non-resistant 0 2 1 material, Si after
! 2 years of operation D Non-resistant 35 12 3 materials no SI E Cracked, 1 0 0 reinforced by weld overlay or mitigated by St F Cracked, 0 0 0 inadequate or no i repair !
G Non-resistant 4 4 45 materials not inspected i TOTALS 161 61 68 Total IGSCC susceptible safety-related welds: 222 l Total IGSCC susceptible non-safety related welds: 68 l Total IGSCC susceptible welds are: 290 1
Explanation of Cateaory Welds l
Category A: The Category A welds are a result of pipe replacement programs which replaced most of the susceptible pipe in the Drywell(PDC 84-56; 83-62) and a pipe replacement program in the RWCU System (PDC 91-39; 92-33). The replacement program will continue in the safety related portion of RWCU.
Welds that are difficult to replace may be Icft, but the major portion of the piping will be changed. Relief is not applicable to these welds.
Category B: There are no Category B welds at PNPS l
l
s Category'C: These two welds are located in RWCU and are a result of mechanical stress improvement of existing welds. They are a 304SS-to-316SS weld for replacement and a pipe-to-penetration weld which was left due to construction difficulties. Relief is not requested for these welds.
Category D: The population of Category D welds will be reduced by about 15 safety related welds when RWCU replacement is complete. This w!:1 put the safety related population at 32. Schedule relief is requested for 11 of these 32 welds. Only Category D welds in the reactor recirculation system are candidates for relief at this time. The non-safety related welds in RWCU will no longer be considered within the scope of examination when the provisions of Generic Letter 88-01, Supplement 1 become applicable during the 1995 refuel outage.
Category E: One cracked and overlayed weld exists at the RPV-N9A-1 safe-end weld. This weld has been examined three times since the overlay with no apparent growth. Relief is not requested for this weld.
Category F: There are no Category F welds at PNPS Category G Eight welds are considered inaccessible for examination. Five of these welds are inside containment penetrations; the other three have space restrictions.
The latter three welds will be upgraded to Category A by replacement. Relief is not applicable to PNPS Category G welds.
- 2. IGSCC Inspection Results PNPS has performed 88 IGSCC (safety and non-safety related) weld examinations since the GL#88-01 inspection schedule was implemented and 24 examinations since HWC was initiated. Four instances of IGSCC (1 safety-related,3 non-safety related) were discovered in July 1991 in RWCU piping welds. This was prior to the continuous use of HWC. No subsequent piping cracks have been observed in any system at PNPS since HWC was placed in continuous service.
B. PNPS Hydroaen Water Chemistry
- 1. Hydroaen Water Chemistry System Description The HWC System consists of Extended Test System (ETS), Electrolytic Hydrogen Water Chemistry System (EHWCS), and the Crack Arrest Verification System (CAVS).
Extended Test System (ETS) provides the means for injecting hydrogen into the reactor via the suction side of the reactor feed pumps and oxygen into the off gas utilizing bottled gas (Hydrogen tube trailers and oxygen tank).
Electrolytic Hydrogen Water Chemistry System (EHWCS) is a permanently installed hydrogen and oxygen injection system utilizing the electrolytic generation of hydrogen and oxygen. This system is currently in preoperational test. This will become the primary means of injecting hydrogen into the feedwater. The ETS system, which is more expensive to operate, will become a backup injection system.
- 2. Crack Arrest Verification System (CAVS) Description The Pilgrim CAVS is an on-line monitoring system used to provide data pertaining to IGSCC behavior in key reactor structural materials. It receives a continuous flow of high temperature reactor water from a sample line connected to the discharge of the
'B' reactor recirculation loop. The CAVS is of modular design consisting of three autoclaves, a water chemistry sample panel and a Data Acquisition System (DAS).
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Autoclave #1 - Provides real-time measurements of crack growth specimens representing three plant components: (1) PRPS1, Pilgrim recirculation piping 304(SS),
fabricated using an actual section of Pilgrim 28-inch cracked recirculation piping; (2)
SSINC, Type 304SS/inconel 82 composite specimen, represents the Pilgrim jet pump instrument nozzle; and (3) INC66, Inconel 182 weld deposit, simulates the recirculation nozzle / safe end inconel 182 butter.
Autoclave #2 - Provides real-time measurements of ECP. The autoclave contains a set of five electrodes. Two of the electrodes (Ag/AgCl & Cu/Cu20) are reference electrodes used to convert the measured potentials to the Standard Hydrogen Electrode (SHE) scale. The remaining three, type 304SS, !nconel 182 and platinum, are working electrodes. The autoclave vesselitself also acts as a workin0 electrode (316SS) The type 304(SS) working electrode reading is used to verify enough hydrogen is being injected. As recommended by EPRI HWC Guidelines, the other working electrodes are used to verify the 304 reading and provide backup readings.
Each electrode is designed to provide the same potential within a range of +30mV.
Autoclave 63 - Designed to provide a backup to autoclave #1. It contains three bolt loaded specimens that are not actively monitored for cracks growth. A destructive examination is required to determine crack growth.
The Water Chemistry Panel accepts water from the same recirculation sample line that supplies the autoclaves and directs it past sensors which measure dissolved oxygen, dissolved hydrogen, conductivity and sample temperature. This panelis designed to monitor recirculation loop water chemistry parameters.
The Data Acquisition System (DAS) calculates and displays the crack growth, ECP and water chemistry measurements, stores the data and provides for retrieval of data.
CAVS Operational Data CAVS data as presented in attached graphs is divided into three periods as follows:
Period I: February 1990 to March 1991 (Cycle 8): Figures 1,2,3,4 Testing and turnover of the HWC System Initial baseline CAVS data
. Period II: September 1991 to April 1993 (Cycle 9): Figures 5,6,7, 8 First continuous cycle on HWC CAVS placed into continuous service following repairs in January 1992 Percent availability per BWROG topical report starts January 1992
. Period Ill. May 1993 to June 1994 (Cycle 10): Figure 9 Second and current cycle on HWC (a) Electrochemical Corrosion Potential (ECP) Measurements As a metal corrodes, cathodic reactions and anodic reactions occur creating a potential that results in an electric current flow. This potential establishes the ECP of a metal. To measure the ECP, a metal of interest electrode and a reference electrode are placed in an aqueous solution. At PNPS, the reference electrode is the Cu/Cu20 electrode, and the solution is recirculation water flowing through the autoclave. A voltmeter is connected between tne two electrodes, and a potentialis measured. This potential can then be compared to the standard reference base, the standard hydrogen electrode (SHE), since the electrical potential between the reference electrode (Cu/Cu20) and the SHE is known.
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t
., l Trie ECP measurement is the primary indicator of the effectiveness of HWC on l mitig'ating IGSCC at PNPS. The primary source of error in the ECP values is the '
difference between the actual potential (SHE) and the theoretical calculated potential (SHE) of the Cu/Cu20 reference electrode.
ECP measurement verification is done as follows.
(i) ECP measurements are verified weekly, if in use, by comparing the reference probe's measured value against the theoretical value and adding the difference to the working electrode ECP measurement. l (ii) Also, reactor water chemistry parameters can be used to verify ECP values.
PNPS expenence has shown that if ECP increases to >-230mV (SHE) reactor conductivity, dissolved oxygen and chromate willincrease. The conductivity j increase is due to the equilibrium condition as follows. j i
5H2O + Cr2 03 <--> 2HCr04 +8H++6e- '
l (ECP<-230mv) (ECP>-230mv).
Thus, oxygen, conductivity and chromate levels can be used to indicate if the ECP is <-230mV SHE.
(b) Correlation of ECP to IGSCC The electrochemical corrosion potential attained by a metal (when immersed in a '
solution) determines the metal's corrosion characteristics for the existing environment.
It was determined during extensive laboratory tests performed by both GE and EPRI, that at a potential less negative than -230 mv (SHE), stainless steel IGSCC will not ;
occur. By monitoring ECP, a determination can be made as to whether or not IGSCC l is occurring. !
(c) Crack Growth Measurement Autoclave #1 has been loaded with three (3) specimens that have known cracks in ;
them. These cracks are monitored for growth using a technique known as reverse DC electrical potential. This method consists of passing a known DC current through each specimen, reversing that current every half second and then measuring the voltage drop across the opening. At the end of each hour, the average of the measurements is taken, and the crack length is calculated.
- 3. Hydroaen Water Chemistry System Operation (a) Implementation of Hydroaen Water Chemistry at PNPS PNPS found IGSCC indications in the reactor recirculation system in 1983. As a ,
result, the IGSCC susceptible 304SS reactor coolant piping was replaced with IGSCC '
resistant 316SS in 1984 (RFO 6). However, remaining alloy 182 (INC66) safe-end weld material was still susceptible to IGSCC as well as 304SS recirculation inlet thermal sleeves.
Hydrogen Water Chemistry was implemented to control the reactor water chemistry environment for inhibiting IGSCC. HWC was implemented in stages as presented below.
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l (b)' " Mini-test"of May 1985 General Electric (GE) conducted a " mini-test" at PNPS and determined that 23 SCFM i hydrogen should be injected at 100 percent power to reduce the ECP in the j recirculation system to less than -230 mV (SHE). This is equivalent to 0.87 ppm j hydrogen in the feed water. The mini-test also showed that the main steam line radiation monitor readings increased = 4 times normal background. The recirculation water conductivity decreased significantly from a normal conductivity of -0.2us/cm to j 0.09us/cm, and oxygen decreased from 280 ppb to 2 ppb.
i
) (c) CAVS Operation Since January 1990 j The CAVS system was placed in operation on January 15,1990 for "line conditioning"
' The normal water chemistry baseline crack growth rates for the three crack growth
' specimens were established prior to the use of hydrogen water chemistry. There were j two periods where crack growth rates were collected under normal water chemistry i (NWC) conditions 0-300 hrs and -1400-2400 hrs. (See Figure 2,3 and 4). The alloy
- 182 specimen (INC66) initially showed a typical growth rate of 11.7 micro-in/hr when compared to laboratory date (See Fig 10). By the end of the NWC baseline period.
INC66 showed a steady crack growth rate of 10.6 micro-in/hr (93 mils /yr).
- The 304SS/ alloy 82 specimen (SSINC) initially showed a crack growth rate of 14.3 i micro-in/hr that is close to the rate of 21.4 micro-in/hr, which the GE PLEDGE crack j growth model predicts for type 304SS under the initial conditions of stress intensity, EPR value, conductivity and ECP value. By the end of the NWC baseline period SSINC growth rate leveled off to an average of 2.4 micro-in/hr (21 mils /yr).
The 304SS Pilgrim recirculation piping specimen (PRPS1) initially showed insignificant j growth at 0.9 micro-in/hr. By the end of the NWC baseline period the growth rate
{ leveled off to 1.4 micro-in/hr (12 mils /yr).
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! The low crack growth rates for the two 304SS specimens reflect the very good water 4 chemistry condition maintained at PNPS during the baseline period i.e. reactor j conductivity of 0.09 us/cm. The ECP measured during the baseline period was in a
. range of +120 to +150 mV (SHE).
i (d) First Cycle with HWC Since September 1991 !
i
{ PNPS started Cycle 9 with HWC. The H2 injection rate was set at 23 SCFM. The
! ECP autoclave was out of service for required repairs. Reactor water conductivity and
] dissolved oxygen decreased to -0.065 us/cm and ~2 ppb respectively. Even though CAVS was out of service, there was a high confidence that ECP values were <-230 1 mV (SHE). This was based on achieving reactor water chemistry parameters maintained during the mini-test. This was later confirmed when the CAVS was placed into service in January 1992, and ECP values of <-400 mV (SHE) were recorded.
Since January 1992, Cycle 9 HWC availability, as recommended by GE Topical Report NEDC-31951P, was > 90% as shown in Table 3 (Page 12).
All three crack growth specimen readings remained viable through July 1992 at which time the SSINC and PRPS1 specimen values became unstable. This is believed to be due to the specimens reaching the end of their design life. Until that time, crack growth data indicated a decrease in the crack growth rate under HWC conditions.
(See Figures 6,7,8 and Table 2).
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d Table 2. CRACK GROWTH RATES Specimen Normal Water Chemistry Hydrogen Water Chemistry Micro-in/hr Mils /yr Micro-in/hr Mils /yr PRPS1 1.4 12 <<1 <1 a 2 i INC66 10.6 93 <<1 <1 SSINC 2.4 21 <<1 < 1 -+ 1 PRPS1: Type 304(SS); Pilgrim Recirculation Piping INC66: Inconel 182; Safe-end weld butter
! SSINC: Type 304(SS)/Inconel 82 Composite: PNPS Jet Pump Instrument )
Nozzle ,
The first cycle under HWC presented an opportunity to measure ECP and crack growth under various plant power levels and operating conditions. ECP and crack growth is presented here for various plant conditions.
. Normal steady-state Full Power Operation - PNPS operated at full power utilizing HWC the majority of the time. The programmed rate for hydrogen injection at 100% was 23 SCFM. Typical ECP measured values are <-400mV 4 (SHE ) decreasing to <-500 mV (SHE )during extended periods of operation. .
The crack growth rates fell to the range of <2 mils /yr. l
. Less than Full Power Operation - The rate of hydrogen injection is programmed to follow power. The required hydrogen necessary to maintain
<-230 mV (SHE) decreases as power decreases. The ECP has been measured at 50% power during a condenser backwash and was still <-230 mV SHE.
. ECP at Full Power vs H2 Injection Rate - The H2 injection rate was kept at 23 SCFM at 100% power. During surveillances and maintenance periods in steam J
- affected areas, the hydrogen injection rate was manually lowered to 13.5 i SCFM to reduce dose to individuals, however, ECP was still maintained i
<-230mV (SHE). This is generally for very short periods, typically less than 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. No increase in crack growth occurred, and reactor water chemistry remained unchanged.
. Hydrogen Interruption Pen'ods - At certain times, the hydrogen injection system (ETS) was secured at 100% power. During this period, ECP rapidly rose to positive levels indicative of normal water chemistry following an interruption of hydrogen. Typically, reactor water conductivity increased to a level of 0.3 us/cm, and pH decreased to about 6.2. This is due to the same chromate ion phenomenon as reported by other BWRs with HWC. Chromate levels typically
, increase from a HWC level of <11 ppb to between 80 and 150 ppb depending on the length of time hydrogen has been injected prior to the interruption.
f!o formal evaluation was done at PNPS to determine when crack growth rates return to normal water chemistry levels after a hydrogen interruption.
However, crack growth for specimen INC66 over Cycle 9 (approximately 2 mils) indicated that the effect was minimal.
. Effect of Conductivity on ECP - There has been no documented effect of conductivity on ECP at PNPS. Introducing anionic contaminants such as sulfate, nitrate, chromate and chloride have no effect on the ECP of type 304SS as repoded by GE.
. Plant Shutdown - Hydrogen injection is normally secured at 100% power on a shutdown. It takes approximately 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> for the plant to cool down to a cold 11
, shutdown condition of 212 F. During this period, ECP levels return to those observed during normal water chemistry conditions at ~+150mV (SHE).
(e) Second Cycle with HWC-June 1993 I
In June 1993, after Refueling Outage 9, PNPS started its second cycle (Cycle 10) with l HWC. The HWC availability for 1993 was 93.8% and as of July 1994 is 98.2%. ECP l is typically maintained in the <-400 mV (SHE) range. All crack growth specimen readings were unstable showing indeterminate crack lengths indicating they had reached the end of their usefullives. PNPS has no plans to replace or repair the l specimens since their usefulness has been realized in demonstrating the benefit of '
HWC in reducing crack growth in susceptible plant materials.
In July 1993, improvements in measuring dissolved oxygen yielded reactor water values of <0.5 ppb which are typical of current values. l (f) Hydroaen Water Chemistry Availability The percentage of Hydrogen availability at greater than 60% power is calculated since 1991 and is shown below in Table 3.
Table 3. Percentaae of HWC Availabilities l l
Year Estimated recirc HWC Recire <-230mV % HWC l temp > 212 Deg F on SHE (hours) Availability (hours) (hours) j 1992 7464 7133 6724 94.3 1993 7154 6482 6079 93.8 1994 4740 4382 4303 98.2
- % AVAIL = ECP@<-230mV SHE (hrsj Hydrogen injection @>60% Power (hrs)
Since January 1992, Cycle 9 HWC availability was > 90%. The data is tabulated accordingly to years vs. cycles because PNPS tracks HWC availability as part of an annual goal. (95%).
(g) Current Status of HWC Hydrogen is injected virtually 100 percent of the time PNPS is above 60% reactor power, and the HWC availability to date is 98.2%
ECP is measured using autoclave #2 approximately 100% of the time hydrogen is injected.
(h) Future Status of HWC Testing and turnover of the Electrolytic HWC System is underway and is expected to be ready to inject H2 and O2 later this year. This system is less expensive to operate and will allow an increased injection rate to 45 SCFM. Plans are to raise the H2 injection rate to 45 SCFM to extend protection to some RPV internals.
- 4. Conformance with BWROG Topical Report. NEDC-319151P BWROG Topical Report NEDC-319151P suggests that to justify revised inspection schedules for a piping system, full HWC conditions are to be achieved in the system.
The report further suggests that three criteria be satisfied to assure full HWC condition: (i) the availability of HWC be > 90%, (ii) ECP measurements should be less 12
-th'an -230mVSHE, and (iii) ECP measurements are to be representative of the ECP in the piping system to be protected. PNPS has implemented an effective HWC program that provides reasonable assurance that the above criteria are met.
(a) HWC Availability > 90%
HWC at PNPS has been available > 90% of the time since ECP was first monitored by the CAVS in 1992 under HWC conditions (See Table 3). Also, included in Table 3 for information are the estimated hours when the recirculation system water temperature was > 212 F. The 212 F was used for convenience since the time the plant spends between 212 F and 200"F is minimal.
(b) ECP Threshold Value The ECP values were maintained below-230mV(SHE) using an injection rate of 23 SCFM at 100% power and lower injection rates commensurate with lower power l levels. Typical measured values were -400 to -500 mV(SHE).
(c) Representative ECP Measurements The CAVS was installed at PNPS to verify the effectiveness of HWC. While "in-situ" i measurements in the recirculation piping are best at verifying representativeness, there is reasonable assurance at PNPS that autoclave ECP readings are valid in j confirming that enough hydrogen is being injected to protect the recirculation piping system. This is based on the following:
l (i) ECP readings are typically much less than -230mV(SHE). ECP values approaching -500mV(SHE) are typical. General Electric has confirmed that ECP readings as low as these are indicative of a fully protected
! recirculation system (Enclosure).
l (ii) Low Recirculation System Oxygen Levels: Pilgrim's present l
recirculation system oxygen levels of - 0.5 ppb indicates that the recirculation system is " buried" with respect to the amount of H2 being injected. The overabundance of H2 produced very low ECP readings in CAVS.
(iii) "In-situ" ECP Measurements PNPS does not have an "in-situ" probe to measure and verify the autoclave ECP measurements. PNPS is planning to install an "in-situ" ECP measuring system during Refueling Outage 11 (1997) to provide l
data foi confirming the effectiveness of HWC in protecting the reactor l recirculation piping.
C. Reduction in Radioloaical Exposure and Resources Inservice (ISI) examinations and the craft support for staging, weld preparation and insulation removal are a major contributor to the dose budget for each %tage. Sriv6gs have already been realized with the c4cravo icpiacement program which improves Category D welds to Category A status and reduces examinations by 90%.
This request applies to 11 safe-end welds at the recirculation nozzles for which the average previous dose has been 11/3 man-rem for each examination. Since the request would change the frequency of examination for all eleven welds from 2 fuel cycles to 4 fuel cycles, a savings of 3 2/3 man-rem each cycle would be realized. The corresponding savings is in the range of $300,000.00 per each refueling outage.
13
i f *.
1 i j D. Proposed IGSCC Inspection Proaram l
., All 11 Category D reactor recirculation nozzle safe end welds have been examined in the past j two refuel cycles. These welds are identified in Figure 11, ISI drawings. We propose to l lengthen the schedule to examine all welde every four cycles.
The schedule for the balance of IGSCC susceptible welds at PNPS will remain unchanged from the current schedule.
E. Fiaures (Enclosed)
F. Enclosure 14
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Enclosure GE Nuclear Energy December 15,1992 Mr. Roy Anderson Vice President Nuclear Operations Boston Edison Company Pilgrim Station RFD # 1 Rocky Hill Road l
Plymouth, MA 02360 '
Dear Roy:
Following our discussion at the BWR Operational Excellence Meeting in Washington, I reviewed I the factors relevant to the question of"What is being protected by HWC at Pilgrim"?
- 1. At the hydrogen injection rate (23 SCFM) currently being utilized full SCC protection of the recirculation system is being obtained. This includes the recirc inlet thermal sleeves.
Protection is occurring as a result of the low ECP values reported (-490 to -530) by the CAVS ECP sensors.
2 About four years ago calculations were made for Pilgrim using the GE/Harwell water chemistry model (reponed by EPRI). Based on this analysis, some SCC protection is also predicted for the lower half of the down comer region; i e. Jet Pump assembly and supports and the outside of the lower half of the Shroud.
- 3. At the current injection rate no SCC protection is predicted for any other area in the pressure vessel.
I am sure that the above points are well understood by the Pilgrim Engineering and Site StafT.
Also we have discussed with your people the importance of completing the In-Core Measurement Program started six years ago, to determine the amount of hydrogen needed to protect the Reactor Internals at Pilgrim. Unfortunately little real progress has been made.
Finally, I want to emphasize that the Optimum Water Chemistry (OWC) approach that I presented in Wilmington should be very seriously considered and a Pilgrim Plant specific plan be established for its implementation. As I stated, we would be pleased to work with your people to help develop and implement such a plan Very truly yours, Dr. E. Kiss, Manager BWR Technology CC D. R. Wilkins V. L Bain D. Heard, BECO 92-171