ML20129H692
| ML20129H692 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 07/08/1985 |
| From: | Johnson E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| To: | Lee O PUBLIC SERVICE CO. OF COLORADO |
| References | |
| NUDOCS 8507190333 | |
| Download: ML20129H692 (3) | |
Text
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3 DrnB JUL 0 31985 In Reply Refer To:
Docket 50-267 Public Service Company of Colorado ATTN: 0. R. Lee, Vice President Electric Production P. O. Box 840, Denver, Colorado 80201
Dear Mr. Lee:
We have completed our review of the various submittals related to the Fort St.
Vrain Prestressed Concrete Reactor Vessel (PCRV) prestressing tendon wire corrosion problems. The results of our review are contained in the attached {
Safety Evaluation (SE). We have concluded that sufficient assurance of Safe operation of the PCRV exists to allow the resumptien of plant operations, provided the following additional items are implemented:
- 1. Incorporate the modified tendon surveillance program into the Technical Specifications.
- 2. Provide the NRC with the results of the tenden surveillance program every 6 months, including the results of inspections related to determining whether there is any evidence of anchorage stress washer failure.
We have reviewed your May 20, 1985 (P-85176), and June 14, 1985 (P-85199) letters and find that your commitments acceptably resolve the above concerns.
We will include the above in the listing of the various PSC commitments which will be confirmed in connection with authorization of plant restart.
We have also enclosed a copy of tne evaluation report, prepared by our consultants at the Los Alamos National Laboratory, for your information and comment.
RIV-SPES SPES RPB1 PCWanger/lk RElreTand EHJohnson 7/3/85 1/$/85 g/f/85 gio 7 (96333 m
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Public Service Company of Colorado If you have any questions on this subject, please contact the NRC Project Manager - P. Wagner - at (817) 860-8127.
Sincerely, OricM c"vd W E. H. K hr.;cn E.'H. Johnson, Chief Reactor Project Branch 1
Enclosures:
- 1. Safety Evaluation
- 2. LANL Evaluation cc:
Mr. D. W. Warembourg, Manager Nuclear Engineering Division Public Service Company of Colorado
- P. O. Box-840 Denver, Colorado 80201 Mr. David Alberstein, 14/159A GA Technologies, Inc.
P. O. Box 85608 San Diego, California 92138 Kelley, Stansfield & 0'Donnell Public Service Company Building 550 15th Street, Room 900 Denver, Colorado 80202 v
Chairman, Board of County Com.
of Weld County, Colorado Greeley, Colorado 80631 Regional Representative l Radiation Programs Environmental Protection Agency 1860 Lincoln Street Denver, Colorado 80203 Mr. H. L. Brey, Manager Nuclear Licensing / Fuels Div.
Public Service Company of Colorado P. O. Box 840 Denver, Colorado 80201 (cont,onnextpage)
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J. ' W. Gahm, Manager, Nuclear Production Division Fort St. Vrain Nuclear Station
-16805 WCR 191 Platteville, Colorado 80651 L. Singleton, Manager, Quality
- Assurance Division (same address)
Colorado Radiation Control Program Director bec distrib. by RIV:
RPB1 Resident Inspector R. D. Martin, RA SectionChief(SP&ES)
EP&RPB P. Wagner, RPB1 R. Denise, DRS&P RIV File E. Haycraft, DRSP/LA
. E. Butcher, ORB 3 CPB/NRR H. Thompson, D/DL G. Lainas, DL K. Heitner, ORB 3 J. Taylor, IE E. Jordan, IE G. Lear, SGEB/NRR H. Polk, SGEB/NRR
p 5 ""% UNITED STATES y- /, NUCLEAR REGULATORY COMMISSION
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REGION IV S 611 RYAN PLAZA DRIVE, SUITE 1000
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- [ ARLINGTON. TEXAS 76011 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION PUBLIC SERVICE COMPANY OF COLORADO FORT ST. VRAIN DOCKET NO. 50-267 PCRV TENDON CORROSION INVESTIGATION AND PROPOSED REMEDY PCRV Tendon Degradation -
Background:
In 1984 during the scheduled PCRV tendon surveillance, the licensee discovered that certain PCRV tendons had broken and corroded wires. In order to determine the extent of this problem, the licensee increased the number of visual examinations of accessible tendons ends. He also performed a number of lift-off tests. Metallurgical examinations of the tendon wires and tests on the protective grease that were performed by the licensee indicate that the corrosion is the result of microbiological attack on the original tendon grease. The licensee has proposed to halt this degradation by filling the tendon sheaths with an inert nitrogen " blanket".
As a mechanism for monitoring the condition of the tendons, the licensee has proposed a surveillance program that increases the frequency of the visual inspection and lift-off tests. The surveillance program would compare an uncorroded tendon control group with a corroded tendons group to establish the effectiveness of the corrosion arresting method and the trend in the tendon wire degradation. The program would include samples of the longitudinal, circumferential, and crosshead tendons proportional to the population of the tendon types.
Evaluation:
- 1. Monitoring:
The staff evaluated the licensee's proposed program for monitoring the PCRV tendons and finds the proposal acceptable for assuring PCRV integrity in the near term with certain modifications, as discussed below:
The surveillance program, as proposed, would produce a sample of significant size to indicate the trend of the tendon wire degradation and the effectiveness of the corrosion arresting method. However, the information gathered by the licensee from the past and future surveillance. activities should be integrated into a complete visual presentation covering all tendons. The purpose of this presentation format would be to provide better information of the extent and significance of the tendon degradation problem. The licensee has committed to incorporate the modified tendon surveillance requirements into the technical specifications.
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- 2. Corrosion Control:
The staff evaluated the integrity of the PCRV with the degraded tendons in a safety evaluation dated May 16, 1984. The staff findings were that the reactor vessel was capable of withstanding the operating pressures with the degraded tendons as determined at that time. Since May 1984, a few additional wires have broken but the reactor sessel remains able to adequately withstand the operating pressure. The licensee plans to use a nitrogen " blanket" in the tendon sheaths to halt the degradation of tendons; however, our earlier evaluation indicated concern with this approach. Accordingly, we recommend that the licensee carefully evaluate the effectiveness of other techniques, in terms of their ability to remove oxygen and moisture, and their long term effects on tendon corrosion.
- 3. Corrosion Problems at Other Plants:
The corrosion problem at Fort St. Vrain (FSV) appears to be different from the tendon problems recently experienced at some other nuclear plants. In the other plants, tendons are used in the containment structure which experiences ambient temperatures and the tendon sheaths are filled with grease. The tendons at Fort St. Vrain are located in the reinforced concrete reactor vessel. These FSV tendons experience higher temperatures than other plants and are in sheaths not filled with grease. The FSV tendon wires themselves are protected by a grease coating and the tendon sheath annulus is coated on the inside with a layer of grease.
A failure mechanism has been identified at the other plants related to stress corrosion cracking of the tendon wire stress washers when r water was present, predominately in the lower end of the vertical tendons. The stress washers are manufactured from a high strength steel which is susceptible to stress corrosion cracking when exposed to a source of hydrogen.
The tendon wires at FSV appear to be corroding from the attack of formic and acetic acids generated from microbilogical sources. The corrosion and failures seen to date at FSV seem to be limited to the wires themselves with only one incident of corrosion occurring on the stress washer. No evidence of failure of the stress washers has been detected to date. The licensee has visually examined 10 of the 34 accessible bottom stress washers of the longitudinal tendons and reported the results in a letter dated June 7,1985. No evidence of cracking was found. However, the possibility of stress washers failing from corrosion cannot be ruled out. The continued presence of moisture in the tendon tubes could lead to failure of the stress washers as seen at other plants. The licensee has proposed an intensified surveillance program which consists of visual inspection of the anchorages and lift-off tests. The licensee proposed to incorporate these inspection requirements into the plant technical
specifications under Section 3/4.6.4 "PCRV Integrity". This intensified surveillance program will require a visual inspection and a report on a sample of 56 tendons at six month intervals. The surveillance program will also require 37 tendons to be lifted-off their shims to determine the amount of prestress available. A sample of 12 tendons are designated as a control set for visual inspection and 8 tendons are the control set for lift-off tests. The samples of 44 visual inspection and 25 lift off tendons will be rotated thru the tendon population.
4.- Restart and Re-evaluation:
The staff has reviewed the licensee's proposed surveillance and the commitment to incorporate the surveillance requirements into the technical specification. The staff accepts the tendon surveillance proposal and find that the proposal would lead to maintaining the structural integrity of the reactor vessel. The staff also finds that the visual inspection of the tendon wire anchorage washer is currently sufficient to determine if failure of the washers has occurred. The increased tendon surveillance would be sufficient to show any tendency of the stress washers to fail similar to those at another nuclear plant. The staff finds the plant structurally ready for restart.
Principal Contributor:
H. Polk, DE Date: July 2, 1985
t l
t Evaluation of Control Rod Drive Mechanism and Reserve Shutdown System Failures, and PCRV Tendon Degradation Issues Prior to Fort St. Vrain Restart NRC Fin No. A-7290 March 12,1985 Los Alamos National laboratorf Deborah R. Bennett, Q-13 Gerald W. Fly, Q-13 L. Erik Fugelso Q-13 Robert Reiswig,,MST-6 Stan W. Moore, Q-13 Responsible NRC Individual and Division J. R. M111er/0RB3 Prepared for the U.S. Nuclear Regulatory Commission Washington, D.C. 20555 g[gO$ f u.
n DISCLAIMER Tnis report was prepared as a.. account of work sponsored by an agency of the United States Go.ornment. Neither the United States Government nor any agency tnereof, or any of their employees, makes any warranty, expressed or impliec, or assumes any legal liability or responsibility for any third party's use, of any information, apparatus, product or process disclosed in this report or represents that its use by such third party would not infringe privately owned rights.
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Table of Contents
1.0 Background
1.1 Assessment Report Restart Issues 1.2 PCRV Tendon Restart Issues 13 Purpose of tne Technical Evaluation 2.0 Control Rod Drive and Orifice Assemblies 2.1 Failure Mechanisms 2.1.1 Motor Brake Malfunctions 2.1.2 Reduction Gear Mechanism Malfunctions 2.1 3 Motor and Motor Bearing Malfunctions 2.2 Refurbishment Program 2.2.1 CRDOA Refurbishment 2.2.2 Control Rod Cable Replacement 2.2 3 Reserve Shutdown System Material-Related Failure 2.2.4 Purge Flow and Seal Replacement 23 CRDM Temperature Recording and CRDM Requalification 2.4 CRDM Preventive / Predictive Maintenance and Surveillance 2.4.1 CRDM Preventive / Predictive Maintenance 2.4.2 CRDM Interim Operational Surveillance 30 Moisture Ingress Issues 4.0 PCRV Post-Tensioning Tendon System 4.1 Tendon Accessibility, Extent of Known Degradation and Failure Mechanism 4.2 Tendon Corrosion Corrective Measures 43 PCRV Tendon Interim Surveillance 4.4 PCRV Structural Calculations by Los Alamos National Laboratory
$.0 Conclusions 6.0 References ,
Evaluation of Control Rod Drive Mechanism and Reserve Shutdown System Failures, and PCRV Tendon Degradation Issues Prior to Fort St. Vrain Restart
1.0 Background
On June 23, 1984, following a moisture ingress event resulting in a loss of purge flow to the Control Rod Drive Mechanism (CBDM) cavities, 6 of 37 control rod pairs in the Fort St. Vrain (FSV) High Temperature Gas-
. Cooled reactor failed to insert on a scram signal. Subsequently, all six control rod pairs were successfully driven into the core.
In July,1984, an assessment team consisting of Nuclear Re6ulatory Commission (NRC) personnel from Headquarters, Region III and Region IV, and their technical consultant, Los Alamos National Laboratory, conducted an on-site review of the Control Rod Drive Mechanism failures, overall conduct of plant operations, adequacy of technical specifications and a review of tne continued moisture ingress problem. An auditional plant visit in August, 1984, reviewed CBDM instrumentation anomalies.
1.1 Assessment Report Restart Issues Tne results of both assessments were reported in the " Preliminary Report Related to the Restart and Continued Operation of Fort St. Vrain Nuclear Generating Station" , in October, 1984. Tne report concluded that Fort St. Vrain should not oc restarted until modifications and/or other corrective actions had been taken, or until all control rod drive mechanisms had been inspected and refurbished to provide reasonable as-surance that the control rods would insert automatically on receipt of a scram signal. More specifically, and as included in this technical eval-uation, Reference 1 required Public Service Co. of Colorado (PSC) to com-plete tne following, prior to restarts
- a. Tne licensee must identify the CRDM failure mechanism (s) and take corrective actions, or, if the mechansm(s) cannot be posi-tively identified, take other compensatory measures to provide assurance of control rod reliability, which could reasonably include refurbianment of all CRDHs.
O. The licensee must outline and commit to periodic inspection, preventive maintenance and surveillance programs for control rod drive mechanisms ano associated position instrumentation.
A change in the Technical Specifications anall be proposed to implement a weekly control rod exercise surveillance program for all partially or fully withdrawn control rods. A Limiting Condition for Operation should define control rod operability, and the minimum requirements for rod position indication.
- c. The licensee must functionally test one-20 weight 5 boron and one-40 weight 5 boron hopper from tne Reserve Snutdown System
. (RSS), to assure the full availability of the RSS, prior to j restart. The licensee must outline and commit to periodic in-l spection, preventive maintenance and surveillance programs for Reserve Shutdown System material. A cnange in the Technical Specifications shall be proposed to implement the RSS surveil-lance program. A Limiting Condition for Operation snould define and confirm the operability of the Reserve Snutdown System.
l o. Tne licensee should develop a procecure requiring reactor shut-down when high levels of moisture exist in the primary coolant, l or when CRDM purge flow is lost.
- e. The licensee should implement a procedure for recording repre-
- sentative samples of CRDH temperatures at all operating condi-tions, until continuous recording capability is available.
- f. The licensee should implement procedure to prevent overdriving the control rods past the " Rod-In" limit.
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- g. The licensee must develop a plan to implement any modifications l recommended by the PSC Moisture Ingress Committee that are determined, by PSC, to have a high potential for significantly reducing the severity and frequency of moisture ingress events.
1.2 PCRV Tendon Restart Issues
! , As a result of previously identified tendon degradation in the Pre-stressed Concrete Reactor Vessel (PCHV) post-tensioning system, PSC must complete tne following, as confirmed by Reference 2, prior to restprts l
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- a. The licensee should submit documentation evaluating the mechan-ism (s) causing corrosion on and failure of tne PCRV tendon wires, and c'orrective messures to eliminate further tendon degradation, thereby assuring the continued structural integ-rity of the PCRV and its post-tensioning system.
- b. The licensee should propose and implement a tendon surveillance program that determines the extent of current tendon degrada-tion in the PCRV, and that systematically monitors the rate of tendon corrosion.
r 1.3 Purpose of tne Technical Evaluation This document provides a technical review of tne restart issues identified above, and the corrective measures and/or actions proposed by licensee, based on the licensee,'s January 31, 1985 submittals (neferences given as used in this document), and the meeting between the licensee and NRC at the FSV plant site on February 20-22, 1905, as transcribed in References 3, 4 and 5.
2.0 Control Rod Drive and Orifice Assemblies This section includes a review of CHDM failure mechanisms, Control Rod Drive and Orifice Assemblies (CRDOA) refurbishment, CRDM temperature recording and requalification testing, CRDM preventive / predictive main-tenance and surveillanco.
2.1 Failure Mechanisms The failures of control rod pairs to scram, under various operating conditions, has been documented since 1982,0' and are as noted in, ,
Table 1 by region, CRDOA number and CRDM purge flow subheader (total of 8 purge flow subheaders).
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Table 1. Control Rod Failures Date 2/22/82 6/23/84 1/14/85 Region 7 28 6 7 10 14 25 28 28 31 32 CRDOA # 18 44 29 18 14 25 7 44 36 17 15 CRDH Purge 1 1 6 1 7 2 5 1, 1 2 3 Subheader #
Hign moisture content in the primary coolant and loss of purge flow were common modes during the 2/22/82 and 6/23/84 events. Substantial descriptions and operating cnaracteristics of tne drive motor, friction Drage and dynamic braking, the reduction gear meer.anism, the cable drum and cable, and the bearing lubricant are provided in Reference 6. Tne licensee reviewed those CRDM co'mponents that could have caused the fail-ures to scram, and postulated various failure mechanisms that could have interacted on each component, as described below.
2.1.1 Hotor Brake Malfunctions During a scram, the motor brake is de-energized and released, thereby freeing the motor rotor snart and gear train assembly to rotate under the torque applied by the weight of the control rods. In the motor brake assembly, failure of the scram contactor to de-energize dc power to the electromagnet was discounted because the operator had removed tne brake fuses following the CRDH failures to insert the control rod pairs.
According to the licensee, electromagnetic remanence and reduced spring constant in the brake spring plungers (due to elevated tempera-tures) were eliminated as possible failure mechanisms. Some corrosion and ruut was identified on the brake disks of CRDOAs 25,18 and 29 How-ever, the disks of a CRDH motor brake assembly with " discoloration and whatever surface variations"3, p.149, could not be made to stick in an elevated temperature helium environment with high moisture content (test T-228). Tne 11censeo concluded tnat the motor brake was not instrumental in the failures to scrat.
Los Alamos agrees with the licensee that the motor brake asse&bly was probably not related to the CRDH failures.
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2.1.2 Reduction Gear Mechanism Malfunctions The reduction gear train is driven by the motor rotor shaft, and rotates the cable drum with a gear ratio of 1150 between the motor and drum. The condition of the reduction gear mechanism was postulated by the licensee to potentially contribute to a failure to scram through gear tooth or bearing damage, by the presence of large particulate matter pre-venting gear rotation, and/or the presence of particulate matter in the 6 ears or gear bearings reducing the gear train efficiency--i.e., the 1
torque transmitted from the gear train to the motor rotor snart might I have been insufficient to overcome the friction of tne motor bearings.
The licensee stated that no major damage has been identified on sev-eral inspected reduction gear mechanisms, even though some wear and debris were observed. The licensee's analyses indicated that particulates with a size of 0.030 inches in diameter or greater, and with a comparable material composition as the reduction gear mechanism (implying comparable hardness), would be required to inhibit gear or gear bearing rotation.
0 Analyses of CRDOA debris showed the presence of rust, molybdenum di-sulfide and traces of silicon particles, wnich are relatively soft mate-rials. The average particle of 0 020 inches was uniform in size, and tended to be smaller than that thought to innibit rotation, even though rust particles on the order of 0 0625 to 0.125 inches were scraped off the ring gear pinion housing of CRDOA 18. However, the presence of debris in the gears and gear bearings tended to support the licensee's case of reduced gear train efficiency when sensitivity studies indicated tnat the motor Dearings were only three times more sensitive to debris than the first pinion gear mesn of the reduction gear assembly, and 500 times more sensitive to debris than the cable drum bearings.
Los Alamos agrees with the licensee that the presence of debris, especially in the first pinion gear mesh and the gear bearings, could reduce the efficiency of the reduction gear train, and thereby contribute to CHDH failures.
2.1 3 Notor and Motor Bearing Hilfunctions During a scram, the motor is de-energized and does not directly con-tribute to the scram process, even though it operates as an induction generator. However, because 16-20 inch-ounces of resisting torque on the i i motor rotor snart can forestall scram,9 the friction from the motor j j bearings can be a significant contributor to the failure to scraa. Pos- !
sible contributions to increase the friction include debris in the bear-l l
ing race, wear on the bearing ball or race, and changes in the lubricant properties during adverse conditions.
The licensee reported that debris was observed in tne bearing races of CRDOAs 7, 18 and 44, " roughness in rolling the bearing balls was noted l in virtually all of the unrefurbished bearings examined",6 and minor
! race wear was identified. Reference 8 verified tnat the major debris constituents could be attributed to the motor bearing materials (whicn l ,
includes bearing balls, races, and other bearing components), whereas l sinor constituents were indicative of the motor itself. The analysis provided little evidence to support the theory tnat debris had been
! " washed" into the bearing races. Tne licensee also determined, because 1 of the relatively close bearing tolerances and because rod weignt alone alght not produce sufficient "crusning force" to deform bearing particu-late, that bearing oper.ation could be reduced with the presence of par-ticulate matter. Tne licensee therefore concluded tnat internally gener-ated wear byproducts in the CRDM motor bearings contributed significantly to tne failures to scram. .
Los Alamos agrees with the licensee that increased friction in tne motor bearings, caused by the presence of internally generated debris, i could have been a likely contributor to the failures to scram. Los Alamos also agrees with the licensee that the " wash in" theory of debris into the actor bearing races is not supported.
Los Alamos contends that the loss of CRDM purge flow allowed primary coolant with high noisture content to enter the CRDM cavity. An indepen- l dont literature search indicates that tne dry film lubricant, solybdenum disulfide, Mo32 ' **E'"i'" ** "" i" I'"** I" it' ' 'III'A'"t UI IIA'"
tion in the presence of moistureN. Therefore, tne increased frictional co3fficient of the lubricant on the motor bearings, MoS , may 2 have also contributed to the CRDM failures by resisting motor rotor shaft rotation.
2.2 Refurbishment Program, The cause of tne failures to scram could be attributed to several mechanisms sucn as reduced reduction gear train efficiency, internally l
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generated debris in the motor bearings causing increased friction on the motor rotor shaft, and possibly an increased frictional coefficient in j the dry film lubricant in the presence of moisture. Because the CRDH I l failure mechanise cannot be specifically delineated, and because of CRDH cable failures, the licensee has uncertaken a refurbishment program, in.
volving the CRDH motors and reduction gear mechanisms, on all 37 CRDHa. !
1he licensee reported that the CRDH refurbishment process and a testin1 j program will ensure the ability of tne control rods to scram under oper-ating conditions.
- In addition, the licensee has elected to replace the control rod cabling and other connecting hardware in light of recently identified stress corrosion problems, to replace the Reserve Snutdown System material due to the discovery of material " bridging" during hopper discharge, ano f to install seals around certain, penetrations into tne CHDH cavity to f nitigate the effects of primary coolant ingress by natural circulation.
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2.2.1 CRDOA Refurbishment [
Tne licensee has proposed costplete refurbishment of all Control Rod l Drive and Orificing Assemblies to ensure that the CRDOAs will perform L
their intended safety functions, and to avoid potential operability prob-less that could limit plant availability. As specified in Reference 10,
! the following major components are to be inspected, tested, refurbished l
or replaced, as necessary: !
! 1. Control Rod Drive (200) Assembly--shin motor and brake assembly, l bearings, reduction gears, limit switches / potentiometers. l l
- 2. Orifice Control Hochanism--orifice control motor, bearings,
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potentiometer, gears, drive shaft and nut, drive shaft housing.
l 3 Control rod clevis bolts. f 4 Reserve Shutdown System--boron balls, rupture disks, DP switch. ;
! Design modifications include tne replacement of control rod caoles, t
cable end fittings, and cable olevis bolts, the installation of new purge (
seals into tne CRDH cavity, and tne installation of RTDs (Resistance Tom-perature Detectors) in all CHDOAs--the impact of tnese design onanges l Will be evaluated later in tnis report.
Lich CRDOA will undergo the following series of scram tests in tne I refurbichment process 6 a pre-refurbishment, in-core full scram test; a
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pre-refurbishment full scram test in the Hot Service Facility (HSF); a scram test with refurbished reduction gear mechanism and unrefurbished shim motor, using dummy weightsi a full scram test using a " standardized" motor, using dummy weights; a scram test with completely refurbished 200 assembly, using dummy weights; a post-refurbishment, full scram test in the HSF; and finally, a post-returbishment, full in-core scram test.
As designated by the licensee in Reference 6, back-EMF voltage meas-urements from the shim motor will be taken for the series of scram tests conducted before, during and after refurbisnment, and should define the CHDH operating characteristics. From tne back-EHF voltage measurements, the licensee states that they can generate the following information--
voltage versus time, frequency versus time, voltage versus frequency, acceleration versus time, torque versus time, peak angular velocity, time to peak back-EMF and angular velocity, average torque on motor rotor dur-ing acceleration to peak velocity, maximum torque on motor rotor each 10 second interval, maximum deviation of torque values each 10 second inter-val, and gear train efficiency.
Tne licensee has proposed a CRDOA refurbishment acceptance criterion, taking into account the results of the back-DiF voltage measurements and the resulting calculations of acceleration and torque such that : 0
- 1. Tne minimum calculated average torque during acceleration to peak velocity will be 17 0 inch-ouncesI tnis value corresponds to an average acceleration to peak velocity of 96.83 radians /
second 2 ,'
- 2. The maximum torque calculated during " steady-state" will be 7 0 inch-ounces.
According to the licensee, final acceptance of a refurbished CHDOA will bo based upon the results of its in-core full scram test.
Los Alamos agrees with the mechanicil refurbishment of all CHDOAs, as the program la currently being implemented by the licensee. In par-ticular, the replacement of shim motor bearings 3 ' EE' AU"U is con-sidered essential to the refurbishment process. However, the current program of mechanical refurnishment alone cannot ensure ChDOA o perability.
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From the documentation presented by the licensee and reviewed earlier in this section,1,os Alamos believes that the proposed back-EMF testing and acceptance criteria have potential in providing a data base from which control rod operability might be determined. But, an element of uncer-tainty, as to CRD0A operability based on back-EMF testing, is introduced
! because the test method and interpretation of its results are still in the developmental stages, and because in-core full scram testing of re-furbished CRDOAs has not yet taken place.
I Los Alamos recommends that the back-EMF testing method continue to be developed, that the further collection of back-EMF information be used in preparing a statistical data base for possibly defining CRDOA opera-bility, and tnat more attention be paid to the initial, start-up scram ,
characteristics of the CMDOA, in developing a better understanding of brea'<-away torque effects. In line with Region IV's increased inspection I
of the refurbishment process, we suggest a review, by Region IV, of all i testing results pertaining to CRDOA refurbishment acceptability, after in-core testing is complete, but prior to startup. As an additional method to ensure CRDOA operability during scram, a procedure requiring
- control rod run-in is recommended.
As a post-startup item, Los Alamos recommends that a final determina-l tion be made as to the suitability and acceptability of back-EMF testing in defining CRDOA operability. !
I 2 2.2 Control Rod Cable hoplacement i
In September, 1964, the control rod cable on CRDOA 25 was severed in i several places during an investigation of a slack cable indicatio').11 A subsequent metallurgical examination of the austenitic 347 stainless . I steel cable indicated that the cable surface was pitted and cracked, tnat ;
the delta-like material cracks were typical of stress corrosion cracks, and that the fracture surfaces were brittle in nature. Further investi- ,
sation revealed that the 347 SS cable material was susceptible to stress l corrosion when under the existing stressed conditions, and in the presence of chlorides and moisture.
The potential sources of the chlorides in the primary coolant' con- I tributing to the chloride stress corrosion are reviewed in Reference 13 l Tne licensee states that the chlorine occurs as two different species--hcl l
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- 1 gas and a salt; the sources of the gas roecies include tne fuel rods, H-327/H-451 graphite, PGX/HLM graphite and tne Ti sponge, whereas the sources of the salt species include the ceramic insulation, concrete and water, all to varying degrees.
As part of tne overall CRDOA refurbishment program, the licensee elected to replace the control rod cable witn Inconel 625, wnicn is con-sidered resistant to chloride stress corrosion, and has increased strength and fatigue properties over the former 347 ss. Cable components and con-necting hardware that were made from materials susceptible to stress cor-rosion, and are being replaced with materials more resistant to stress corrosion include Component Material _
- l. Cable and rod portion Inconel 625--high strength of tne ball end and resistance to oxidation
- 2. Anchor, set screw Hartensitic steel-high strength, ability to be nitrided, resistance to oxidation
- 3. Spring, connecting bolt Inconel X-750--high yield strength, resistance to oxidation.
Drawing numbers and material information are available in Reference 12.
A safety analysis of tne material changes in tne reactor control rod drive and orificing assembly, wnich are classified as Class 1, Safety Helated and Safe Shutdown components, is included in Reference 14.
Los Alamos metallurgical analyses on a sample of the corroded control rod cable II also indicate pitting on the cable surface, ductile and brittle fracture surfaces, and to a lesser degree than the licensee, cracking indicative of stress corrosion cracking. Qualitative measure-monts confirm the presence of chlorine on fracture surfaces. Therefore ,
Los Alamos agrees tnat chloride stress corrosion contributed to the de-graded condition of the control rod cable. The Los Alamos analysis also observed that a certain particle removed from between the individual cable strands of the Los Alamos sample had a " shaved" appearance, and was t
identified as a 7000 series aluminum alloy--the licensee noted tnat tne control rod cable drum is constructed of 7075 aluminum alloy4 , p.20, and that no excessive drum wear had been noted.
Los Alamos agrees that' the licensee's recommended material changes tend to improve the overall resistance of the CHDOA cable components and connecting hardware to chloride stress corrosion. However, Los Alamos also recommends a continued analysis into the sources of tne chlorine and its effects on other reactor components, especially components potentially subjected to high chlorine concentrations such as the bottom plenum or other areas where water could accumulate.
2.2 3 Reserve Shutdown System Material-Related Failure In Novemoer,19o4, during the required testing of a 20 weight 5 boron and a 40 weight 5 boron hopper in the Reserve Shutdown System, only half of tne RSS material in CRDOA 21 (40 weight $ boron) was disonarged.
The licensee's examination of the undischarged material revealed that the b4 C boronated graphite balls had "oridged" together through a crystal-line structure on the ball surfaces. Analyses on tne crystalline material indicated that it was boric acid. The formation of tne boric acid crystals was caused by moisture reacting with residual boric oxide in the RSS material. It was concluded that tne moisture had entered tne HSS hopper through the CRDOA vent / purge line by " breathing", and/or by water contamination in the helium purge line.
In Reference 16, the licensee proposed a threefold corrective action to the RSS material problems. First, ned RSS material, manufactured by Advanced Refractory Technologies ( ART) in late 1984 and early 1965, has an order of magnitude less residual boric oxide in the D 4C material, and will be installed in all RSS hoppers as part of the overall CADOA refurbisnment program. No effort will be maoe to use AhT blended RSS material currently in stores N' P'32 unless NRC is notified. Second, an expanded HS3 material surveillance program, which Will be incorporateJ into the Technical Specification, will test one 20 weight 3 boron nopper and one 40 weight 1 boron hopper during each refueling outade, and will include visual examinations for borio acid crystal formations, chemical analyses of NSS material for boron carDide and leacnable boron oxide con-tent. Tntrd, efforts will be maae to mitigate or eliminate tne ingress l
l l l
l s
4 of moisture into the RSS hoppers by installing a knock-out pot, aoisture elements, and a back-up helium source for the main CRDOA purge and Reserve Snutdown System purge lines. I Each knock-out pot will be equipped with a sight glass and a high level alarm in the Control Room. ,
i Los Alamos concurs that the crystalline structures on the surface of '
the B4C RSS balls is meta-boric acid,10 most probably formed by nois-f ture reacting with leachable boric oxide in the B C material. In light 4
of the new RSS natorial to be used, the increased surveillance efforts, and seasures to mitigate the ingress of moisture in tne RSS hoppers, Los Alamos agrees that the refurbished RSS should be able to reliably perform [
its function.
l 2.2.4 Purge Flow and Seal Replacement Just prior to the June 23. 1984 event when 6 of 37 control rod pairs i failed to insert on a scram signal, a high moisture content in the primary coolant resulted in the loss of purge flow into tne CtWH cavities. The .
loss of purge flow may have allowed the additional ingress of moist pri- .
l mary coolant into tne CRDH cavities, resulting in mechanisms tnat may have contributed to the CRDH failures, because the exact CRDM failure mechanism has not been determined, and to alleviate the poss10111ty of purge flow loss and/or hign moisture content in the primary coolant con- f tributing to future CADM failures, the licensee has proposed several cor- I rective measures 19 as part of the overall CADOA refuroishmunt program. ;
To provido an accurate measure of the purge flow into tne CRDM cavi- {
ties, tne licensee has proposed snat new flow indicators witn a range of 0-20 scrm be installed on each helium purge line, providing local indica-tion, remote indication in the control Hoom, anu an alarm in the Control l Hoom to indicate low flow conditions.20 A minimum of 8 bypass lines (one line serviced by eacn of tne 6 purge (Aow subheaders) will be in- !
stalled prior to restart. Tne licensee intends to install the flow instrumentation ' E6 3* on these subheaders wnen tne devices are availacle.
As mentioned in section 2 2 3, to rouuce tne possibility of moisture ingress into tne CHDM cavities via tne helium purge lines, the lic,ensee will install a knock-out pot, moisturo elements and a back-up helium '
source for the main ChDOA purge and hus purge lines, prior to criticality '
12 -
t l
following the fourth refueling outage"' pg 5 The knock-out pots will be equipped with a sight glass and a high level alarm in the Control Room.
The helium trailer, which will act as the back-up source of dry helium i for purge, can provide helium at a rate of 7 4 acfm (4 5 lbms/hr per pen-etration at 700 psig) for approximately 2 nours.N '
To mitigate the ingress of primary coolant, wnich could contain moisture, into the CRDM cavity, seals will be installed on four larde i flow passages into tne ChDM cavity--the two passages in too reserve shut-down tube holes, and the two passages over the eye bolts that penetrate the floor of the CHUM cavity.21 Cover plates witn inte aral gasxots will also be installed on the four access openings on the lower ChDH housing. Thermal and mecnanical analyses 22 nave determined that the seal acditions will not interfere with the Rs3 performance under tne in-fluence of mechanical, thermal or seismic loadings. Tne flow calculations
~
in Reference 22 conclude tnat addition of tne mechanical seals to tne Nas pressure tubes and the liftina eyebolts will reduce naturally convective ini,ress of primary coolant into the CHDH cavity from a flow rate of 0.68 acfm to less than 0.006 actm. Additional calculations have confirmed that the seals are able to withstand both a design basis slow depressuri-l zation transient and a design basis rapid depressurization transient.
The licenses has proposed a procedure in heference 23 tnat basically I requires reactor shutcown in the event CRDit purge flow is lost, or if high moisture content is present in tne primary coolant.
Los Alamos agrees with tne efforts of tne licensee in monitoring the flow and moisture content of tne helium purge into tne ChDM cavities, in restricting the ingress of moisture into the ChD cavities via tne pura#
lines, and in providing a bacd-up source of helium in case of purge flow loss. From tne review of the provided documentation, Los Alamos agrees that the addition of seals and coverplates with integral gaskets will indeed mitigate the ingress of primary coolant and moisture into tne ChD
, cavities through penetrations.
1 In nualtion, Los Alamos believes tnat the procedure requiring reactor ;
shutdown with loss of purge flow or hign moisture levels in the primary I
coolant fulfills the requirements of the assessment report . Tne licensee definen "nign moisture levels" in Hoference 23.
I
23 ChDM Temperature becording and cRDH Roqualification i
Tne lack of direct measurements of Chum temperatures curing tno Juns 23 event, and during steady state and other transient operating condi-tions, has prompted tne installation of hTDs to monitor tna ChDH cavity closure plate (ambient), orifice valve motor plate and control roo orive motor temperatures. Strip cnart recorders will continuously record tne tnrev ten.peratures far each CRDM,2 anc will provice a CatDM operating temperature data base. The old data collection surveillance prococurv 0 will be modified to collect cata on a continuous basis. , p.60 Tne licensee intends to install tne permanent recordera prior to restart.4, p.57 The licensee postulates in Reference 20 that "Lne maximus. temperature j rating of tne orive mecnanism which might innibit the scram function is 274'F", and in monitoring CHDH, temperatures "tne maximum temperature rating of 272'F anould not be exceeded curing power operation".
Tne licensee has also proposed a CADOA requalification testing pro- I gram that is designeo to establian a temperature at whicn the CatDUA is qualified for operation.27 The helium test environment W111 be operated at 220*F, 200'F, 270'F, 200*F, 290'F and 300*F witn a goal of qualifying all ChDUA components for 300'F operation. hosults of too requalification ,
testing are unticipated by the end of 1905 Los Alamos agrees tnat the placement of CitDOA tnormocouples, and tne continuous cata monitoring at all operating conditions la sufficient to provide a CRDOA temperature data base curing steady state and transient operating conditions.
In adoition, Los AAamos believen tnat tne CnDOA is currently only qualified to operate up to 215'F based on the original mechanical ChDOA .
qualification tests, an Nhc recommendation,20 arid fa evious Los Alamos calculations. Tne licensee's argument that the CNDOA is qualified for 272*r operation cased on analytical calculations"' P'#9 is not sub-stantiated. Tnerefore, Los Alamos recommends tnat CNDOA operation be l
limited to 215'F until mechanical requalification supports a higner oper-nting temperature.
= 14 = i
(
2.4 CRDM Surveillance and Preventive / Predictive Maintenar n l i The licensee han proposed a set or preventive / predictive m.ninte- '
nance tests and surveillance inspection proccoures that are intended to monitor the performance of the CNDOAs and to determine the overall opera- :
bility of the CRDOAs during reautor operation. Initial development of l these operating tests arc considered part of the CliDOA refuroishment pro- I gram, and will utilize the data base and resultant trends formulated dur-ing refuroishment. !
l 2.11.1 CRL+1 Preventive / Predictive M.aintenance Tae licensee's CRDOA preventive / predictive maintenance program is proposed in Beforence 30. According to the licensee, the normal preven-tive m.sintenance (PM) program will be implemented on a refueling basis rotational c/cle for CHD0As tnat would normally be removed for refueling, l unless the predictive maintenance (PbH) program indicates tne need for j more frequent maintenance. Tne PH program woulo empnasize the mecnanical h examination and refurbishment of tne anim motor / brake assemoly, the drive train, control rod cable, reserve shutdown system, position potentio:n-l eters, limit switchen, orifice drive motor assemoly, orifice drive lead screw, assorted seals, valves, eAectrical components, bolts and the so- l uorber atring.
On tne otner hano, the predictive maintenance techniques woulo os used to monitor the most important aspect of ChDOA performance--the l
" scram capability"--Dy determining the shim motor /orake and gear train performance . The testa proposed in the PDH program include wattabe
! reluirements, back-EHF voltages, delivered torque at tne motors, scram I times, rod drop rates and torques to rotate motor / brake assemblies. Cer- .
tain aspects of the PuH program would be implemented on a weedly basis to '
1 I l determine scram capability and temperaturo performance during power oper- :
ation. The licensee nas also proposea that testing information be f acquired during reactor snutdown for trending purposes. l Los Alamos concurs with the proposea preventive maintenance pro- !
gram an out1 Aned by the licensee, on the assumption tnat cata acquired during reactor operation will show that predictive maintenance tecnniques can be used to detoot a reduction in CNDOA performance. Tne PDM testin a, tecnniques are closely linkou to the techniques that are being used for
- 1$ =
L I
i
the acceptance criteria in the refurbisnment program, and will tnerefore be dependent on the auttability and acceptability of back-EMF testing for determining CRDOA operability, as discussed in section 2.2.1.
2.4.2 CRDM Interim Operations 1 Surveillance Tne licensee's ChDOA interim surveillance program is proposed in Reference 31. The surveillance tests are scheduled on a weekly basis, using a 10" rod drop method on all witndrawn and partially inserted con-trol rods, except the regulating rod.5, pg 82 Tne surveillance tests will obtain data for analysis and long term trending, exercise the rod, test selected circuitry, verify FSAh (Final Safety Analysis heport)9 assumed scram times, and confirm control rod operaoility. In addition, ChDOA touperature and purge flow information will be collected.
For a fully witharawn rod, analog and digital position information will be obtained, " Hod-Out" lights will be verified on, "Had-In" ano " Slack Cable" lights will oc verified off, and tne rod will be dropped approximately 10" by de-energizing tne braxe, wn11e bacx-EMF data are obtaineo for future trending. 1ne " Rod-Out" 11gnt indication will be verified off, and analog and digital information will be compared, witn an acceptaDie deviation of 10 incnos between position indications.
1he rod will then be withdrawn to the full out position, so that analog and digital positions can again be ootained. Control rods that are par.
tially or fully inserted will undergo variations of this method.
Quarterly surveillance tests are intended to supplement weekly sur-veillance information, and to verify redundancy of selected control rod position limit switenes. hefueling snutdown surveillance wt11 acquire the same information as the weex1y and quarterly testa, except full strode insertion tests will be performed.
1he operdollity acceptance criteria, according to the licensee, will be based on distance and time rod drop data used to calculate a conserva-tive averato full length scram time. A CHbOA will be considered inoper.
able if it does not meet the maximum scram time of 100 seconda as defined in the FsAh9 . Such an indication would warrant back-Lir testing in i confirmine scram operability.
Los Alamos agroos that the basic surveillance mutnodology is uffi.
clont to exercise tne control rod, verify FSAN acram times, and to test l
(
10
e
, i I
U selected circuitry. However, references to 274 F as the maximum ChDOA operating temperature are still considered inappropriate as discussed in section 2 3, and a 10 inch deviation is not considered acceptable between digital and analog position indications--such a deviation could inadver-tently lead to control rod overdrive through a misinterpretation of rod I position. Also, tne back-EMF testing methods and interpretation of tne results are still in the developmental stages, and an engineering deter-mination of tne suitacility and accepta0111ty of tnis testing metnoa in
( determining continued ChDOA opera 0111ty will need to be maos cofore the l licensee can ranalize tnis portion of tne surveillance program.
l .
t l 30 Hotature ingrens fasuen Tne licensee nas submitted 32 a listing of tne issues considered, and actions taxon, by the FSV (mprovement Committee (formerly the F3V ,
Moisture Ingress Committee) in significantly reducing the frequency and severity of moisture ingress events. The issues were divided into four categories:
- 1. Issues currently under consideration by tne Fort St. Vrain Irn-i provement Committoo.
- 2. Circulator Auxiliary System modifications yet to be completed prior to startup.
3 Circulator Auxiliary Systeta modifications to be completed prior to startup, provided material availability and scheuule permits.
- 4. Items identitled by tne hoisture Ingress Committee wnion are installed and operational.
Los Alamos believes that a listing of intenced and installed modifi-cations does not provide any indication as to wnat any given moutfication really iue wn/ tney contribute to too reduction in potentiaA for moisture ingress events, nor which improvements will substantially reduce tuo neverity ano frequency of moisture ingress events. 1ne licensee nas com-mitted to submit a more eAplanatory version of tne actions to mitigate moisture ingress, prior to restartN ' E' 08 40 PChv Pont-Tensionintiendon Systeem ,
In the spring of 1904, during scheduled PChV tendon surveillance, tendons with corroded and broken wires were found. Since tnat time, the
17
. . t 11cehJe a has evaluated the corrosion mecnanian, has performed lift-off ,
tests on aclected tendons to determine their 1tud-carrying capability, Mnd proposed corrective actions and an increased surveillance procedures.
l ..
' / . ,
4.1 Tendon Accessibility. Extent of Known Iwaradation ano Failure e l Hochanism .
- he licensee, in determinina tne extent of tendon corrosio.-in tne
- 1 PCHV, determined what fraction of the tencons were available for visual ' '
examination and lift-off testa. Tne teudon system is subdivided into four major groups: the 90 longituoinal (vertical) tendons tt.we 169 wires per tendont tne 210 circumferential tendons an tne PGhV aidewall have 192 wires per tendon, and tra S0 c'.rcamfgrential tencons in botn tne top ano bottom heads nave 16) wires prir tencang tng 24 t,ottom cross _-reeaJ tencons, ana 24 top cross-nead tendons nave 169 wires per tendon. Of tne 'Itwr t
groups, tne licensee states tne following accessibilityN
! lendon Group batn 1;ncs Acces. Uno End Acces. $ tner
, :s Acces.
Qngitudinal vasual 20 69 1 Lif t-o ff <
0 74 16 l
l qi rcumferen tini, -
Visual 2d1 27 2 1.i f t-o ff 236 62 W fiottomcroasggy visual 20 4 0 Lift-off Ib 4 h-Inp crynn-neaa Vaauel 17 7 o -
Lift-off 16 b i Tne nuuber of tendone witn xnown broden warna as identifaen an in., i ticonsee's 1909 surveillance,b anbludeo 10 lor,gitudinal tendona witn 1 to 22 broWen wires, 2 circu'oferential tendon.1 with 2 and 1$ brogsn wires, b bottom crons nead tencons with 1 to 19 broren wires, and no top cr,oss-l head tendann with broNon wires. In some cases, the total number of cor-roded, bronen wires inclaue wires braken during 111't-off tests, te during, ,
retensioning., '
.18 -
f ,
l The results of 74 longitudinal lift-off tests 35 indicated tnat tendons with identified broken wires generally had a slightly smaller lift-off value tnan intact tendons. Thirty lift-off tests on circumfer-ential tendons snowed little change in lift-off value. Some of the fif-l teen bottom cross-head tendon lift-off tests showed a definite reduction in lift-off value for tendons with multiple wire breaks. 1ne value of
, tne lift-off test on one top cross-head tendon was nominal. All lift-off 1
test values exceeded tne minimum limits.
l The licensee conducted uetallurgical investigations into the cause of the corrosion, and determined that microbiological attacg on tne tendon NO-0X-ID CH organic grease caused the formation of formic and acetic acids. ' Tne acids, in conjunction with moisture in tne tendon tube, vaporized and recondensed on the cooler portions of the tendons--in this caso, usually toward the tendon, ends. Tne acidic attack resulted in re-cuced cross-sectional wire area, stress corrosion cracking, localized 1
tensile overload anc wire breakage.
Los Alamos believes, based on the documentation presented by the licenseo, that microbiological attack of the tendon grease and tne resul-tant formation of acetic and formic acids, in the presence of moisture, is a probable cause for tne currently observed tendon corrosion, and has led to the subsequent wire breakage througn tensile overload. However, I
Los Alamos believes tnat the extent of known tendon corrosion, breakage and previous suryeillance have not been clearly defined by the licensee.
Los Alamos tnerefore recommends that a complete map oc maue tnat lists each tenoon, its visual examinations and lift-off values, and tno number ana location of corroded and broken wires. An indication of tne degree of wiro corrusion would also bo desiraule.
4.2 Tendon Corrosion Corrective Hoasures The licenseo evaluated several methods for arresting the corrosion process, ' includin8 the use of ozone as a biocide to kill the micru-organisms, the use of an alkaline grease wnich should not be conducive to microbiological growth, and tne use of an inert blanxet consisting of nitrogen gas. The licensee's consultants found that the nitrogen atmo-spnero arrested the growth of the microbos in the 10-0X-ID CM organic
grease 34,35, and eliminated the oxygen wnich is necessary for the cor-rosion process to continue. based on these results, and as a snort term action, the licensee has proposed tnat nitrogen blankets ce establisneu on the longitudinal and bottom cross-nead tencons. Long term actions would include further investi6ations into tne corrosion process and ar-resting techniques, and the possible installation of acoltional load cells in monitoring the PCRV behavior.
Los Alamos believes that the use of a nitrogen blanket to halt tne corrosion process may be suitable, but difficult to implement as proposed.
The tendon tubes are not likely to be leaktight, and maintaining an inert gas atmosphere at a set over-pressure may prove difficult. Consideration might be given to maintaining an intermittent or continuous purge flow [
through the tendon tubes, as needed, rather than to maintaining a speci-Tied overpressure. However, Lo,s Alamos recommends that initially the nitrogen be purged through the individual tendon tubes to remove as much moisture as possible, and that gas samples be used to monitor moisture and oxygen recuction. Further investigation into the long term effects l of a nitrogen blanket on tendons, tne corrosion process and currently available corrosion acids are also recommended.
43 PCRV Tencon Interim Surveillance Because the total extent of tendon corrosion in tne PCRV is unknown, because the rate of existin6 corrosion is unknown, and because tne use of a nitrogen blanket as an arrest to tne corrosion process is an unxnown, tne licensee has proposed an interim surveillance program desi6ned to address eacn of tnese issues.5, pp.164-7 The interim terdon Surveil-lance program would incluae increased visual and lift-off surveillance for tnree years, or until effective corrosion control naa been estao-lished. Two populations of tendons would be inspected -a population of tendons that have not been previously identifieo as being corroded, ano a control population with known corrosion. On a six-month frequency, visual surveillance of both tendon ends, when accessible, woulo include:
t Tendon Group No. of New Tendons No. of Control Tencons Longitudinal 24 6 l Circumferential 13 3 Bottom cross-head 6 2 Top cross-nead 1 1 Lift-off tests would be performed on two frequencies--an lo montn frequency for the population of new tendons, and a 6 montn frequency for
~
the control population. The number of tendons for lift-off will include:
~
lendon Group .ho. of New Tendons No. of Control Tendons Longitudinal 12 3 Circumferential 13 3 Bottom cross-head - 3 1 Top cross-head 1 1 As an acceptance criteria, tne licensee proposed enat, based on vis-ualexaminations,amandatorye[sineeringevaluationbeconductedonany tencon that has 20% of its wires broken, for any tendon tnat has only one accessible end, the mandatory engineering evaluation will be con-ducted when any tendon has lo) of its wires broken. ine control tendon population will include those tendons with the worst known corrosion with reaoy accessibility.
Los Alamos agrees that the increased tendon surveillance program of the nature proposed by the licensee will provide more information on the extent of corrosion in the PCRV by inspecting new tendons each surveil-lance, and at the same time, monitor the rate of corrosion with the con-trol tendon population. Tne increased surveillance should also determine the effectiveness of the nitrogen blanket in arresting corrosion, or any other corrective measure the licensee may propose. Los Alamos recommends tnat the licensee submit an outline of the intended mancatory engineering evaluation, which should include all lift-off, load cell and relaxation data incorporated into a safety evaluation. The licensee should define the extent of. tne visual and lift-off testing procecures, ano coula use US/NRC Regulatory Guide 1 35 37 for guidance. 4 r1
4.4 PCRV Structural Calculations by Los Alamos National Laboratory The PCRV tendons are intended to apply sufficient compression in tne concrete to balance or exceed the circumferential and vertical tension in the concrete that results from the internal pressure. A combined analyt-ical and numerical study 39 was undertaken by Los Alamos National Laboratory to evaluate the evolution of these stresses, both to the Isi-tial prestressing and to subsequent partial and total rupture of tnese tendons. At the stress levels anticipated in the concrete, and for the anticipated operating life span of tne PCRV, the concrete benavior was modeled as a linear viscoelastic solid with tne creep strain varying pro-portionally with the logarithm of time at constant stress tnrougnout tue projected reactor lifetime.
A one-dimensional model of a lons concrete column of rectangular cross-section, with an embedded,prestressing tendon along the length, was used to evaluate the concrete ano steel stresses as well as tne nold-down and lift-off forces. Tnese were evaluated for the intact tendons and the degraded tendons. The degree of tendon degradation is described tnrough the ratio of the number of unbroken strands to tne original numoer of s tranos . Initial time of rupture was varied from the time of initial prestressing to 400 days after emplacement. Tne formulation led to an integral equation, which was solved numerically. The hold-down forces decayed approximately with the logarithm of time and for both the extreme observed degradation (21 broken strands) and for a more extreme case (40 broken stranas), the hold-down force still exceeded the minimum safety design requirements.
In addition, several finite element calculations, using tne finite element code NONSAP-C, were made to evaluate complete tendon failure in a 60 sector of the Fort St. Vrain PCRV. This code nas an extensive material library of constitutive relations to model the various properties of concrete, tobether with a . specialized element model to simulate pre-stressing tendons. Two rows of vertical and an arc row of circumferential tendons were incorporated in the model as a baseline calculation. Tne tendons were prestressed to 70) of the ultimate and an internal pressure of 775 psi was applied (this pressure is the internal pressure.of tne h ,
helium coolant in the HTGR) and the creep of the concrete and slow decay S of tne tenoon stresses were evaluated out to 30,000 days. Tnen, tnree w,
cases wherein one tendon was removed at one day were evaluated. Firs t the middle vertical tendon in the outer row and in line with the outer buttress was removed. Second, an inner vertical tendon opposite the thinnest portion of the PCRV wall was removed. Finally, an inner layer circumferential tendon at michei6ht was removed. Stress redistributions at 300 days after ruptures were calculated and snifts or tne remaining tendon loads to accommodate the broken tendon were calculated. Regions of local tensile and snear stress in the concrete portion of tne PCRV were identified and related to overall structural integrity.
With all tendons present, the mean vertical stress was aDout -760 psi, the radial stress decreased from the applied internal pressure of
-705 to about -1200 psi at the ring of circumferential tendons and tne tangential stress ranged from -2400 psi at the inner wall to about -2200 psi at the same place. Hemoval of a vertical tendon reauced the mean axial stress by about +40 psi, the local tangential stress by -10 psi and did not materially affect the radial stress. Removal of a circumferential tendon reduced the mean tangential stress by +30 psi and the local axial stress by -80 psi. The vertical hold-down force from zero days tnrough 30,000 days decreased linearly and remained above the prescribed safety limit, as did the circumferential hold-down force.
Comparison of the analytical solution and a small finite element problem simulating the analytical proolem was made to verify the visco-elastic creep models and the tendon element in the NONSAP-C code. Excel-lent agreement for stresses, strains and hold-down forces was ootained.
5.0 Conclusions Los Alamos concludes that tne licensee, Public Service Co. of Colorado, has made a conscientious effort to acdress all of the restart issues listed in the assessment report. The refurbishment program on all CRDOAs provides confidence in CRDOA operability during reactor opera-tion and the ability to scram, even if the exact " failure to scram" mecn-anism has not been defined. Questions concerning the reliability of the back-EMF testing procedure on the shim motor / brake assembly in determining control rod operational acceptability still exist, but further method development, more experience with result interpretation, and in-core J
testing may alleviate the questions. Until CRDOA operability can defin-itely be ascertained with these methods, we recommend that the licensee have backup measures such as rod run-in following scram.
Control rod caole and connecting hardware material replacement, alon6 with replacement of tne Reserve Shutdown System material, serve to rectify the material proolems brought on by corrosive mechanisms.
In light of chloride stress corrosion problems, Los Alamos also recommends that all reactor components exposed to the primary coolant be reviewed for susceptibility to chloride attack, especially the PCRV liner.
Review should continue into the source of enlorine and methods to elimi-nate its generation and presence.
The effects of purge flow loss have not been determined to be in-strumental in CRD0A failures to scram, yet the licensee has committed to maintaining purge flow by external means, and to reducing the effects of primary coolant naturally convecting into the CRD0A cavity with extra seal installation.
Even though current qualified CRDOA operating temperatures are very much in question, the licensee is in the process of requalifing the mech-anism for temperatures more in line with those anticipated during reactor operation.
From a meenanical standpoint, CdD0A preventive / predictive mainte-nance proceaures are certainly reasonable, but like the proposed surveil-lance program, tney are depenaent on back-EMF testing methods wnica are still in tne developmental stages.
Evaluation of moisture ingress corrective measures was cifficult due to the lack of information with whicn to understand the measures taken.
The licensee has committed to submit a more explanatory version of the actions to mitigate moisture ingress prior to restart.
Tne extent of PCRV tendon degradation is not well known, even if tne licensee may nave determined the cause of the corrosion. Further investi-gation into arresting measures is definitely required, especially because the nitrogen blanket technique may be so difficult to employ. However, the interim surveillance program should provide information on the cegree and rate of corrosion, in addition to establishing a tendon wire loss acceptance criteria. The tendon acceptance criteria snould ensure' PCRV margins to safety.
6.0 References
- 1. " Preliminary Report Related to the Restart and Continueo Operation of Fort St. Vrain Nuclear Generating Station," Docket No. 50-267, Public Service Co. of Colorado, October,1964.
- 2. " Review of Dallas Meeting (1/15/85) and Restart Committments",
letter from Martin, NRC/ Reg IV, to Lee, PSC,1/17/85.
3 " Fort St. Vrain Meeting, NRC-PSC, February 20, 1985," Volumes I, II and III, recorded and transcribed by Koenig & Patterson, Inc.
- 4. " Fort St. Vrain Meeting, NRC-PSC, February 21, 1985," Volumes I and II, recorded ano transcribed by Koenig & Patterson, Inc.
- 5. " Fort St. Vrain Meeting, NRC-PSC, February 22, 1985," Volumes I and II, recorded ano transcribed by Koenig & Patterson, Inc.
- 6. " Engineering Report on CRD.0A Failures to Scram-Control Rod Drive and Orifice Assemblies," PSC suomittal P-85037,1/31/85.
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- 8. " Bearing Deoris Analysis," PSC submittal P-85017,1/18/85.
- 9. " Fort St. Vrain Nuclear Generating Station, Upoated Final Safety Analysis Report," Public Service Co. of Colorado.
- 10. "CRDOA Refurbishment Program Report," PSC submittal P-85040-2, 1/31/d5.
- 11. " Control Rod Drive Cable Replacement," PSC submittal P-85032-2, 1/20/85.
- 12. " Control Rod Drive Cable Replacement Report," GA Technologies Document 907822, Attachment 1 to PSC submittal P-85032-2, 1/31/85.
13 " Investigations into Sources of Chloride in FSV Primary Circuit,"
PSC submittal P-86036,1/31/85
- 14. " Safety Analysis Report--Change in. Material of tne FSC Control Rod and Orifice Assemblies," Attachment 2 to PSC submittal P-85032-2, 1 /31/85.
15 "FSV Control Rod Cable Metallurgical Examinations," craft report from Los Alamos National Laboratory, 3/85
- 16. " Report on Reserve Snutdown AbsorDer Material," PSC submittal P-85027, 1/28/85 ,
17 " Moisture Control in CRDOA Purge Lines," PSC submittal P-65032-9, 1/20/85.
l l
L.
9 f
- 18. "FSV Reserve Snutoown System Material Metallurgical Examinations,"
draft report from Los Alamos National Laocratory, 3/85 19 "CRDOA Moisture / Purge Flow," PSC submittal P-85032-6,1/20/85
- 21. " Control Rod Drive Cavity Seals," PSC submittal P-85032-7,1/20/85.
- 22. "FSV CRD Cavity Seals Design Report," GA Tecnnologies Dccument 907604, Attachment 1 to PSC submittal P-obO32-7,1/20/85.
23 " Operations Order No. 84-17 Describing Operator Actiont Upon a Loss of Purge Flow and or Detection of Hign Moisture Levels in Primary Coolant," PSC submittal P-85040-8,1/31/85
- 25. " Current CED Temperature Data Collection Procedure Which Requires Station Manager Notification Upon Discovery of r. Measured CRD Temperature in Excess of 250*F," PSC submittal P-85040-9,1/31/85.
- 26. " Control Rod System Operaoility Evaluation Report," PSC suomittal P-85040-1, 1/31/85.
- 27. "CEDOA dechanism Temperatures Environmental Requalification," PoC submittal P-85032-1, 1/20/65
- 28. Letter from Robert A. Clark, Cnier, ORd3, to 0. R. Lee, PdCo.,
December 2, 1982.
29 heier, K., " Fort St. Vrain Reactor Control Roo Drive Mechanism Over-Temperature Problem," Los Alamos National Laboratory,1982.
- 30. "CEDOA Proposed Preventive / Predictive Maintenance Program Report,"
PSC submittal P-85040-3,1/31/o5
- 31. "CEDOA Interim Surveillance Program Report," PSC submittal P-85040-5, 1/31/85.
- 32. "fSV Improvement Committee Actions," PSC submittal P-85022,1/24/85.
33 " Tendon Accessibility Report," PSCo. letter from Warembourg, PSC, to Jonnson, NhC/ Reg IV, PSC submittal P-84523,12/14/d4.
- 34. " Lab Report No. 52--Examination of Failed Wires from Fort dt. Vrain Unit No. 1," PSC submittal P-o4543-4, 1/24/85.
35 "Lif toff Tests," Attacnment 1 to " Engineering Report on Fort St.
Vrain Tenoons," PSC submittal P-84543,12/31/d4. *
- 36. Thurgood, Roberts and Epstein, " Evaluation of tne Causes of Corrosion in the Fort St. Vrain Post-Tensioning. Tendon Wires," GA Tecnnologies, PSC submittal P-84543-5,1/24.85.
37 US/NRC Regulatory Guide, Rev. 2, January 1976.
- 38. Clauss, F. J., " Solid Lubricants ar.o Self Lubricating Solids",
Academic Press, 1972.
39 Fugelso, E. and Anderson, C., " Evaluation of Concrete Crrep and Stress' Redistribution in the Fort st. Vrain PCRV Following hupture of Prestressing Tendons", Los Alamos National Laboratory, Octooer 31, 1984 e
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