ML20132G502
| ML20132G502 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 03/12/1985 |
| From: | Bennett D, Fly G, Fugelso L LOS ALAMOS NATIONAL LABORATORY |
| To: | John Miller Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20132G456 | List: |
| References | |
| CON-FIN-A-7290 TAC-54639, TAC-55294, NUDOCS 8507190345 | |
| Download: ML20132G502 (30) | |
Text
S 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 Laboratory 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 anc Division J. R. M111er/ ORB 3 Prepared for the U.S. Nuclear Regulatory Commission Washington, D.C. 20555 e
8507190345 B50700 PDR ADOCK 05000267 S PDR u
DISCLAIMER
, Tnis report was prepared as an account of work sponsored by an agency of the United States Government. 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.
2 3 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 Surveil.'.ance 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 50 conclusions 6.0 References ,
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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 (CRDM) 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, J
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 additional plant
. visit in August, 1984, reviewed CRDS 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"1 , in October, 1984. Ine report concluded that Fort St. Vrain should not ce 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.the following, prior to restart:
- 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 refurbisnment of all CRDMs.
- b. The licensee must outline and commit to periodic inspection, preventive maintenance and surveillance programs for control rod drive mechanisms and associated position instrumentation.
A change in the Technical Specifications shall 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 the Reserve Shutdown System
. (RSS), to assure the full availability of the RSS, prior to restart. The licensee must outline and commit to periodic in-spection, preventive maintenance and surveillance programs for Reserve Shutdown System material. A change 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.
- d. The licensee should develop a procecure requiring reactor shut-down when high levels of moisture exist in the primary coolant, or when CRDM purge flow is lost.
- e. The licensee should implement a procedure for recording repre-sentative samples of CRDM 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.
- g. The licensee must develop a plan to implement any modifications 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 (PCRV) post-tensioning system, PSC must complete the following, as comfirmed by Reference 2, prior to rest, art:
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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 measures 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.
13 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 (deferences 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 surveillance.
2.1 Failure Mechanisms The failures of control rod pairs to scram, under various operating conditions, has been documented since 1982,6,7 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 l
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 CRDM Purge 1 1 6 1 7 2 5 1 1 2 3 Subheader #
High mois.ture 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 brage and dynamic braking, the reduction gear mechanism, 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 Motor Brake Malfunctions During a scram, the motor brake is de-energized and released, thereby-freeing the motor rotor sneft and gear train assembly to rotate under the torque applied by the weigtt of tne control rods. In the motor brake assembly, failure of the scram contactor to de-energize de power to the electromagnet was discounted because the operator had removed tne b'ake fuses following the CRDM failures to insert the control rod pairs.
According to the licensie, electromagnetic remanence and reduied spring constant in the brake spring plungers (due to elevated tempara-tures) were eliminated as possible failure mechanisms. Some corr >sion anc rust was identifiec on the brake disks of CRDOAs 25,18 and 29 How-ever, . the disks of a CRDM motor brake assembly with "discoloratica and whatever surface variations"3, p.149, could not be mace to stick in an elevated temperature helium environment with high moisture contert (test T-228). Tne licensee concluded that the motor brake was not instrumental in the failures to scram.
Los Alamos agrees with the licensee that the motor brake assesbly was probably not related to the CRDM failures.
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 gears or gear bearings reducing the gear train efficiency--i.e., the torque transmitted from the gear train to the motor rotor snart mignt have been insufficient to overcome the friction of tne motor bearings.
The licensee stated that no major damage has been identified on sey-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.
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 0125 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 bearinds, could reduce the efficiency of the reduction gear train, and thereby contribute to CRDM failures.
I 2.1 3 Motor and Motor Bearing Malfunctions l
During a scram, the motor is de-energized and does not directly con- l tribute to the scram process, even though it operates as an induction generator. However, because 16-20 inch-ounces of resisting torque on the
motor rotor snart can forestall scram,9 the friction from the motor
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bearings can be a significant contributor to the failure to scram. Pos-sible contributions to increase the friction include deDris in the bear-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
-in virtually all of the unrefurbished bearings examined", and minor race wear was identified. Reference 8 verified that the major debris constituents could be attributed to the motor bearing materials (wnich includes bearing balls, races, and other bearing components), whereas minor constituents were indicative of the motor itself. The analysis provided little evidence to support the theory that debris had been .
" washed" into the bearing races. The licensee also determined, because of the relatively close bearing tolerances and because rod weignt alone might not produce sufficient " crushing force" to deform bearing particu-late, that bearing operation could be reduced with the presence of par-ticulate matter. Tne licensee therefore concluded that 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 the motor bearings, caused by the presence of internally generated debris, 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 motor bearing races is ,not supported.
Los Alamos contends that tne loss of CRDM purge flow allowed primary coolant with high moisture content to enter tne CRDM cavity. An indepen-dent literature search indicates that toe dry film lubricant, molybdenum disulfide, MoS experiences an increase in its coefficient of fric-2e
' tion in the presence of moisture 38 . Therefore, the increased frictional coefficient of the lubricant on the motor bearings, MoS , may have also 2
contributed to the CRDM failures by resisting motor rotor shaft rotation.
2.2 Refurbishment Program Tne cause of tne failures to scram could be attributed to sev ral mechanisms suen 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 the dry film lubricant in the presence of moisture. Because the CRDM failure mechanism cannot be specifically delineated, and because of CRDM cable failures, the licensee has uncertaken a refurbishment program, in-volving the CRDM motors and reduction gear mechanisms, on all 37 CRDMs.
The licensee reported that the CRDM refurbishment process and a testing program will ensure the ability of the 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 Shutdown System material due to the discovery of material " bridging" during hopper discharge, ano to install seals around certain, penetrations into the CRDM cavity to mitigate the effects of primary coolant ingress by natural circulation.
2.2.1 _CRDOA Refurbishment The licensee has proposed complete refurbishment of all Control Rod Drive and Orificing Assemblies to ensure that tne CRDOAs will perform their intended safety functions, and to avoid potential operability prob-lems that could limit plant availability. As specified in Reference 10, the following major cooponents are to be inspected, tested, refurbished or replaced, as necessary:
- 1. Control Rod Drive (200) Assembly--shim motor and brake assembly, bearings, reduction gears, limit switches / potentiometers.
- 2. Orifice Control Mechanism--orifice control motor, bearings, potentiometer, gears, drive shaft and nut, drive shaft housing.
3 Control rod clevis bolts.
- 4. Reserve Shutdown System--boron balls, rupture disks, DP switch.
Design modifications include the replacement of control rod caoles, cable end fittings, and cable clevis bolts, the installation of new purge seals into tne CRDM cavity, and the installation of RTDs (Resistance Tem-perature Detectors) in all~ CRDOAs--the impact of tnese design cnanges will be evaluated later in tnis report.
Each CRD0A will undergo the following series of scram tests ib tne refurbishment process : a pre-refurbishment, in-core full scram test; a l
i pre-refurbishment full scram test in the Hot Service Facility (HSF); a l scram test with refurbished reduction gear mechanism and unrefurbished shim motor, using dummy weights; 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-refurbishment, 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 refurbisnLent, and should define the CRDM operating characteristics. From tne back-EMF 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.
The licensee has proposed a CRDOA refurbishment acceptance criterion, taking into account the results of the back-EMF voltage measurements and the resulting calculations of acceleration and torque such that :
- 1. Tne minimum calculated average torque during acceleration to peak velocity will be 17.0 inch-ounces; tnis value corresponds to an average acceleration to peak velocity of 98.83 radians /
second .
- 2. The maximum torque calculated during " steady-state" will be 7.0 inch-ounces.
According to the licensee, final acceptance of a refurbished CRDOA will be based upon the results of its in-core full scram test.
Los Alamos agrees with tne mechanical refurbishment of all CtlDOAs, as the program is currently being implemented by the licensee. In par-
.ticular, the replacement of snim motor bearings , pp.174-75 3
is con-sidered essential to the refurbishment process. However, the current program of mechanical refurbishment alone cannot ensure CRDOA
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operability.
~From the documentation presented by the licensee and reviewed earlier in this,section, Los Alamos believes that the proposed back-EMF testing and acceptance criteria have potential in providing a data base from which a control rod operability might be determined. But, an element of uncer-tainty, as to CRDOA operability based on back-EMF testing, is introduced A
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.
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 that more attention be paid to the initial, start-up scram characteristics of the CRDOA, 'in developing a better understanding of break-away torque effects. In line with Region IV's increased inspection of the jrefurbishment process, we suggest a review, by Region IV, of all testing results pertaining to CRDOA refurbishment acceptability, after in-core testing is complete, but prior to startup. As an additional metnod 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-tion be made as to the suitability and acceptability of back-EMF testing in defining CRDOA operability.
l 2.2.2 Con'brol Rod Cable Replacement In September, 1984, the control rod cable on CRDOA 25 was severed in i
several places during an investigation of a slack cable indication.
1 A subsequent metallurgical examination of the austenitic 347 stainless steel cable indicated that the cable surface was pitted and cracked, that
- j. the delta-like material cracks were typical of stress corrosion cracks, and that the fracture surfaces were brittle in nature. Further investi-gation revealed that the 347 Ss cable material was susceptible to stress corrosion when under the existing stressed conditions, and in the presence of chlorides and moisture.
Tne potential sources of the chlorides in the primary coolant' con-tributing to the chloride stress corrosion are reviewed in Reference 13 Tne licensee states that the chlorine occurs as two different species--hcl gas and a salt; the sources of the gas species include the fuel rods, H-327/H-451 graphite, PGX/HLM graphite and the 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, tne licensee elected to replace the control rod cable witn Inconel 625, wnien 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
- 1. Cable and rod portion Inconel 625--high strength of tne ball end and resistance to oxidation
- 2. Anchor, set screw Martensitic 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, which are classified as Class I, Safety Related and Safe Shutdown components, is included in Reference 14.
Los Alamos metallurgical analyses on a sample of the corroded control 0
rod cable 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-ments confirm the presence of chlorine on fracture surfaces. There fore ,
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
4 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 November,1964, 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 5 boron) was disonarged.
The licensee's e.xamination of the undischarged material revealed that the S4 C boronated graphite balls had " bridged" 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 the moisture had entered tne RSS 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, new 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 B 4C material, and will be installed in all RSS hoppers as part of the overall CRDOA refurbisnment program. No effort will be mace to use ART blended RSS material currently in stores , p.32 unless NRC is notified. Second, an expanded HSS material surveillance program, which will be incorporated into the Technical Specification, will test one 20 weight % boron hopper and one 40 weight 5 boron hopper during each refueling outage, and will include visual examinations for boric acid crystal formations, chemical analyses of RSS material for boron carDide and leacnable coron oxide con-tent. Tnied, efforts will be mace to mitigate or eliminate tne ingress
of moisture into the RSS hoppers by installing a knock-out pot, moisture
. elements, and a back-up helium source for the main CRDOA purge and Reserve Snutdown System purge lines.1I Each knock-out pot will be equipped with a' sight glass and a high level alarm in the Control Room.
Los~ Alamos concurs that the crystalline structures on the surface of the B4C RSS balls is meta-boric acid,10 most probably formed by mois-ture reacting with leachable boric oxide in the B C material. In light 4
of the new RSS material to be used, the increased surveillance efforts, and measures to mitigate the ingress of moisture in the RSS hoppers, Los Alamos agrees that the refurbished RSS should be able to reliably perform its function.
2.2.4 Purge Flow and Seal Replacement Just prior to the June 23 .1984 event when 6 of 37 control rod pairs
-failed to insert on a scram signal, a high moisture content in the primary coolant resulted in the loss of purge flow into tne CRDM cavities. The loss of purge flow may have allowed the additional ingress of moist pri-mary coolant into the CRDM cavities, resulting in mechanisms tnat may have contributed to tne CRDM failures. Because the exact CRDM failure mechanism has not been determined, and to alleviate the possibility of purge . flow loss and/or hign moisture content in the primary coolant con-tributing to future CEDM failures, the licensee has proposed several cor-rective measures 19 as part of the overall CRDOA refuroishment program.
To provide an accurate measure of the purge flow into the CRDM cavi- -
ties, tne licensee has proposed tnat new flow indicators with a :ange of 0-20 scfm be installed on each helium purge line, providing local indica-tion, remote indication in the Control Room, ano an alarm in the Control Room to indicate low flow conditions.20 A minimum of 8 bypass lines (one line serviced by each of tne 8 purge flow subheaaers) will be in-stalled prior to restart. Tne licensee intends to install the flow instrumentation ' M on tnese subheacers wnen tne devices are availaDie.
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As-mentioned in section 2.2.3, to recuce tne possiDility of moisture ingress into tne CRDM cavities via the helium purge lines, the lic,ensee I will install a knock-out pot, moisture elements and a back-up helium source for the main CBDOA purge and RSS purge lines, prior to criticality l
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4 following the fourth refueling outageN ' 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 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 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
To mitigate the ingress of primary coolant, wnich could contain moisture, into the CRDM cavity, seals will be installed on four large flow passages into tne CRDM cavity--the two passages in tne reserve shut-
[downtubeholes,andthetwopassagesovertheeyeboltsthatpenetrate the floor of the CRDM cavity.21 Cover plates with integral gaskets will also.be installed on the four access openings on the lower CRDM
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housing.. Thermal and mechanical analyses have determined that the seal acditions will not interfere with the RSS performance under tne in-fluence'or' mechanical,-thermal or seismic ioadings. Tne flow calculations in Reference 22 conclude tnat addition of tne mechanical seals to tne RsS pressure tubes and the lifting eyebolts will reduce naturally convective in&ress of primary coolant into the CRDM cavity from a flow rate of 0.68 acfm to less than 0.006 acfm. Additional calculations have confirmed that the seals are able to withstand both a design basis slow depressuri-zation transient and a design basis rapid depressurization transient.
The licensee has proposed a procedure in Reference 23 tnat basically requires reactor shutcown in the event CRDM 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 the CRDM cavities, in restricting the ingress of moisture into the CRD cavities via the purge
' lines, and in providing a back-up source of helium in case of purge flow loss. From the 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 CBD cavities tnrough penetrations.
In acdition, Los Alamos believes tnat the procedure requiring reactor shutdown with loss of purge flow or hign moisture levels in the primary
. coolant fulfills the requirements of the assessment report . Tne licensee defines "high moisture levels" in Reference 23 1
~2 3 CRDM Temperature Recording and CRDM Requalification Tne lack of direct measurements of CROM temperatures curing tne June 23 event, and during steady state and other cransient operating condi-tions, has prompted the installation of RTDs to monitor tne CRDM cavity closure plate (ambient), orifice valve motor plate and control roc crive motor temperatures. Strip'cnart recorders will continuously record tne tnree temperatures for each CRDM, and will provice a CnDM operating temperature data base. The old data collection surveillance procedure25 will be modified to collect data on a continuous basis.#' p.60 The licensee intends to install tne permanent recorders prior to 4, p.57 restart.
The licensee postulates in Reference 26 that "the maximum temperature rating of tne crive mecnanism which might innibit the scram function is 272'F", and in monitoring CRDM, temperatures "the maximum temperature ratind of 272*F should not be exceeded curing power operation".
The licensee has also proposed a CRDOA requalification testing pro-gram that is designec to establisn a temperature at which the CRDOA is qualified for operation.27 The helium test environment will be operated at 250*F, 260'F, 270*F, 200*F, 290'F and 300'F with a goal of qualifying all ChDOA components for 300*F operation. Results of tne requalification testing are anticipated by tne end of 1965 Los Alamos adrees tnat the placement of CnDOA tnermocouples, and tne continuous cata monitoring at all operating conditions is sufficient to provide a CRDOA temperature data case curing steady state and transient operating conditions.
In adcition, Los Alamos believes tnat tne CRDOA is currently only qualified to operate up to 215'F based on the original mechanical CRDOA qualification tests, an KRC recommendation,28 and previous Los Alamos calculations. 9 Ine licensee's argument tnat tne CRDOA is qualified for 272*F operation cased on analytical calculations4 , p.49 is not sub-stantiated. Tnerefore, Los Alamos recommends tnat CRDOA operation be limited to 215'F until mechanical requalification supports a higher oper-ating temperature.
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2.4 CRDM Surveillance and Preventive / Predictive Maintenance The licensee has proposed a set or preventive / predictive mainte-s nance tests and surveillance inspection procedu'res that are intended to monitor the performance of the CRDOAs and to determine tne overall opera-bility of the CRDOAs during reactor operation. Initial development of these operating tests are considered part of the CRDOA refurbishment pro-gram,.and will utilize the data base and resultant trends formulated dur-ing refuroishment.
2.4.1 CRDM Preventive / Predictive Maintenance Tne licensee's CRDOA preventive / predictive maintenance program is proposed in Reference 30. According to the licensee, the normal preven-tive maintenance (PM) program will be implemented on a refueling basis
, rotational cycle for CRDOAs that would normally be removed for refueling, unless the predictive maintenance (PDM) program indicates tne need for more frequent maintenance. The PM program woula emphasize the meenanical examination and refurbishment of the anim motor / brake assemoly, the drive train, control rod cable,' reserve shutdown system, position potentiom-eters, limit switenes, orifice drive motor assecoly, orifice drive lead screw, assorted seals, valves, electrical components, bolts anc the ao-sorcer string.
On the otner hano, the predictive maintenance techniques would oe used to monitor the most important aspect of CRD0A performance--the
" scram capability"--Dy determining the shim motor /orake and gear train performance . The tests proposed in the PDM program include watta6e requirements, bacx-EMF voltages, delivered torque at tne motors, scram times, rod drop rates and torques to rotate motor / brake assemblies. Cer-tain aspects of 'the PDM program would be implemented on a weekly 6 asis to determine scram capability and temperature performance during power oper-ation. The licensee has also proposec that testing information be acquired during reactor snutdown for trending purposes.
Los alamos concurs with the proposec preventive maintenance pro-gram as outlined by the licensee, on the assumption that cata acquired during reactor operation will show that predictive maintenance tecnniques can be used to detect a reduction in CRD0A performance. TnePDMtIstin e tecnniques are closely linkea to tne techniques tnat are being usec for
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p the acceptance criteria in the refurbisnment program, and will tnerefore be dependent on the suitability and acceptability of back-EMF testing for
-determining CROOA operability, as discussed in section 2.2.1.
2.4.2 CRDM Interim Operational Surveillance Tne licensee's CRDOA interim surveillance program is proposed in Reference 31. The surveillance tests are scheduled on a weekly basis, a- using a 10" rod drop method on all withdrawn and partially inserted con-trol. rods, except the regulating rod.5, pg 82 The surveillance tests will~ obtain data for analysis and long term trending, exercise the rod, test selected circuitry, verify FSAR (Final Safety Analysis Report)9 assumed scram times, and confirm control rod operaoility. In addition, ChDOA teuperature and purge flow information will be collected.
For a fully withdrawn rod, analog and digital position information will be obtained, " Rod-Out" lights will be verified on,
" Rod-In" and " Slack Cable" lights will be verified off, anc tne rod will be dropped approximately 10" by de-energizing the brake, while back-EMF data are obtainea for future trending. Tne " Rod-Out" light indication will be verified off, and analog and digital information will be compared, witn an acceptaDie deviation of 10 inches between position indications.
The rod will then be withdrawn to the full out position, so that analod 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 roc position limit switches. Refueling snutdown surveillance will acquire the same information as the weex1y and quarterly tests, except full stroke insertion tests will be performed.
The operaoility acceptance criteria, according to the licensee, will be based on distance and time rod drop data used to calculate a conserva-tive average full lengtn scram time. A CdDOA will be considered inoper-able if it does not meet the maximum scram time of 160 seconds as defined in the FSAR9 . Such an indication.would warrant back-EMF testing in jonfirming scram operability.
Los Alamos agrees that the basic surveillance methodology is suffi-cient to exercise tne control rod, verify FSAR scram times, and to test selected circuitry. However, references to 272 F as the maximum CRDOA operating temperature are still considered inappropriate as ciscussed in section 2 3, and a lo inch deviation is not considered acceptable between digital and analog position indications--such a deviation coula inadver-tently lead to control rod overdrive through a misinterpretation of rod position. Also,' tne bacx-EMF testing methods and interpretation of the results are still in the developmental stages, and an engineering deter-mination of tne suitaDility and acceptability of tnis testing methoc in determining ~ continued ChDOA operability will need to be mace Defore the licensee can finalize tnis portion of tne surveillance program.
30 Moisture Ingress Issues The licensee nas submitted 32 a listing of tne issues considered, and actions taken, by the FSV Improvement Committee (formerly the FSV Moisture Ingress Committee) in significantly reducing the frequency and severity of moisture ingress events. The issues were dividec into four categories:
- 1. Issues currently under consideration by tne Fort St. Vrain Im-provement Committea.
- 2. Circulator Auxiliary System mocifications yet to be completed prior to startup.
3 Circulator Auxiliary System modifications to be completed prior
_to startup, provided material availability and schecule permits.
- 4. Items ideitified by tne Moisture Ingress Committee wnicn are installed and operational.
Los Alamos believes that a listing of intenced and installed mocifi-cations does not provide any indication as to what any given mooification really is, wny tney contribute to tne reduction in potential for moisture ingress events, nor which improvements will substantially recuce tne severity anc frequency of moisture ingress events. Tne licensee nas com-mittea to submit a more explanatory version of tne actions to mitigate e p6 80 moisture ingress, prior to restart ,
4.0 PCRV Post-Tensioning Tendon System .
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In tne spring of 1984, during scheduled PCRV tendon surveillance, tendons with corroded and broken wires were found. Since that time, the
licensee has evaluated the corrosion mecnanism, has performed lift-off tests on selected tendons to determine their load-carrying capability, and proposed corrective actions and an increased surveillance-procedures.
4.1 Tendon Accessibility, Extent of Known Degradation anc Failure Mechanism The licensee, in determinin6 the extent of tendon corrosion in tne PCRV, determined what fraction of the tencons were available for visual examination and lift-off tests. The tendon system is subdivided into four major groups: the 90 longitudinal (vertical) tendons have 169 wires per tendon; tne 210 circumferential tendons in tne PCRV sidewall have 152 wires.per tendon, and tne 50 c.ircumferential tendons in both tne top ano bottom heads have 169 wires per tendon; tne 24 bottom cross-heaa tendons, anc 24 top cross-head tendons nave 169 wires per tendon. Of tne four groups, tne licensee states the following accessibility 33 Tendon Group both Ends Acces. One End Acces. Neither End Acces .
Longitbdinal Visual 20 69 1 Lif t-o ff 0 74 16 Circumferential Visual 2dl 27 2 Lif t-off 236 62 12 Bottom cross-heac Visual 20 4 0 Lif t-off 16 4 4 Top cross-nead Visual 17 7 0 Lif t-off 16 6 2 Tne numoer of tendons witn gnewn broxen wires as identified in fue licensee's 19o4 surveillance,3" includea 10 longitudinal tendons witn 1 to 22 broken wires, 2 circumferential tendons witn 2 and 15 broxen wires, 8 bottom cross-nead tencons with 1 to 19 broxen wires, and no top cc,oss-head tendons with Droxen wires. In some cases, tne total number of cor-roced, broken wires incluce wires brcken during lift-off tests, or during retensioning.
The results of 74 longitudinal lift-off tests 35 indicated tnat tendons with identified broken wires generally had a slightly smaller lift-off value than intact tendons. Thirty lift-off tests on circunfer-ential tendons -snowed little change in lift-off value. Some of tne fir-teen bottom cross-head tendon lift-off tests showed a definite reduction '
in lift-off value for tendons with multiple wire breaks. Ine value of tne lift-off test on one top cross-head tendon was nominal. All lift-off test values exceedeo tne minimum limits.
The licensee conducted metallurgical investi6ations into the cause of the corrosion, and determined that microbiological attacx on tne tendon NO-0X-ID CM 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 case, usually toward the tendon ends. Tne acidic attack resultec in re-cuced cross-sectional wire area, stress corrosion cracking, localized
-tensile overload ana wire breakage.
Los Alamos believes, based on the documentation presented by the licensee, that microbiological attack of the tendon grease and the resul-tant formation of acetic and formic acids, in the presence of moisture, is a probable cause for the currently observed tendon corrosion, and has
, . led to the subsequent wire breakage througn tensile overload. However, Los Alamos believes tnat the extent of known tendon corrosion, breakage and previous surye111ance have not been clearly defined by the licensee.
Los Alamos tnerefore reco=cends that a complete map oe mace tnat lists each tencon, its visual examinations and lift-off values, and tne number anc location of corroded anc broken wires. An indication of tne degree of wire corrosion would also be desiracle.
4.2 Tendon Corrosion Corrective Measures The licensee evaluated several methods for arresting tne corrosion process,34,36 including the use of ozone as a biocide to kill the micro-organisms, the use of an alkaline grease wnich should not be conducive to microbiological growth, and tne use of an inert blanket consisting of nitrogen gas. The licensee's consultants found that the nitrogen too-sphere arrested the growth of the microbes in the h0-0X-ID CM organic
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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 snat nitrogen blankets ce establisnea on the longitudinal and bottom cross-nead tencons. Long term actions would include further investigations into tne corrosion process and ar--
resting techniques, and the possible installation of accitional load cells in monitoring tne 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-fled 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 reduction. Further investigation into the long term effects of a nitrogen blanket on tendons, tne corrosion process and currently available corrosion acids are also recommended.
43 PCRV Tencon Interin Surveillance Because the total extent of tendon corrosion in tne PCRV is unknown, because the rate of existing corrosion is unxnown, and because tne use of a nitrogen blanket as an arrest to tne corrosion process is an unxnown, the licensee has proposed an interim surveillance program designed to address eacn of tnese issues.5, pp.164-7. The interim tendon surveil-lance program would incluce increased visual and lift-off surveillance for tnree years, or until effective corrosion control nas been estao-lished. Two populations of tendons would be inspected--a population of tendons that have not been previously identifieo as being corroced, ano a control population with known corrosion. On a six-month frequency, visual surveillance of both tendon ends, when accessible, woulo include:
c Tendon Group' No. of New Tendons No. of Control Tencons Longitudinal 24 6 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:
Tendon 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 proposec tnat, based on vis-ual examinations, a mandatory engineering evaluation be conducted on any 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 10% of its wires broken. Ine control tendon population will include those tendons with tne worst known corrosion with reacy accessibility.
Los Alamos agrees that the increased tendon surveillance program of the nature proposed by the licensee will provide more information on tne extent of corrosion in the PCRV by inspecting new tendons eacn surveil-lance, and at the same time, monitor tne rate of corrosion with the con-trol tendon population. Tne increased surveillance snould also determine .
the effectiveness of the nitrogen blanket in arresting corrosion, or any other corrective measure the licensee may propose. Los Alamos recommends that the licensee submit an outline of tne 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, anc could use US/NRC Regulatory Guide 1 35 37 for guidance. <
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 numer1 cal study 39 was undertaken by Los Alamos National '
Laboratory to evaluate the evolution of these stresses,' both -to the ini-tial prestressing and to subsequent partial and total rupture of tnese
, tendons.. At the stress levels anticipated in tne concrete, ano for.the ,
anticipated operating life span of tne PCRV, the concrete benavior was modeled as a linear viscoelastic solid with the creep strain varying pro--
portionally witn the logarithm of time at constant stress tnrougnout tne I projected reactor lifetime.
. A one-dimensional mocel of a long 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 tne degraded tendons. The cegree of tendon degradation is oescribed tnrou6h the ratio of the number of unbroken strands to tne original numoer of s trancs . Initial time of rupture was varied from the time of initial prestressing to 400 days after emplacement. Tne form 01ation 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 strancs), the hold-down force still exceeded the minimum safety design requirements.
In adcition, 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, together 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 tend,ons were prestressed to 70) of the ultimate and an internal pressure of 775 psi was applied (tnis pressure is the internal pressure,of tne ,
nelium coolant in tne HTGR) and the creep of the concrete and slow decay of tne tencon stresses were evaluated out to 30,000 days. Tnen, tnree cases wherein one tendon was removed at one day were evaluated. First 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 micheight was removed. Stress redistributions at 300_ days after ruptures were calculated and snitts of 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 tne inner wall to about -2200 psi at the same place. Removal of- a vertical tendon reouced 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 tne analytical solution and a small finite element proolem 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 nold-down forces was ootained.
50 conclusions Los Alamos concludes.that tne licensee, Public Service Co. of Colorado, has made a conscientious effort to aadress all of the restart issues listed in the assessment report. The refurbishment program on all CRLOAs provides confidence in CRDOA operability during reactor opera-tion and the ability to scram, even if the exact " failure to scram" meen-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 i development, more experience with result interpretation, and in-core i l
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i 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, along 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 purde flow loss have not been determined to be in-strumental in CRDOA 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 CRDOA 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 necnanical standpoint, CHDOA preventive / predictive mainte-nance procecures are certainly reasonable, but like the proposed surveil-lance program, tney are depencent 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 Re ferences
- 1. " Preliminary Report Related to the Restart and Continueo Operation of Fort St. Vrain Nuclear Generating Station," Docxet 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 and transcribed by Koenig & Patterson, Inc.
- 5. " Fort St. Vrain Meeting, NRC-PSC, February 22, 1985," Volumes I and <
II, recordeo ano transcribed by Koenig & Patterson, Inc.
t .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.
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- 9. " Fort St. Vrain Nuclear Generating Station, Upcated Final Safety Analysis Report," Public Service Co. of Colorado.
- 10. "CRDOA RefurDishment Program Report," PSC submittal P-85040-2, 1/31/85
- 11. " Control Rod Drive Cable Replacement," PSC submittal P-85032-2, 1/20/85. .
- 12. " Control Rod Drive Cable Replacement Report," GA Technologies Document 907622, . 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," draft report from Los Alamos National Laboratory, 3/85
- 16. " Report on Reserve Snutdcun Absoroer Material," PSC submittal P-85027, 1/28/85. ,
17 " Moisture Control in CRDOA Purge Lines," PSC submittal P-65032-9, 1/20/85.
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- 18. "FSV Reserve Snutoown System Material Metallurgical Examinations,"
draft report from Los Alamos National Laooratory, 3/65 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 Document 907604, Attachment 1 to PSC submittal P-d5032-7,1/20/85.
23 " Operations Order No. 84-17 Describing Operator Actions 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 CRD Temperature Data Collection Procedure Wnich Requires Station Manager Notification Upon Discovery of a Measureo 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. "CRD0A dechanism Temperatures Environmental Requalification," pac submittal P-85032-1, 1/20/65
- 28. Letter from Robert A. Clark, Cnier, ORd3, to O. R. Lee, PdCo.,
December 2, 1982.
29 heier, K.,
" Fort St. Vrain Reactor Control Roo Drive Meenanism Over-Temperature Problem," Los Alamos National Laboratory,1982.
- 30. "CHDOA Proposed Preventive / Predictive Maintenance Program Report,"
PSC submittal P-85040-3, 1/31/o5
- 31. "CHLOA 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 suomittal P-84523, 12/14/84
- 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 " Liftoff Tests," Attacnment 1 to " Engineering Report on Fort St.
Vrain Tencons," PSC submittal P-84543,12/31/84.
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- 36. Thurgood, Roberts and Epstein, " Evaluation of tne Causes of Corrosion in the Fort St. Vrain Post-Tensioning Tendon Wires," GA Tecnnologies, f PSC submittal P-84543-5,1/24.85 37 Us/NRC Regulatory Guide, Rev. 2, January 1976.
- 38. Clauss, F. J., " Solid Lubricants anc Self Lubricating Solids",
Academic Press, 1972.
-39 Fugelso, E. and Anderson, C., " Evaluation of Concrete Crrep anc Stress Redistribution in the Fort st. Vrain PCRV Following Rupture-of Prestressing Tendons", Los Alamos National Laboratory, Octooer 31, 1984 0
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