ML20132C975

Safety Evaluation Re CRD Mechanisms & Reserve Shutdown Sys. Unresolved Issues Noted

ML20132C975
Person / Time
Site:	Fort Saint Vrain
Issue date:	05/21/1985
From:	Office of Nuclear Reactor Regulation
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ML20132C967	List: ML20132C964 ML20132C969 ML20132C975 ML20132C978
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NUDOCS 8507310104
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' ' Attachment 1 UNITED STATES

' f( S NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20655 5 "[

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION FORT ST. VRAIN NUCLEAR GENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO CONTROL ROD DRIVE MECHANISMS AND RESERVE SHUTDOWN SYSTEM Docket No. 50-267 1.0 Introduction and Background On June 23, 1984, a failure occurred at the Fort St. Vrain Station (FSV) in which 6 of 37 control rod pairs failed to insert on the receipt of a scram signal. As a result of this incident and other problems at FSV, the Director of the Office of Nuclear Reactor Regulation asked his staff to assess several aspects of operations at FSV. The resulting assessment (Reference 1) defined actions required of the licensee before the plant could be restarted. It also defined longer term issues for the licensee to address.

Subsequent to the June 23, 1984 event, on November 5, 1984, a reserve shutdown hopper failed to fully discharge during a routine Surveillance Test. Further meetings were held with the licensee on November 28 and 29, 1984 and on January 15, 1985. During the January 15 meeting, additional commitments made by the licensee and requirements for restart were clarified (References 2 and 3). Key commitments made by the licensee included refurbishment of all control rod drive mechanisms (CRDMs) and replacement of all reserve shutdown system (RSS) material.

In addition, the licensee provided a number of submittals in response to the restart issues addressed in the assessment report. These submittals I

are referenced in the Technical Evaluation Report (TER) (Reference 4),

which is enclosed with this evaluation. The TER was prepared by Los Alamos National Laboratory under contract to the NRC.

This evaluation has been perfonned by the Office of Nuclear Reactor Regulation in partial fulfillment of Item 1 of Technical Interface Agreement (TIA) 85-01 dated January 2,1985. This TIA was formulated in response to a request from Region IV (Reference 5). The key issues covered in this evaluation are:

- Control Rod Drive Mechanism Failures

- CRDM Refurbishment Program

- CRDM Temperature Recording and Requalification

- CRDM Surveillance and Preventative Maintenance i 2.0 Evaluation of Individual Issues Restart of the Fort St. Vrain Nuclear Generating Station is dependent on satisfactory evaluation of several major issues identified in the assessment report (Reference 1). The following evaluations are based on the enclosed TER (Reference 4).

8507310104 850712 PDR ADOCK 05000267 PDR l _ _

2.1 Control Rod Drive Failure Mechanisms (Reference 4-Section2.1)

Failure of the control rods to insert on a scram signal has been attributed to two mechanisms. The first failure mechanism is the accumulation of debris in both the CRDM motor bearings and gear mechanism. This debris could eventually bind the mechanical drive and prevent control rod insertion. The second failure mechanism is the potential effect of temperature and moisture on the CRDMs. Loss of CRDM purge flow can result in the CRDM binding due to differential expansion effects as well as a change in properties of the molybdenum disulfide lubricant.

Since the failure mechanism (s) has not been definitely identified, the licensee has proposed multiple measures in response to each failure mechanism. First, each of the 37 CRDMs will be completely refurbished prior to restart.

Second, the temperature of the CRDMs will be monitored. Third, backup source of purge flow capable of two hours of operation will be installed. Fourth, the licensee will institute a program to qualify the CRDMs for a temperature of 300 F.

We find that the measures proposed by the licensee address each of the proposed failure mechanisms. However, we conclude that further work must be done to assure that the CRDMs are operable over the remaining plant life. These long tenn issues will be discussed in Sections 2.3 l and 2.4. We conclude the work to establish the CRDM failure mechanism is adequate and an acceptable basis for plant restart.

2.2 CRDM Refurbishment Program (Reference 4 - Section 2.2)

In view of the mechanical deterioration of the CRDM motor bearings and gear trains, the licensee has elected to perfonn a complete refurbishment of the CRDMs. Additionally, since the licensee also found deterioration in the CRDM cables and the Reserve Shutdown System (RSS) material, he elected to replace these items with new materials.

CRDM Refurbishment The licensee's program for refurbishment of the CRDMs involves inspection, testing, refurbishment or replacement of all major components. We find l that the refurbishment process is thorough and represents an effort to restore the CRDMs as closely as possible to as-new conditions.

1 Furthennore, the licensee has proposed a testing program to establish the acceptability of the CRDMs following refurbishment. This program involves a multiple step testing program, with eventual tests in the reactor core. Certain aspects of this testing procedure are still in development and cannot be reviewed at this time.

l We conclude that the licensee's mechanical refurbishment program is l

adequate and provides an acceptable basis to support plant restart.

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t However, we also conclude that an improved testing and surveillance program should be formulated to assure continued CRDM operability during plant operation. We have discussed these long term items in Sections 2.3 and 2.4.

Other Refurbishment Items The licensee has elected to replace the control rod cables which were found to be failing from stress corrosion. The licensee has selected different materials for portions of this refurbishment to improve the resistance of these materials to future stress corrosion failures. We have reviewed the replacement materials proposed by the licensee and found them acceptable.

We recommend continued investigation by the licensee of the sources of chlorine in the reactor and its potential effects on other reactor components.

The licensee has also elected to completely replace the materials contained in the RSS hoppers. The replacement was in response to the failure of a hopper to discharge due to formation of boric acid crystals on the RSS material.

The licensee selected a new material with inproved properties to reduce the likelihood of future boric acid crystals. The licensee has also taken measures to reduce the possibility of moisturr: ingress into this system.

i We have reviewed the licensee's program for replacement of the control rod cables and the RSS materials and found it acceptable.

! 2.3 CRDM Temperature Recording and Requalification (Reference 4 - Section 2.3) l The licensee has proposed to upgrade the CRDM temperature measuring systems to provide continuous records. He intends to monitor weekly the CRDM temperature, in all operating conditions. We have reviewed this monitoring program and find it is acceptable for steady state operation.

However, we recommend it be supplemented with a transient monitoring program to provide an increased monitoring frequency during transient events that could lead to high CRDM temperatures.

The licensee has proposed that the CRDMs are currently qualified to 272*F, and reconnends plant operation with an administrative temperature limit of 250 F. In addition, the licensee is pursuing a program to requalify the CRDMs to temperatures of 300*F. Our evaluation of the CRDM qualification based on test data only supports an average operating temperature of 215'F. The licensee has indicated that the 215'F temperature limit would impact on plant operation.

We conclude that pending the receipt of new qualification data, the temperature of the CRDMs be limited to 215 F as an administrative limit. In the event that 215'F is exceeded, continuous monitoring of the affected CRDM must be initiated and the results reported to the NRC on a monthly basis. The CRDMs, until requalified, should not be operated at higher than 250*F. We require that the licensee provide a commitment to operate within these temperature limits until further i requalification test data is available or provide another method of assuring CRDM operability. These temperature limits cannot be changed l

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without NRC approval of new temperature limits or alternative methods of assuring CRDM operability.

2.4 CRDM Surveillance and Preventative Maintenance (Reference 4-Section2.4)

The licensee has proposed a program of surveillance and preventative maintenance to assure continued operability of the CRDMs and RSS during plant operation. The surveillance program is an interim program, based mainly on weekly ten inch control rod drops. Our evaluation of this surveillance program is that it may not provide the detailed information needed to predict potential failures of the CRDMs to insert on a scram ,

signal over the long tem. The licensee is cognizant of the limitations of the current tests, and is developing more thorough surveillance tests.

We conclude that the licensee's proposed interim surveillance procedures provide an acceptable basis for plant restart. However, we require the licensee to submit an improved (long tem) surveillance program within six months of plant restart for NRC review.

The licensee has also proposed a preventative maintenance program for the CRDMs and RSS. Certain of these components are nomally removed from the reactor for refueling. At each refueling, the systems and their com-ponents would be thoroughly inspected and refurbished as needed. Additionally, as more sensitive surveillance tests are developed, surveillance data could be used to detemine additional candidates for preventative maintenance activities. We conclude that the licensee's preventative maintenance program is an acceptable basis for plant restart. However, the licensee must provide information on the interaction between improved surveillance program (noted above) and the preventative maintenance activities. This infomation should be provided with the revised surveillance program.

3.0 Conclusions on Restart Issues l

We have evaluated the licensee's proposed programs to address the issues covered by this evaluation and related commitments made to the NRC (References 2 and 3). We find that these issues, as discussed in Section 2.0, have been adequately resolved to allow plant restart with the exception of the following:

1. The licensee must provide a comitment to operate the plant within .

the CRDM temperature limits accepted by the NRC. These limits cannot be changed without NRC approval of new temperature limits or alternative methods of assuring CRDM operability.

2. The licensee must provide a comitment to submit an improved CRDM surveillance and preventative maintenance program within six months of plant restart.

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These open issues should be resolved prior to plant startup.

4.0 Long Term Issues Continued plant operation beyond one refueling cycle should be contingent on resolution of the longer term items identified in the Assessment Report of October 16, 1984 (Reference 1). We request that the licensee commit prior to restart to resolution of these long term items as outlined in the Assessment Report (Page viii).

Enclosure:

LANL TER Date: May 21, 1985 Principal Contributor:

K. Heitncr 4

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REFERENCES

1. TIA 85-01 dated January 2, 1985

2. Memorandum from J. R. Miller and E. Johnson to F. Miraglia dated February 5, 1985

3. Preliminary Report Relating to the Restart and Continued Operation of Fort St. Vrain Nuclear Generating Station, Docket No. 50-267, October 16, 1984

4. Evaluation of Control Rod Drive Mechanism and Reserve Shutdown System Failures and PCRV Tendon Degradation Issues Prior to Fort. St. Vrain Restart. LANL FIN No. A-7290, March 12, 1985 (Enclosed) l l

E Attachment 2 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 Besponsible NRC Individual and Division ,

J. R. M111er/0RB3 Prepared for the U.S. Nuclear Regulatory Commission Washington, D.C. 20555

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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 thereof, 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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P Table of Contents

1.0 Background

1.1 Assessment Report Restart Issues 1.2 PCRV Tendon Restart Issues 13 Purpose of the Technical Evaluation 2.0 Control Rod Drive and Orifice Assecblies 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 50 Conclusions 6.0 References I

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 Regulatory 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 the continued moisture ingress problem. An additional plant visit in August, 1984, reviewed CRDM instrumentation anomalies.

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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 j

Nuclear Generating Station" , in October,1984. Tne 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 beramsignal. More specifically, and as included in this technical eval-uation, Reference 1 required Public Service Co. of Colorado (PSC) to com- 7 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 acasures to provide assurance of control rod reliability, which could reasonably include refurbisnment of all CRDMs.

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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 should define and confirm the operability of the Reserve Snutdown System.

d. The licensee should develop a proceoure 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 confirmed by Reference 2, prior to restart:

a. 'The licensee should submit documentation evaluating the mechan-ise(s) causing corrosion on and failure of the PCRV tendon wires, and corrective 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.

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 (seferences given as used in this document), and'the meeting between the licensee and NRC at the FSV plant site on February 20-22, 1965, as transcribed in References 3, 4 and 5.

2.0 Control Rod Drive and Orifice Assemblies This section includes a review of CHDH failure mechanisms, Control Rod Drive and Orifice Assemblies (CRD0A) refurbishment, CRDH 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, ' and are as noted in Table 1 by region, CRDOA number and CRDM purge flow subheader (total of 8 purge flow subheaders).

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 #

i High 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 4 descriptions and operating cnaracteristics of the drive motor, friction brake and dynamic braking, the reduction gear mechanism, the cable drum l and cable, and the bearing lubricant are provided in Reference 6. Tne licensee reviewed those CRDM components that could have caused the fail-l 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 shaft and gear train assembly to rotate under the i

torque applied by the weight of the control rods. In the motor brake j assembly, failure of the scram contactor to de-energize de power to the electromagnet was discounted because the operator had removed the brake

! fuses following the CRDM 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 rust was identified on the brake disks of CRDOAs 25,18 and 29. How-l l ever, the disks of a CRDM 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 soisture content (test l T-228). Tne licensee concluded that the motor brake was not instrumental in the failures to scram.

l Los Alamos agrees with the licensee that the motor brake assembly l

was probably not related to the CRDM 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 gears or gear bearings reducing the gear train efficiency--i.e., the torque transmitted from the gear train to the motor rotor shaft might 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, which 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 l

reduced gear train efficiency when sensitivity studies indicated that the motor bearings were only three times more sensitive to debris than the first pinion gear mesh 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, f especially in the first pinion gear mesh and the gear bearings, could reduce the efficiency of the reduction gear train, and thereby contribute to CRDH failures.

2.1 3 Motor and Motor Bearing Halfunctions 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 l

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motor rotor shaft can forestall scram,9 the frictiori from ch's motor

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bearings can be a significant contributor to the failure to scrtna. Pos-sible contributi6ns to increase the friction include debris in th3 bear-ing race, wear on the bearing ball or race, and changes in the

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properties during adverse conditions.,

1he licensee . reported that debris was observed in the bearing races of CRDOAs 7,18 and 4k " 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 tnat the major debris constituents could be sti.ributed to the motor bearing materials (which includes bearing ballo, races, and other bearing components), whereas minor constituents were andicative of the motor itself. The an'alysis provided little evidence. to support the theory that debris had been

" washed" into the bearing ' races. Trae licensee also determinsd, because of the relatively close bearing fiolerances and because rod weight alone

" might not produce cufficient-" crushing force" to deform bearing particu-late, that bearing operation could be: reduced with the prese.nce 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 t'nat increased friction in the motor bearings, caused by the presence of internally generated debris, ,

-l could have been a likely contributor to the failurea to scram. les Alamos also agrees with the licensee _.that the bash in" theory of debris into

! the motor bearing races is not supported;

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IAs Alamos contends that the loss of CRDM purge flow allowed primary ~ ,

coolant with high moisture content to enter the CRDM cavity. An indepen-dent literature search indicates that tne dry film lubricant, molybdenum disulfide, MoS2 , experiences an increase in its coefficient of fric-tion in the presence of moisture 38 Therefore, the increased frictional coefficient of the lubricant on the motor bearings, MoS p, may have also contributed to the CRDM failures by resisting motor rotor shaft rotation.

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2.2 Refurbishment Program '

The cause of the failures to scram could be attributed to several mechanisms such as reduced reduction gear train efficiency, internally l -

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9 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 CRDH failure mechanism 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 CRDMs.

The licensee reported that the CRDH 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, ana 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 the CRDOAs will perform i

their intended safety functions, and to avoid potential operability prob-lems that could limit plant availability. As specified in Reference 10, i the following major components 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,

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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 CRDH cavity, and the installation of RTDs (Resistance Tem-perature Detectors) in all CRD0As--the impact of tnese design onanges will be evaluated later in this report.

I Each CRDOA will undergo the following series of scram tests in the refurbishment process : a pre-refurbishment, in-core full scram test; a

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 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 ano after refurbisnment, and should define the CRDM operating characteristics. From the 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 CRD0A 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 the mechanical refurbishment of all CRDOAs, as the program is currently being implemented by the licensee. In par-ticular, the replacement of shim motor bearings3 , pp.174-75 is con-sidered essential to the refurbishment process. However, the current program of mechanical refurbishment alone cannot ensure CRDOA 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 control rod operability might be determined. But, an element of uncer-tainty, as to CRDOA optrability 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.

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

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of the refurbishment 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 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-tion be made as to the suitability and acceptability of back-EMF testing in defining CRDOA operability.

2.2.2 Control Rod Cable Replacement In September, 1984, the control rod cable on CRDOA 25 was severed in several places during an investigation of a slack cable indication. -

A subsequent metallurgical examination of the austenitic 347 stainless steel cable indicated that the cable surface was pitted and cracked, that 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.

The 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

e 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 tne sources of the salt species include the ceramic insulation, concrete and water, all to varying degrees.

As part of tne overall CRDOA refuroishment 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 5

rod cable also indicate pitting on the cable surface, ductile and i brittle fracture surfaces, and to a lesser degree than the licensee, cracking indicative of stress corrosion cracking. Qualitative measure-l ments 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 I strands of the Los Alamos sample had a " shaved" appearance, and was i

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identified as a 7000 series aluminam 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,1904, 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 disenarged.

The licensee's examination of the undischarged material revealed that the by C boronated graphite balls had "oridged" together through a crystal-line structure on the ball surfaces. Analyses on the 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 RSS hopper through the CRD0A 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 C material, 4

and will be installed in all RSS hoppers as part of the overall CRDOA refurbisnment program. No effort will be maae to use ART biended 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 5 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 caroide and leachable Doron oxide con-tent. Tnird, 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. 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 B C RSS balls is meta-boric acid,1 most probably formed by mois-4 ture reacting with leachable boric oxide in the B 4C material. In light of the new RSS material to be used, the increased surveillance efforts ,

and measures 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.

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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 CdDM 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 contriouted 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 tne CRDM cavi-ties, the licensee has proposed tnat new flow indicators witn a range of 0-20 scfm be installeo 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 oy eacn of tne 8 purge flow subheaaers) will be in-stalled prior to restart. Tne licensee intends to install the flow instrumentation , ps 3-4 on tnese subheaaers wnen tne devices are availaDle.

As mentioned in section 2.2 3, to recuce tne poss1D111ty of moisture ingress into tne CRDM cavities via the helium purge lines, the licensee will install a knock-out pot, moisture elements and a back-up helium source for the main ChDOA purge and RSS purge lines, prior to criticality

?

i

e pg 5 The knock-out pots will following the fourth refueling outage 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 heliun 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.

To mitigate the in6ress of primary coolant, wnich could contain moisture, into the CRDM cavity, seals will be installed on four lar6e flow passages into tne CRDM cavity--the two passages in tne reserve shut-down tube holes, and the two passages over the eye bolts that penetrate the floor of tne CRUM cavity. Cover plates with intee ral gaskets will also be installed on tne four access openings on the lower ChDM 22 housing. Thermal ano mecnanical analyses have determined that the seal aoditions will not interfere with the RSS performance under tne in-fluence of mechanical, thermal or seismic ioadings. Tne flow calculations in Reference 22 conclude tnat acdition of tne mechanical seals to tne nss pressure tubes and the lifting eyebolts will reduce naturally convective ingress of primary coolant into the CRuM cavity from a flow rate of 0.68 acfm to less than 0.006 acfm. Additional calculations have confirmec 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 shutoown 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 bac4-up source of helium in case of purge flow loss. From tne review of the provided documentation, Los Alamos agrees that the adoition of seals and coverplates with integral gaskets will indeed mitidate the ingress of primary coolant and moisture into tne CRD cavities tnrough penetrations.

In aooition, Los Alamos believes tnat the procedure requiring reactor l

shutdown with loss of purge flow or hign moisture levels in the primary coolant fulfills the requirements of the assessment report . Tne licensee defines "nigh moisture levels" in Reference 23

23 ChDM Temperature hecording and CRDM Requalification Tne lack of direct measurements of CHOM temperatures curing tne June 23 event, and during steady state and other transient operating condi-tions, has prompted the installation of hTDs to monitor tne Cahi cavity closure plate (ambient), orifice valve motor plate and control roo orive motor _ temperatures. Strip cnart recorders will continuously record tne tnree temperatures for each CRDM, and will provioe a CnDM operating temperature data base. The old data collection surveillance proceoure 25 will be modifiec tc collect cata on a continuous basis.4, p.60 Tne licensee intends to install tne permanent recorders prior to 4 p.57  ;

restart.  ;

The licensee postulates in R.eference 26 that "tne maximum temperature rating of tne orive mecnanism which might innibit the scram function is 272*F", ano in monitoring CRDM temperatures "the maximum temperature rating of 272*F snould not be exceeoed ouring power operation".

Tne licensee has also proposed a CRDOA requalification testing pro-gram that is designeo to establisn a temperature at which the ChDOA is qualified for operation.27 The helium test environment will be operated at 230*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 eno of 1965 Los Alamos agrees tnat the placement of CnDOA toermocouples, and tne continuous cata monitoring at all operating conditions is sufficient to provide a CEDOA temperature data case curing steady state and transient operating conoitions.

In adoition, Los Alamos believes tnat tne CRD0A is currently only ,

gualified to operate up to 215'F based on the original mechanical CRDOA qualification tests, an NRC recommendation,28 and previous Los Alamos calculations. 9 Tne licensee's argument tnat tne CRD0A is qualified for 272*F operation oased on analytical calculations4 ' E*#9 is not sub-stantiated. Tnerefore, Los Alamos recommends tnat CHDOA operation be limitea to 215'F until mechanical requalification supports a higher oper-ating temperature.

2.4 CRDM Surveillance and Preventive / Predictive Maintenance The licensee has proposed a set of preventive / predictive mainte-nance tests ano surveillance inspection procedures that are intendeo to monitor the performance of the CHDOAs and to determine the overall opera-bility of the CRDOAs during reactor operation. Initial development of these operating tests are considered part of the CRD0A refuroishment pro-gram, and will utilize the data base and resultant trends formulated dur-in6 refurDishment.

2.4.1 CRDM Preventive / Predictive Maintenance Tae licensee's CRDOA preventive / predictive maintenance program is proposed in Reference 30. According to the licensee, the normal preven-tive maintenance (Pd) program will be implemented on a refueling basis rotational c/cle for CRD0As tnat woula normally be removed for refueling, unless the predictive maintenance (PDM) program indicates tne neeo for more frequent maintenance. Tne PM program woulo empnasize the mecnanical examination and refurbishment of tne snim motor / brake assemoly, the drive train, control rod cable, reserve shutdown system, position potentiom-eters, limit switenes, orifice drive motor assemoly, orifice drive lead screw, assorted seals, valves, electrical components, bolts ano the ao-sorDec. string.

On tne otner hana, the predictive maintenance techniques woulo De used to monitor the most important aspect of ChDOA performance--the

" scram capability"--Dy determining the shim motor / Drake and gear train performance . The tests proposed in the PDM program include wattabe requirements, back-EMF voltages, deliverec torque at tne motors, scram times, rod drop rates and torques to rotate motor / brake assemblies. Cer-tain aspects of the PuM program would be implemented on a weexly basis to determine scram capability and temperature performance during power oper-l ation. The licensee nas also proposeo that testing information be acquired during reactor snutdown for trending purposes, i Los alamos concurs witn the proposeo preventive maintenance pro-j gram as outlined by the licensee, on the assumption tnat cata acquireo during reactor operation will show tnat predictive maintenance tecnniques I can be useo to detect a reauction in CRDOA performance. Ine PDM testine tecnniques are closely linkeo to tne techniques tnat are being usea for I

i l

tne acceptance criteria in the refurbisnment program, and will tnerefore be dependent on the suitability and acceptability of back-EMF testing for determining CRDOA 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, using a 10" roa drop method on all withdrawn and partially inserted con-trol rods, except the regulating rod.5e pg 82 Tne surveillance tests will obtain data for analysis ana long term trending, exercise the rod, test selected circuitry, verify FSAh (Final Safety Analysis heport)9 assumeo scram times, and confirm control rod operaoility. In addition, ChDOA teuperature and purge flow information will be collected.

For a fully witharawn rod, analog ano digital position information will be obtained, " Rod-Out" lights will be verified on,

" Rod-In" ano "diack Cable" lights will oe verifled off, anc tne rod will be dropped approximately 10" by de-energizing tne brake, vnile back-EMF data are obtainea for future trending. 1ne " 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 tnen be withdrawn to the full out position, so that analog ano digital positions can again be ootained. Control roos 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 roa-position limit switches. Refueling snutdown surveillance will acquire the same information as the weexly and quarterly tests, except full stroke l

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 aver ae full lengtn scram time. A CnDOA will be considered inoper-able if it does not meet the maximum scram time of 100 seconos as defined in the FSAh9 . Such an indication would warrant back-EHF testing in i confirminh scram operability.

Los Alamos agrees that the basic surveillance metnodology is suffi-cient to exercise the control rod, verify FSAR scram times, and to test i

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r' selected circuitry. However, references to 272 F as the maximum CRDOA operating temperature are still considered inappropriate as alscusseo in section 2 3, and a 10 inch deviation is not considered acceptable between cigital and analog position indications--such a deviation coulo inadver-tently lead to control rod overdrive througn a misinterpretation of rod position. Also, tne bacx-EMF testing methods and interpretation of tne results are still in the developmental stades, and an engineering deter-mination of tne suitability ano acceptability of tnis testind metnoo in determining continued ChDOA operability will need to be maoe before the licensee can finalize tnis portion of tne surveillance program.

30 toisture j Ingress Issues Tne licensee nas submitted 32 a listing or tne issues considered, and actions taken, by tne FSV Improvement Committee (formerly the FSV Hoisture Ingress Committee) in significantly reducing the frequency and severity of moisture ingress events. The issues were civideo into four cate6ories:

1. Issues currently under consideration by tne Fort St. Vrain Im-provement Committee.

2. Circulator Auxiliary System moaifications yet to be completed prior to startup.

3 Circulator Auxiliary System modifications to be completea prior to startup, provided material availability ano schecule permits.

4. Items identified by tne Moisture Ingress Committee wnien are installeo and operational.

Los Alamos believes that a listing of intenced and installea moalfi- ,

cations does not provide any indication as to wnat any given mooification really is, wn/ tney contribute to tne reduction in potential for moisture ingress events, nor which improvements will substantially reauce tne severity ano frequency of moisture ingress events. Ine licensee nas com-mittea to submit a more explanatory version of tne actions to mitigate 4, p6 80 moisture ingress, prior to restart ,

4.0 PChV Post-Tensioning Tendon System In tne spring of 1964, during scheduled PCRV tendon surveillance, tendons with corroded and broken Wires Were found. Since tnat time, the licensee has evaluated the corrosion meenanism, 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 ano Failure Mechanism The licensee, in determining tne extent of tenden corrosion in tne PCRV, determined what fraction of the tencons were available for visual examination ano lift-off tests. Tne tendon system is subdivided into four major groups: the 90 longitudinal (vertical) tendons nave 169 wires per tendon; tne 210 circumferential tendons in tne PCRV sicewall have 132 wires per tendon, and tne 50 circumferential tencons in botn tne top ano bottom heacs nave 169 wires per tendon; tne 24 bottom cross-heaa tenoons, ano 24 top cross-nead tendons nave 169 wires per tendon. Of tne four groups, the licensee states the followind accessibility 33 Tendon Group botn Ends Acces. One End Acces. Neitner End Acces.

Longitudinal Visual 20 69 1 Lift-off 0 74 16 Circumferential Visual 2d1 #7 2 Lift-off 236 62 12 Bottom cross-heau Visual 20 4 0 Lif t-off lb 4 4 Top cross-neaa Visual 17 7 0 Lift-off 16 6 2 Tne numoer of tendons witn xnown broxen wires as identifieo in tue licensee's 19o% surveillance,34 incluceo 10 longituoinal tendons witn 1 to 22 broken wires, 2 circumferential tencons witn 2 and 15 broxen wires, 8 bottom cross-nead tenoons with 1 to 19 broxen wires, and no top cross-head tendons with Droxen wires. In some cases, tne total number of cor-rodeo, broken wires incluce wires broken durin6 lift-off tests, or durin6 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 circumfer-ential tencons snowed little enange in lift-off value. Some of tne fif-teen bottom cross-head tendon lift-off tests showed a definite reauction 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 exceedea tne minimum limits.

The licensee conducted metallurgical investi 6ations into the cause of the corrosion, ano determined that microbiological attacg on tne tenoon NO-0X-ID CM or6anic grease caused the formation of formic and acetic acids.3 ' Tne acias, in conjunction with moisture in tne tendon tube, vaporized and recondensed on the cooler portions of the tendons--in this case, usually towara the tendon enas. Tne aciaic attack resulteo in re-ouced cross-sectional wire area, stress corrosion cracking, localized tensile overload ano wire creaka6e.

Los Alamos Delieves, based on the documentation presented by the licensee, that microbiological attack of the tendon grease and tne resul-tant formation of acetic and formic acias, 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, Los Alamos believes tnat tne extent of known tendon corrosion, breaka6e and previous sur,yeillance have not been clearly defined by the licensee.

Los Alamos tnerefore recommends that a complete map oe mace tnat lists each tenoon, its visual examinations and lift-off values, and tne number ano location of corroded anc broken wires. An inaiCation 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 blanget consisting of nitrogen gas. The licensee's consultants found that the nitro 6en atmo-sphere arrested the growth of the microbes in the h0-0X-ID CM organic

,,, ,. . . .- ~. .. . . _ _ ~ . . . . . . . - _ - -

9' grease 3 .35 , and eliminated the ox/ gen wnich is necessary for the cor-rosion process to continue, sased on these results, and as a snort term action, the licensee has proposed tnat nitrogen blankets ce establisneo i on the longituoinal and bottom cross-nead teccons. Long term actions would include furtner 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 tnat the use of a nitrogen blanket to halt tne corrosion process may be suitable, but difficult to implement as proposea.

The tendon tubes are not likely to oe leaktight, and maintainin6 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, Los Alamos recommends that initially the nitrogen be purged through the individual tendon tubes to remove as much moisture as possible, and tnat gas samples be used to monitor moisture ano oxygen reduction. Further investigation into the long term effects i of a nitrogen blanket on tendons, tne corrosion process and currently available corrosion acids are also recommended.

43 PCRV Tencon Interin Surveillance because tne total extent of tendon corrosion in tne PCRV is unknown, because tne rate of existins corrosion is unxnown, and because tne use of a nitrogen blanket as an arrest to tne corrosion process is an unknown, tne licensee has proposed an interim

• surveillance program designed to address eacn of tnese issues.5, pp.164-7 The interim tendon surveil- ,

lance program woulo incluce increased visual and lift-off surveillance

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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 botn tendon ends, when accessible, woulo include:

i . _. . -- - _ _ . - - _ . . - , _ - - _ _ _ _ - - - - - . - . - , - - - - _ . - - - - _ - _ _ .

Tenoon 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 oe performed on two frequencies--an lo montn frequency for the population of new tendons, and a O montn frequency for the control population. The number of tendons for lift-off will include:

~4endon Group ho. of hew Tenaons No. of Control Tendons Lon61tudinal 12 3 Circumferential 13 3 Bottom cross-heaa 3 1 Top cross-head 1 1 As an acceptance criteria, tne licensee proposea snat, based on vis-ual examinations, a mandatory engineering evaluation be conducted on any tenoon tnat nas 20% of its wires broken. For any tendon tuat has only one accessiDie end, the mandatory engineering evaluation will be con-ducted when any tencon has lo) of its wires broken. 2ne control tendon population will include those tendons with tne worst known corrosion with reacy accessiollity.

Los Alamos agrees that tne 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 the rate of corrosion with the con-trol tendon population. Tne increased surveillance snould also determine the effectiveness of the nitrogen blanket in arrestin 6 corrosion, or any other corrective measure the licensee may propose. Los Alamos recommends that the licensee submit an outline of the intended mancatory engineerins evaluation, which should incluae all lift-off, load cell and relaxation data incorporated into a safety evaluation. The licensee snould define the extent of tne visual and lift-off testing procecures, ano coulo use U3/NHC Regulatory Guide 1 35 37 for guidance.

4.4 PCRV Structural Calculations by Los Alamos National Laboratory Tne PCRV tendons are intended to apply sufficient compression in tne concrete to balance or exceed the circumferential and vertical tension in the concrete tnat results from the internal pressure. A combined analyt-ical ano numerical study 39 was undertaken by Los Alamos National Laboratory to evaluate the evolution of these stresses, Doth to the ini-tial prestressins and to subsequent partial ano total rupture of tnese tendons. At the stress levels anticipated in tne concrete, ano for the anticipated operating life span of tne PCnV, tne concrete benavice was modeled as a linear viscoelastic solid with tne creep strain varying pro-portionally witn tne logarithm of time at constant stress tnroubnout tne projected reactor lifetime.

A one-dimensional mocel of a long concrete column of rectangular cross-section, with an embeaded prestressins tendon along tne length, was used to evaluate the concrete ano steel stresses as well as tne nold-down ana lift-off forces. Tnese were evaluated for tne intact tendons and tne degraded tenoons. Tne ce5ree of tendon degradation is cescribeo tnrough tne ratio of tne numoer of unbroken strancs to tne original numoer of stranos. Initial time of rupture was varied from the time of initial prestressin5 to 400 days after emplacement. Tne formulation led to an integral equation, which was solved numerically. The hold-down forces decayed approximately with tne 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 adoition, 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 PChV. Inis code nas an extensive material library of constitutive relations to model the various properties of concrete, todether with a specialized element model to simulate pre-stressing tencons. Two rows of vertical and an arc row of circumferential tendons were incorporated in the model as a baseline calculation. Tne tenoons were prestresseo to 70) of the ultimate and an internal pressure of 775 psi was applied (this pressure is tne internal pressure of tne nelium coolant in the HIGh) and the creep of the concrete and slow decay of tne tenuon stresses were evaluated out to 30,000 days. Inen, tnree O

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 miche16ht was removea. Stress redistributions at 300 days after ruptures were calculated and snifts of tne remaining tendon loads to accommodate the broxen 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, tne mean vertical stress was aoout -760 psi, the radial stress decreased from the appliec 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. Tne 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 coacludes 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 CHbOAs provides confidence in CRDOA operability during reactor opera-tion and tne 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 anim motor / brake assembly in determining control rod operational acceptability still exist, but further methoa development, more experience with result interpretation, and in-core testing may alleviate the questions. Until CRDOA operability can defin-Ately 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 enloride attack, especially the PChV liner.

Review should continue into the source of enlorine and methoos to elimi-nate its generation and presence.

The effects of purse 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

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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 mecnanical standpoint, CHD0A preventive / predictive mainte-nance proceoures 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 cue to the lack of information with whien to understand the measures tagen. .

The licensee has committed to submit a more explanatory version of tne actions to mitigate moisture ingress prior to restart.

Tne extent of PCAV 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.

b.0 References

1. " Preliminary Report Related to the Restart and Continueo Operation of Fort St. Vrain Nuclear Generating Station," Docxet No. 50-207, Public Service Co. of Colorado, October,1904.

2. " Review of Dallas Meeting (1/15/85) and Restart Committments",

letter from Martin, NRC/ Reg IV, to Lee, PSC,1/17/65.

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, NHC-PSC, February 21, 1985," Volumes I and II, recorded ano transcrioea by Koenig & Patterson, Inc.

5 " Fort St. Vrain Meeting, NRC-PSC, February 22, 1985," Volumes I ana

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II, recordeo ano transcribed by Koenig & Patterson, Inc.

6. " Engineering Report on CRDOA Failures to Scram-Control 2od Drive and Orifice Assemblies," PSC suomittal P-65037,1/31/82

7. " Failure of Three CRDOAs to SCRMi," PSC submittal P-85029,1/28/e5.

8. "eearing Deoris Analysis," PSC submittal P-65017,1/16/65.

9 " Fort St. Vrain NJelear Generating Station, Upoated Final Safety Analysis heport, Puolic Service Co. of Coloraco.

10. "CRDOA RefurDishment Program Report," PSC submittal P-85040-2, 1/32/d5

11. " Control Rod -Drive Cable Replacement," PSC suomittal P-85032-2, 1/20/85.

12. " Control Roo Dr,1ve Cable Replacement Report," GA Technologies Document 907d22, Attachment 1 to PSC suomittal P-85032-2,1/31/85. .

13 " Investigations into sources of Cnloriae in FSV Primary Circuit,"

PSC submittal P-66036,1/31/85

14. " Safety Analysis Report--Change in Material of tne FsC Control Roo and Orifice Assemolies," 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 Snutdown Absoroer Material," PSC submittal P-65027, 1/28/65 17 "doisture Control in CRDOA Purge Lines," pdc submittal P-65032-9, 1/20/85.

18. "FSV Reserve Snutoown System Material Metallurgical Examinations,"

draft report from Los Alamos hational Laooratory, 3/05 19 "CRDOA Holsture/ Purge Flow," PSC submittar P-8503e-6,1/20/85

20. " Modifications to CHDOA Helium Purge Supply," PSC suomittal P-85032-8, 1/20/o5.

21. " Control Roa Drive Cavity Seals," PSC suomittal P-85032-7,1/20/85.

22. "FSV CRD Cavity Seals Design heport," GA Tecnnologies Document 907604, Attachment 1 to PSC submittal P-65032-7,1/20/85.

23 " Operations Order No. 64-17 Describing Operator Actions Upon a Loss of Purge Flow and or Detection of Hign Moisture Levels in Primary Coolant," PSC suomittal P-85040-8, 1/3A/85

24. "CND Temperature and Helium Purge Flow Recorders," pdc suomittal P-85032-3, 1/20/65 25 " Current CHD Temperature Data Collection Procedure Wnich Hequires Station hanager Notification Upon Discovery of a Measureo CRD Temperature in Excess of 250*F," PSC suomittal P-65040-9,1/31/65

26. " Control hod System Operaoility Evaluation Report," PSC suomittal P-65040-1, 1/31/62

27. "ChDOA dechanism Temperatur_es Environmental Requalification," pac submittal P-85032-1, 1/20/b5.

2d. Letter from Robert A. Clark, Cnier, Oho 3, to O. R. Lee, PdCo.,

December 2,1982.

29 heier, K., " Fort St. Vrain Reactor Control hoo Drive Mechanism Over-Temperature Problem," Los Alamos National Laboratory,1982

30. "ChDOA Proposed Preventive / Predictive Maintenance Program heport,"

PSC submittal P-85040-3, 1/31/o5

31. "ChLOA Interim Surveillance Program Report," PSC submittai P-85040-5,

• 1/31/85.

32. "fSV Improvement Committee Actions," PSC submittal P-85022, 1/24/85.

33 "lendon Accessibility Report," PSCo. letter from Warecbourg, PSC, to Jonnson, NnC/nes IV, pdc submittal P-84523,12/14/d4.

34. " Lab Report No. 52--Examination of Failed Wires from Fort st. brain Unit No.1," PSC submittal P-o4543-4,1/24/65.

. 35 " Liftoff Tests," Attacnment 1 to " Engineering Report on Fort St.

Vrain Tenoons," PSC submittal P-64543,12/31/d4.