ML20197B361
| ML20197B361 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 10/21/1986 |
| From: | Mangan C NIAGARA MOHAWK POWER CORP. |
| To: | Kane W NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| References | |
| (55(E)-86-18, (55(E)-86-18), (NMP2L-0920), (NMP2L-920), NUDOCS 8610280399 | |
| Download: ML20197B361 (34) | |
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N3F##ae NBAGARA RA0 HAWK POWER CORPORAT10N/300 ERIE BOULEVARD WEST. SYRACUSE, N.Y.13202/ TELEPHONE (315) 474-1511 October 21, 1986 (NMP2L 0920)
Mr. W. Kane, Director U.S. Nuclear Regulatory Commission Region I Division of Reactor Projects 631 Park Avenue King of Prussia, PA 19406 Re: Nine Mlle Point Unit 2 Docket No. 50-410
Dear Mr. Kane:
Enclosed is a final report, in accordance with 10CFR50.55(e), for two problems concerning the Main Steam Isolation Valves. The first problem concerns a cracked latching roller found in one MSIV. This condition was reported per telecon to G. Meyer of your staff on August 8, 1986 and followed with an interim report (55(e)-86-18) on September 11, 1986.
The second problem concerns the closure time of the MSIVs. This condition was reported to J. Linville of your staff on August 15, 1986 and followed by an interim report (55(e)-86-19) on September 12, 1986.
Very truly yours, C. V. Ma an Senior Vice President GAG /ca xc: J. M. Taylor, Director of Inspection and Enforcement U.S. Nuclear Regulatory Commission Washington, DC 20555 M. Haughey, NRC Project Manager W. Cook, NRC Senior Resident Inspector Project File paho2gggggggg;o s 1 i
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UNITED STATES OF AMERICA NUCLEAR REGULATORY COMMISSION In the Matter of )
Niagara Mohawk Power Corporation ) Occket No. 50-410 (Nine Mlle Point Unit 2) )
AFFIDAVIT C. V. Mangan , being duly sworn, states that he is Senior Vice President of Niagara Mohawk Power Corporation; that he is authorized on the part of said Corporation to sign and file with the Nuclear Regulatory Commission the documents attached hereto; and that all such documents are true and correct to the best of his knowledge, information and belief.
PcuYA.AAL&f m '
O Subscribed and sworn to before me, a Notar P bile in and fpr the State of New York and County of dI?XV//7n2. , this day of [fBie/#A , 1986.
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FINAL REPORT 10CFR50.55(e) .
MSIV ACTUATORS Nine Mile Point Unit 2 Niagara Mohawk Power Corporation October 1986 I
e TABLE OF CONTENTS
1.0 INTRODUCTION
1.1 Objectives 1.2 Executive Summary 1.3 Safety Evaluation
2.0 BACKGROUND
2.1 valve Description 2.2 Original Actuator Design 3.0 PROBLEM DESCRIPTION AND EVALUATION 3.1 Initial Problem Discovery.
3.1.1 Jobsite Testing 3.1.2 Prototype Testing 3.2 Mechanical Latch Evaluation 3.2.1 Finite Element Analysis 3.2.2 Load Cell Measurements 3.2.3 Strain Gage Measurements 3.2.4 Additional Testing 3.2.5 Summary 4.0 CORRECTIVE ACTION AND TECHNICAL JUSTIFICATION 4.1 Description of Corrective Action 4.1.1 Basis 4.1.2 Design Concept t
2 9
4.1.3 Mechanical Equipment 4.1.4 Instrumentation and Controls 4.2 Technical Justification i
4.2.1 Safety Evaluation 4.2.1.1 Seismic Qualification 4.2.1.2 Equipment Qualification 4.2.1.3 Failure Analysis 4.2.2 Operability Testing 4.2.3 In-service Testing 5.0 ADDITIONAL CONFIRMATORY TESTING r
6.0 CONCLUSION
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1.0 INTRODUCTION
During August 1986 the- eight NMP2 Main Steam Isolation Valve (MSIV) actuators underwent testing to verify conformance to Technical Specification Sections 3/4.4.7 and 3/4.6.3 which require that the MSIVs be capable of closing within 3 to 5 seconds. During this time several deficiencies were identified, including cracking of the mechanical latching roller on one of the actuators, closure durations in excess of the 5 second limit, and on occasion, failure of the actuator to trip and allow the valve to move from its open position. An extensive investigation into the cause of the problems was initiated which included evaluation and testing of various potential actuator design modifications to implement as corrective action. The problems were reported to the NRC on August 8, 1986 by 55(e)-86-18 and on August 15, 1986 by 55(e)-86-19.
1.1 Objectives This report describes the original design features of the actuator, explains how the actuator problems were discovered, presents the design modifications being implemented to resolve these problems, and discusses the testing performed to justify the resolution of the actuator problems. Justification for the corrective action is based on a design evaluation of the simplified actuator design, together with the testing results to date and the additional planned confirmatory testing. This document is the final report required 4
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j by 10CFR50.55(e) addressing the MSIV actuator problems which were '
reported as construction deficiencies 55(e)-86-18 and 55(e)-86-19.
This report supplements information contained within the Application for Schedular Exemption Related to Further Analysis of and Possible Modification to the Main Steam Isolation Valves submitted to the NRC on October 10, 1986.
1.2 Executive Summary The problem resolution presented in this report includes sufficient testing and analyses to demonstrate that the MSIVs will close reliably. Original testing did not identify a time dependent closure problem (i.e. the longer the latch time the greater the force required for closure). Preoperational testing and subsequent shop testing verified a time dependent closure problem. Although the mechanism which caused the initial deficiencies has not been fully determined, the investigation and testing have absolutely established that the problems are associated with the mechanical
, latching and actuating device. These components have been removed and the latching and closure actuation are now being performed in an entirely different manner. The actual opening and closing mechanisms, however, have not been substantively altered.
The design modifications which have been made to the actuating mechanism have been thoroughly evaluated and tested to ensure the operability and reliability of the MSIVs. The modified actuating mechanism has been put through a rigorous operability testing 5
program at the manufacturer's facility. The design has been evaluated to assure that the seismic qualification has not been degraded.
The mechanical components in the hydraulic system have been tested and evaluated to assure they are qualified for the revised service conditions. The failure mechanisms of the new and modified components have been evaluated to assure that the safety function of the MSIVs has not been diminished.
Niagara Mohawk is committed to continued testing and contingency programs regarding these valves and actuators. The long-range testing programs provide further assurance that the cause of previous failures will be determined and confirmed.
In conclusion, the closure problems associated with the MSIVs have been resolved, and the valves, including their actuators, are appropriate for normal and emergency plant operation.
1.3 Safety Evaluation The design function of the MSIVs is to provide rapid isolation of the main steam lines at the primary containment in the event of a break in a steam line outside containment, a design basis loss of coolant accident (LOCA), or other events requiring main steam line or containment isolation. In the case of a steam line break, the isolation valves are designed to terminate reactor coolant blowdown by closing within 3 to 5 seconds, thereby preventing
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6
an uncontrolled release of radioactivity from the reactor vessel to the environment.
Failure of the MSIVs to close as required could result in release of offsite radiation doses in excess of 10CFR100 guidelines and excessive doses in plant occupied areas. Therefore, had this condition remained uncorrected, it could have adversely af fected the safety of plant operations.
2.0 BACKGROUND
2.1 Valve Description NMP2 is provided with two MSIVs on each of the four main steam lines; one valve is close to the inside of the primary containment and the other is located just outside the containment.
Valve operation, under normal conditions, takes approximately five minutes to open and twenty seconds to close. Under emergency situations the valves are designed to rapidly achieve containment isolation by closing within 3 to 5 seconds. Additional description of the valve is provided in the final report, 10CFR50.55(e), MSIV Leakage. Figure 2-1 shows a sketch of the actuator assembly.
2.2 Original Actuator Design The original actuator design included a hydraulic subsystem to open and test the MSIV, a mechanical latch to hold the valve open 7
until required to close, and a spring canister assembly to close the valve. The major components of the hydraulic subsystem include a motor operated pump, a hydraulic cylinder / piston assembly, one 3/8" diameter and two - 2" diameter solenoid operated valves (SOVs),
and a fluid reservoir as shown in Figure 2-2. To open the MSIV, the SOVs are closed while the pump pressurizes the system causing the hydraulic cylinder / piston to rotate the connected clevis drive shaft, opening the MSIV (See Figure 2-3). This rotation also causes compression of springs in canisters connected to the drive shaft. When the MSIV reaches the fully open position, the mechanical latching roller is engaged to hold the valve in the open position, the hydraulic pump is shutoff, and the SOVs are opened (See Figure 2-4). To close the MSIV, two solenoid spring plungers are deenergized. The plungers impact the pivot plate which rotates the latch arm, removes the latching roller from its position against the clevis drive shaft, and allows the springs in the spring canister assembly to close the valve (See Figure 2-5).
The original actuator design employed a de-energize to trip control scheme designed in a manner to ensure reliability. The control logic was designed with a two channel arrangement such that loss of a single channel did not trip the MSIV. These two logic channels were fed from separate Class IE uninterruptible power supplies.
Both logic channels for the inboard valves were supplied from Division II and both logic channels for the outboard valves were supplied from Division I. For reliability purposes, wiring and cabling were routed separately.
8
The two - 2" diameter SOVs and two solenoid spring plungers were supplied power such that one SOV and one solenoid spring plunger was supplied from ea'ch logic channel. Once the MSIV was opened and resting on its mechanical latch, it required the de-energization of both solenoid spring plungers to initiate a closure of.the MSIV. Only one of the two SOV's was required to be de-energized (open) to ensure a hydraulic vent path for MSIV closure.
The separation requirements of Regulatory Guide 1.75 were applied and implemented to separate the different divisional power supplies and cabling used for the Division I outboard MSIV's and Division II inboard MEIV's. The single failure criterion of Regulatory Guide 1.53 was adhered to and assured by virtue of the total indepen-dence of the inboard and outboard MSIV's, their components, and their power supplies. No single failure within the logic circuitry of an MSIV could propagate to its redundant MSIV because of the power supply independence.
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. 3.9 PROBLEM DESCRIPTION AND EVALUATION 3.1 Initial Problem Discovery 3.1.1 Jobsite Testing Problems with the actuator operation were initially discovered during testing at the jobsite. The Technical Specification requires that MSIV closure be complete within 3 to 5 seconds
. of solenoid spring plunger deenergization. Testing at the site revealed closure durations in excess of this limit, and the excessive closure time was related to the length of time the valve had been latched open. This testing revealed that in some cases the force produced by the solenoid spring plungers was inadequate to trip the latch, and that the latching roller on one valve's actuator had cracked. Detailed analysis was begun to determine the cause of these failures.
Similar valves and valve actuators are installed in the Beaver Valley Unit 2 Nuclear Power Plant which is under construction l near P i ttsburgh, PA. During preoperational testing, cracked latching rollers were also discovered in those actuators.
i 3.1.2 Prototype Testing In light of the site problems, further testing was initiated i using a test actuator at the valve manufacturer's facilities.
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This testing confirmed the site results of excessive closure durations and insufficient solenoid spring plunger forces.
Measurements wers made to determine the force required to trip the latch mechanism. The results indicated that holding the actuator latched open for a 29 hour3.356481e-4 days <br />0.00806 hours <br />4.794974e-5 weeks <br />1.10345e-5 months <br /> period resulted in a tripping force approximately 1.5 times that required for tripping immediately after opening. Holding the valve open for a week resulted in a tripping force approximately 3 times the original force.
3.2 Mechanical Latch Evaluation The original mechanical latch design initiated a valve trip when the trip solenoid spring plungers were deenergized, allowing the release of stored energy, which caused the plungers to impact a pivot plate. This force on the pivot plate caused it to rotate and function like a cam moving the latch arm such that the clevis drive shaft released. The valve then closed mechanically under the action of the spring canister assemblies. Additional details are contained in Gulf & Western Topical Report No. G&W FSD 2538 submitted to the NRC on January 24, 1979. Extensive evaluation, including design modifications, additional testing and detailed analysis of the mechanical latch components, was undertaken in an attempt to determine the root cause of the actuator problems.
Some of these activities are explained in more detail in the following paragraphs.
16
3.2.1 Finite Element Analysis The mechanical latch components were analyzed using finite element methods to evaluate component stresses, deflections and load distributions. Specific areas of stress reviews were lever arm loading, stresses in the pivot plate applied by the solenoid spring plungers, and latching roller loading.
The analysis did not provide an explanation for the delatching problems.
3.2.2 Load Cell Measurements To further evaluate the mechanical latching roller, a roller pin load cell was installed. Of particular interest in the roller testing was whether it would experience a time dependent load increase. The testing showed that the roller did not experience excessive loadings and that the loadings did not increase with extended durations "on the latch".
3.2.3 Strain Gage Measurements Strain gages were used extensively on the test actuator and several of the valves installed at NMP2. The latch arm and other mechanical latch components were instrumented and the actuator was operated to provide information on the time dependent load and stress distributions. Review of the data did not reveal the cause of the tripping problems.
1 17
3.2.4 Additional Testing In an effort to identify the problem, the pivot plate and cam wear pad were redesigned to improve ~the overall latch efficiency and thereby increase both reliability and speed i
of operation. Testing of the initial redesign resulted in a 10 second trip time (unacceptable) and also resulted in bending of the lower pivot plate pin. The lower pin was reworked, with the addition of a lower bearing. The assembly was again instrumented and tested. Trip times remained excessive and the force required to trip the latch continued to show an increase with increasing durations "on the latch".
3.2.5 Summary In the course of this evaluation Niagara Mohawk was able
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to duplicate the time dependent phenomenon of increased latch tripping force with increased latched open time, but was not able to isolate the root cause. It was concluded that the only problem with actuation was within the latching mechanisim and that a revised latching system should be provided.
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4.0 CORRECTIVE ACTION AND TECHNICAL JUSTIFICATION 4.1 Description of Corrective Action 4.1.1 Basis The testing discussed in the preceding section did not reveal any plausible cause for the time dependent aspect of the actuator closing problems. It appeared that resolution of the mechanical latch problems would not be possible in the near term. It was, therefore, decided to investigate the feasibility of, removing the mechanical latch and related mechanisms and modifying the existing actuator's hydraulic system to perform the function of holding the valve in the open position.
4.1.2 Design Concept The MSIV actuator with the modified hydraulic system continues to use the spring canisters to provide the force for valve closure; it also uses the hydraulic system to open the MSIV.
The solenoid spring plungers and the mechanical latch have been eliminated. The solenoid operated valves (SOVs) which in the original design were closed only when the MSIV was being opened and were then put in the normally open position are retained. Their function is revised, however. They are 19
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now maintained in the closed position during normal operation i
and the hydraulic system is kept pressurized to hold the
- MSIV open. When the MSIVs are required to close, the SOVs are opened, the hydraulic system is depressurized and the fluid in the cylinder which holds the valve open is exhausted
, into the reservoir (See Figure 4-1).
4.1.3 Mechanical Equipment The hydraulic latch design required minimal additional equipment to enable the hydraulic system to perform the latching and tripping functions. A jockey pump and accumulator were j added to reduce cycling times of the main pump and keep the l system pressurized. New hydraulic cylinders were ordered with the following design enhancements (Figure 4-2):
l o Piston with a lip seal to minimize fluid loss and associated system pressure drop i
o A mechanical stop in the cylinder end cap to control
., piston position and MSIV open position l
Additional controls and hydraulic tubing and fittings complete 4
tha modifications.
1 All components added to the system (jockey pump, accumulator, tubing, pressure switches and cylinder modifications) are 20
8 4 classified as non safety related. These items are.not required to function during an MSIV closure.
Further, their failure will not preclude a.: essential actuator component from function-
- ing.
4.1.4 Instrumentation and Controls The control system was modified to accommodate the new actuator design. Two new control panels are being designed to house new relays. The major philosophical differences between the old design and the new design lie in the total elimination of the two solenoid spring plungers and the mechanical latch, and the modification of the normal operating modes of the three hydraulic SOVs to one of being energized (closed) while the MSIV is open.
To enhance the reliability of the design, both 2" diameter SOVs will be supplied from either of the two RPS uninterruptible power supplies through appropriate auctioneering circuitry, such that a loss of either supply will not de-energize (open) either SOV. Enhanced monitoring of the hydraulic system will be added to detect and alert the control room of abnormal operation.
The safety related aspects of the original and new actuator design are essentially unchanged. Specifically, the inboard 21
and outboard valves are powered from different divisions and the separation criteria of Regulatory Guide 1.75 are still implemented'. Also, with the redundancy of the inboard and outboard MSIVs, the single failure criterion is met with no significant desig, differences.
The following is a brief discussion of the modes of operation of the new actuator design:
Normal Closing This mode of operation remains essentially unchanged. Placing
, the main control switch in "close" de-energizes (opens) the 3/8" diameter SOV which will slowly exhaust hydraulic fluid from the piston allowing for a slow close of the MSIV. The two - 2" diameter SOVs remain energized and closed throughout the closing cycle.
Normal Opening This mode of operation also remains essentially unchanged.
Placing the main control switch in "open" energizes all Sovs and starts both hydraulic pumps which generates sufficient hydraulic pressure to open the valve. Upon reaching a specified high pressure within the hydraulic system both pumps are shutoff. The jockey pump will then cycle between specified setpoints to maintain the valve in a full open position.
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The setpoints for cycling of the jockey pump will be selected such that the valve will remain fully open with no valve move-ment between cycles of the jockey pump. The travel stops added to the hydraulic cylinders prevent over-travel of the valve during the opening cycle.
In-service Test Actuation of the RPS trip test pushbutton will de-energize (open) the 3/8" diameter SOV allowing a slow exhaust of hydraulic fluid, initiating closure of the MSIV. When the MSIV closes approxi-mately 6*, the 3/8" diameter SOV will energize (close) and both pumps will start reopening the MSIV.
Emergency Close Upon receipt of an emergency signal, all SOVs will de-energize (open), allowing fast closure of the MSIVs. Both pumps will i stop on receipt of an emergency signal. An emergency signal will override all modes of operation.
4.2 Technical Justification A properly functioning mechanical latch has the advantage that .
there is a positivo latching force applied to the valve to hold it open. However, the new hydraulic latch design has several ,
3 23
inherent advantages over the original actuator design. First, the actuator design is simplified by the elimination of the solenoid spring plungers and 'the mechanical latch assembly. Also, the system is inherently fail safe, since any event which results in the loss of hydraulic system pressure will cause the MSIV to close. Further, a loss of power to either of the two 2" SOVs i
will cause the MSIV to close within 3 to 5 seconds. More detailed discussion of the technical justification for these modifications is provided in the following sections. The technical justification is based on a safety evaluation of the design, the operability
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testing already performed, and the in-service testing that will continue during plant operation.
4.2.1 Safety Evaluation The safety evaluation of the revised MSIV hydraulic actuator 4
takes advantage of the fact that considerable testing and analysis had already been done with the original mechanical latch actuator, and the revised design contains very little substantive change.
1 The safety evaluation focuses on the changes that have been made and their potential impact on the qualification of actuator resulting from previous testing and evaluation. This evaluation 4
specifically considers the seismic and equipment qualification I
24
of the revised actuator, and the possible failure mechanisms of the new and modified components.
4.2.1.1 Seismic Qualification The qualification of the SOVs will include the results of existing analyses and tests, and new operability testing.
The existing documentation consists of seismic analysis and dynamic testing performed for a similar valve for seismic and hydrodynamic loads. These will be supplemented by a static deflection test in which the maximum anticipated load, developed from the analysis and testing of the. actuator, will be statically applied. The valve's operability will be verified during this test. Through these analyses and tests, the capability of the NMP2 MSIV SOVs will be verified and documented. This qualification will be completed prior to fuel load.
4.2.1.2 Equipment Qualification The actuator design has undergone significant analysis and prototype testing to ensure acceptable operability under seismic and dynamic operating conditions. Since the actuator changes are minimal, and the mass of the removed mechanical latch and solenoid spring plunger assemblies is negligible compared to the overall actuator mass, the existing qualification results are still valid.
25
The most significant change in actuator operability qualification involves the solenoid operated valves (SOVs). In the original actuator design, the SOVs were normally maintained in the open position and were not required to operate during an emergency event. The current " hydraulic latch" operator design maintains the SOVs in a closed position during normal plant operations and requires the SOVs to open to release hydraulic system pressure and allow the MSIV to close. To confirm SOV operability under seismic and dynamic operating conditions, extensive analysis is being performed, as discussed in Section 4.2.1.1.
The remaining mechanical components in the hydraulic system are required to open the MSIV, hold the valve open during normal plant operation and perform inservice operability testing. To provide assurance of operating reliability these hydraulic system mechanical components have been evaluated for suitability under the revised service conditions. Functional operability testing of each new cylinder at the vendor's facilities has been performed and documented. Welder qualification in accor-dance with ASME Section IX has been verified, and certified material test reports for the cylinder materials have been obtained.
26
4.2.1.3 Failure Analysis As discussed above, the SOVs must reliably function for the MSIVs to close as required by Technical Specifications.
These SOVs will be seismically and dynamically qualified for their intended service as discussed in the preceding sections. Single failure protection for the isolation function is provided by the redundant MSIVs in each steam line. Further redundancy is provided by the fact that each MSIV actuator contains 2 SOVs, either of which is capable of relieving hydraulic pressure rapidly enough for the spring to close the MSIV in 3 to 5 seconds. The closure tim'e is controlled by the flow of ' fluid through the end cap fitting of the hy-draulic cylinder. The hydraulic system is also designed such that the MSIV will close in the required 3 to 5 seconds even if the two hydraulic pumps continue to run.
4.2.2 Operability Testing Initial testing of the hydraulic latch design on the prototype revealed that the original SOVs in the new, conti.nuously pressurized system arrangement were not capable of opening quickly enough to support the 3 to 5 second MSIV closure requirement. Investigations determined the cause to be sticking of the EP rubber 0-ring seals (See Figure 4-3) in the Sovs.
Various combinations of SOV seat disc and ring materials were tested. Based on the successful test results on the 27
prototype actuator at Crosby and environmental properties the valve was modified as shown on Figure 4-3.
Testing at Crosby has also verified the need for a lip seal on the piston in the hydraulic cylinder to minimize hydraulic system pressure decay and the associated excessive jockey pump cycling times. Testing of the hydraulic cylinder with the lip seal indicates improved jockey pump cycle times in excess of 3 days. Ongoing testing will provide additional verification of system design parameters, including pump cycle times and actuator trip times.
4.2.3 In-service Testing In order to test the Reactor Protection System on a monthly basis, as required by the Technical Specifications, the following in-service test of the actuator will be performed:
The 3/8" SOV is opened, the MSIV slowly closes approximately 6*, and the hydraulic system then reopens the MSIV.
28
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t 5.9 ADDITIONAL CONFIRMATORY TESTING A prototype test program is being developed. This test configuration will duplicate the valve and actuator configuration installed at NMP2. Specific details of this program are contained in Niagara Mohawk's Final Report, 55(e)-86-20, MSIV Leakage.
6.0 CONCLUSION
S This report demonstrates that the MSIV actuators are now capable of performing the required design functions. When the modifications discussed in this report have been completed and the valves have successfully passed their preoperational tests, the MSIVs will be appropriate for normal operation of the plant. The normal in-service testing of these valves will assure their continuing ability to perform their safety function. Furthermore, Niagara Mohawk is committed to a long-term testing program to further assure reliable service throughout the plant operating life. 32
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