ML18057B275
| ML18057B275 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 09/09/1991 |
| From: | Slade G CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9109190009 | |
| Download: ML18057B275 (8) | |
Text
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consumers Power PDWERINli MICHIGAN'S PRDliRESS Palisades Nuclear Plant:
27780 Blue Star Memorial Highway, Covert, Ml 49043 September 9, 1991 Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 DOCKET 50-255 - LICENSE DPR PALISADES PLANT -
GB Slade General Manager REQUEST FOR RELIEF FROM SPECIFIC ASME B&PV CODE REQUIREMENT - CONTAINMENT AIR COOLER CODE REPAIR At the end of the 1990/1991 refueling and steam generator replacement outage, Consumers Power Company (CPCo) requested relief from the zero leakage requirement of the ASME B&PV Code, for the plant's containment air coolers.
In letters dated January 16, February 11, and February 27, 1991, we stated and clarified our request.
The NRC's March 6, 1991 letter granted the relief with certain conditions. Two of the conditions were that the relief from zero leakage would expire upon the completion of refueling outage number 9, (1992 refueling outage), or April 30, 1992, which ever occurs first, and that if repairs other than those meeting the code are to be accomplished to limit leakage, relief from the ASME B&PV Code,Section XI is required.
The Palisades containment air cooler cooling coil manifold to tube connections are brazed and are configured such that they are difficult to access for repair. Attempts at rebrazing the leaking connections have resulted in adjacent tube to manifold connections becoming heat affected and creating additional leaks on previously non-leaking connections. In this regard it has been determined that, due to the design, geometry and materials of construction of the Palisades containment air coolers cooling coils, the ability to complete a code repair as required by ASME B&PV Code,Section XI, is impractical.
Because we want to assure that the repair or modification of the coolers also involves a long term look at reliability, we are reviewing the total cooler design and considering cooler replacement as one of the modification options.
Since this evaluation is still in progress, we have not finalized the best possible modification option. Without a final modification design we are unable to complete a cooler repair by the end of the 1992 refueling outage.
A CMS ENERGY COMPANY
Since the final modification design is incomplete and the relief request granted from the zero leakage requirement expires at the end of the 1992 refueling*outage, we have designed an "engineered clamp" to stop identified leakage until a permanent cooler modification can be installed. We have reviewed the ASME B&PV Code for guidance and have determined that submitting a relief request for the NRC's approval of our use of the "engineered clamp" to stop the leakag~ on the containment air coolers is appropriate.
Therefore, pursuant to 10 CFR 50.SSa(g), a relief request from certain provisions of Section XI of ASME B&PV Code is submitted herewith for NRC review and approval.
10 CFR S0.55a(g)(4) states that components which are classified as ASME Code Class 1, Class 2 and Class 3 shall meet the requirements of Section XI of the Code, to the extent practical.within the limitations of design, geometry and materials of construction of the components.
10 CFR S0.55a(g)(5&6) explain that if the licensee does determine that conformance with certain code requirements is. impractical, the Commission may grant relief.
A discussion of the requested relief and the basis for the request are presented in the Attachment to this letter. This request has been reviewed and approved by plant management and will be incorporated into the Palisades Nuclear Plant 40-year Master Inservice Inspection Plan upon approval by the NRC, pursuant to Palisades Technical Specification 4.3.e.
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Gerald B Slade General Manager CC Administrator, Region III, USNRC NRC Resident Inspector - Palisades Attachment
ATTACHMENT Consumers Power Company Pa 1 i sades Pl ant Docket 50-255 PROPOSED REQUEST FOR RELIEF FROM THE PROVISIONS OF ASME B&PV CODE SECTION XI CONTAINMENT AIR COOLER CODE REPAIR Pursuant to 10 CFR 50.55a(g) 5 Pages
1 INTRODUCTION The Palisades Nuclear plant containment air coolers (VHX-1-4) are air handling and cooling units located entirely in the containment building.
Plant service*
water from the critical service water header is circulated through the units' cooling coils. The units are designed to control containment temperature during normal operation and units VHX 1,2 & 3 are designed to condense steam during certain accident scenarios to help limit peak containment pressure.
The coolers are classified as safety class 3 per Regulatory Guide 1.26. 10 CFR 50.55a(g)(4) states that components which are classified as ASME Code Class 1, 2 and 3 shall meet the requirements of Section XI of the Boiler and Pressure Vessel (B&PV) Code, to the extent practical within the limitations of design, geometry, and materials of construction of the components.
The applicable sections of the B&PV Code, referenced above, require a Class 3 system to be examined and to exhibit zero leakage during post-repair hydrostatic testing.
Each cooler consists of eight sets of coils composed of 5/8-inch tubes piped to manifolds for supply and return connections to the critical service water system.
Some of the coils' manifold-to-tube connections have exhibited leakage. Attempts have been made to repair these leaks. While some of these leak repairs have been successful, CPCo has stated that limitations of the cooler design, and configuration make achievement of the post-repair zero leakage requirement impractical.
IWA-4000 of the B&PV Code requires that repairs shall be performed in accordance with the owner's design specification and construction code of the component for defects found in Class 1, 2, and 3 systems. The cooler design calls for a brazed joint. Experience at Palisades has shown that such a repair on the containment air cooler tube to manifold joint causes heat-induced leaks on tubes adjacent to the tube under repair.
In letters dated January 16, 1991, February 11, 1991 and February 27, 1991, we requested relief from the zero leakage requirements of the ASME B&PV Code for the containment air coolers. The NRC's March 6, 1991 letter granted the relief for one operating cycle.
In response to the relief request, a project team was organized to affect an appropriate modification to achieve zero leakage. After an extensive problem solving-decision making process was completed, the team presented to plant management a proposed parallel paths approach consisting of 1) pursuing redesign of the coolers and 2) completion of a design package for an "engineered clamp" to stop leakage until the path 1 modification can be engineered and installed.
During the 1990/1991 Refueling Outage, one of the coolers, VHX-4, was replaced with essentially the same design as had been previously used except that the cooling coil headers had been modified in an attempt to solve the tube to manifold connection problem.
In our previous relief request, dated February 11, 1991, we stated that this repair appeared to be the best long term solution to the cooler problem.
We further stated that we would monitor the operation of the coils, and given satisfactory experience with the modified coils, CPCo intends to continue modifying and replacing coils in future outages for the remaining coolers. Following further reviews CPCo has
determined that the present design and ~he design used in the VHX-4 modification is not the optimum design, because it results in excessive flow resistance due to the insertion of the cooler tube into the manifold. This results in a decreased service water margin. CPCo believes the correct approach to the overall cooler problems should include consideration of a design which minimizes the required service water flow rate, thus increasing the available margin.
This design may consist of a waterbox configuration which would improve margin and enhance any future maintenance activities.
Several vendors have been contacted for redesign suggestions and projected lead times for supplying replacement coolers. All reasonable estimates show delivery times of 30 to 34 weeks from order placement.
In addition, prior to the 1992 refueling outage, there is no available opportunity for the detailed engineering and constructability walkdown which would be required in the containment prior to completing the detailed engineering phase.
2 Consideration in redesigning or repair of the coolers must also be given to those components of the current design which are showing age-related degradation.
Destructive examinations of the remaining cooler parts performed by CPCo's laboratory show that the expected remaining life for the coolers is
- at or near the expiration date of the plant license. Therefore, CPCo is proposing utilizing the recently instituted three year modification process to complete our investigations and finalize a new cooler design or permanent repair. The three year modification process allows one year to complete conceptual design, one year for detailed design and the third year for procurement of materials and installation. For this particular project conceptual engineering and walkdowns would be completed by the end of the 1992 refueling outage, detailed engineering would continue through the 1993 refueling outage, and the first phase installation of new coolers or replacement components would occur during the 1994 refueling outage.
In later refueling outages the remaining coolers will be repaired or replaced, with a final estimated completion date of sometime in 1997. The coolers sit on the lowest level of containment and their repair or replacement present many space and logistic problems.
Until the replacement with an appropriate design can be effected, CPCo proposes installation of an "engineered clamp" for both the tube-to-manifold joint and the manifold miter joint.
In support of this approach, studies have shown pinhole leaks have developed in the braze material of the tube-to-manifold joints, but no evidence exists to suggest that defects are migrating to the base metal.
It has also been shown that joint separation for the tube-to-manifold joint is not, a credible scenario. The manifold miter joint clamp is being qualified as a contingency plan should any leaks occur in this area.
The plan for the engineered clamp includes a comprehensive pre-installation testing and qualification program which consists of confirming material compatibility, chemical composition, testing of the clamp on a mock-up test manifold assembly, and pressure testing in accordance with the requirements of AMSE Section XI.
After extensive consultation and engineering, CPCo is confident the clamp will ensure desired system integrity until the time that the final modification can be implemented.
3 RELIEF REQUESTED It is requested that relief be granted from the repair techniques as specified in ASME Boiler and Pressure Vessel (B&PV) Code,Section XI, Subsection IWA 4000, 1983 Summer 83 edition.
As an alternative to the code repair techniques described by the code, Palisades will or has performed the following actions:
Engineered Clamp Design Design and qualification testing an "engineered clamp" which will stop identified leakage is in progress.
Experience has shown the majority of leaks to be at the tube-to-manifold joint; however, there have been a few manifold miter joint leaks in plant life. Therefore, a clamp design has been developed for both the tube-to-manifold joint and the miter joint.
Testing The clamps are designed with a drilled and tapped port to accommodate injection of a sealing compound (GE RTF 85108, a silicone rubber material), into an annulus at the leak location. The injection pressure of the "injectable gasket" will be equal to or slightly greater than the service water system pressure at the leak location to ensure that a significant amount of sealant does not enter the piping system.
The sealant exhibits a gel time of approximately 2.5 minutes from the time mixing is initiated.
An engineering analysis was developed to evaluate the clamp designs and it shows the clamp design to be acceptable. A second analysis was written to evaluate the seismic impact of the clamp assemblies on the Containment Air Coolers. This analysis used a value of 3% (312 lbs) added cooler weight and showed this addition to be acceptable.
The tube-to-manifold clamps weigh 0.5 lbs each and the miter joint clamp weighs 2.5 lbs. The clamp also has a design rating of 150 lbs. at 300 °F and will be shown to be able to withstand a boric acid solution, to assure the clamp will remain functional in a post LOCA environment.
- A qualification test has been developed which will be performed on specially made test rigs which mimic the coolers in both design and leak locations. The test procedure consists of two sections, a flow test and a pressure test. The flow test is designed to ensure the clamps are unaffected by flow induced vibration and to ensure the clamp/gasket configuration is not weakened by the induced vibration.
The pressure test is designed to ensure the clamp/gasket assembly can contain the leak (which has been induced by drilling a one sixteenth hole in the brazed joint), under Section XI test pressures. The flow portion of the test is to be run for a minimum of 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> with the clamps installed, but it is our intention to run for an extended period to provide assurance that the clamps will not degrade due to any flow-induced phenomenon.
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Component Remaining Life Extensive measurements have been performed on the cooler which was removed during the 1990 ~efueling Outage.
From the most limiting of those measurements, calculations have been developed which show the remaining life of the components which make up the cooler units (straight tubes, cleanable U-bends, non-cleanable U-bends).
The calculations, based on measured wear rate and minimum allowable wall thickness, show the following minimum remaining life:
Straight Tubes Cleanable U-Bends Non-cleanable U-Bends 20 years 14 years 17 years 4
It is proposed the remaining component, the header/manifold assemblies, be replaced by the 1997 Refueling Outage since this component has been the source of the joint leaks.
Since the engineered clamps will see no wear due to erosion, the material is similar to the cooler material, and since the source of the leaks, will be replaced no later than 1997, we have concluded that the clamp life will be adequate to seal leaks on the cooler joints for as long as the current manifolds and headers are in place. It*should be noted, however, that the clamps are designed for the remaining plant life.
Inspection Prior to the start-up from the 1992 refueling outage and every refueling thereafter until the cooler repair or modification is completed, each non-repaired cooler will be inspected for leakage and clamps applied where needed. During each refueling outage subsequent to clamp installation, the clamps will be visually inspected to ensure integrity and replaced if necessary. In the event a leak is detected during power operation, a containment entry can be made to locate the leak and install a clamp at the affected joint. The installation vendor estimates that clamps can be installed at a rate of 20*per shift once preparation and set up are completed. It is estimated that each clamp could be installed in less than 30 minutes, and since the general area radiation levels in the vicinity of the coolers is low, exposure to installing personnel would be minimal.
Operability Limits If we discover a containment air cooler leak during power operation the limiting condition for operation in Technical Specification 3.4.2 for containment cooling will be entered. This allows one cooler to be out of service for up to seven days before a plant shutdown is initiated.
If the cooler leakage cannot be stopped within the allowable LCO time frame, the plant will be shutdown in accordance with the requirements of Technical Specification 3.4.2.
5 Additional Information In response to the previous relief request for the containment air coolers in which we requested relief from the zero leakage requirements of the Code (March 6, 1991), the NRC had questioned how we had addressed the structural integrity of the brazed tube joints, what the root cause of the failure was, and what confidence we had that the brazed joints will not fail during a MSLB or LOCA event. CPCO's February 27, 1991 letter provided the response to these questions. Our review of that response finds that information still to be current.