ML20207D675

Assessment of Design Info on Piping Restraints for Point Beach Nuclear Plant,Units 1 & 2.Staff Concludes That Licensee Unable to Retrieve Original Analyses That May Have Been Performed to Justify Removal of Shim Collars

ML20207D675
Person / Time
Site:	Point Beach
Issue date:	02/22/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20207D673	List: ML20207D666 ML20207D675
References
NUDOCS 9903100039
Download: ML20207D675 (4)

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UNITED STATES j' , :r NUCLEAR REGULATORY COMMISSION L WASHINGTON, D.C. 30000 4001 ASSESSMENT OF THE DESIGN INFORMATION ON PIPING RESTRAINTS WISCONSIN ELECTRIC POWER COMPANY POINT BEACH NUCLEAR PLANT. UNITS 1 AND 2 l

l DOCKET NOS.60-266 AND 50-301

1.0 INTRODUCTION

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By letter dated April 16,1998, the staff informed Wisconsin Electric (WE or licensee) that it wan l performing a review of records related to the design of restraints on the main steam (MS) piping

and primary loop auxiliary piping systems at the Point Beach Nuclear Plant (PBNP). ' Specific i requests for information were included in an enclosure to that letter. On June 15,1998, the licensee responded to the request for information based on availab!e records and the licensee i staff's recollection of original design aspects requested in the NRC staff's inquiry. As a follow-j up to this assessment activity, the staff visited WE corporate offices on October 15 and 16,

! 1998, to review relevant design records and interview licensee representatives. Duke

Engineering and Services (DE&S) was recently contracted by WE to perform an independent j evaluation of the existing MS piping at PBNP to confirm the adequacy of restraint design in the

MS piping system.

2.0 DISCUSSION

! 2.1 Main Steam Pioina i

j The licensee analyzed the MS piping for high energy line break (HELB) scenarios inside the

. Unit 1 and Unit 2 containments at PBNP. This evaluation was performed to assess the impact

of removing various shim collars from the original Unit 2 pipe whip restraint design. The 4 evaluation utilized criteria contained in Generic Letter (GL) 87-11 (" Relaxation in Arbitrary l Intermediate Pipe Rupture Requirements") to determine postulated break Irt.ations on the

! piping. In accordance with GL 87-11, licensees may eliminate intermediate .o re breaks when i pipe stresses are within specified acceptance limits. Review of the pipin'a @ ass analysis 4 reports for the MS piping inside containment indicates that the stresses in affected piping are j below the limits established in GL 87-11, and consequently, only the terminal endpoints required evaluation for HELBs in the MS piping inside containment.

The methodology utilized to evaluate the consequences of an HELB in the MS piping in this

analysis is consistent with the criteria established by Bechtel for the original plant design. This

method employs a concentrated blowdown force (originally developed by Westinghouse) equal i to the pressure multiplied by the intemal area of the pipe that is applied statically at the break ~

4 location. The piping system is then evaluated for plastic hinge formation as well as for the

effects on the piping system.-

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i 9903100039 990222 PDR ADOCK 05000266 -

P pop e ENCLOSURE'

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DE&S utilized the "AutoPIPE" computer program to perform an elastic analysis of the MS piping for a guillotine failure at the SG MS nozzle and the containment penetration anchor. This ,

computer program has been adequately benchmarked and has been accepted by the NRC for I similar applications. The analytical results were reviewed to determine whether plastic moments are developed in the system. If no plastic hinges are formed, then the system is stable and the piping is acceptable. If a plastic hinge is formed, then a moment-less hinge connection is modeled at that point, and the analysis is repeated based on the modified geometry and boundary conditions. The force applied to the terminal end for the second run is l equal to the original blowdown load minus the load required to form the initial plastic hinge. The l results of the second run are superimposed upon the results of the initial analysis, and then reviewed for development of additional plastic hinges. If no additional hinges are formed, the piping model is considered stable and deemed acceptable. If an additional hinge forms, further reanalysis would be required.

In the evaluation of the MS piping for a guillotine break at the containment penetration, the Unit 2 Loop B MS piping system was used as an enveloping case. A plastic hinge developed near one whip restraint due to a load less than the Bechtel tJowdown force. The result from the second run (including the plastic hhge in the model) was superimposed onto the results from the initial analyses. The combined results indicate that no additional plastic hinges will form in the piping. Therefore, the piping system was determined to be acceptable for the containment penetration guillotine break scenario.

Evaluation of the MS piping for a guillotine break at the SG MS nozzle addressed the deleted shim collars in the Unit 2 failure restraints. The Loop B MS piping system was again used as an enveloping case. A plastic hinge developed at an elbow as a result of a load less than the Bechtel blowdown force. Further evaluation identified that an additional plastic hinge will form at the snubber support upstream of the first hinge. This condition required additional analyses to be performed, as described below.

The jet loads required to develop the initial hinge on each MS piping loop (Unit 1 A/B and Unit 2 ,

A/B) were also determined. These loads were used to perform a nonlinear inelastic analysis to '

determine the rotation of the plastic hinge using an energy balance method. An energy balance of the plastic moment capacity of the hinge is equated to the energy in the system due to the fore applied to the overhung segment of piping from the break location at the SG nozzle to the location near the snubber supports where the initial plastic hinge formed.

Two cases were considered for the energy balance evaluation:

I Case 1 assumed no singla active failures occur and a limited fluid reservoir is available in the system; j

• Case 2 assumed a single active failure of the MS check valves,in the SG foop. This scenario provided an essentia!!y unlimited fluid reservoir from the opposite SG which j caused a sustained jet force to exist.

The energy balance evaluation used a 3-step time history considering an " initial" blowdown force, a " transitional" blowdown force, and a

• steady state" blowdown force. The blowdown forces developed in the time history were based on theoretical and testing data available with DE&S, as well as in supplemental technical procedures used for pipe rupture analysis and design in the 1978

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through 1983 time period. This data has been previously reviewed and found to be acceptable by the NRC staff. The initial and transitional blowdown forces were dependent on the pipe dimensions, and the pipe configuration from the break location to the fluid reservoir or an intermediate flow restriction. The steady-state blowdown force is dependent on the system frictional flow losses from the break location to the flow reservoir or an intermediate flow restriction.

The time history was used as a basis for the development of a single blowdown thrust force to be ,

used for the two phases of pipe whip motion: (1) the fluid accelerating the pipe from zero to a  !

maximum velocity, and (2) the hinge decelerating the piping from a maximum velocity back to

]j zero. These pipe movements determined the input inelastic energy and this energy was compared 1

to the energy needed for hinge rotation where all energy is conservatively absorbed. The overhung beam model was used to determine the rotation of the plastic hinge.

'l The snubber loads generated in Case 1 were determined to be less than the design loads provided by Bechtel. The system is therefore considered stable and the piping is acceptable. The rotational path of the plastic hinge was reviewed to verify that the jet loads do not impact critical ,

l systems / equipment. However, the energy developed as a result of the initial blowdown force in Case 2 is suffacient to rotate the plastic hinge in the piping system. This second case also includes a steady-state load remaining in the system after rotation of the hinge. Hence, a supplemental analysis was performed to determine additional formation of hinges. The results from this i 1

supplementary analysis determined that no further plastic hinges will form in the system.

Therefore, the system was considered stable and the piping was determined to be acceptable for the guillotine break at the SG nozzle.

i Based on its review of the licensee's summary of reanalysis of the MS piping and interviews with  !

licensee staff, the staff concludes that the licensee provided adequate justification that the existing l MS piping system and restraints inside containment at PBNP Units 1 and 2 are structurally j adequate and meet current licensing and design-basis requirements. On the basis of its assessment as discussed above, the staff believes the licensee provided a reasonable justification regarding the deletion of shim collars in the MS piping.

2.2 Auxiliary Pioina The staff examined records relating to the licensee's implementation of IE Bulletin 79-14 (" Seismic Analyses for As Built Safety-Related Piping Systems") for the auxiliary piping system. The records indicate that the licensee has completed the piping reverification program to satisfy the requirements of IE Bulletin 79-14 for safety-related piping at PBNP. The purpose of this program was to compare and reconcile the as-built pipo routing and support configurations against the l

design records. A total of 139 subsystems have been evaluated since the program was initiated in 1991, comprising almost 3000 piping supports and more than 36,000 feet of pipe. Fourteen subsystems in the original scope were dropped from the analysis effort in 1996 when the piping ,

was identified as nonsafety-related, independent contractors performed the piping reverification programs related to IE Bulletin 79-14 for the licensee. As-built walkdowns of selected relrod piping were completed by the licensee in 1996, with the exception of some of the RHR (residual heat removal) subsystems, which were added to the scope and completed in 1997. The majority of piping analyses were completed by the first half of 1998. The NRC regional staff followed the licensee's implementation program for IE Bulletin 79-14 and found it acceptable as discussed in a number of reports issued during the

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' -4 period between 1991 and 1998. The overall modification rate for the program, which includes support upgrades, was approximately 8 percent.

The staff believes the implementation of thic program provides reasonable assurance to conclude that the as built auxiliary piping and support installation are structurally adequate.

3.0 CONCLUSION

in summary, the staff concludes that the licensee was unable to retrieve the original analyses that may have been performed to justify the removal of the shim collars in the early 1970s, and hence, the staff cannot conclude whether or not their removal was justified in consideration of the licensing basis requirements at that time. However, based on the recent evaluations of the MS piping examined by the staff, the licensee has provided adequate technical justification for the removal of certain shim collars in the MS piping restraints. The staff also finds that the licensee '

has provided reasonable basis to confirm the structural adequacy of the current as built configuration of the MS and auxiliary piping systems.

Principal contributor: Jai Rajan Date: February 22, 1999 i

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