ML20136D675

Safety Evaluation Re Elimination of Arbitrary Intermediate Pipe Breaks in High Energy Piping Sys.Pipe Rupture Postulation & Associated Effects Are Adequately Considered in Design.Deviation from SRP Acceptable

ML20136D675
Person / Time
Site:	Seabrook
Issue date:	04/30/1985
From:	Office of Nuclear Reactor Regulation
To:
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ML20136D673	List: ML20136D667 ML20136D675
References
NUDOCS 8601060240
Download: ML20136D675 (12)

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SEABROOK NUCLEAR STATION UNIT 1 SAFETY EVALUATION FOR THE ELIMINATION OF ARBITRARY -

INTERMEDIATE PIPE BREAKS Docket No. 50-443 and 50-444 -

April 1985 U.S. Nuclear Regulatory Comission Office of Nuclear Reactor Regulation e

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TABLE OF CONTENTS Page I. INTRODUCTION ................................................. 1 II. APPLICANT'S BASES FOR THE ELIMINATION OF ARBITRARY I NTERMED I ATE P I PE B REA KS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 III. STAFF EVALUATION OF THE BASES FOR THE ELIMINATION ,,

O F A RB I TRARY B REA KS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 IV. REFERENCES .................................................... 9 9

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SEABROOK NUCLEAR STATION UNIT 1 SAFETY EVALUATION FOR THE ELIMINATION OF ARBITRARY INTERMEDIATE PIPE BREAKS

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I. INTRODUCTION In the " Background" to Branch Technical Pos'ition (BTP) MEB 3-1 as presented in Standard Review Plan (SRP) Section 3.6.2 (Ref.1), the staff position on pipe break postulation acknowledged that pipe rupture is a rare event which may only occur under unanticipated conditions such as those which might be caused by possible design, construction, or operation errors, unanticipated loads or un-anticipated corrosive environments. The BTP MEB 3-1 pipe break criteria were intended to utilize a technically practical approach to ensure that an adequate level of protection had been provided to satisfy the requirements of 10 CFR Part 50 Appendix A, General Design Criterion (GDC) 4. Specific guidelines were developed in MEB 3-1 to define explicitly how the requireinents of GDC 4 were to be implemented. The SRP guidelines in BTP.MEB 3-1 were not intended to be absolute requirements but rather represent viable approaches considered to be acceptable by the staff.

The SRP provides a well-defined basis for performing safety reviews of light water reactors. The uniform implementation of design guidelines in MEB 3-1 assures that a consistent level of safety will be maintained during the licens-ing process. Alternative criteria and' deviations from the SRP are acceptable provided an equivalent level of safety can be demonstrat'ed. Acceptable reasons for deviations from SRP guidelines include changes in emphasis of specific I

guidelines as a result of new developments from operating experience or plant-unique design features not considered when ~the SRP guidelines were developed.

The SRP presents the most definitive basis available for specifying NRC's design criteria and design guidelines for an acceptable level of safety for light water reactor facility reviews. The SRP guidelines resulted from many years of experience gained by the staff in establishing and using regulatory requirements in the safety evaluation of nuclear facilities. The SRP is part of a continuing regulatory standards development activity that not only docu-ments current methods of review, but also provides a basis for an orderly modification of the review process when the need arises to clarify the content, correct any errors, or modify the guidelines as a result-of technical advance-

! ments or an accumulation of operating experience. Proposals to modify the I guidelines in the SRP are considered for their impact on matters of major

) safety significance.

l The staff has recently received requests from the applicant for Seabrook Nuclear Station, Unit I to consider an alternate approach to the existing guidelines in SRP 3.6.2,~ MEB 3-1 regarding the postulation of intermediate pipe breaks (References 2, 3 and 6). For all high energy piping systems at Seabrook, the applicant proposes to eliminate from design Mon: 4crations those breaks w . - .- . .

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j generally referred to as " arbitrary intermediate breaks" (AIBs) which are t

defined as those break locations which, based on piping stress analysis results, are below the stress and fatigue limits specified in BTP MEB 3-1, but are <

selected to provide a minimum of two costulated breaks between the terminal

ends of a piping system. The applicant has documented the cost savings and reduced radiation exposure benefits resulting from the elimination of the  ;

structures associated with the protection against the effects of pipe rupture.

I The applicant has further stated that all dynamic effects associated with

! previously postulated arbitrary intemediate pipe breaks will be excluded from the plant design basis and that pipe whip restraints and jet shields associated with previously~ postulated arbitrary intermediate breaks will be eliminated.

g lowever, the. applicant has stated that the elimination of AIBs will not

downgrade the environmental qualification levels of Class IE equipment. The

break postulation for environmental effects is performed independently of break postulation for pipe whip and jet impingement.

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In the early 1970's when the pipe break criteria in MEB.'3-1 were first drafted, I

the advantages of maintaining low stress and usage factor limits were clearly recognized, but it was also believed that equipment .n close proximity to'the i

piping throughout its run might not be adequately designed for the

' environmental consequences of a postulated pipe break if the break postulation proceeded on a purely mechanistic basis using only high stress and teminal end .

breaks. As the pipe break criteria were implemented by the industry, the.

impact of the' pipe break criteria became apparent on plant reliability and costs as well as on plant safety. Although the overall criteria ~in MEB 3-1 have resulted in a viable method which assures that adequate protection has been provided to satisfy the requirements of GDC 4, it has become apparent.

• that the particular criterion requiring the postulation of arbitrary inter-mediate pipe breaks can be overly restrictive and may result in an excessive number of pipe' rupture protection devices which do not. provide a compensating

. level of safety.

At the time the MEB 3-1 criteria were first drafted, high energy leakage cracks were not being postulated. In Revision 1 to the SRP (July 1981), the concept of using high energy leakage cracks to mechanistically achieve the environment desired for equipment qualification was introduced to cover areas which are below the high stress / fatigue limit break criteria and which would otherwise not be enveloped by a postulated break in a high energy line. In the proposed elimination of arbitrary intermediate breaks, the staff believes that the essential design requirement of equipment qualification is not only being re-tained but is being improved since all safety-related equipment is to be quali-fied environmentally, and furthermore certain elements of construction which may lead to reduced reliability are being eliminated.

In addition, some requirements which have developed over the years as part of l j the licensing process have resulted in additional safety margins which overlap l

,' the safety margin provided in the pipe break criteria. For example, the cri-  !

teria in MEB 3-1 include margins to account for the possibility of flaws which '

might remain undetected in construction and to account for unanticipated piping steady-state vibratory loadings.not readily determined in the design process.

However, inservice inspection requirements for the life of the plant to detect flaws before they become critical, and staff positions on the vibration monitoring of safety-related and high energy piping systems during preoperational testing, further reduce the potential for pipe failures i occurring from these causes.

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Because of the recent interest expressed by the industry to eliminate the arbitrary interinediate break criteria and, p'articularly, in response to the detailed submittals provided by several utilities besides PSNH, the staff has reviewed the MEB 3-1 pipe break criteria.and approved deviations from SRP criteria for Catawba Unit 2 and Vogtle Units 1 and 2 (References 4 and 5).

II APPLICANT'S BASES FOR THE ELIMINATION.0F ARBITRARY INTERMEDIATE PIPE BREAKS In a letter from PSNH dated February 7,1985 (Ref. 2) and supplemental letters

~ dated February 19,1985 (Ref. 3) and April 12, 1985 (Ref. 6), the applicant presents a request for the elimination of arbitrary intermediate breaks and the technical bases for the request. The applicant's submittals suggest a general consensus in the nuclear industry that current knowledge and experience support the conclusion that designing for the. arbitrary intermediate pipe breaks is not justified. The reasons given for this conclusion are discussed in the following paragraphs.

1) Operating Experience Does Not Support Need for: Criteria PSNH states that the combined operating history of. connercial nuclear plants (extensive operating experience in over 80 operating U.S. plants and a number of similar plants overseas)* has not shown the need to provide. protection from the dynamic effects of arbitrary intermediate brea.ks.

2) Piping Stresses Well Bel'ow ASME Code Allowables Currently, AIBs are postulated to provide a minim 0m of two pipe breaks at the two highest stress locations between piping terminal ends. Consequently, arbitrary intermediate breaks are postulated at locations in the piping system where pipe stresses and/or cumulative usage factors are well below ASME Code allowables. Such postulation ne' cessitates the. installation and maintenance of complicated mitigating devices to afford protection from dynamic effects such as pipe whip and/or jet impingement. When these selected break Iccations have stress levels only slightly greater than the rest of the system, installation of mitigating devices not only lends little to enhance overall plant safety, i

but also provides the potential for inadvertent thermal restraint of piping.

3) Arbitrary Intermediate Breaks Complicate the Design Process

' The applicant states that the design of piping systems is an iterative process and the location of the highest stress points usually change several times

' during design. Although SRP Section 3.6.2 (Ref.1) provides criteria intended to reduce the need to relocate the intermediate break locations when high stress points shift due to piping reanalysis, in practice, these criteria provide little relief from moving arbitrary break locations since the revised break locations must still be evaluated as to their effects on essential equipment and structures.

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• 'The information within parentheses is provided by the NRC staff and not by PSNH.

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4) Substantial Cost Savings The cost benefits to be realized from the elimination of,the arbitrary inter-mediate break locations center primarily on the elimination of the associated pipe whip restraints and jet shields. While a substantial reduction in capital and engineering costs for these restraints and structures can be realized in the design and construction stages of the plant, there are also significant operational benefits to be realized over the 40 year life of the plant, as reduced manhours for inservice inspection and maintenance will result.

,5) Improved Inservice Inspection Pipe whip restraints are normally located adjacent to or surrounding the welds at changes in pipe direction. Access during plant operation for inservice inspection activities can be improved due to the elimination of congestion created by these pipe rupture protection devices and the supporting structural framing associated with arbitrary pipe breaks.

6) Reduction in Radiation Exposure In the event of a radioacti've release or spill inside the plant, decontamination operations could be more effective 1,f the pipe whip restraints and jet shields associated with AIBs and the large structural frameworks supporting the restraints were eliminated. Recovery from unusual plant conditions would also be improved by reducing the congestion in the plant. A significant reduction in man-rem exposure can be realized through fewer man-hours spent in radiation a reas.

The applicant, as part of its justification for the elimination of arbitrary intemediate breaks has estimated that the reduction in operational radiation exposure due to elimination of arbitrary intermediate pipe breaks and the resulting decrease in pipe whip restraints and jet deflectors over the 40 year life of the plant will be about 80-160 person-rems (References 2 and 6).

7) Improved Operational Efficiency The elimination of pipe whip restraints associated with arbitrary breaks will preclude the requirement for cutback insulation or special insulating assemblies near the close fitting restraints. This will reduce the heat loss to the surrounding environment, especially inside containment.

III STAFF EVALUATION OF THE BASES FOR THE ELIMINATION OF ARBITRARY BREAKS The technical bases for the elimination of the arbitrary intermediate break criteria as discussed in the preceding section of this report provided many arguments supporting the applicant's conclusion that the current SRP guidelines on this subject should be changed. However, it is not apparent that a unila-teral position by the utility concluding an unconditional deletion of the ar-bitrary intermediate break criteria can be justified without a clear understanding of the safety implications that may result for the various classes of high energy piping systems involved. In this section, we will discuss the bases behind the current arbitrary intermediate break criteria from an ASME Code design standpoint and put into perspective the uncertainty factors on which the need to postulate arbitrary intermediate breaks should be

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evaluated. We further evaluate the' acceptability of the applicant's proposed i deviation from SRP Section 3.6.2. In the " Background" to Branch Technical Position MEB 3-1 in Section 3.6.2, of the SRP (NUREG-0800), the staff position  :

recognizes that pipe rupture is a rare event which may only occur under ,

i unanticipated conditions such as those which might be caused by possible design,  :

construction, or operation errors, unanticipated loads, and unanticipated I corrosive environments. Furthemore, the staff recognizes that on those rare  ;

i occasions when piping failure does occur the failure is expected to occur at  ;

4 locations of high stress and fatigue such as at teminal ends of piping systems ,

and at local welded attachments to the piping wall. This generalization does not cover situations in which stress corrosion cracking is prevalent. Thus, the staff believes that pipe breaks should be postulated at locations where  ;

there exists a relatively higher potential for . failure to assure a practical level of protection. The preceding staff positions are not new and are stated in the " Background" to BTP MEB 3-1 of the SRP.

The SRP guideline which requires that two intermediate breaks be postulated even when the piping stress is low resulted from the need to assure that equip-ment qualified 'for the environmental consequences of a postulated pipe break 4

was, provided over a greater portion of the high energy piping run. The staff j now proposes to dispense with arbitrary intermediate breaks on the condi-j tion that al,1 equipment in the spaces traversed by the fluid system lines, for l

l intemediate breaks are being eliminated, is qualified for the '

i which arbitrary (non-dynamic) conditions that would result from a non-mechanis environmental l

j break with the greatest consequences on surrounding equipment. .

ASME Code Class 1 Piping Systems i

3 In accordance with BTP MEB 3-1 (paragraph B.1.c.(1)), breaks in ASME Code  !

Class 1 piping should be postulated at the following locations in each piping

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i (a) at teminal ends;  !

} (b) at intemediate locations where the maximum stress range as {

j calculated by Eq. (10) and either Eq. (12) or (13) of ASME Code  !

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(c) at intermediate locations where the cumulative usage factor exceeds j

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! (d) If two intemediate locations cannot be detemined by (b) and (c)

I above, two highest stress locations based on Eq. (10) should be i

! selected. i l The AIB criteria are stated in (d) above. It should be noted that the request j for alternative criteria does not propose to deviate from the criteria in (a),

(b),and(c)above. Pipe breaks will continue to be postulated at terminal

ands irrespective of the piping stresses.

! Pipe breaks are to be postulated at intemediate locations where the maximum j

stress range as calculated by Eq. (10) and either (12) or (13) exceeds 2.4 S,.

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The stress evaluation in Eq. (10) represents a check of the primary plus

secondary stress intensity range due to ranges of pressure, moments, thermal gradients and combinations thereof. Equation (12)isintended.topreventfor- ,

mation of' plastic hinges in the piping system caused only by moments due to i i themal expansion and thermal anchor movements. - Equation (13) represents a j limitation for primary plus secondary membrane plus bending stress intensity

' i l excluding thermal bending and thermal expansion stresses; this limitation is

! intended to assure that the K factor (strain concentration factor) is con-servative. The K factor waI developed to compensate for absence of elastic

, shakedown when prTsary plus secondary stresses exceed 3 S,.

l 11th respect to piping stresses, the pipe break criteria were'not intended to imply that breaks will occur when the piping stress exceeded 2.4 S

{ primary plus secondary stress limit). It is the staff's belief, h8we(80%

ver, of the that

' if a pipe break were to occur (in one of those rare occasions), it is more likely to occur at a piping location where there is the least margin to the ,

ultimate tensile strength. -

} Similarly, from a fatigue strength standpoint, the staff believes that a pipe i break is more likely to occur where the piping is expected to experience large cyclic loadings. Although the s'taff concurs with the industry belief that a i ', cumulative usage factor of 0.1 is a relatively low limit, the uncertainties '

• involved in the design considerations with respect to the actual cyclic load-

, ings experienced by the piping tend to be greater than the uncertainties i involved in the design considerations used for the evaluation of primary and i

i secondary stresses in piping systems. The staff finds that the conservative fatigue considerations in the current SRP guidelines provide an appropriate margin of safety against uncertainties for those locations where fatigue failures are likely to occur (e.g. at local welded attachments).

l In its presentation to the ACRS on June 9,' 1983 and in an October 5,1983 meet-

) ing between a group of PWR near-term operating license utilities, the staff indicated that the elimination of arbitrary intermediate breaks was not to i

apply to piping systems in which stress corrosion cracking, large unanticipated dynamic loads such as steam- or water-hanner, or thermal fatigue in fluid mixing situations could be expected to occur. In addition, the elimination of arbitrary intermediate breaks was to have no effect on the requirement to environmentally aualify safety-related equipment and in fact this requirement was to be clarified to assure positive qualification requirements.

For Class 1 piping, a considerable amount of quality assurance in design, analyses, fabrication, installation, examination, testing, and documentation is l provided which ensures that the safety concerns associated with the uncertain-i ties discussed above are significantly reduced. Based on the staff evaluation j

i of the design considerations given to Class 1 piping, the stress and fatigue limits provided in the MEB 3-1 break criteria, and the relatively small degree I

of uncertainty in the loadings, the staff finds that the need to postulate arbi-trary intermediate pipe breaks in ASME Code Class 1 piping in which large l , unanticipated dynamic loads, stress corrosion cracking, and thermal fatigue i

such as in mixing situations are not present and in which all equipment has been environmentally qualified is not compensated for by an increased level of j safety. In addition, systems may actually perform more reliably for the life l

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of the plant if the reque'st to postulate arb'itrary intermediate break criteria 1 for ASME Code Class 1 piping is eliminated. The staff has concluded that

! these requirements are present for those ASME Code Class 1 piping systems l identified in the applicant's submittals of February 7,1985 and February 19, 1985 (References 2 and 3).

( ASME Code Class 2 and 3 Piping Systems l In accordance with MEB 3-1 [ paragraph B.I.c.(2)] breaks in ASME Code Class 2 and 3 piping should be postulated at the following locations:

! (a) at teminal ends l (b) at intermediate locations selected by one of the following criteria:

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(1) at each pipe fitting, welded attachment, and valve (ii) at each location where the stresses exceed 0.8 (1.2 Sh + SA ) bUI i at not less than two separated locations chosen on the basts of highest stress.

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In its request, the applicant has not proposed changing criterion (a) above.

i Postulation of pipe breaks at teminal ends will not be eliminated in the i requested SRP deviation for Class 2 and 3 piping systems. Breaks are required to j be postulated at teminal ends irrespective of piping stresses.

' The " arbitrary intemediate break criteria" is stated in (b)(ii) above where breaks are to be postulated at intermediate locations where the stresses exceed 1

0.8 (1.2 S S.) but "at not less than two separated locations chosen on the

basisofhIg+eststress."

h The stress limit provided in the above pipe break l .

criterion represents the. stress associated with 80% of the combined primary and

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i secondary stress limit. Thus, a break is required to be postulated where the

! maximum stress range as calculated by the sum of Equation (9) and (10) of NC/

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ND-3652 of the ASME Code,Section III, exceeds 80% of the combined primary and '

l secondary stress limit, when we consider those loads and conditions for which t

} 1evel A and level B stress levels have been specified in the system's design ,

4 specification (i.e. sustained loads, occasional loads, and thermal expansion) f i

including an operating basis earthquake (08E) event. However, the Class 2 and

! 3 pipe break criteria do not have a provision for the postulation of pipe breaks based on a fatigue limit since an explicit fatigue evaluation is not {

i required in the ASME Code for these classes of construction because of l

favorable service experience and lower levels of operating cyclic stresses.

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For those Class 2 and 3 piping systems which ex (

cycles (e.g., main steam and feedwater systems)perience a large

, the ASME Code hasnumber provisions of stress E which are intended to address these types of loads. The rules governing con-siderations for welded' attachments in ASME Class 2 and 3 piping which do pre- 3 clude fatigue failure are partially given in paragraph NC/ND-3645 of the ASME  ;

Code. The Code states.

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" External and internal attachments to piping shall be designed so as not I*

to cause flattening of the pipe, excessive localized bending stresses, or hamful thermal gradients in the pipe wall. It is important that such attachments be designed to minimize stress concentrations.in applications i

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where the number of stress cycles, due either t'o pressure or thermal effect, is relatively large for the expected life of the equipment."

i Code rules governing the fatigue. effects associated with general bending stresses caused by thermal expansion are addressed in NC/ND-3611.2(e) and are

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generally incorporated into the piping stress analyses in the form of an allowable stress reduction factor.

Thus it can be concluded that when the piping designers have appropriately-considered the fatigue effects for Class 2 and 3 piping systems in accordance

,with NC/ND-3645, the likelihood of a fatigue failure in Class 2 and 3 piping caused by unanticipated cyclic loadings can be significantly reduced.

Because of the susceptibility of main feedwater systems in other plants to water hamer, the applicant has jncorporated several water hamer prevention /

minimization features into the design of the feedwater piping at the.Seabrook Station. The Seabrook steam generators (SG) are Westinghouse Model F type in which feedwater enters at an elevation above the top of the U-tubes through 3

the 16-inch feedwater nozzle. The water is distributed circumferential1y within the SG by means of a feed ring. The feedwater enters the ring via a welded thermal sleeve connection and leaves it through inverted J-tubes located at the flow holes that are at the top of the ring. The J-tubes are arranged to distribute the bulk of the colder feedwater to the hot leg side of the tube

,' bundle. The feedring is designed to minimize conditions within the SG which could result in water hamer occurrences in the feedwater piping.

Complying with the Westinghouse recomendations regarding the routing of the feedwater piping, each SG nozzle utilizes a 90-degree elbow connected imediately to a near vertical run of pipe to minimize potential steam voids.

Under normal operating ' conditions, the feedwater flow arrangement ensures that the line is kept filled with water, thus preventing steam from leaking back .

into the feedwater piping. Also, feedwater control valves (FCV) instabilities have been minimized by ensuring that all components in the system are compatible.

To stabilize operation of low power levels, a small bypass control line is l

provided.

As stated in the Seabrook SER Section 10.4.7 the applicant has agreed to perform tests to verify that unacceptable water hammer will not occur when the plant operating procedures are being used for normal-and emergency restoration of SG water level following a loss of normal feedwater. The staff concurs with the applicant's conclusion that the design features and operating procedures

, described above will minimize the potential for water hammer occurrence in the main feedwater piping system.

Based on~ the staff evaluation of the design considerations given to Class 2 and ,

3 piping, the stress limits provided in the SRP break criterion, and the degree ,

of uncertainty in unanticipated loadings, the staff finds that dispensing with arbitrary intermediate pipe breaks is justified for Class 2 and 3 piping in ,

which stress corrosion cracking, large unanticipated dynamic loads, or thermal '

fatigue in fluid mixing situations are not expected to occur provided 1) the

' piping designers have appropriately considered the effects of local welded s

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. 1 attachments per NC/ND-3645, and 2) all safety-related equipment in the vicinity of Class 2 and 3 piping systems have been environmentally qualified for the non-dynamic effects of a non-mechanistic pipe break with the greatest conse .

quences on the equipment. The staff has concluded that the above described requirements are present for those ASME Code Class 2 and 3 piping systems identified in the applicant's letters dated February 7, 1985. February 19, 1985 I and April .12,1985 (References 2, 3 and 6).

Piping Systems Not Included in Proposal For those piping systems, or portions thereof, which are not included in the ipplicant's submittals (References 2, 3 and 6), the staff requires that the existing guidelines in BTP MEB 3-1 of the SRP (NUREG-0800) Revision 1 be met. However, should other piping lines which are not specifically identified in the applicant's submittals (References. 2, 3 and 6) subsequently qualify for the conditions described above, the implementation of the proposed elimination of the arbitrary intemediate break criteria may be used provided those additional piping lines are appropriately identified to the staff.

Conclusion

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The applicant has requested a deviation from the current guidelines of the SRP by proposing relief from postulating arbitrary intermediate pipe breaks in some high energy piping systems. The applicant has consnitted to perfonn pre-operational testing of all the systems identified in References 2, 3 and 6 and also monitor those systems' for vibration during preoperational and startup testing. The staff has evaluated the technical bases for the requested deviation with respect to satisfying the requirements of GDC 4. Furthermore, the staff has considered the potential problems identified in NUREG/CR-2136 (Reference 7) which could impact overall plant reliability when excessive pipe whip restraints are installed. Based on its review, the , staff finds that when- those piping system conditions as stated above are met, there is a sufficient basis for concluding that an adequate level of safety exists to accept the requested deviation.

Thus, based on the piping systems having satisfied the above conditions, the staff concludes that the pipe rupture postulation and the associated effects are adequately considered in the design of the~Seabrook Nuclear Station Unit I and, thus, the deviation fro:n the Standard Review Plan is acceptable.

• IV REFERENCES ~

1) " Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants", NUREG-0800 (Revision 1) dated July 1981.

2) Letter from J. DeVincentis, PSNH, to G. W. Knighton, NRC, subject.

" Elimination of Arbitrary Intemediate Pipe Breaks," dated February 7, 1985.

3) Letter from J. DeVincentis, PSNH, to G. W. Knighton, NRC,

Subject:

" Elimination of Arbitrary Intermediate Pipe Breaks; Retransmittal i of Attachment D. Potential for Stress Corrosion Cracking," dated i February 19, 1985. l h_ _ _ _ ___ _ _ _ _ _ . . _ . _ _ ._ _ _ _ --

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l.  ! t l . , Ia I 4) ' Letter dated April 2,1984 from T. Novak to H. 'B. Tucker, subject, j " Catawba Nuclear Station, Unit 2 - Transmittal of a Safety Evaluation j Report" ,

l 5) Letter dated June 28, 1984 from T..Novak to D. O. Foster,' subject, "Vogtle Electric Generating Plant - Units 1 and 2, Arbitrary Intemediate Pipe Breaks."

6) Letter from J. De Vincentis, PSNH, to G. W. Knighton, NRC, subject, j " Elimination of Arbitary Intermediate Pipe Breaks in the Feedwater System", April 12, 1985.

7) "Effect of Postulated Event Devices on Nomal Operation of Piping Systems in Nuclear Power Plants", NUREG/CR-2136 dated May 1981. '

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