ML20215F916
| ML20215F916 | |
| Person / Time | |
|---|---|
| Site: | Washington Public Power Supply System |
| Issue date: | 10/01/1986 |
| From: | UNITED ENGINEERS & CONSTRUCTORS, INC. |
| To: | |
| Shared Package | |
| ML20215F914 | List: |
| References | |
| NUDOCS 8610160418 | |
| Download: ML20215F916 (33) | |
Text
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DOCKET NO. 50-460 f'*_
WNP-1 POSITION ON NON-MECHANISTIC PIPING FAILURES IN MAIN STEAM AlO FEE 0 WATER ISOLATION AREA (MSFIA)
WASHINGT05 PUBLIC POWER SUPPLY SYSTEM NUCLEAR PROJECT NUMBER 1 (WNP-1) i PREPARED BY UNITED ENGINEERS & CONSTRUCTORS INC.
30 SOUTH 17TH STREET, PHILADELPHIA, PA 19101 8610160418 861001 PDR ADOCK 05000460 A
PDR 3096R/jah J
C TABLE OF CONTENTS PAGE ABSTRACT i
1.b INTRODUCTION 1
2.0 DISCUSSION OF LARGE-80RE PIPING SYSTEM FEATURES 3
A.
Main Steam System 3
8.
Main Feedwater System 6
C.
High Pressure Condensate Orain 8
3.0 LICENSING EVALUATION 9
A.
Background
9 8.
" Exempt Piping" Design Features 13 C.
MSFIA Building Design Features 16 D.
Industry Evaluations of Piping 18 Failure Mechanisms 4.0 St.NMARY 20
5.0 REFERENCES
22 FIGURES
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A8STRACT High-energy fluid system piping in containment penetration areas is given special considerations with respect to postulated failure events. Design features to protect such piping and the containment isolation valves against failure effects that might occur in piping either upstream or downstream of the isolation valves as well as features to provide adequate protection to essential components and equipment are contained in NRC Standard Review Plans 3.6.1 and 3.6.2.
A discussion of the specific design features incorporated into the WNP-1 Main Steam and Feedwater Isolation Area (MSFIA) to address these concerns is contained herein.
The measures taken to provide a high level of assurance that a large-bore high-energy piping failure would not occur and the extent of compliance of the design relative to the above guidelines are presented.
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e EXECUTIVE
SUMMARY
The design and construction of the Main Steam and Feedwater Isolation Area (MSFIA) high-energy fluid system piping has embodied features which provide a high level of' assurance against the possibility of a large-size piping failure.
Thesi features were provided in order to meet the criteria of Standard Review Plans 3.6.1 and 3.6.2 of NL. REG-75/087.
Included in this 4
category erithe design and installation of boundary restraints on the eair.
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steam and main feedwater piping at the transition point from ASE III, Class 2 to Non-Nuclear Saf.ety'(NNS), special pipe forgings at these restraints, simplified piping geometry within the MSFIA and containment penetrations with integrally-forged flued heads. A planned program to perform augmented in-service inspection (ISI) on the welds within the fluid pressure boundary
' will further enhance the level of protection provided.
Current SRP criterion, specifically BTP ASB 3-1 of NUREG-0800, states that a
'1.0 ft2 "non-mechanistic" failure be postulated in the main steam and l
feedwater system piping and the ensuing jet impingement and environmental i
l effects addressed. This criterion is typically directed towards portions of main steam and feedwater piping that have satisfied all,of the
" break-exclusion" requirements delineated in BTP WB 3-1 of NUREG-0800'so that environmental considerations on essential equipment can be addressed. A back-fit of this criterion to the existing MSFIA design was not practical at the time of publication of NUREG-0800, given the advanced stage of construction.
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e The MSFIA structure and the internal essential equipment and components are or will be structurally and environmentally qualified to withstand fluid releases from a 4" diameter steam line circumferential failure. Such a failure represents the most severe " mechanistically-derived" break postulated for high-energy piping within the MSFIA, and is the design-basis break for the MSdIA.
In view of the protective features incorporated with regard to large-bore piping and tne implementation of mechanistic break criteria on the small-bore pipirg, it is felt that the basic concern of assuring plant safe shutdown through the functioning of essential equipment and components is adequately addressed in the WNP-1 design.
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1.0 INTRODUCTION
The Main Steam and Feedwater Isolation Area (MSFIA) is a reinforced concrets structure, designed to Seismic Category I criteria and with the capability to resist tornado-generated missiles. This structure houses, among other items, the main steam and feedwater piping and associated isolation valves, code safety valves and modulating atmospheric dump valves.
Inasmuch as this. fluid system piping is
. categorized as high-energy, consideration must be given in the plant design to the potential for a piping failure.
j Standard Review Plans 3.6.l( ) and 3.6.2(2) were used as a basis for establishing criteria for pipe break considerations within the MSFIA. BTP APCSB 3-1, attached to Reference (1), provided for the incorporation of pipe whip restraints upstream and downstream of the
" break-exempt" piping such that any extraneous loadings would not impair the operability of the isolation valves nor, affect the leaktight integrity of the containment. The identification of terminal ends of piping runs beyond the " break-exempt" piping and the maintenance of piping classification until beyond the outboard l
restraint was also stipulated.
o BTP MEB 3-1, attached to Ref. (2), also provided criteria relative to qualifying certain piping in the " break-exempt" category. Stress allowables for the plant normal and upset conditions as well as for the plant faulted condition were specified.
The WNP-1 design incorporated those features necessary to comply with the then-current SRP criteria, contained in References 1 and 2, for exempting certain fluid system piping from failure considerations.
Subsequent to this activity, revisions to the afore-mentioned Standard Review Plans and identified as References (3) and (4) were published in NUREG-0800.
Within this document, specifically Subsection B.l.a of BTP AS8 3-1, a provision was included for the 2
postulation of a 1.0 ft longitudinal break in the main steam and feedwater lines.
Per this criteria, this break was to be evaluated in the above piping even after having satisfied the criteria stipulated in BTP MEB 3-1 for " break-exemption" status.
This break is commonly termed as "non-mechanistic", in that its origin is not explicitly attributable to high stress levels or fatigue factors or to complex piping geometry. Postulation of this break was stipulated primarily to provide assurance that essential equipment, if located in this area, could withstand the adverse environment produced by such a piping failure.
2.0 DISCUSSION OF LARGE-BORE PIPING SYSTEM FEATURES Large bore high-energy piping runs within the MSFIA that have been considered in the " break-exempt" category are the four (4) 28" I.D.
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main steam lines, the four (4) 6" modulating steam dump lines, the two (2) 22" dia, feedwater lines and the two (2) 10" dia.
high-pressure drain lines. The physical layout of these lines is illustrated in Figures 1 through 4.
Each system is discussed separately below:
A.
Main Steam System Each of the four exempt 28" main steam lines is bounded by the containment penetration on its upstream side and by a " boundary restraint" on its downstream side. On each line, between the bounced portions is an isolation valve and a free-ended branch which contains the five code safety valves. Except for the safety / relief valve branch connection, the piping main runs between the containment penetration and the boundary restraint do not contain any elbows or otherwise change direction. The downstream boundary restraint, located in the exterior wall of l
l the MSFIA, is close to and in line with each isolation valve.
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This restraint has been analyzed and designed so as to prevent l
the downstream piping from transmitting horizontal and vertical shear forces, bending moments about either axis and torsional l
moments into the exempt piping section. As such, this restraint l
l functions to react five out of a possible six components of load, and has been termed a five degree-of-freedom (5-00F)
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restraint, or boundary restraint. Axial expansion (and contraction) of the exempt pip'ing is permitted, inasmuch as the upstream end (at the containment penetration) is a full an' hor.'
c Thus, only axial loads produced by occasional events, such as seismic or hydraulic transients, could be transmitted into this MSS piping. However, due to the configuration of the piping and the design of the containment penetration, such loads would have a minimal effect on piping stresses and would not impair isolation valve operability.
Loads originating in the piping upstream of the exempt section in the MSFIA are reacted by the containment penetration, which has been designed as a full anchor. This isolates all inside containment loads and effects, except for anchor displacements, from the exempt piping section. Stresses within the exempt piping section have been calculated due to the loadings imposed by deadweight, thermal, seismic and safety / relief valve actuation effects.
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Piping classification, as required by Regulatory Guide 1.26, would normally change at the isolation valve.
However, in accordance with BTP APCSB 3-1, the code transition has been 1
extended to just beyond the 5-DOF restraint.
This piping segment between the isolation valve and the restraint has been analyzed and designed in accordance with ASME Section III, Class l
2 standards. Likewise, its procurement, fabrication and i
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installation are to the same standards. For.any in-service inspection considerations, this piping segment will be treated
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in a manner identical to the piping upstream of the isolation valve.
The boundary restraint has been designed and analyzed as a Class 2 Component Support, in accordance with the provisions of Subsection NF.
In the consideration of loads that might be applied to this restraint, normal and upset, as well as postulated event (seismic, pipe breaks in the downstream Non-Nuclear Safety.(NNS) Section) effects were included.
The stresses created by these sustained and occasional loadings, when combined in accordance with the guidance in SRP 3.6.2, produce values which at all locations in the exempt piping, are below the prescribed limits contained in this SRP. Figure 1
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illustrates the lay-out of two (2) main steam runs which are considered exempt.
Also within the " break-exenpt" category are four 6" lines between the 28" main steam headers and the normally-closed modulating atmospheric dump (MAO) valves.
There are two such lines per steam generator train. Discharge from these valves exits the MSFIA via a roof stack which is supported independently from the valve exhaust line.
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In as much as the downstream side of each valve is normally at atmospheric pressure, no boundary restraint is required at the Class 2/NNS interface. All piping between the connection t'o the 28" main steam header and the MAO valve is ASME II'I, Class 2; all supports are per Subsection NF.
Welds comprising the fluid pressure boundary will be included in the augmented ISI program. The stress analysis of each line includes the effects of the movement of the 28" main steam header as well as the consideration of dynamic loads originating from MAD valve actuation. The calculated pipirg stresses are less than the prescribed limits given in Reference (2).
The physical lay-out
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of these lines for one steam generator train is illustrated in Figures 1 an'd 4.
8.
Main Feedwater System There are two (2) 22" 0.D. feedwater piping runs located inside the MSFIA which have been considered " exempt" from non-mechanistic failure postulation. Each run contains two (2)
ASME III Class 2 isolation valves.
The exempt portion is bounded on the upstream side by a boundary restraint and on the downstream side by the containment penetration.
The boundary restraint, located in the exterior wall of the MSFIA, is in close proximity to and in line with the outermost isolation valve. This restraint is configured similar to that employed on the main steam lines, such that all components of piping loads except for axial force are reacted.
Such loads would originate -
t in ' he upstream NNS piping in the Water Treatment Building and t
would include normal operational loads as well as loads from postulated events. The boundary restraint, as well as the piping between the outermost isolation valve and this restraint,
.were treated in the same manner as that employed on the MSS runs.
The containment penetration, anchored into the primary containment structure, bounds the downstream side and serves to isolate the " exempt", piping from all inside containment piping effects.
Within each exempt piping run, an offset was provided because of space requirements associated with the operators on the isolation valves. To minimize loads that might be induced into i
this offset section by axial forces, an additional pipe support / whip restraint was provided inside the MSFIA. Figure 2 illustrates the routing of one piping train.
Stress analysis of these feedwater piping sections has produced values which are below those prescribed in BTP ME8 3-1.
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C.
High Pressure Condensate Drain There are two piping headers (sizes 10" and 6" diameter) inside the MSFIA which have been considered " exempt" from non-mechanistic failure postulation. Each header contains an electro-hydraulic Class 2 containment isolation valve.
The exempt portion is bounded by the containment penetration on the upstream side and by a boundary restraint on the downstream side which is close to and in line with the containment isolation valve (See Figure 3).
It is configured to react loads in a manner identical'to those on the main steam and feedwater lines. The analysis and design approach for this restraint, as well as for the piping segment between the isolation valve and the restraint, is similar to that used for the main steam and feedwater systems.
Figure 3 illustrates the routing of these piping headers.
Values of piping stress in these " break-exempt" portions are less than the prescribed limits contained in BTP MEB 3-1 of Ref.
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3.0 LICENSING EVALUATION A.
Background
The'WNP-1 design for the MSFIA complies with the Standard Review Plan criteria contained in References (1) and (2) relative to the exemption from postulated failure of certain high-energy piping.
These criteria permitted exclusicn from failure considerations those portions of fluid system piping that were essentially located between the containment penetration and the containment isolation valve. BTP MEB 3-1, attached to Ref. (2),
delineated the features that such piping should possess in order to be so categorized.
For ASPE III Class 2, fluid system piping, these features are summarized ~as follows:
(1) satisfaction of maximum stress ranges under plant normal / upset conditions as well as of a stress limit under a plant faul'ted condition.
-(2) avoidance of welded attachments, for pipe supports or other purposes, to such piping except where detailed stress analyses or tests demonstrate compliance with (1) above.
(3) minimization of pressure boundary welds and branch connections.
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(4) reduction, to the extent practicable, of the length of exempt piping.
(5) utilization of pipe anchors or restraints which do not require welding directly to the fluid pressure boundary except where these welds are accessible for examination and a detailed stress analysis is performed to comply with (1) above.
BTP APCSB 3-1, attached to Ref. (1), likewise provided guidelines for fluid system piping in containment penetration areas. Briefly, these provisions were as follows:
(1) satisfaction of the stress limits of B.l.b of BTP MEB 3-1 and incorporation of pipe whip restraints capable of withstanding the loadings resulting from postulated piping failure beyond these (i.e., break-exempt) portions of piping so as to assure the operability of containment isolation valves and the leak-tight integrity of the containment.
(2) satisfaction of certain design pressure, temperature, stress level and access features on guard pipes, if used (applicable to dual barrier containment structures).
(3) identification of points adjacent to the required pipe whip restraints located inside and outside containment as terminal ends (for purposes of pipe break postulation). -
(4) retention of the piping classification, as defined in Regulatory Guide 1.26, until beyond the outboard restraint.
In 1981, NUREG-0800 was published; revisions to SRP's 3.6.l(3) and 3.6.2 were included in this document.
BTP MEB 3-1, attached to Ref. (4), was essentially unchanged relative to the original issue with respect to piping in containment penetration areas. A supplementary guideline was added for augmented in-service inspection of pressure boundary welds during each inspection interval as defined by Section XI of the ASME Code.
A commitment to adhere to this guideline is contained in Subsection 6.6.8 of the WNP-1 FSAR.
BTP ASB 3-1, attached to Ref. (3) and formerly identified as BTP APCSB 3-1, contained a major perturbation for the large-bore
" break-exclusion" piping. This BTP called for the postulation 2
of a 1.0 ft longitudinal break in the main steam and feedwater piping in the containment penetration area and a
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subsequent evaluation of the jet impingement and environmental effects on essential equipment and structures, i
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The primary intent of this stipulation wcs to address concerns regarding essential equipment in such areas,and the proximity of such piping to the control room. For compliance, essential equipment would either have to be separated from the piping so as not to experience the environmental effects of such a failure or would have to be qualified to function in the adverse environment.
The structure enclosing such piping and equipment would have to be capable of sustaining the elevated pressures-and temperature levels produced by the fluid release. ' For control room considerations, such large-bore high-energy piping should be sufficiently remote from it such that the functionality and habitability of the control room would be unimpaired.
At this stage of construction, modifications to the MSFIA structure to accommodate the overpressure produced by a 1.0 2
ft piping failure are not practical. The structure is capable of withstanding an overpressure of 2 psig, which is greater than that produced by the limiting mechanistic piping failure, a 4" steam line break.
The as-designed, as-constructed condition of the MSFIA has been re-evaluated in detail with regard to the effects produced by mechanistic piping failures.
As a result, modifications involving the relocation or supplemental qualification of 4 ~
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essential equipment will be implemented so as to assure functionality of this ecpipment in the adverse environment resulting from the design basis mechanistic pipe break. These are discussed further in Section 4.0 of this report.
The extent to which protective measures and features have been incorporated on an overall basis into the MSFIA are described below.
B.'
" Exempt Piping" Design Features The high-energy portions of piping within the MSFIA for which non-mechanistic failures have not been postulated share several common design features.
These features are:
Simplified Geometry: These sections are short and, with th6' exception of the feedwater lines, are straight.
The feedwater lines required an offset in order to provide physical space for the isolation valve operators.
Use of Seamless Piping for the Fluid System Pressure Boundary: All piping portions comprising the exempt sections are constructed either from SA-106, Gr.B or SA-106, Gr.C.
Such piping, suitable for high-temperature service, is formed with no longitudinal welds.
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of such material has minimized the number of welds within the pressure boundary. -
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Flued Integrally-Forged Heads at Containment Penetrations:
The means of anchorage of the process piping to the containment structure is achieved through the use of a forged flued head / process pipe which is welded to an embedded sleeve in the containment wall.
This forging is of a length such that the weld ends on the process pipe are readily accessible for in-service volumetric examination.
Low Stress Levels: These piping portions satisfy all applicable ASME III criteria as well as the more restrictive NRC criteria regarding combined stresses
. under normal / upset plant conditions as well as for a plant faulted condition.
Boundary Restraint: Each piping portion contains a NF support designed and fabricated to protect the ASME III portion from the effects of postulated events which might.
originate in the NNS portion.
Extension of the Class 2 Requirements to the Boundary Restraint: Piping between the outside containment isolation valve and the boundary restraint was procured l
in accordance with ASME III standards even though the functional boundary between ASME III and NNS occurs at the' isolation valve.
In the main steam and feedwater systems, these portions were procured with substantially l
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greater wall thicknesses than those used in other portions of the exempt piping. This was done to provide further assurance that any postulated event loadings occurring in the NNS piping could be transferred from the piping at the boundary restraint into the boundary restraint. On the main steam system, the wall thickness of this p'iping is 2-5/8", versus the 1-1/4" wall thickness piping used elsewhere within the MSFIA. On the feedwater system, the wall thickness is 2-7/16", versus the wall thickness of 1-3/8" employed on remaining exempt piping.
Augmented In-Service Inspection (ISI): All piping welds in the exempt portions will be subjected to volumetric examination during each inspection interval, as defined
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in Section XI of the ASME Code.
Remoteness from the Control Room: All " break-exempt" piping is distant from the control room and there is no physical interconnection between the MSFIA and the Control Room.
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C.
MSFIA Building Design Features As the result of an extensive study of the effects arising from small-bore postulated piping failures in the MSFIA, changes are planned which will significantly up-grade the level of protection provided in the plant design against such events.
These modifications, while formulated primarily to address small-bore piping failure concerns, would also come to bear in addressing failures, albeit less than the ASB guideline value of 2
1.0 ft,in large-bore high-energy piping.
These improvements include the following:
Structural Integrity: The MSFIA has been designed to withstand the over-pressure produced by mechanistic piping failures.- The limiting failure is that of a guillotine rupture in a 4" steam line. To accommodate the elevated pressure associated with this failure, the tornado check valves currently in place will be removed to provide additional vent area. Certain doors will be designed to resist the over-pressure, and the structural steel. framing will be modified to resist temperature effects associated with this steam line break.
The pressure and temperature time history envelopes pertaining to this break are contained 1
in Figures 5 and 6.
Qualification / Protection of Equipment for Harsh Environment:
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Isolation valve operators and associated equipment.for the Main Steam and Feedwater lines will be requalified to the environment produced by the limiting mechanistic piping failure.
The actuators for the Modulating 1.
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Atmospheric Oump valves will be qualified to this limiting environment.
o The vault housing instrumentation and control equipment will be isolated from the rest of the MSFIA and be serviced by an independent HVAC system.
Instrument racks presently located in various compartments of the MSFIA will be re-located to this imune I&C vault.
o The Reactor Coolant Pump Monitors will be relocated away from the MSFIA.
Enhancement of Cold Safe Shutdown Capability Relative to Small-Bore Piping Failures: To augment existing plant features in coping with isolable and non-isolable small-bore piping failures, redundant isolation valves are to be installed in the high pressure condensate lines.
In addition, a break detection, annunciation and automatic 2
isolation system is to be incorporated and physical separation of the Train "A" room from the Train "B" room will be inclemented.
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1 D.
Industry Evaluations of Piping Failure Mechanisms Considerable attention and effort has been directed towards the examination of failure mechanisms in piping which might obviate the criteria for postulation of catastrophic failures and subsequent evaluation of the associated effects. Evidence has been gathered, for large-diameter thick-walled Reactor Coolant System (RCS) piping, which shows that such piping would not experience the types of failures as had been previously postulated, given the level of inservice inspection performed and the leakage detection measures employed.
This information
. has been used by licensees in support of exemption from General Design Criterion 4 (GDC-4) for LOCA load issues.
These leak-before-break (LBB) evaluations typically involve fatigue flaw growth, flaw stability and limit load analyses to demonstrate that any partial through-wall flaw would propagate in a through-wall direction, rather than longitudinally or circumferentially and that the piping would remain stable if normal operating plus SSE loads are applied to the postulated leakage flow.
A portion of the LBB effort involves the use of piping material fracture toughness properties for performing fatigue flaw growth analyses and elastic-plastic fracture mechanics analysis.
Initial industry efforts were directed towards stainless steel piping of the type used in a Westinghouse NSSS and these results have formed the basis for plant specific requests for exemption
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from GOC-4. A recent study (Reference 5) was performed on low-alloy ferritic steels typically used in a B&W NSSS, in support of the LB8 program for B&W Owners Group plants.
This report has been reviewed by the NRC staff; it was concluded that an acceptable technical basis was provided to eliminate, as a design basis, dynamic effects arising from ruptures in RCS primary loop piping (Reference 6).
Piping materials used in these loops are low alloy ferritic steels and wrought stainless steels; included in the former category is SA-106, Gr. C, which is also utilized in the MSS " break-exempt" piping.
While results of this study are not directly transferable to the large-bore high energy piping within the MSFIA, the major
' parameters used in the staff-approved LBB evaluation (e.g.,
stress levels under normal and faulted plant conditions, potential cracking mechanisms, leakage-size crack stability and material fracture toughness properties) would bear favorably in a similar evaluation of the subject piping.
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4.0
SUMMARY
The features incorporated into the WNP-1 MSFIA piping design, provide a high level of assurance that the large-bor'e high-energy piping contained therein would not sustain a large breach in its pressure boundary. Restraints to protect this piping against extraneous effects, simplified geometry, material of high quality, low inherent stresses and augmented ISI have been the means to establish this level of safety. This approach was consistent with the S3P guidance in effect at the time of design and construction.
Current SRP 2
guidelines, which call for the postulation of a 1 ft non-mechanistic longitudinal break, would necessitate extensive modifications which are impractical to back-fit in the MSFIA.
An ongoing effort within industry, both through analytical investigations and test programs, has been demonstrating that the fracture toughness of piping materials typically used in RCS loops falls in a range such that the susceptibility of an undetected flaw in such materials to propagate to a point where a large breach would
, occur is implausible. These investigations have been conducted in support of the " Leak Before Break" concept on large-bore RCS stainless steel piping. A recent study (Reference 5) was conducted on low-alloy ferritic steel RCS piping; the same overall conclusions for this family of material as for stainless steel were reached.
Included in that evaluatin was SA-106, which is the material used on the WNP-1 " Break-Exempt" piping.
This study has been approved by the
' NRC for use as a basis to eliminate the consideration of dynamic effects associated with the rupture of RCS main loop piping.
The MSFIA design has been evaluated with respect to the effects produced by a small-bore mechanistic piping failure.
Through existing features or through modifications to be incorporated upon construction restart, the MSFIA structure, building internal steel, essential equipment and instrumentation will be qualified to withstand the adverse environment and all w for plant safe shutdown.
In light of these considerations, it is felt that a significant 2
increase D1 safety would n6t be acnieved by imposition of a 1 ft non-mechanistic high-energy piping failure criterion in the WNP-1 MSFIA.
In addition, such a change in criterion would be impracticabletoimplement,}oecausaoftheessentiallycompleted design and construction.
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5.0 REFERENCES
(1) Standard Review Plan 3.6.1, dated 11/24/75, with attached Branch
. Technical Position APCSB 3-1, " Protection Against Postulated Piping Failures in Fluid Systems Outside Containment."
(2) Standard Review Plan 3.6.2, dated 11/24/75, with attached Branch Technical Position MEB 3-1, " Postulated Break and Leakage Locations in Fluid System Piping Outside Containment."
(3) Standard Review Plan 3.6.1, Rev.1, July 1981, with attached Banch Technical Position ASB 3-1, " Protection Against Postulated Piping Failures in Fluid Systems Outside Containment."
4 (4) Standard Review Plan 3.6.2, Rev. 1, July 1981, with attached Branch Technical Position EB 3-1, " Postulated Rupture Locations in Fluid System Piping Inside and Outside Containment."
(5) B&W Owners Group Leak-Before-Break Report BAW-1847, " Evaluation of the Margins Against Full Break for RCS Primary Piping of B&W Designed NSS", Rev.1, September 1985.
k (6) Letter, J. F. Stolz, NRC to D. W. Mazur, Supply System, "WPPSS Nuclear Project No.1 (WNP-1), Safety Evaluation of B&W Owners Group Reports Dealing with Elimination of Postulated Pipe Breaks in PWR Primary Main LOOPS", dated February 18, 1986.
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