ML19221A937
| ML19221A937 | |
| Person / Time | |
|---|---|
| Issue date: | 08/31/1976 |
| From: | NRC OFFICE OF STANDARDS DEVELOPMENT |
| To: | |
| References | |
| REGGD-01.121, REGGD-1.121, NUDOCS 7907110128 | |
| Download: ML19221A937 (8) | |
Text
August 1976 O.S. NUCLEAR REGULATORY CO U
OFFICE OF STANDARDS DEVELOPMENT REGULATORY GUIDE 1.121 BASES FOR PLUGGING DEGRADED PWR STEAM GENERATOR TUBES A.
INTRODUCTION General Design Criteria 14, " Reactor roolant Pressure Boundary," and 31, " Fracture Prevention of Reactor Coolanc Pressure Boundary," of open-dix A, " General Design Criteria for Nuclear Power Plants," to 10 3A Part 50, " Licensing of Production and Utilization Facilities,"
e that the reactor coolant pressure boundary have an extremely pro - ility of abnormal leakage, of rapidly propagating failure, and of o.
rup ure.
General Design Criterion 15, " Reactor Coolant System Des n'
4 res that the reactor coolant system and associated auxiliar o
01, and protection systems be designed with sufficient margin sur that the y
design conditions of the reactor coolant pressure d-Je not exceeded during any condition of normal operation, includint, h c ated operational occurrences.
Furthermore, General Design Crit
' Inspection of Reactor Coolant Pressure Boundary," reauires hI nents that are part of the reactor coolant pressure bounda-be r ig.
to permit periodic inspection and testing of critical arcac go ~
their structural and t-
's leaktight integrity.
NN Rupture of the steam genera -
t L
's,
hich constitute a portion of l
uld permit flow of reactor coolant the reactor coolant pressure bount 1
e versa.
In addition, the weakening into the secondary coolant s; of these tubes due to servic ind ed tube degradation processes could, in the event of a postulated lo L of oolant accident (LOCA), result in rupture of tubcs and release ci uid energy from the secondary system into the containment or the reactor vessel.
The rupture of a number of single tube wall b v 2rs between primary and secondary fluid has safety consequences only resulting fluid flow exceeds an acceptable amount f
and rate.
Q
" Rupture' k s dhinedasanyperforationofthetubepressureboundaryaccom-l b, hfM of fluid either f rom the primary to the secondary side of panig the W or vice versa, depending on the differential pressure condition preva1 4 'g during normal plant operation or developed in the event of pos-tulated ipe break accidents within either the primary reactor coolant pres-sure boundary or the steam system pressure boundary.
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This guide describes a method acceptable to the NRC staff for establish-ing the limiting safe conditions of tube degradation of steam generator tubing, beyond which defective tubes as established by incervice inspection should be removed from service by welding plugs at each end of the tube.
This guide applies only to pressurized water reactors (PWRs).
B.
DIS CUSSION The heat transfer area of the steam generators associated w'th pressurized wster reactors can comprise well over 50' of the total primary system pressure-retaining boundary.
The steam generator tubing therefore represents an integral part of a major barrier against fission product release to the environment. The steam generator tubing also represents a barrier against steam release to the containment in the event of a postula-ted LOCA.
The design criteria used to establish the structural integrity of the steam generator tubing should include analyses that define the mini-mum tube wall thickness that can sustain, with adequate margins and under normal plant operating conditions, the pressure and thermal load resulting conditions, including a safe shutdown earthquake fromgostulatedaccident (SSE) in combination with a LOCA break, a steam line break, or a feedwater line break.
Regulatory Guide 1.83, " Inservice Inspection of Pressurized Water Reactor Steam Generator Tubes," defines defective tubes (i.e., tubes with wall thickness less than the r in' mum acceptable thickness) as being unaccept-able for continued service aad racommends that these and leaking tubes be plugged. Partially degraded tubes with a wall thickness greater than the minimum acceptable tube wall thickness are acceptable for continued service, provided the minimum required tube wall thickness includes an operational allowance for tube degradation that may occur before the next scheduled tube inspection.
Calculations and analytical procedures and the operational history of the steam generator are used to arrive at the minimum acceptable tube wall thickness and thus are the basis for defining the plugging criteria.
For degraded steam generator tubes, plugging criteria have been developed by licensees on a case-by-case basis, using analyses and tests to establish the maximum tube degradation that can be tolerated. This maximum is such that the degree of loading required to burst or collapse a tube wall is consistent with the safety f actor in Section III of the American Society of Mechanical Engineers' Boiler and Pressure Vessel Code.3 Tests have demonstrated" that degraded steam generator tubes have a safety margin against burst or collapse, because new steam generator tubes 2 As defined in Appendix A, " Seismic and Geologic Siting Criteria for Nuclear Power Plants," to 10 CFR Part 100, " Reactor Site Criteria."
Copies may be obtained from the American Society of Mechanical Engineers, 3
345 East 47th Street, New York, N Y.
10017.
4 "The Effect of Wall Degradation on the Burst and Collapse Pressure of Inconel 600 Steam Generator Tubes," presentation by Combustion Engineering on October 25, 1973, at Bethesda, Md.
1.121-2 142 046
are manuf actured with a wall thickness much greater than the minimum thick-ness indicated by the design rules of Section III of the ASME Code. Heavier wall thicknesses than required by design rules at used in procurement documents for steam generator tubes primarily to at ;ommodate f abrication procedures and installation and handling requirements.
For certain cases, analytical results indicate that steam generator tubes that are locally thinned or cracked will remain intact under loads postulated f rom a LOCA in combination with an SSE.5 However, to establish an operational limit for a steam generator whose tubes have been subject to degradation, three factors should be considered:
(1) the minimum tube wall thickness needed in order for tubes with defects to sustain the imposed loadings under normal operating conditions and postulated accident conditions, (2) an operational allowance for degrada-tion between inspections, and (3) the crack size permitted to meet the leakage limit allowed per steam generator by the technical specifications of the license.
The chemical environment of the secondary side of the steam generator has been identified as one of the prime sources of steam generator tube degradation, and plants experiencing chemical imbalance have exhibited corrosion-induced defects that manifest themselves as wastage, inter-granular penetration, and cracking. Mechanical and flow-induced vibra-tions have been known to cause fretting and fatigue damage that also leads to degradation of steam generator tubes.
The latter effects have been less severe than corrosion effects.
Remote and rapid probing of steam generator tubes using eddy-current techniques has proven to be a successful means for establishing the depth of imperfections in degraded steam generator tubes. Tubes with imperf ec-tions located through eddy-current probing that exceed the minimum accept-able tube wall thickness and the operational limit can be taken out of service by blocking both ends of the tube in the tube sheet with welded plugs. Two methods are presently available for plugging:
(1) manual and automatic welding and (2) explosive welding.
C.
REGULATORY POSITION As noted in Regulatory Guide 1.83, applicants or licensees may submit plugging criteria to NRC for approval.
In any event, this information will be needed when degraded steam generator tubes are detected through eddy-current inspections (conducted according to Regulatory Guide 1.83) in order to indicate to NRC the bases for determining the number of tubes to be plugged.
To define minimum acceptable wall thickness and una ceptable defects, both analytic and experimental justification is necessary.
5 Westinghouse Report WCAP-7832, " Evaluation of Steam Generator Tube, Tube Sheet and Divider Plate Under Combined LOCA Plus SSE Conditions."
147
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L 1.121-3
1.
Unacceptable Defects Unacceptable defects fall into the following three broad categories:
Thru-wcil cracks that do not have adequate margins of safety a.
during either normal operation or postulated accident conditions and that could lead to tube rupture. Eddy-current inspection and radiation monitoring of the reactor coolant fluid leaking into the feedwater through a steam generator tube crack should be used to detect thru-wall cracks. The limit of reactor coolant in-leakage to the secondary coolant system stated in the plant's technical specifications should be of such magnitude that the corresponding single crack size through which this leakage is shown to occur under normal operating conditions meets Regulatory Positions C.2. (a)(3),
(4), and (5) b.
Part thru-wall cracks and wastage, occurring together or separately such that the remaining wall thickness is less than the minimum acceptable wall thickness.
c.
Thru-wall and part thru-wall cracks, wastage, and combinations of these that exceed the operational limit.
2.
Minimum Acceptable Wall Thickness Information should be developed to provide a basis.or ene aring a.
that tube integrity will be maintained during postulated design basis accidents such as a LOCA or a steam line break in combination with an SSE.
Such infctuation should be developed by performing analyses that demon-strate that the following goals are met:
(1) Tubes with detected part thru-wall cracks should not be stressed during the full range of normal reactor operation beyond the clastic range of the tube material.
(2) Tubes with part thru-wall cracks, wastage, or combinations of these should have a factor of safety against failure by bursting under norn'l operating conditions of not less than 3 at any tube locacion.
(3)
If thru-wall cracks with a speelfied leakage limit occur either on a tube usll with normal thickness or in regions previously thinned by wastage, they should not propagate and result in tube rupture under postulated accident conditions.
(4) The margin of safety against tube rupture under normal operating conditions should be not less than 3 at any tube location where defects have been detected.
(5) Any increase in the primary-to-secondary leakage rate should be gradual to provide time for corrective action to be taken.
1.121-4 0
42 048
(6) The margin of safety against tube failure under postulated accidents, such as a LOCA, steam line break, or feedwater line break con-current with the SSE, should be consistent with the margin of safety determined by the stress limits specified in NB-3225 of Section III of the ASME Boiler and Pressure Vessel Code.
b.
An additional thickness degradation allowance should be added to the minimum acceptable tube wall thickness to establish the operational tube thickness acceptable for continued service.
An imperfec ion that e
reduces the remaining tube wall thickness to ass than the sum of the minimum acceptable tube wall thickness plus the operational degradation allowance is designated as an unacceptable defect. A tube containing this imperf ection has exceeded the tube wall thickness Ibnit for continued aervice and should be plugged before operation of the steam generator is resumed.
3.
Analytical and Loading Criteria Applicable to Tubes with Either Part Thru-Wall or Thru-Wall Cracts and Wastage Conservative analytical models should be used to establish the a.
minimum acceptable tube wall thickness generally applicable to those areas of tube length where tube degradation is most likely to occur in service due to cracking, wastage, intergranular attack, and the mechanisms of f atigue, vibratian, and flow-induced loadings.
The wall thickness should be such that sufficient tube wall will remain to meet the design limits specified by Section III of the ASME Boiler and Pressure Vessel Code for Class 1 cocponents, as well as the following criteria and loading conditions:
(1) Loadings associated with normal plant conditions, including stnrtup, operation in power range, hot standby, and cooldown, as well as all anticipated transients (e.g., loss of electrical load, loss of offsite power) that are included in the design specifications for the plant, should not produce a primary membrane stress in excess of the yield stress of the tube material at operating tesperature.
(2) The margin between the maximum Laternal pressure to be contained by the tubes during normal plant conditions and the pressure that would be required to burst the _ tubes should remain consistent with the margin incorporated in the design rules of Section III of the ASME Code.
(3) Loadings associated with a LOCA or a steam line break, either inside or outside the containment and concurrent with the SSE, should be accommodated with the margin determined by the stress liaits specified in NB-3225 of Section III of the ASME Code and by the ultimate tube burst strength determined experimentally at the operating temperature.
b.
(1) The stress calculations of the thinned tubes should consider all the stresses and tube deformations imposed on the tube bundle during
)4} Of}
1.121-5
the most adverse loadings of the postulated accident conditions. The dynamic loads should be obtained f rom the modal analysis of the steam generator and its support structure. All major I.ydrodynamic and flow-induced forces should be considered in this analysis.
(2) The f atigue ef fects of cyclic loading forces should be considered in determining the minimum tube vall thickness. The transients considered in the original design of the steam generator tubes should be included in the f atigue analysis of degraded tubes corresponding to the ministas tube wall thickness established. The magnitude and frequency of the temperature and pressure transients should be based on the estimated ntatber of cycles anticipated during normal operation for the maximum service interval expected between tube ina.,ection periods. Notch effects resulting from tube thinning should be taken into account in the fatigue evaluation.
The combination of loading conditions for the postulated acci-c.
dent conditions should include, but not be limited to, the following sources:
(1)
Impulse loads due to rarefaction waves during blowdown, (2) Loads due to fluid friction from mass fluid accelerations, (3) Loads due to the centrifugal force on U-bend and other bend regions caused by high velocity fluid motion, (4) Loads due to the dynamic structural response of the steam generator components and supports, (5) Faismic loads, (6) Transient pressure load differentials.
d.
For tubes with thru-wall cracks on either walls of normal thickness or regions previously thinned by wastage, the following goals should be met:
(1) The maximum permissible length of the largest single crack should be such that the internal pressure required to cause crack propaga-tion and tube rupture is at least three times greater than the normal operating pressure. The length and geometry of the largest permissible crtck size should be determined analytically either by tests or by refined finite element or f racture mechanics techniques. The material stress-strain characteristics at tec:perature, fracture toughness, stress intensity factors, and materfal flow properties should be considered in making this determination.
1.121-6 O
142 050
(2) Adequate margin should be provided between the loadings associ.ated with a large steam line brerk or a LOCA concurrent with an SSE and the loading required to initiate propagation of the largest permissible longitudinal crack resulting in tube rupture. The Ivadinga associated with the postulated accident conditions should include the transient hydraulic and dynamic loads listed in C.3.(c).
(3) The primary-to-secondary leakage rate limit under normal operating pressure is set forth in the plant technical specifications and should be less than the leakage rate determined theoretically or experi-mentally from the largest single permissible longitudinal crack. This would ensure orderly plant shutdown and allow sufficient time for remedial action if the crack size increases beyond the permissible limits during service.
When appl. cants or licensees present plugging criteria to NRC, e.
a summary of the analysis should be provided. This should include at least the following:
(1) Stress allowables used in the analyses, including justifica-tion for those which differ from the limits listed in C.3.(a).
(2) The geometrical configuration of the tube bundle and the support structure and the mathematical model_used in the dynamic computer analysis.
(3) The assumptions made in the elastic and elastic / plastic analyses.
(4) The nature and development of the loads outlined in C.3.(c),
including pressure-time histories of the loadings.
(5) The postulated LOCA or steam line breaks, including bra.ak opening time and duration of the pulses.
(6) The structural and thermal-hydraulic computer codes used in the dynamic analysis.
(7) The critical arcas of the tube b6ndle and the primary membrane and bending stresses due to the most adverse load components.
(8) The analytical or experimental determination of the largest permissible crack length based on the most adverse loadings as described in C.3. (d)(1) and (2).
(9) Experimental or theoretical justification for the primary-to-secondary leakage rate data used in meeting Regulatory Position C.3.(d)(3).
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(10) Experimental verification of the design bases and safety margins, if available, and f atigue ef fects.
f.
The basis used in setting the operational degradation allowance, as added to the minimum tube wall thickness established for continued operation of steam generators, should be provided.
It should include:
(1) The maximum number of tubes allowed to have a wall thickness less than the minimum acceptable thickness, (2) The method and data used to predict continuing degradation, (3) Consideration of measurement error and any other eignificant eddy-current testing parameters.
D.
IMPLEMENTATION The purpose of this section is to provide information to applicants and licensees regarding the NRC staff's plans for using this regulatory guide.
Except in those cases in which the applicant or licensee proposes an acceptable alternative method, the staff vill use the methods described herein in evaluating an applicant's or licensee's capability for and perfc,rmance in complying with specified portions of the Ccmission's regulations after April 1, 1977.
If an applicant or licensee wishes to use the method described in this regulatory guide on or before April 1,1977, the pertinent portions of the application or the licensee's perforiaance will be evaluated on the basic of this guide.
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