ML20245E102
| ML20245E102 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 12/06/1988 |
| From: | Sniezek J Office of Nuclear Reactor Regulation |
| To: | Jordan E Committee To Review Generic Requirements |
| Shared Package | |
| ML20245E106 | List: |
| References | |
| NUDOCS 8812210333 | |
| Download: ML20245E102 (20) | |
Text
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'o, UNITED STATES 4 k" ),
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NUCLEAR REGULATORY COMMISSION N*
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- E CASHINGTON, D. C. 20555 N
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DEC ~ 61988 MEMORANDUM FOR:
Edward L. Jordan, Chairman Committee to Review Generic Requirements FROM:
James H. Sniezek, Deputy Director Office of Nuclear Reactor Regulation
SUBJECT:
REQUEST FOR REVIEW 0F A DRAFT BULLETIN REGARDING PRESSURIZER SURGE LINE DEFLECTION DUE TO THERMAL STRATIFICATION i
On October 7,1988, the NRC staff issued Information Notice No. 88-80, "Un-expected Piping Movement Attributed to Thermal Stratification." The information notice alerted addressees to unexpected movement of the pressurizer surge line that has been attributed to thermal stratification. The thermal stratification occurs during heatup and cooldown when significant temperature differences can exist between the pressurizer and the reactor coolant system. The staff's concern is that the stratification can cause stress and fatigue on the surge line to exceed design limits.
As a result, compliance with GDCs 1, 4 and 14 and 10 CFR 50.55a may be suspect.
The proposed bulletin requests licensees to enter into a program which incor-porates the appropriate thermal parameters into the analysis which supports the safe operation of the surge line. The program begins with a physical examina-tion and a bounding analysis.
If these actions do not support regulatory com-pliance for the duration of the license, further actions are requested which may be based on realistic estimates of the fatigue life obtained from actual temperature measurements. The bulletin emphasizes the urgency for action on plants that have operated for more than 10 years. The schedules proposed for the actions and reports would ensure that the concern would be resolved within 2 years after issuance of the bulletin.
The staff requests that the proposed updating of the surge line fatigue evalu-ation include thermal stratification effects and be performed in accordance with the latest referenced ASME Section III Code requirements which incorporate high cycle fatigue. Note that code requirements on piping fatigue analysis did not exist prior to the issuance of ANSI B-31.7 in 1967. The requirement was specified after the issuance of ASME Section III in 1971 and was further revised in later editions of the code.
Since accurate fatigue evaluation for the surge line is. essential to assure integrity of the reactor coolant pressure boundary and is also necessary to comply with 10 CFR 50.55a, the use of an up to date technique on fatigue evaluation will provide the necessary assurance of surge line structural reliability. Accordingly, pursuant to 10 CFR 50.109 (a)(4), even if this is considered a backfit, a backfit analysis as delineated in 10 CFR 50.109 (a) and (3) is not required because the proposed new requirement is neces-sary to bring a facility into conformance with the rules (GDC 1,4 and 14) of the Commission, and to ensure that the facility poses no undue risk to the public health and safety. The above discussion is supported by the findings resulting from the evaluadon of surge line thermal stratification in Enclosure 3.
CONTACT:
N. P. Kadambi, NRR 49-21153 M
2A6 333 20y.
l't D.-
Edward L. Jordan 2-
.The enclosed draft bulletin, addressed to all pressurized water reactors, would' ensure compliance with GDC 1, 4 and 14. Hence, no further staff evaluation -
pursuant to 10 CFR 50.54(f) is required.
-The proposed bulletin and background information, including information recently
.obtained from Trojan, Comanche Peak, Beaver Valley Unit 2, and the three PWR Owner's Groups, are enclosed as required by the CRGR Charter. We request that review of this package be scheduled at CRGR's earliest convenience.' The bulletin is sponsored by Lawrence Shao, Director, Division of Engineerig
.and Systems Technology.
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4 es H. SnNz,e(, W @ Dire h
Depu or Office of Nuclear React Reg l' ation
Enclosures:
1.
NRC Bulletin No. 88-XX, " Pressurizer Surge Line Deflection i
Due to Thermal Stratification" 2.
CRGR Item IV.B.
3.
References submitted to CRGR e
NRC BULLETIN NO. 88-XX: PRESSURIZER SURGE LINE DEFLECTION DUE TO THERMAL STRATIFICATION Addressees:
All holders of operating licenses or construction permits for pressurized water nuclear power reactors.
Purpose:
The purpose of this bulletin is to, (1) request that addressees establish and implement.a program to confirm pressurizer surge line integrity in view of the occurrence of thermal stratification, and (2) require addressees to inform the staff of the actions taken to resolve this issue.
Description of Circumstances:
The licensee for the Trojan plant has observed unexpected movement of the pressur-izer surge line during inspections performed at each refueling outage since 1982 when monitoring of the line movements began. During the last refueling outage, the licensee found that, in addition to unexpected gap closures in the pipe whip restraints, one restraint was in actual contact with the piping. Although the licensee had repeatedly edjusted shims and gap sizes, based on analysis of various postulated conditions, the problem had not been resolved. The most recent investigation by the licensee confirmed that the movement of piping was caused by thermal stratification in the line. This phenomenon was not considered in the original piping design.
l L Discussion:
Unexpected piping movements are highly undesirable because of potential high piping stress that may exceed design limits for fatigue and stresses.
The problem can be more acute when'the piping expansion is restricted, such as through contact with pipe whip restraints.
Plastic deformation can result, which can lead to high local stresses, low cycle fatigue and functional im-pairment of the line. The Trojan report indicated _ that thermal stratification occurs in the pressurizer surge line during heatup, cooldown, and steady-state operations of the plant.
l During a typical plant heatup, water in the pressurizer is heated to about 440'F; a steam bubble is then formed in the pressurizer. Although the exact phenomenon is not thoroughly understood, as the hot water flows (at a very low flowrate) from the pressurizer through the surge line to the hot-leg piping, the hot water rides on a layer of cooler water, causin;; the upper part of the pipe to be heated to a higher temperature than the lower part (see Figure 1). The differential temperature could be as high as 300*F, based on expected conditions during typical plant operations. Under this condition, differential thermal expansion of the pipe metal could cause the pipe to deflect significantly.
For the specific configuration of the pressurizer surge line in the Trojan plant, the line deflected downward and when the surge line contacted two pipe whip restraints, it underwent plastic deformation, resulting in permanent deformation of the pipe.
On the basis of the evaluation of the Trojan event, the staff concludes that the thermal stratification hypothesis for the pressurizer surge line movement is supportable by the observations. The licensing basis according to 10 CFR 50.55a
' for all PWRs requires that the licensee meet the American Society of Mechanical j
l Engineer:; Boiler and Pressure Vessel Code Sections III and XI, and to reconcile the pipe stresses and fatigue evaluation when any significant differences are observed between the measured data and the analytical resultr. for the hypothe-sized conditions. Staff evaluation ir,dicates that the thermal stratification phanomenon could occur in all PWR surge lines and may invalidate the analyses supporting the integrity of the surge line. The staff's concerns include un-expected Landing and thermal striping (rapid oscillation of the thermal boundary interface along the piping inside surface) as they affect the overall integrity of the surge line for its design life (e.g., the increase of fatigue).
Action Requested:
Each addressee is requested to take the follo> nf actions:
1.
For all licensees of operating PWRs:
Licensees are requested to conduct a visual inspection (ASME, Section a.
XI, VT-3) of the pressurizer surge line at the first available cold shutdown after rece'pt of this bulletin which exceeds seven days. This inspection should determine any discernable distress or structural damage in the entire pressurizer surge line, including piping, pipe supports, pipe whip restraints, and anchor bolts.
b.
Within four months c receipt of this Bulletin, licensees of plants in operation over ten years (i.e., low power license prior to November 1, 1978) are requested to demonstrate that the pressurizer surge line meets the applicable design codes
- and other FSAR and regulatory commitments for
- Fatigue analysis should be performed in accordance with the latest ASME Section III requirements incorporating high cycle fatigue.
u l,
1 the licensed life of the plant, considering the phenomenon of thermal stratification and thermal striping in the fatigue and stress evalu-ations. This may be accomplished by performing a plant specific or I
generic bounding analysis.
If the latter option is selected, licensees should demonstrate applicability of the referenced generic analysis.
Licensees of plants in operation less than ten years (i.e., low power license after November 1, 1978), should complete the foregoing analysis within one year of receipt of this bulletin. Since any piping distress j
observed by addressees in performing action 1.a may affect the analysis, the licensee should verify that the bounding analysis remains valid.
f I
If the opportunity to perform the visual inspection in 1.a does not occur in the periods specified herein, incorporation of the results of the visual inspection into the analysis may be performed in a supple-l mental analysis as appropriate.
l Where the analysis shows that the sarge line is acceptable for a I
period of operation which is less than the plant life, the licensee should submit a justification for continued operation or bring the i
plant to cold shutdown, as appropriate, and implement Items 1.c and j
l 1.d below to develop a detailed analysis of the surge line.
l c.
If the analysis in 1.b does not show regulatory compliance for the I
duration of the operating license, the licensees are requested to i
obtain plant specific data on thermal stratification, thermal striping, j
and line deflections.
The licensee may choose, for example, either to install instruments on the surge line to detect temperature distribu-tion and thermal movements, or to obtain data through collective I
efforts, such as from other plants with a similar surge line design.
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.- If the latter option is selected, the licensee should demon-strate similarity in geometry and operation.
d.
Based on the applicable plant specific or referenced data, licensees ain requested to update their stress and fatigue analyses to ensure compliance with applicable Code requirements, incorporating any observations from 1.a above. The analysis should be completed no later than two years after receipt of this bulletin.
If licenscss are unable to show compliance with the applicable design codes and other FSAR and regulatory commitments, the licensees are requested to-submit a justification for continued operation and a description of the proposed corrective actions for effecting long term resolution, q
2.
For all applicants for PWR Operating Licenses:
a.
Before issuance of the low power license, applicants are requested to demonstrate that the pressurizer surge line meets the applicable design codes and other FSAR and regulatory connitments for the licensed life of the plant. This may be accomplished by performing a plant specific or generic bounding analysis. The analysis should include consider-otions of. thermal stratification and thermal striping to ensure that fatigue and stresses are in complience with applicable code limits.
The analysis and hot functional testing should verify that piping thermal deflections result in no adverse consequences, such as contacting the pipe whip restraints.
If analysis or test results a
show Code noncompliance, conduct of all actions specified below is requested.
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"b Applicants are requested to' evaluate operational alternatives or piping modifications needed to reduce fatigue and stresses to an acceptable l-1-
value.
Applicants.are requested to either monitor the surge line for the effects c..
of thermal stratification, beginning with hot functional testing, or obtain data through collective efforts to assess the extent of thermal stratification, thermal str! ping and piping deflections.
d.
Applicants are requested to update stress and fatigue analyses, as necessary, to ensure Code compliance.* The analyses should be completed.
4 no later than one year after issuance of the icw power 1icense.
3 Addressees are requested to generate records to document the development and implementation of the program requested by Items 1 or 2, es well as any subsequent corrective actions, and maintain these records in accordance with 10 CFR Part 50, Appendix B and plant procedures.
i Repurting Requirements:
1.
Addressees shall report to the NRC any discernable distress and damage observed in Action.1.a along with corrective actions taken or plans and schedules for repair before restart of the unit.
- 1f compliance with the applicable codes is not demonstrated for the full duration of an operating license, the staff may impose a license condition such that normal operation is restricted to the duration that compliance is actually demonstrated.
.. 2.
Addressees who cannot meet the schedule described'in Item 1 or 2 of Actions Requested are required to submit to the NRC within 60 days of receipt of this bulletin an alternative schedule with justification for the requested schedule.
3.
Addressees shall submit a letter which notifies the NRC that the actions requested in Items 1 or 2 of Actions Requested have been performed and that the results are available for inspection within 30 days after the completion of these actions.
The letter shall include a justification for continued operation, if appropriate, a description of the analytical approaches used j
and a summary of the results.
Although oot requested by this bulletin, addressees are encouraged to work collec-tively to address the technical concerns associated with this issue, as roll as to share pressurizer surge line data and operational experience.
The letters required above shall be addressed to the U.S. Nuclear Regulatory Commission ATTN: Document Control Desk, Washington, D.C.
20555, under oath or affirmation under the provisions of Section 182a, Atomic Energy Act of 1954, l
as amended.
In addition, a copy shall be submitted to the appropriate Regional l
Administrator.
e
. i This request is covered by Office of Management and Budget Clearance Number 3150-0011 which expires December 31, 1989. The estimated average burden hours is approximately 3000 person-hours per licensee response, including assessment of the new requirements, searching data sourcer, gathering and analyzing the data, and preparing the required reports. These estimated average burden hours pertain only to these identified response-related matters and do not include the time for actual implementation of physical changes, such as test equipment installation or component modification, consistent with the requested actions.
The estimated average radiation exposure is approximately 3.5 person-rems per licensee response.
Comments on the accuracy of this estimate and suggestions to reduce the burden
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may be directed to the Office of Management and Budget, Room 3208, New Executive Office Building, Washington, D.C.
20503, and to the U.S. Nuclear Regulatory Commission, Records and Reports Management Branch, Office of Administration and Resource Management, Washington, D.C.
20555.
If you have any questions about this matter, please contact one of the technical et ntactslisted below or the Regional Administrator of the appropriate regional office.
Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: S. N. Hou, NRR (301) 492-0904 S. S. Lee, NRR (301) 492-0943 N. P. Kadambi, NRR (301) 492-1153
4 Surge Line Stratification O
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Hot Flow from Pressurizer Thot = 425 F
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Stagnant Cold Fluid Tcold = 125 F l
Figure 1
CRGR Item IV.B Contents of Packages Submitted to CRGR (Rev. 4, Stello to List 042387, das 41860 342 ff)
The following requirements apply for proposals to reduce existing requirements or (regulatory) positions as well as propocals to increase requirements or
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(regulatory) positions. Each package submitted to the CRGR for review shall include twenty (20) copies of the following information:
SUBJECT *: BULLETIN REGARDING PRESSURIZER SURGE LINE DEFLECTION DUE TO THERMAL STRATIFICATION Question (1):
The proposed generic requirement or staff position as it is proposed to be sent out to licensees.
Response
The proposed staff position is set forth in the bulletin (Enclosure 1).
Question (ii):
Draft staff papers or other underlying staff documents supporting the require-ments or staff positions.
(A copy of all materials referenced in the document
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shall be made available upon request to the CRGR staff. Any committee member may request CRGR staff to obtain a copy of any referenced material for his or heruse.)
Response
- 1) NRC Information Notice No. 88-80.
2 Trojan Submittal of 5-31-88.
3 Comanche Peak and Beaver Valley Unit 2 Presentation slides of 9-7-88.
(4 B&W, Westinghouse and CE Owner's Group Slides of 9-7-88.
(S) ASME Section III, Figure I-9.2 (197/) and Figures I-9.2.1 and I-9.2.2.2 (1986).
Question (iii)-
Each proposed requirement or staff position shall contain the sponsoring office's position as to whether the proposal would increase staff require-ments or staff positions, would implement existing staff requirements or positions, or would relax or reduce existing requirements or staff positions.
l
Response
Action items in the proposed bulletin will implement existing regulatory requirements as follows:
4 10 CFR 50.34(a)(3)(1) requires that principal design criteria meet or exceed requirements established in Appendix A to 10 CFR 50 and that these criteria be identified in the preliminary safety analysis report (PSAR). Appendix A, General Design Criterion (GDC) 14, requires that the reactor coolant pressure boundary be designed so as to have an extremely low probability of abnormal leakage, of rapidly propagating failure, and of gross rupture.
10 CFR 50.34 (a)(4) requires that the PSAR include a preliminary analysis of the design of systems with the objective of assessing the risk to public health and safety.
10 CFR 50.34(b)(4) requires that the final safety analysis report (FSAR) in-clude a final analysis of the design of' systems with the same objective.
Notwithstanding these existing regulatory requirements, the reactor coolant pressure boundary in the pressurizer surge line was found to experience bending in ways inconsistent with the analyses supporting the design.
Initial indications show that the Trojan experience may be characteristic of other PWRs as well.
- The ASME Code requirements which apply to the surge line are indicated in Sub-section NB of the Boiler and Pressure Vessel Code, Sectica III. Reference 5
. shows the fatigue curves incorporated in the Code in 1977 and 1986. 10 CFR 50.55a (g)(6)(ii) allows the Commission to require licensees to follow an augmented inservice inspection program for systems and components for which the Commission deems that added assurance of structural reliability is necessary.
Thus, we conclude that the action items in the proposed bulletin implement existing regulatory requirements by ensuring that addressees comply and remain in compliance with GDC 14, and by minimizing (through early detection of poten-tially damaging effects) the likelihood of a breach of the reactor coolant boundary.
Question (iv):
The proposed method of implementation with the concurrence (and any comments) of OGC on the method proposed.
Response
The method of implementation will be the proposed bulletin (Enclosure 1).
A copy of this bulletin has been forwarded to OGC.
Comments received were incorporated prior to issuance.
Question (v):
Regulatory analyses generally conforming to the directives and guidance of NUREG/BR-0058 and NUREG/CR-3568.
i R
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Response
This is a compliance issue. No value/ impact analysis was performed.
It is the staff's position that although licensees may have to perform analyses not previously required, such analyses address a safety concern which has been embodied in GDCs 1, 4 and 14 as well as 10 CFR 50.55a. Hence, the tenns of 10 CFR 50.109(a)(4)(1) apply and a value/ impact analysis is not required.
Question (vi):
Identification of the category of reactor plants to which the generic require-ments or staff position is to apply (that is, whether it is to apply to new plants only, new OLs [ operating licensees] only, OLs after a certain date, all OLs, all plants under construction, all plants, all water reactors, all PWRs
[ pressurized water reactors] only, some vendor types, some vintage types such as BWR 6 and 4, jet pump and nonjet pump plants, etc).
Response
The proposed bulletin would apply to all holders of operating licensees or construction permits for pressurized water reactors.
Question (vii):
For each such category of reactor plants, an evaluation which demonstrates how g
the action should be prioritized and scheduled in light of other ongoing regulatory activities. The evaluation shall document for consideration information available concerning any of the following factors as may be appropriate and any other information relevant and material to the proposed action:
(a) Statement of the specific objectives that the proposed action is designed to achieve...
Response
The observations at Trojan demonstrates that operating conditions can exist at PWRs that may result in noncompliance with GDC 14. The objective of the pro-posed action is to ensure that licensees comply and remain in compliance.
Continuation of Que'stion (vii):
(b) General description of the activity that would be required by the licensee or applicant in order to complete the action...
I i
Response
I The activity required of the licensees depends on the design and age of the plant. The actions represent a graded response to the concerns raised by an unexpected mechanism that potentially increases significantly the extent of material fatigue experienced by the surge line over the life of the plant.
l j
. l The first action is a physical observation for signs of distress in the piping and associated structures. The corrective actions depend on the nature and extent of any observed distress. The second action is a screening analysis si.nilar to that performed at Trojan. All licensees would have to perform this bounding analysis based on simplified assumptions and conditions.
It is expected thdt this analysis will be highly conservative. Based on the outcome of the analysis at Trojan, the results may indicate the need for more realistic analyses in tne future.
The results of the physical examination and the screening analysis determine the need for less conservative and more realistic analyses which would havE to be-based on measured parameters. The measurements could be specific to a plant or based on a representative sample which can conservatively serve as a prototype.
The realistic analyses that would be used to support regulatory compliance must be codipleted no later than two yea.3 after issuance of this bulletin.
Continuation of Question (vii):
(c) Potential change in the risk to the public from the accidental offsite release of radioactive material...
Response
At present, the risk to the public, from the accidental offsite release of radioactive material due to a LOCA, exceeds that which is implicit in GDC 14.
Compliance with the regulations, as required by the proposed bulletin, would reduce the risk to that intended by promulgation of GDC 14.
Continuation of Question (vii):
(d) Potential impact on radiological exposure of facility employees and other l
onsite workers...
Response.
3 The radiological exposure would be associated with the physical examination of
'the piping and supports, and installation of monitoring instruments.
In some
]
cases, the examination may require removal of insulation in specific locations j
such as pipe whip restraints. Most of the examination would involve observation I
of the insulation on the p;pe because the appearance of the insulation is expected to be a sensitive indicator of any unexpected deflections. Based on the experience at Trojan, the average exposure is expected to be 3.5 man-rem per unit.
Continuation of Question (vii):
(e) Installation and continuing costs associated with the action, including the cost of facility downtime or the cost of construction delay...
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Re'sponse:
I The costs associated with the actions involve analyses, as well as instrumentation and recording costs if plant specific measurement is the chosen method to justify the thermal stratification and thermal striping anclyses.
Based on contacts with the industry groups, it appears, that at least three Westinghouse plants, one B&W plant and one CE plant will be instrumented.
In addition, it is possible that a few licensees may choose to develop their own plant specific information. Assuming that three plants may choose to develop their own data, the total number of plants L
that directly use instrumentation is estimated to be eight. Based on estimates obtained from Trojan, the cost is estimated to be $1.2 million for a plant that pursues a similar approach. On the basis of 84 licensees with PWRs, the average cost per licensee is estimated to be $0.8 million. No continuing costs are expected once the analysis of record for the surge line incorporates consider-ation of thermal stratification and thermal striping.
If the actions are efficiently scheduled into the outages, no increased downtime is expected. No licensing delay is expected for NT0Ls.
Continuation of Question (vii):
(f) The potential safety impact of changes in plant or operational complexity, including the relationship to proposed and existing regulatory requirements and staff positions...
Response
(
Some increased 1perational complexity may result from changes in operating procedures whicit decrease the magnitude of the temperature differential sustained by the surge line. The significance of the complexity is expected to be minimal when compared with the increased assurance of pipe integrity.
Continuation of Question (vii):
(g) the estimated resource burden on the NRC associated with the proposed action and the availability of such resources...
Response
Licensees would he required to submit letters confirming that the requested actions have been taken.
NRR has identified a lead project manager to coordinate the review of licensee's responses.
The estimated staff effort to perform limited audit reviews of licensee responses is approximately 2000 person-hours. Some review and inspection responsibility may be allocated to the regional staff.
In addition, selected licensees may be requested to present the results of their analysis to the staff in a meeting format.
Continuation of Question (vii):
(h) The potential impact of differences in facility type, design, or age on the relevancy and practicality of the proposed action...
-4
.c r
_ 'Responsq:
The concerns regarding thermal stratification in~ pressurizer' surge lines are significant to all FWRs. The magnitude of the effect appears to be similar in the'three. types of PWRs' built in.the U.S.. The age of plants is. considered only to the-extent that a' more rapid response is sought for those which are 10 years or older.
Continuation of Question (vii):
(1). Whether the proposed action is interim or final, and if interim, the E
justification for imposing the proposed action on an interim basis.
Response
The proposed action is final.
Question (viii):
For each evaluation conducted pursuant to 10 CFR 50.109, the proposing Office Director's determination together with the rationale for.the determination 1
'basedonthe'considerationsofparagraph(i)through(vii)above.that:
(a) there is a ~ substantial increase in the overall protection-of publir, health l
and safety or the common defense and security to be derived from the proposal; (b) the direct and indirect costs of implementation, for the facilities affected, are-justified in view of this increased protection.
Response
f(ot applienble.
Question (ix):
For each evaluation conducted for proposed relaxations or decreases in current requirements or staff positions, the prrposing Office Director's determination.
together with the rationale for the determhation based on the considerations of paragraphs (i) through (vii) above, that:
(a) the public health ad safety and the common defense and security would be adequately protected if the proposed reduction in requirements or positions were implemented, and (b) the cost savings' attributed to the action would be substantial enough to justify taking the action.
Response
Relaxations or decreases in current requirements or staff positions are not proposed.
Background Information
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UNITED STATES NUCLEAR REGULATORY COPPl!S$10N 0FFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C.
20555 October 7, 1988 NRC INFORMATION. NOTICE NO. 88-80: UNEXPECTED PIPING MOVEMENT TO THERMAL STRATIFICATION Addressees:
All holders oV operating licenses or construction permits for pressurized water nuclear power reactors.
Purpose:
The purpose of this infomation notice is to alert addressees to unexpected thermal movement of the pressurizer surge line attributed to thermal strati-It is expected that recipients will review the information for fication.
applicability to their facilities and consider actions, as appropriate, to However, suggestions contained in this infomation avoid similar problems.
notice do not. constitute NRC requirements; therefore, no specific action or written response,is required.
Description of Circumstances:
The licensee for the Trojan plant has observed the results of unexpected move-ment of.the pressurizer surge line at every refueling outage since 1982 when The monitoring program was implemented monitoring of the line movements began'.
after removal of a thermal sleeve that was found to have cracked attachmen During the last refueling outage, the licensee found that, in addition to.unex-pected gap closures in the pipe whip restraints, one restraint was in actual contact with the piping although the shims and gap sizes had been adjusted on the basis of previous analysis. The recent investigation indicated that the movement of piping was caused by thermal stratification in the line. This phe-nomenon was not considered in the original piping design.
Discussion:
Unexpected piping movements are highly undesirable because of potential high The piping stress that may exceed design limits on fatigue and stresses.
problem can be more acute when the piping expansion is restricted, such as through contact with pipe whip restraints. Plastic deformation could result, which could lead to functional impairment of the line Tha Trojan licensee's report indicated that thermal stratification might have occurred in the pres-surizer surge 'line during heatup, cooldown, and steady-state operation of the plant.
8810030255 -
_ _ = _ _ _ _ - - _ _ _
IN 88-80 October 7, 1988 Page 2 of 2 During a typical plant heatup, water in the pressurizer is heated to about 440*F.
A steam bubble is then formed in the pressurizer. Although the exact phenomenon is not thoroughly understood, as the hot water flows (at a very low flowrate) from the pressurizer through the surge line to the hot-leg piping, the hot water rides on a layer of cooler water, causing the upper part of the pipe to be heated The differential temperature could to a higher temperature than the lower part.
be as high as 300'F based on limitations on plant operation. Under this con-dition, analysis has shown that differential thennal expansion of the pipe metal causes the pipe to deflect.
In the specific configuration of the pressurizer surge line in the Trojan plant, the line deflected downward, contacted two lipe whip restraints, and t
underwent plastic deformation, which resulted in tie permanent deformation of the pipe.
On the basis of the evaluation of the Trojan event, the staff concludes that the thermal stratification hypothesis for the pressurizer surge line movement is supportable by the observations. The licensee for Trojan is required by the American Society of Mechanical Engineers Boiler and Pressure Vessel Code to reconcile the pipe stresses and fatigue evaluation if any significant dif-ferences are observed between the measured data and the analytical results for the hypothesized conditions. The staff evaluation indicates that the thermal stratification phenomenon could occur in all PWR surge lines. The staff's con-cerns include unexpected bending, and thermal striping as they affect the overall integrity of the surge line for its design life (e.g., the increase of fatigue).
No specific action or written response is required by this information notice.
However, the staff may consider further generic communications on the subject.
If you have any questions about this' matter, please contact the technical contacts listed below or the Regional Administrator of the appropriate regional office.
Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts:
S. N. Hou, NRR (301)492-0904 N. P. Kadambi, NRR (301) 492-1153
Attachment:
List of Recently Issued NRC Information Notices
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