ML20136G046
| ML20136G046 | |
| Person / Time | |
|---|---|
| Issue date: | 05/21/1985 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Stello V NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| Shared Package | |
| ML20136A677 | List: |
| References | |
| TASK-B-19, TASK-OR NUDOCS 8506040556 | |
| Download: ML20136G046 (5) | |
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UNITED STATES
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MAY 21385 MEMORANDUM FOR: Victor Stello, Deputy Executive Director for Regional Operations and Generic Requirements FROM:
Harold R. Denton, Director, Office of Nuclear Reactor Regulation
SUBJECT:
CLOSE OUT GENERIC ISSUE fB THERMAL-HYDRAULIC STABILITY The staff has been studying BWR Thermal-Hydraulic Stability characteristics for several years. The generic issue #B THERMAL-HYDRAULIC STABILITY -
has recently been prioritized and the staff was directed to take appropriate action (Attachment 1) to complete this issue. We have recently completed our technical evaluation (Attachment 2) of General Electric topical report NEDE-22277-P, " Compliance of the General Electric Boiling Water Reactor Fuel Designs to Stability Licensing Criteria," which was submitted as Amendment 8 to GESTAR. This SER concludes that General Electric methods for calculation of core stability decay ratio are uncertain by 20% in predicting the onset of limit cycle oscillations (decay ratio = 1.0). Thus a core having a calculated decay ratio of 0.80 may in fact be on the verge of limit cycle oscillations within permissible operating space. The SER further concludes that operating limitations-which provide for the detection and suppression of flux oscillations in operating regions of potential instability, consistent with the recommendations of General Electric SIL-380, are acceptable to demonstrate compliance with GDC-10 and GDC-12 for cores loaded with approved GE fuel designs. The basis for accept-ability is that the regions of potential instability have been conservatively defined based on exten:,fve generic and plant specific calculations, test data, and operating experience.
In sumary, the following options are available to demonstrate that a core design is in compliance with the regulations relating to thermal-hydraulic stability:
(1) Calculated stability decay ratio < 0.80 using approved methods of NEDE-22277-P-1.
(2) A core design consisting of approved General Electric fuel bundles in conjunction with SIL-380 operating recomendations enforced by technical specifications.
(3) Options similar to (1) and (2) based on other approved calculation methods and/or operating restrictions.
The SER on NEDE-22277-P-1 and the technical data developed for evaluation of similar stability calculational methods provide the technical resolution for Generic Issue #B Thermal-Hydraulic Stability.
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8 Victor Stello 2
MAY 21985 We have concluded that the #B-19 technical resolution is entitely consistent with the existing Section 4.4 of the Standard Review Plan acceptance criteria for thermal-hydraulic stability which is being applied to license applications.
The SRP criteria are stated as follows:
"The reactor should be demonstrated to have suffic'ient margin to be free of
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undamped oscillations and other themal-hydraulic instabilities for all conditions of steady-state operation (including part loop operation) and for anticipated operational occurrences."
To show compliance with this criterion the SRP further states that:
"The reviewer determines that the applicant has used approved analysis methods in the manner specified by topical reports describing the methods and by staff i
reports approving the. methods. The analysis methods addressed are to include core themal-hydraulic calculations to establish local coolant conditions, departure from nucleate boiling or boiling transition calculations,.and thermal-hydraulic stability evaluation."
A similar finding has been made with regard to EXXON'S analysis methods (Attachment 3)'and should other vendors develop BWR fuel reload capability, a similar review would be performed.
As a result of these findings we plan to take the following actions:
1.
For. all new licensing actions which affect thermal-hydraulic stability, the licensee must show compliance with GDC 10 and GDC 12 by either of the following methods.
I (a) Show thermal-hydraulic instabilities are not possible by design.
This would be done using a best estimate analysis which includes approved analysis uncertainties.
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(b) Show that proper detection and suppression capabilities (consistent with GE recommendations) are in the plant Technical Specifications.
2.
Inform all BWR licensees by letter (Attachment 4) of the staff's technical resolution of dB-19 and its safety significance to evaluation of core reloads.
3.
Take no ininediate action on operating plants but assure that new core reloads are properly evaluated for conformance to GDC 12 in accordance with item.(1).
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The staff concludes our technical findings concerning themal-hydraulic stability do not pose an imediate safety concern for continued BWR operation j
prior to. orderly examination and possible change of Technical Specifications for the reasons which follow:
(a) Current BWR Technical Specifications pla*ce restrictions'on i
operation under conditions of natural circulation or single i
loop operation such that the frequency of operation in regions i
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-with low stability margin is low.
In addition, the core designs for most operating reactors are sufficiently stable that limit cycle operations are believed to be unlikely even in the-less stable permissible o Even if such oscillations should occur, peration regimes.
i it is believed that the high
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power level APRM scram protection would be adequate in most cases where. large local oscillations are not involved.
(b) The magnitude of thermal-hydraulic instability induced neutron flux oscillations is considerably higher than the oscillations l
in the average cladding heat flux because of delays caused by l
the fuel thermal time constant. General Electric Company calculations for the case where the neutron flux oscillates
-up to 120 percent of rated (RPS trip point) indicate that th,e surface heat flux has a peak oscillation amplitude of only 5 percent of its rated value.
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(c) The oscillations are detectable with proper monitoring j
and can be easily suppressed by inserting control rods.
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(d). General Electric Company has provided all its operating plants guidance on the proper methods to monitor for i
themal-hydraulic instabilities and on the actions that should be taken to suppress such oscillations if they should occur. BWR owners have been made aware of the problem and of pending actions, i
The impact of this position on operating plants is as follows:
1.
As described in our SER (Attachment 2), the BWR 1, 2, & 3s utilizing current fuel designs have been found to have sufficient margins to thermal-hydraulic instability and will not be impacted.
2.
All BWR 4, 5, and 6s which are past the first few cycles of operation will calculate decay ratios > 1.0 after application of approved un-certainties.
For such plants the most feasible approach would be to implement technical specifications which enforce GE SIL 380 recomen-dations.
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e In fact, in response to GE and GE Owners group recommendations, most of these owners have either submitted revised stability technical specifications or intend to do so. Attachment 5 lists the current status of these plants based on a telephone survey by the staff.
The actual impact of the GE SIL-380 operating procedures or related Technical Specifications on plant operation is significant only in the low flow (< 45% of rated) region of the power / flow operating map. This region may be encountered during startup/ shutdown, during rod sequence exchanges and as a result of a recirculation pump (s) trip event. During normal startup operations, small delays in power escalation may be experienced to accommodate rod position restrictions and for neutron flux noise level monitoring requirements and to establish baseline neutron flux and flow noise levels at least once per fael cycle. During operation at low flow (usually due to recirculation pump trip),
control rods must be inserted to 80% of the 100% rated rod line. However, most plants currently have technical specifications which restrict operation at power to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or less with one or both recirculation pumps out of service.
Restrictions on continuous operation in the single loop mode may be removed for plants having the revised stability technical specifications in place.
In conclusion, we believe the position outlined in this memo resolves generic issue #B.19. THERMAL-HYDRAULIC STABILITY. We believe this position is an acceptabl,e application of GDC 10 and GDC 12 and does not constitute a new requirement. We have also reviewed Standard Review Plan 4.4 and find this position to be entirely consistent with current instructions to reviewe'rs.
For these reasons we do not believe CRGR review is needed. We would expect toissuethegenericletter(Attachment 4)inapproximately2-3weeksunless we hear otherwise.
By copy of this memorandum, the Division of Licensing is requested to prepare and issue the appropriate generic letter as indicated in the preceding paragraph and the Division of Safety Technology is requested to close out Generic Issue No. B_19.
Original signed by Darrello.zisenhut
[/HaroldR.Denton, Director g Office of Nuclear Reactor Regulation DISTRIBUTION EENTRAL FILE D. FIENO cc:
R. Lobel CPB:R/F G. SCHWENK D. Fieno H. DENTON L. RUBENSTEIN R. Clark D. EISENHUT L. PHILLIPS H. THOMPSON R. LOBEL T. SPEIS R. BERNER0 NR
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References:
1.
G. A. Watford, " Compliance of the General Electric Boiling Water o
Reactor Fuel Designs to Stability Licensing Criteria". NEDE-22277-P-1, October 1984.
2.
L. A. Nielsen, et.al., " Stability Evaluation of Boiling Water Reactor Core,"XN-NF-691(P)(A), August 1984.
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ATTACHMENT 1 i
UNITEJSTATES f
p NUCLEAR REGULATCM COMMISSION i
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e MEMORANDtJM FOR:
Robert H. Bernero, Director Division of Systems Integration
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FROM:
Harold R. Denton, Director l
Office of Nuclear Reactor Regulation l
SUBJECT:
SCHEDULE FOR RESOLVIllG AND COMPLETING GENERkC ISSUE NO. B THERMAL-HYDRAULIC STABILITY The technical resolution for Generic Issue B-19. " Thermal-Hydraulic Stability" is available. This memorandum approves NRR staff taking appropriato actions necessary to complete this issue. The evaluation of the subject issue is'provided in Enclosurt 1.
In accordance with NRR Office letter No. 40, " Management of Proposed Generic Issues," the resolution of this issue will be monitored by the Generic Issue
!!anagement Control System (GIMCS). The information needed for this system is indicated on the enclosed GIMCS infomation sheet (Enclosure 2). Your schedule for resolving and completing this generic issue should be comensurate with the high priority nature of the work and consistent with the NRR Operating Plan.
Nomally, as stated in the Office Letter, the infomation needed should be provided within six weeks.
The enclosed prioritization evaluation will be incorporated into NUREG-0933 "Prioritization of Generic Safety Issues," and is being sent to other NRC offices, the ACRS, and the PDR for coments on the technical accuracy and completeness of the prioritization evaluation. Any changes as a result of coments will be coordinated with you. However, the schedule for the resolution of this issue should not be delayed to wait for these coments.
The infomation requested should be sent to the Safety Program Evaluationeq Branch, DST. Should you have any questions' pertaining to the contents of this memorandum, please contact Louis Riani (24563).
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Harold R. Denton, Director Office of Nuclear Reactor Regulation Enclosu.res:
1.
Prioritization Evaluation 2.
Generic Tssue Management Control System j
cc: See next page l'
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ENCLOSURE 1 r
PRIORITIZATION EVALUATION GENERIC ISSUE B-19
" THERMAL-HYDRAULIC STABILITY" t
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ISSUE NO. B-19: M ERMAL - HYDRAULIC STABILITY DESCRIPTION i
j Historical Backcround I
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The possibility of thermal-hydraulic instability in a BWR has been investigated i
by GE since the start-up of early BWRs (references 2, 3). Analytical methods j
and c~ odes were formulated on the basis of these early investigations (references 7, 8, 9, J0,' 11,12,13, and 15) with which to predict the stability
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characteristics of B'WRs.
Eventually the licensing basis and analytical methods used by the General Electric Company to evaluate the stability of BWRs were
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documented and presented in January 1977 in NED0-21506 (reference 1). Since 1977, significant effort has been expende'd on developing an understanding of i
i BWR instability. Testing at operating BWRs (references 4, 5, and 6) has added I
to the information obtained from single channel and control rod escillator
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tests in the early BWRs (reference 3).
In addition, improved state-of-the-art thermal-hydraulic methods and fuel rod performance studies have permitted l.
greater definition of the stability phenomenon and criteria for prevention of instability (references 14,17).
Developments along these lines have i
resulted in updated methods and models for the assessment and evaluation of BWR stability limits for licensing purposes-(references 18 and 19). However, recent data from a high power-density foreign BWR unexpectedly indicated that i
scram protection based on the average power range monitor signals would not necessarily prevent violation of the critical heat flux limits if local instabilities occur.
As a result of these findings the staff proposed the issuance of a Board Notification (reference 20)..
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At the request of the licensees the NRC staff has reviewed two submittals l
and has recogtly approved technical specification changes for two BWRs to resolve the concerns related to the thermal-hydraulic stability in these plants (references 21 and 22) 1 i
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SAFETY SIGNIFICANCE
-.r. 93 Hydrodynzmic how instabilities'nay occur in a BWR when two phase flow exists in a channel with critical dimensions and particular f1ow parameters. The instability can cause power esci11ations and'1ead to local violation of the critical heat fl,ux.
POSSIBLE SOLUTIONS The proposed resolution is Technical Specifications that will restrict operation of the rea'ctor in regior.s of potential thermal-hydraulic instability and/or provide for surveillance and corrective measures under conditions of marginal stability.
CONCLUSION
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Updated analytical methods and analyses based on the recent experimental results have been made available to address thermal-hydraulic instability concerns. These metheds are being reviewed by the hRC staff to determine their acceptability for evaluating the stability.of core designs and for delineating the power / flow regions of potential instability for which reactor operation will be restricted by appropriate sodification of the plant technical specifications (references 21 and 22). Therefore, the resolution of this issue has been identified.
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