ML20217A846
| ML20217A846 | |
| Person / Time | |
|---|---|
| Issue date: | 01/09/1998 |
| From: | Robert E. Uhrig Advisory Committee on Reactor Safeguards |
| To: | Advisory Committee on Reactor Safeguards |
| References | |
| ACRS-3082, NUDOCS 9803250274 | |
| Download: ML20217A846 (9) | |
Text
RI.lWH,:'""Lepvz ""~"o) r' 1/9/96 i
Q_OTIFIED BY:
Robert E. Uhrig - 1/13/98 f
ADVISORY COMMITTEE ON REACTOR SAFEGUARDS MINUTES OF THE ACRS SUBCOMMITTEE ON PLANT OPERATIONS
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DECEMBER 2,1997 ROCKVILLE, MARYLAND j
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INTRODUCTION The Advisory Committee on Reactor Safeguards (ACRS) Subcommittee on Plant Operations held a meeting on December 2,1997, in Room T-2 B3,11545 Rockville Pike, Rockville, Maryland, with representatives of the U.S. Nuclear Regulatory Commission (NRC). The meeting was held to gather information conceming the staffs safety evaluation report on the industry-sponsored utility resolution guidance for the issue of blockage of the suction strainer in the emergency core cooling system (ECCS). The Subcommittee also reviewed the status of licensee responses to NRC bulletins to address strainer blockage, research associated with debris generation and transport, and the staffs proposals to resolve these issues. The entire meeting was open to the public.
Mr. Amarjit Singh was the cognizant ACRS staff engineer for this meeting. The meeting was convened at 8:30 a.m. and adjourned at 4:30 p.m.
ATTENDEES:
ACRS Members R. Uhrig, Acting Chairman R. Seale W. Shack D. Miller D. nowers T. Kress Princioal NRC Seeakers G. Holahan, Office of Nuclear Reactor Regulation (NRR)
R. Elliott, NRR M. Marshall, Office of Nuclear Regulatory Research (RES) g/
A. Serkiz, RES D. Rao, Science and Engineering Associates, Inc. (SEA)
[JQ No written comments or requests for time to make oral statements were received from members of the public. A complete list of meeting attendees is kept in the ACRS Office File and will be made available upon request. A list of those who registered is available in the ACRS Office. The presentation slides and handouts used during the meeting are j
attached to the office copy of these minutes.
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Plant' Operations December 2,1997 Subcommittee Minutes Chairman's Opening Remarks Dr. Robert E. Uhrig, Acting Chairman of the Plant Operations Subcommittee, convened the meeting at 8:30 a.m. He stated that the purpose of the meeting was to discuss the sta#s draft safety evaluation report (SER) on the industry-sponsored utility resolution guidance (URG) and stafs review of licensee, responses to NRC bulletins to address the strainer blockage, research associated with debris generation and transport, and proposed actions taken by the staff to resolve these issues.
NRC Staff Presentation Overview - Mr. Gary M. Holahan, NRR Mr. Holahan, NRR, presented the overview and updated the Committee on the status of stafs activities to resolve the Boiling Water Reactor (BWR) ECCS Strainer Clogging Issue.
Mr. Holahan stated that the stafs presentation was divided into five following areas:
Drywell Debris Transport Study: Purpose /Results Drywell Debris Transport Study: Methodology and Description Status of Licensee Response to NRC Bulletin 96-03 Safety Evaluation Report on BWROG (BWR Owners Group) Utility Resolution Guidance Document ECCS Purnp Debris ingestion Study Puroose and the Results of the Drvwell Debris Transoort Studv (DDTS). - Mr. Michael L.
Marshall, NRR Mr. Marshall, presented the purpose and results of the DDTS. The DDTS was performed by SEA, who has a technical assistance contract with RES. The purpose of the DDTS was to provide a reasonable basis for making an informed engineering judgment on the magnitude of debris transport from the drywell to the wetwell. The objective of the study was to develop a methodology for estimating the fraction of loss-of-coolant-accident (f OCA) generated fibrous insulation debris that would be transported from the location of
~ their generation in the drywell to the suppression pool. The DDTS provided a basis by which NRC staff could judge the appropriateness of the debris transport factors used in the utility strainer blockage analyses.
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I Plant Operations December 2,1997 Subcommittee Minutes i
The first step of the DDTS was to perform an end-to-end scoping calculation to gain an understanding the thermal and hydraulic conditions that would govem debris transport.
The findings of the DDTS were used to delineate plant features and transport phenomena that dominate debris transport in the drywell. Three such plant features were identified:
(1) number and arrangement of gratings with respect to break; (2) duration of unthrottled ECCS flow; and (3) vent and drywell floor design. The experimental results clearly illustrated that during blowdown, the drywell floor gratings provided the largest potential for capturing of both small and large debris, with a capture efficiency between 15 percent and 30 percent for small debris and 100 percent for large debris. The small pieces captured on gratings can be easily re-entrained by ECCS water flow during washdown. The only mechaniam available for washdown of large pieces was erosion, which was found to be a constant rate process. Therefore, time assumed for unthrottled operation of ECCS pbyed a key role in determining the fraction of large pieces that would be washed down.
A total of 20 accident scenarios covering three plant types (Mark I,11, and Ill), two break types (main steam line break and recirculation line break), and a variety of assumptions regarding ECCS response were analyzed. For each scenario, two types of transport factors were obtained. The upper bound estimates were obtained by compounding bounding estimates for the effect of each transport pathway. It is extremely unlikely that transport following a LOCA would exceed upper bound estimates. The central estimates were judged to be more realistic representation of debris transport. The DDTS concluded that URG-recommended transport factors for Mark ll containments underestimate debris transport. For Mark I and Mark lli drywells, URG appeared to present reasonable estimates, NRR staff used the findings that the plant contains a continuous lower grating free of large holes. The results of the DDTS has been used by the NRR staff during the review of BWROG URG and will use them during the review of plant-specific assessments.
Drvwell Debris Transoort Study Methodoloav and Descriotion - Dr. Dasari V. Rao, SEA Dr. Rao discussed the description and the methodology used in performing the PDTS. He stated that, during the performance of the DDTS, a series of experiments wh designed i
and conducted to address the data needed related to inertial capture on dryweh structures and washdown of debris by ECCS flow. The DDTS also contained detailed computational fluid dynamics (CFD) simulations taken to determine the flow pattems that exist on the drywell floor and the likelihood of debris sedimentation. The DDTS relied primarily on this database to quantify the importance of each transport pathway. Analytical tools (e.g.,
computational fluid dynamics codes and MELCOR) were used as needed either during the design flow of the experiment to establish thermal-hydraulic conditions to be simulated in the experiments, or in the post experimental stage to scale the experimental data to plant conditions. Significant points considered in the methodology used in DDTS follow:
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u Plant Operations December 2,1997 Subcommittee Minutes I
'Quantification Method transport problem broken down into components that are amenable to simulation 1
components solved using experimental data or analytical models l
engineering judgments applied when experimental data were not available and analytical models were too complex Quantification is tractable and can be easily modified to accommodate future data Transport Fraption Estimates The two types of estimates used for debris transport fractions were:
Upper Estimate These estimates were obtained by compounding the bounding estimates for each individual logic-tree branch. It is extremely unlikely that actual drywell transport J
factor in a BWR following a LOCA would be greater than the upper bound estimate.
The upper bound estimates can be treated as generic estimates that will most likely bound all accident scenarios and operating conditions for that plant type.
Central Estimate These estimates were obtained by compounding more realistic estimates for each individual branch. The central estimates are judged to be closer to reality, but without the assurance that they can not be exceeded under any accident condition.
Central estimates should be applied on a plant-specific basis after assuring that -
actual plant systems behavior is consistent with the accident scenarios analyzed and that the underlying modeling assumptions were representative of the particular plant.
Main Steam Line Break "MSLB" Small Debris Blowdown Transoort i
The accident scenario was analyzed in a postulated MSLB in a Mark i drywell. The break was assumed to be in the mid-region of the containment where it was likely to generate
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maximum amount of debris. Debris generation occurs within seconds after LOCA. Debris i
generation was limited to a zone of influence surrounding the break. Further destruction a
Plant Operations December 2,1997 Subcommittee Minutes or degradation of the debris outside the zone of influence was negligible. Steam was the medium of transport. Medium of transport affects pathways available for transport.
The debris were nearly dry. Wet debris tends to be heavier and will be more likely to impact the structural impediments in their path and adhere to them. A maximum increase in weight of 20 percent was noted in these tests with all the condensed water film on the outer surface; the bulk of the fragment volume was essentially dry. Debris size distribution is uncertain. The typical sizes of the debris as they exited the zone ofinfluence and the i
mass of debris belonging to each size group were vital components necessary to estimate overall transport factor.
Approximately 35 percent of the small debris were advocted to the vents within first two J
seconds.15 percent of the debris were captured at floor gratings and the remaining 85 percent would be transported downwards.
MSLB Erosion and Washdm l
The ECCS flow rates and the drywell layout information were coupled to calculate the j
water flow impinging on each structure. The experiments were conducted to evaluate potential for washdown and erosion of debris corresponding to these conditions. The experiments also established that the spray water would not result in significant erosion of large pieces. Small debris deposited on structures above were subjected to condensate drainage. The upperbound estimate was obtained on these assumptions. In thirty minutes, the containment sprays would washdown 100 percent of the small debris deposited on structures. At the end of blowdown, approximately 11 percent of the small debris were retained on the floor gratings, which were exposed to containment spray flow for thirty minutes. The containr.1ent sprays had washdown 1 ' percent of the large debris.
The data showed that about 8 and 27 percent of the debris were eroded in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> respectively for the central estimate during the full flow of ECCS 12 and 30 percent for the upper bound estimate.
Status of Licensees Resoonse to NRC Bulletin 96-03: Potential Pluaaina of Emeroencv Core Coolina Suction Strainers by Debris in Boilino-Water Readors - Mr. Robert B. Elliott, NRR Mr. Elliott briefly presented the status of the licensees response to NRC Bulletin 96-03 issued on May 6,1996. He stated that the licensees who received the bulletin were required to respond to the bulletin. The licensees were given three options to resolve this issue: (1) installation of a large passive strainer; (2) installation of a self-cleaning strainer and (3) backfiush of the system. Most of the licensees have concluded that the
' installation of a new larger strainer does not constitute an unreviewed safety question in
Plant Operations December 2,1997 Subcommittee Minutes accordance with 10 CFR 50.59. Installation of new strainer does not require prior staff approval. All the licensees chose option to install the large-capacity-passive strainers.
Mr. Elliott stated that the staff has planned to conduct 6 to 8 inspections starting in the i
spring of 1998. He concluded that all the licensees have made good progress toward implementing of NRC Bulletin 96-03. The staff has developed a closure plan consisting of URG.
Safetv Evaluation Renort on the BWROG Utility Resolution Guidance Document - Mr.
Elliott, NRR Mr. Elliott presented the background of the safety evaluation report on the URG document.
He stated in his summary that the Boiling Water Reactor Owners Group (BWROG) submitted the URG document for the staffs review. The document gave BWR licensees guidance on complying with NRC Bulletin 96-03. The major focus of the URG was to provide detailed guidance on performance of plant-specific analyses consistent with Regulatory Guide 1.82 (RG 1.82), Revision 2, " Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant-Accident." Before reviewing the URG, the staff had closely followed the generic effort conducted by the BWROG for resolving the ECCS strainer clogging issue.
The staff identified the following open items in the safety evaluation report:
Standard Review Plan Section 3.6.2 and Branch Technical Position (MEB) 3-1 are unacceptable for demonstrating compliance with 10 CFR 50.46.
Licensees should verify experimentally that the strainer solution under consideration is not susceptible to thin bed effects. The experimental data should demonstrate that no thin bed effects up to and including debris loadings are consistent with a medium break for the licensees's plant.
Large breaks with the largest " particulate debris to insulation debris" ratio by weight should be considered.
Weaknesses in the scaling of the BWROG air jet impacts test data.
On the Basis of the staffs analysis of a reference BWR 4 with a Mark I containment for NUREG/CR-6224, the use of values given in the URG for dirt and rust are acceptable; however, licensees should evaluate the URG for applicability to their plants.
Plant Operations December 2,1997 Subcommittee Minutes Licensees should assume 100-percent transport of unqualified / indeterminate coatings to the strainers.
The BWROG did not establish an adequate basis for assuming different transport factors for Mark lis.
The suggested transport factors for large debris below the lowest grating lacked adequate justification. A 100-percent transport factor should be used for large.
fibrous debris below the lowest grating.
On the basis of the confirmatory calculations, the staff concluded that the BWROG head loss correlation for fibrous debris beds is unacceptable.
~ Since the ECCS must be capable of performing its safety function throughout the entire accident, not positive suction head (NPSH) should be evaluated to ensure that adequate NPSH is available at all times.
The staff concluded that, in general, the URG is a good guide, and made the following comments:
Where staff concems have been identified, the licensees should address those concerns when applying the URG.
if a licensee desires to use a portion of the URG guidance on which the staff has been unable to reach a conclusion on adequacy--due to an insufficiency in the URG document-the licensee should address application of the guidance and the staff's concerns in a plant-specific submittal.
Since several staff concems have been identified with the URG document, the draft safety evaluation will be issued to BWROG to respond to the previously stated concems.
The staff is planning to issue the final safety evaluation report after it receives the responses from the BWROG in April 1998.
ECCS Pumo Debris Inaestion Studv - Mr. Aleck W. Serkiz, RES Mr. Serkiz presented the background _and summarized the effects of debris ingestion on ECCS pump performance. He stated that the purpose of this study was to assess the effect of ingesting LOCA-generated materials into ECCS pumps' and the subsequent
. impact of this debris on the pumps ability to provide long-term cooling to the reactor core.
-The study discussed the potential effects of ingested debris on pump seals, bearing
Plant Operations December 2,1997 Subcommittee Minutes assemblies, cyclone debris separators, and seal cooling water subsystems.
The evaluations indicated the following:
Pump shaft seals have a fairly high likelihood of failing the assumed 100-day post-LOCA mission time. Seal failure should not disable the pump or degrade its performance to the point at which the pump is considered failed, unless there is a lot of leakage.
Pump bearing assemblies, particularly those designs that depend on availability to provide hydrodynamic or hydrostatic balance to the pump impeller and other rotating components, are suspectable to debris blocking the coolant passage.
SUBCOMMITTEE RECOMMENDATIONS The subcommittee recommended that the staff brief the full committee on these issues.
FOLLOWUP ACTIONS The staff plans to send the ACRS a copy of the final safety evaluation report on the URG by the end of April 1998. The Subcommittee also recommended that the staff should investigate PWR's to see if they have concerns that are similar to BWR concems.
CONCLUSION This was for information only. No Committee action is required.
BACKGROUND MATERIAL PROVIDED TO THE SUBCOMMITTEE:
Transmittal of BWR Owners' Group utility Resolution Guidance for ECCS Suction Strainer Blockage Drywell Debris Transport Study, NUREG/CR-6369, Draft Final Report dated August 29,1997 Reassessment of Debris ingestion Effects on Emergency Core Cooling System Pump Performance, Final Report dated July 31,1997
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Plant Operations December 2,1997 Subcommitt.ee Minutes Drywell Debris Transport Study: Computational Work Final Report dated September 26,1997 NRC Bulletin 96-03: Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling-Water Reactors dated May 6,1996 Report dated February 26,1996, from T. S. Kress, Chairman, ACRS, to Shirley Ann Jackson, NRC Chairman,
Subject:
Proposed Final Bulletin 96-XX, " Potential Plugging -
l-of Emergency _ Core Cooling Suction Strainers by Debris in Boiling Water Reactors" and an associated Draft Revision 2 of Regulatory Guide 1.82, " Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident" Memorandum dated June 18,1996, from Gary M. Holahan, Director, Division of Systems Safety and Analysis, Office of Nuclear Reactor Regulation
Subject:
Reconciliation of Staff response to Advisory Committee on Reactor Safeguards Comments on Bulletin 96-03, Potential Plugging of Emergency Core Cooling Suction Strainers by Debris in Boiling-Water-Reactors" Executive Summary of the Draft Safety Evaluation on the BWR Owners Group Utility Resolution Guidance for ECCS Suction Strainer Blockage Generic Letter 97-04: Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps dated October 7, 1997 Project Status Report: Assurance of Sufficient Net Positive Suction Head Final Report BWR Drywell Debris Transport Phenomena Identification and Ranking Tables (PIRTs) dated September 1997 NOTE:
Additional details of this meeting can be obtained from a transcript of this meeting available in the NRC Public Document Room,2120 L Street, N.W.,
Washington, D.C. 20006, (202) 634-3274, or can be purchased from Neal R.
Gross & Co., Inc., Court Reporters and Transcribers,1323 Rhode Island Avenue, N.W., Washington, D.C. 20005, (202) 234 4433.
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