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Generic Safety Issue (GSI) 191Pressurized Water Reactor (PWR)

Sump PerformancePresented by:

Donnie HarrisonOffice of Nuclear Reactor Regulation

Presented to:Advisory Committee on Reactor Safeguards

October 2, 2008

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Today’s Discussions

• Generic Letter (GL) 2004-02 Closure Process and Overview of Current Status

• Discussion of Selected Technical Areas– Emergency Core Cooling System (ECCS)

Sump Strainer Head Loss Testing– Chemical Effects– In-vessel Downstream Effects

• Fuel inlet blockage TRACE Calculation

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Background

• GSI-191– Assessment of Debris Accumulation on PWR Sump Performance

• Bulletin 2003-01– Licensees who chose not to confirm regulatory compliance were

asked to describe any interim compensatory measures that would be implemented to reduce risk until the analysis could be completed

– All licensees responded to Bulletin 2003-01, but it was recognized that the methodology to perform the evaluations was not available at the time

• GL 2004-02– Most licensees requested and received extensions to GL 2004-

02 to support the completion of testing, analyses, and implementation of corrective actions

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Current Status of GSI-191

• All licensees have installed significantly larger ECCS sump strainers

• Licensees have done, or will do, other modifications, for example:– insulation modifications– replace sump buffer– debris interceptors– water management

• Strainer testing activities have been performed for nearly all licensees

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Current Status (Continued)

• Most licensees requested extensions beyond December 2007 to complete certain corrective actions– Integrated head loss testing, including

chemical effects– Downstream effects analyses– Plant modifications

• The staff is nearing completion of the review of the licensees’ initial supplemental responses

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Closure Process

ManagementDecision

Licensee SubmitsGL 2004-02Information

Detailed StaffReview

IntegrationReview Team

DocumentLicensee Closure

IRTDetermination

Draft RAIs

Request AdditionalInformation (RAIs)

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License Submittal

• Each licensee will provide, as applicable:– Initial response to GL

• All plants submitted supplemental responses to GL in February/March 2008

– Responses to RAIs on the licensee’s GL supplemental submittals

– Responses to open items identified in NRC staff audits

– Final supplemental response after all testing and evaluations completed

– Submittal addressing in-vessel downstream effects after WCAP-16793-NP issued, if appropriate

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Detailed Staff Reviews

– Break selection– Debris generation– Debris characteristics– Latent debris– Debris transport– Head loss and vortexing– Net positive suction head

– Coatings– Debris source term– Screen modifications– Structural analysis– Upstream effects– Downstream effects– Chemical effects

• Technical staff performs area-specific detailed reviews

• Reviews involve 10 staff members from DSS, DCI, & DE• Output of initial review is draft RAIs

- 60% of plants through detailed reviews

• Plan to have completed initial reviews of all plants by

end of October

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Integration Review Team

• Consists of 3 senior technical staff (including senior level scientist)

• Performs holistic review of licensee information and staff detailed review and draft RAIs– Interactions with detailed technical reviewers to ensure staff views and

IRT recommendations understood

• Makes determination regarding need for RAIs/issue closure– Recommendation to Management includes minority opinions– Detailed reviewers can appeal IRT recommendation to Management– 50% of plants through IRT phase– Staff has informed several licensees with “low-fiber” that the staff has

few RAIs– Most other plants have received, or will receive, RAIs– Most plants receive a “Placeholder” RAI for in-vessel downstream

effects if they are relying on WCAP-16793-NP

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Closure Activities

• The staff reviews supplemental information/RAI responses in accordance with the closure process

• The Regions inspect implementation of modifications and other commitments

• The staff will issue a closure letter to each licensee when sufficient information is provided to close the issue for that plant

• After all licensees have been issued closure letters, GL 2004-02 will be formally closed

• Some modifications will be made after planned issue closure– NRC will track all commitments to completion

• The staff expects to complete all technical review activities to support closure next year

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ECCS Sump Strainer Testing

Presented by: Stephen Smith

Office of Nuclear Reactor Regulation

Presented to:Advisory Committee on Reactor Safeguards

October 2, 2008

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Strainer Testing Overview

• Strainer testing is being conducted to ensure adequate net positive suction head (NPSH) margin for emergency core cooling system (ECCS) and containment spray system (CSS) pumps under accident conditions

• The staff’s assessment of testing has been refined as observations of additional testing allowed understanding of how various test parameters affect results

• To be discussed:– Staff observation and review of strainer testing– Lessons learned regarding head loss testing– Review guidance for head loss testing and evaluation– Staff review of GL 2004-02 responses in the head loss area– Path forward

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Head Loss TestingStaff Observations

• Staff has witnessed a number of head loss tests at each vendor – Lessons learned have been incorporated into review guidance

for testing, staff review of licensee test activities, and staff review of GL 2004-02 submittals

• Most strainer vendors/testers have now developed procedures that the staff agrees are capable of producing conservative head loss results

• Some vendors have not provided adequate assurance that their current protocols are conservative

• Some licensees may be able to justify the use of head loss results from testing that does not meet the current guidance

• Some licensees will likely have to retest using procedures that meet staff guidance

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Head Loss TestingLessons Learned

• Debris Preparation– Fibrous debris sizing– Debris sizing should match transport evaluation

• Debris Introduction– Agglomeration

• Thin Bed Test Protocol– Debris introduction order– Debris amounts not conservative– Debris sizes not conservative

• Test Flume Flow Patterns– Stirring– Similarity to plant (e.g. floor or sump location, volume, circumscribed

velocity)• Lessons learned are reflected in the staff review guidance for

strainer testing

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• Example of inappropriate debris addition• Excessive agglomeration due to high fibrous debris

concentration• Excessive settling of debris could occur• Agglomerated debris less likely to uniformly cover strainer

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Video of Appropriate Debris Preparation and Introduction

• Finer debris, more uniformly covering strainer will lead to higher head losses

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Head Loss TestingReview Guidance

• Staff Issued Updated Head Loss Testing Review Guidance in March 2008– Incorporates recent lessons learned from

industry head loss testing discussed previously

– Publically available– Tests and evaluations conducted per this

guidance should result in conservative results that may be used for plant strainer qualification

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Path Forward

• Plants that have RAIs will have to provide acceptable responses, additional analyses, or retest to assure adequate strainer performance

• Some licensees that have had unacceptable test results using conservative protocols are “testing for success” by identifying and testing several contingency plans until success is achieved, e.g.:– Analytical changes to reduce calculated debris loading– Physically removing debris sources from containment– Installing debris interceptors or other plant modifications– Plant will be modified at upcoming outage to be consistent with

successful test condition

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Conclusions

• Strainer testing methods have improved• Some licensees have demonstrated acceptable

strainer performance as shown by conservative tests

• Some licensees are working to reduce debris loads– Retesting with reduced debris loads is required

• Some licensees will attempt to stand on current test results by responding to staff RAIs– Staff will consider the additional information provided

on a case by case basis

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Chemical Effects

Presented by: Paul Klein

Office of Nuclear Reactor Regulation

Presented to:Advisory Committee on Reactor Safeguards

October 2, 2008

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Chemical Effects Activities

• NRC Staff evaluation of industry chemical effects testing– Different approaches by multiple industry vendors– Pre-mixed precipitates, precipitates formed in-situ, and

“evolving chemistry” tests– Staff commenting on test procedures and observing testing– Issued review guidance in chemical effects, Sept. 2007– Issued safety evaluation report for WCAP-16530, Dec. 2007

• Technical Support From Argonne National Laboratory, Dr. Robert Litman– Relative head loss from various precipitates – Investigate aluminum solubility in alkaline, borated water– Licensee GL 2004-02 supplement review

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Argonne Vertical Loop Test Results

• Industry Test Precipitates Characteristics – Per unit mass of Al removed from the solution, the WCAP AlOOH

surrogate caused greater head loss than aluminum hydroxide precipitate from aluminum coupon dissolution.

• Increasing head loss for ANL test loop conditions: – WCAP AlOOH– WCAP Sodium Aluminum Silicate – “tap water”– In-situ formation of aluminum hydroxide by chemical addition – 6061 Aluminum, 1100 Aluminum– WCAP Sodium Aluminum Silicate – high purity water

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Long-term Al Hydroxide Precipitation Tests: Al Hydroxide Precipitation Map

• Long-term solubility test results for various pHs and Al concentrations represented in a Al hydroxide precipitation map that plots pH and Al concentration vs. temperature.

• Solubility increases with pH and temperature

• Loop tests with Al alloy plates seem to suggest lower solubility than the chemical Al tests. This may be due to heterogeneous nucleation of Al hydroxide on intermetallic particles and/or on the surfaces of preexisting precipitates

10 ppm

50 ppm

100 ppm

pH of 8 at 140F

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Summary Chemical Effects Status

• NRC Staff has observed tests at each vendor facility • Vertical head loss loop tests are typically more

susceptible to large head losses from chemical precipitates compared to larger scale strainer tests

• Most plants are using test methods that are acceptable to the staff, although some technical issues remain that will be resolved with individual licensees

• Testing at Argonne and at vendor facilities continues to indicate that the WCAP-16530-NP methodology is conservative with respect to the amount and the properties of precipitates

• NRC staff to perform a few chemical effects audits to assess overall evaluations at selected plants

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In-Vessel Downstream EffectsFlow Resistance due to Potential

Debris AccumulationPresented by: Stephen Smith

Office of Nuclear Reactor Regulation

Presented to:Advisory Committee on Reactor Safeguards

October 2, 2008

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Introduction

• Background

• Debris in the core

• How debris loads are determined

• Diablo Canyon testing

• PWROG testing

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Debris Effects on Reactor Core - Background

• WCAP-16793 issued to provide guidance to plants on in-vessel debris effects

• ACRS raised concerns with adequacy of the WCAP methodology and assumptions– Only one significant set of tests for fuel head loss had been

conducted– Some assumptions used in WCAP evaluation were not validated

• PWROG is working to provide more rigorous guidance in the WCAP

• An outstanding concern is potential head loss within core due to debris accumulation

• PWROG is conducting testing with representative fuel inlet types and varying debris loads

• Staff will review revised WCAP when it is completed

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Debris at Fuel Inlet

• Debris load is plant specific• Fibrous bypass (pass through) determined by

strainer testing• Fibrous test debris characteristics are similar to

actual bypassed debris• Testing to date has assumed no filtering of

particulate or chemical debris by strainer– This is a conservative assumption because some

debris will filter out on the strainer– Chemical loading determined per WCAP-16530

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Vendor Fiber Bypass Testing

• Vendor, and in some cases, plant specific

• Downstream sampling methods

• Sample is dried and weighed to determine mass

• Size distribution of sample is determined

• PWROG to provide fiber bypass data

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Diablo Canyon Fuel Testing

• Westinghouse Alternate P-grid• Testing Performed at CDI • Witnessed by staff• Prototypical Debris for Plant• Varied Debris Loads• Bottom Nozzle and One Intermediate Grid Strap• Hot Leg and Cold Leg Flows Tested• Tested head losses were within allowable limits

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PWROG Test Observations

• Testing has just begun• Staff observed early testing at

Westinghouse• Standard P-grid was tested• Testing used hot leg break flow rate• Debris preparation and introduction was

appropriate• Observations indicate that test program

can result in conservative results

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PWROG Test Plans

• Other Westinghouse fuel designs will be tested to ensure that results are bounded

• Testing of Areva fuel designs to be conducted in the future

• PWROG plans to increase debris loads to bound as many plants as possible

• Staff will continue to review data and test information as it becomes available

• PWROG to determine limiting core head losses allowable for various breaks to be applied as acceptance criteria for tests

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Conclusions

• Westinghouse and CE fuel testing is currently underway

• Areva fuel testing is scheduled to begin later this year.

• Testing will determine acceptable debris loading for various fuel designs, postulated conditions, and debris mixtures

• WCAP-16793 to be revised based on test results

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Core Inlet Blockage AssessmentGSI-191 In-Vessel Review

Presented by: Ralph R. Landry

Senior Level AdvisorOffice of New Reactors

Presented to:Advisory Committee on Reactor Safeguards

October 2, 2008

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Core Blockage Assessment - Background

• March 2008 T/H Subcommittee– Presented TRACE analysis based on 95%

core inlet blocked– Core heatup calculated to be less than 300 ºF– Questions raised concerning coolant dispersal

beyond core entrance plane

• RES Performed Additional Calculations– TRACE based on porous medium at core

entrance– “Hand calc” using Excel spreadsheet solution

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Core Blockage Assessment -TRACE Model

• Core entrance flow passing through porous medium

• Uniform Nukon/CalSil debris bed instantaneously present– Debris bed form loss developed from test

data in NUREG-1862 and NUREG/CR-6917;

Δpbed = f(bed thickness, approach velocity)– Debris bed thicknesses of 1.2, 2.4, and 4.8

inches

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Core Blockage Assessment -Core Collapsed Level

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Core Blockage Assessment -PCT

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Core Blockage Assessment -Hand Calc Model

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Core Blockage Assessment -Hand Calc

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Conclusion

• The staff has established a process for closure of GL 2004-02

• Licensees have implemented significant modifications to prevent unacceptable strainer blockage

• Guidance has been developed to ensure conservative test protocols and evaluations

• In-vessel downstream effects will be resolved as part of WCAP-16793 review