ML20199E421

Informs That NRC Planning to Issue in Discussing Recent Axial Offset Anomaly at Plant.Requests Review of Encl Draft to Ensure Accuracy of Technical Info Re Event.Comments Requested by 971121

ML20199E421
Person / Time
Site:	Callaway
Issue date:	11/14/1997
From:	Dennig R NRC (Affiliation Not Assigned)
To:	Randolph G UNION ELECTRIC CO.
References
NUDOCS 9711210197
Download: ML20199E421 (6)

Text

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L November 14, 1997 Mr. Garry L. Randolph Vice President and Chief Nuclear Officer Union Electric Company Post Office Box 620 Fulton, Missourl 05251

SUBJECT:

REQUEST FOR A TECHNICAL REVIEW OF A DRAFT INFORMATION NOTICE REGARDING THE EFFECTS OF CRUD BUILDUP ON POWER DISTRIBUTION AND SHUTDOWN MARGIN

Dear Mr. Randolph:

The U.S. Nuclear Regulatory Commission is planning to issuo an information notice (IN) discussing the recent axial offset anomaly at Callaway. We ask that you review the ene'osed draft of that IN to ensure the technical information regarding the event is accurate. Your cooperation in this matter is appreciated. Please return any comments you may have by close of business on November 21,1997. A copy of this request ano your response will be placed la the Public Document Room for review by the public. Yocr response should be mailed to:

U.S. Nuclear Regulatory Commission ATTN: William Burton MAllSTOP: 0-11E4 Washington, D.C. 20555-0001 Please address any questions you may have on this matter to William Burton of my staff.

Mr. Bur.on can be reached at 301-415-2853. If no comments are received ty ",e close of business on November 28,1997, we will assume the technical information in the notice is ,

correct.

Sincerely,

[0riginal sigt.ed by]

Robert L. Dennig, Section Chief Events Assessment, Generic Communications, and SpecialInspections Branch Division of Reactor Program Management Office of Nuclear Reacto: Regulation Docket No. 50-483

Enclosure:

Draft Information Notice p]' .

cc w/ encl: See next page DISTRIBUTION: diusleemlos PUBLIC PD42 Rdg/ File i PECB R/F RLDennig JWRoe SARichards EGAdensam (' y C k l WFBurton BCWestreich WHBateman EPeyton OGC ACRS WDJohnson, RIV DOCUMENT NAME: G:\WFB%OA *SEE PREVIOUS CONCURRENCES To receive a copy of this document. indicate in the box C= Copy w/o attachment / enclosure E= Copy with attachment! enclosure N = No copy OFFICE PM:PD42 PECB SC:PECB C:PECB b NAME BWestreich* WBurton:jkd* RDenning* SRichards

,DATE 11/13/97 11/12/97 11/13/97 11H/97 5@

OFFICIAL RECORD COPY

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9711210197 971114 PDR ADOCK 05000483 P PDR ll -ll".ll4}ll fll}ll.ll.

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l November 14, 1997

. . Mr. Garry L. Randolph Vice President and Chief Nuclear Officer Union Electric Company Post Office Box G20 '

Fulton, Missouri 65251

SUBJECT:

REQUEST FOR A TECHNICAL REVIEW OF A DRAFT INFORMATION NOTICE REGARDING THE EFFECTS OF CRUD BUILDUP ON POWER DISTRIBUTION AND SHUTDOWN MARGIN

Dear Mr. Randolph:

The U.S. Nuclear Regulatory Commission is planning to issue an information notice (IN) discussing the recent axial offset anomaly at Callaway. We ask that you review the enclosed draft of that IN to ensure tha technical information regarding the event is accurate. Your cooperation in th!s matter is appreciated. Please retum any comments you may have by close of business on November 21,1997.- A copy of this request and your response will be placod in the Public Document Room for review by the public. Your response should be mailed to:

U.S. Nuclear Regulatory Commission ATTN: William Burton MAIL STOP: 0-11E4 Washington, D.C. 20555-0001 Please address any questions fou may have on this matter to William Burton of my staff.

Mr. Burton can be reached at 301-415-2853. If no comments are received by the close of business on November 28,1997, we will assume the techn:0,alinformation in the notice is correct.

Sincerely,

[0riginal signed by)

Robert L. Dennig, Section Chief Events Assessment, Generic Communications, and SpecialInspections Branch Division of Reactor Program Management Office of Nuclear Reactor Regulation Docket No. 50-483

Enclosure:

Draft Information Notice cc w/ encl: See next page DISTRIBUTION: Docket Files PUBLIC PD42 Rdg/ File PECB R/F JWRoe SARichards EGAdensam RLDennig WFBurton BCWestreich WHBateman EPeyton OGC ACRS WDJohnson, RIV DOCUMENT NAME: G:\WF8%OA *SEE PREVIOUS CONCURRENCES TD receive a copy of this document, Indicate in the box C= Copy w/o attachment / enclosure E= Copy with attachmer.t/ enclosure N a No copy OFFICE PM:PD42 PECB SC:PECB C:PECB E NAME BWestreich* WBurton:jkd* RDenning* SRichards DATE 11/13/97 11/12/97 1/13/97 11ki/97 ~

OFFICIAL RECORD COPY

4

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l ,

Mr. G:rry L. R:nd:lph l cc w/encis:

Professional Nuclear Mr. Otto L. Maynard Consulting, Inc. President and Chief Executive Offic.er 19041 Raines Drive Wolf Creek Nuclear Operating Corporation Derwood, Maryland 20855 P.O. Box 411 Burlington, Kansas 66839 Gerald Chamoff, Esq.

Thomas A. Baxter, Esq. Mr. Dan I. Bolef, President Shaw, Pittman, Potts & Trowbridge Kay Drey, Representative 2300 N. Street, N.W. Board of Directors Coalition Washington, D.C. 20037 for the Enviremnt 6267 Delmar Boulevard Mr. H. D. Bono University City, Missouri 63130 Supervising Engineer Quality Assurance Regulatory Support Mr. Lee Fritz Union Electric Company Presiding Comrnissioner Post Office Box 620 Callaway County Court House Fu4on, Missouri 65251 10 East Fifth Street Fulton, Missouri 65151 U.S. Nuclear Regulatory Commission Resident inspector Office Mr. Alan C. Passwater, Manager 8201 NRC Road Licensing and Fuels Steedman, Missouri 65077-1302 Union Electric Company Post Office Box 66149 Mr. J. V. Laux, Manager St. Louis, Missouri 63166-6149 Quality Assurance Union Electric Company Post Office Box G20 Fulton, Missouri 65251 Manager - Electric Department Missouri Public Service Commission 301 W. High Post Office Box 360 Jeffersen City, Missouri 65102 Rogional Administrator. Region IV U.S. Nuclear Regulatory Commission Harris Tower & Pavilion 611 Ryan Plaza Driv;, Suite 400 Arlington, Texas 76011-8064 Mr. Ronald A. Kucera, Deouty Director Department of Natural Resources P.O. Box 176 Jefferson City, Missouri 65102

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UNITED STATES

• NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555-0001 November ,1997 NRC INFORMATION NOTICE 97-XX: EF 'ECTS OF CRUD BUILDUP AND BORON DEPOSITION ON POWER DISTRIBUTION AND SHUTDOWN MARGIN Addressees-

- All holders'of operating lianses for pressurized-water reactors (PWRs), except those licensees  ;

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who have permarantly ceased operations and have certified that the fuel has been permanently removed from the reactor vessel.

Purcose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to a potentially significant problem pertaining to.i Tialous behavior of the core axial power distribution and erosion of shutdown margin (SDM) attributed to crud buildup on the nuc!sar fuel and subsequent boron deposition in the crud layer, it is expected that recipients-will review the information Mr applicability to their facilities and consider actions, as appropriate,

. to avoid similar probierns <3 wever, suggestions contained in this inforrn:*lon notice are not NRC requirements; therefore, no specific action or written response is required.

Backaround The Callaway Plant is a PWR with a Westinghouse nuclear steam supply system, owr:ed and operated by Union Electric Compai)y (UE). The core contains 193 fuel assemblies and is currently in its ninth cycle of operation. The plant is licensed to operate at 3565 MWt.

, Beginning with operating cycle 4, UE observed unexpected behr.vior in the power distribution at the plarit. This anomaly was characterized by a gradual unexpected power shift toward the bottom of the core and was first detected at a core average bumup of approximately 7000 MWD /MTU in cycle 4. ' The power shift continued until bumup effacia became dominant and caused power to shift back to the too of the core near the end of the cycle, in addition to the anomalous power distribution, deviations were observed in the estimated critical position (ECP) .

of the control rods. Although ECPs for plant restarts that occurred early in each cycle agreed well with measured critical positions, this egreement disappeared for restarts that occurred later in core life. During cycles 4 and 5, ECP deviations increased to over 0.5% ak/k.

After analyzing relevant data, perbrming scoping calculations, and reviewing industry experience, UE and Westinghouse concluded that the power distribution anomaly was most likely initiated by subcooled nucleate boiling. The ECP deviations were another effect of this anomaly.

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1-IN 97-xx November ,1997 Page 2 of 3 It core detector indications of flux depressions between fuel grids, as well as visual examinations showing crud deposits on fuel pins, supported these conclusions.

Discussion Axial offset (AO) is a measure of the difference between power in the upper and lower portions of the core. This difference must remain tithin limits established in the technical specifications to ensure that both SDM end clad local peaking factors are not exceeded. Exceeding these limits could result in the reactor fuel exceeding 10 CFR 50.46 limits on fuel clad temperature (2200 'F). If the raactor approachs these limus, compensatory measures, including a power re:luction, must be taken to maintain the reactor within its operational limits.

On the basis of investigations by UE and Westinghouse, the cause of the axial offset anomaly (AOA) at Callaway is most likely crud buildup on fuel assemblies in the upper portion of the reactor core. This crud buildup is especially pronounced in high-power cores. High core power >

results in increased subcooled nucleate boiling in the upper core, which, in tum, causes greater crud accumulation on the fuel assemblies. Lithium borate is absorbed in the crud layer, reducing the fission rate in the upper portion of the core. The visualiridications of crud buildup and flux depression at the grid straps support these conclusions. As a result of the reduced fissioning in the upper core, the power distribution shifts towards the bottom of the core. This resulting power shift causes a reduction in SDM and an increase in local peaking factors. Near the end of cycle, excess bumup in the bottom of the core causes the power distribution to shift back towards the upper portion of the ccre, pr.rtially restoring the power balance.

A number of Westinghouse plante have experienced AOAs ranging from -3 percent to -15 percent. These plants ine!ude Callaway, Catawba 1, Comanche Peak 2, Millstone 3, Seabrook, Vogtle 1 and 2, and Wolf Creek.

The -15 percent AO at Callaway is the largest to date. In response to the anomaly and continued erosion of SDM the licensee for Callaway reduced reactor power to 95 percent on July 15,1997, and to 70 percent in mid-August 1997. On September 6,1997, with the unit at 70 percent power and with AO at approximately 0 percent, tne licensee, in consultation with Westinghouse, reduced reactor power to 30 percent in the hope of bringing about 75 percent of the lithium borate out of the crud, thereby creating a more positive AO when power wac retumed to 70 percent. Similar results had been seen in a previous cycle following a downpower event at Callaway. The results of this maneuver were not as predicted. After 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at 30 percent power, only 25 percent of the lithium borate had come out of the crud.

When power was increased to 70 percent, approximately 70 percent of the lithium borate that had been released during the power reduction was reabsorbed into the crud. The AO at the conclusion of the maneuver was larger than before (the AO trended several percentage points negative after the maneuver).- Westinghouse and the licensee surmised that the cause of the worsening AO was the release of depleted boron during the power reduction, followed by the crud's reabsorbing fresh boron during the retum to 70 percent power.

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IN 97-xx November ,1997 Page 3 of 3 SDM is decreasing 1.t the rate of 3-4 percent-millitho (pcm) per day. As SDM has decreased (the technical specification limit is 1300 pcm), UE has taken several actions to restore it. These actions include reducing operational flexibility by modifying the rod insertion limits and relaxing the rod worth uncertainties in the SDM calculations (based on an approved Westinghouse topical report).

By reducing power and introducing operational restrictions, the licensee has continued to operate Callaway within the limits of the safety analysis for the plant. UE and Westinghouse are continuing their investigation into this phenomenon.

This information notice requires no speci5c action or written response. However, recipients are reminded that they are required by 10 CFR 50.S5 to take industry-wide operating experience (including information presented in NRC information notices) into consideration, where practical, when setting goals and performing periodic evaluations. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

Jack W. Roe, Acting Director Division of Reacto, ;'rogram Management 04 ice of Nuclear Reactor Regulation Technical Contacts: Margaret S. Chatterton, NRR Edward D. Kendrick, NRR 301-415-2889 301-415-2891 E-mail: msci@nrc. gov E-mail: edk@nrc. gov Will;am F. Burton, NRR 301-415-2853 E-mail: wfb@nrc. gov

Attachment:

List of Recantly issued NRC Information Notices.