ML20024H217

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LER 91-008-00:on 910422,cracking Discovered on Control Rod Drive Housing Flange Cap Screws.Caused by Inadequate Design of Screws.Four New,Original Equipment Cap Screws Installed & Reactor Vessel Hydro performed.W/910522 Ltr
ML20024H217
Person / Time
Site: Monticello Xcel Energy icon.png
Issue date: 05/22/1991
From: Hippe M, Parker T
NORTHERN STATES POWER CO.
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-008, LER-91-8, NUDOCS 9105300237
Download: ML20024H217 (5)
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Text

{{#Wiki_filter:' I Northem States Power Company 414 Nicollet Mall Minneapchs, Minnesota $5401 1927 Telephone (612) 330-5500 May 22, 1991 Report Reg red by 10 CFR Part 50. Section 50.73 Director of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission - Attn: Document Control Desk Vashington, DC 20555 MONTICELLO NUCLEAR GENERATING PLANT Docket No. 50 263 License No. DPR-22 Inadequate Control Rod Lrive Cap Screw Design Postiltinn in Stress Corrosion Crackint Indications The Licensee Event Report for this occurrence is attached. This event was reported via the Emergency Notification System in accordance with 10 CFR Part 50, Section 50.72 on April 22, 1991. 47c4 

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During the 1991 refueling outage Control Rod Drive change-out, 37 of 72 Control Rod Drive housing flange cap screws examined displayed indications of cracking just below the cap screw head. The root cause of the cap screw cracking is attributed to inadequate design of the screws. The cap screws are made of material susceptible to stress corrosion cracking in the under vessel environment. L'pon determination of crack indications , a Safety Review Item addressing safety concerns was prepared. Four new, original equipment cap screws were installed in each Control Rod Drive housing flanga. A reactor vessel hydro was performed. A sample of cap screws is being tested to verify the me' of failure. Improved design cap screws will be installed during the next rt ;ueling outage. I. .

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Monticello Nuclear Cererating Plant oIsjojolol 2l6l2 9l1 _ 0l0l8 0l0 0l2 or 0l 4 v ,w - - . - - ne m .onn DESCRIPTION On April 22, 1991, while changing out Control Rod Drives during a refueling l outage, Quality Control personnel discovered indications of cracking on 37 of ) 72 Control Rod Drive housing flange cap screws just below the cap screw head. ) The indications were discovered as a result of nondestructive examination of l the cap screws during drive change out. Cap screw examination was included in i the Control Rod Drive change out procedure as recommended in General Electric l Service Information Letter 483 "CRD Cap Screw Crack Indications." It is not , knov.s when during the life of the cap screus the crack indicatior.s first l appeared. This condition was reportable 3r 10CFR Part 50, Section 50.12(b)(2)(1) since it represents a degra. ton of the Reactor coolant pressure boundary. , CAUSE In May 1988, General Electric issued a notification (RICSIL 019) of shallow linear indications that were observed in Control Rod Drive housing cap screws at an operating boiling water reactor. The indications were detected during the required ASME Code visual in-service examination of the cap screws, General Electric indicated that there was no safety significance to the event. ' The cracking was attributed to stress corrosion initiating at the bottom of corrosion pits. It is likely that the cracking is aggravated by the presence of manganese sulfide inclusions in the cap screw material. In March 1989, General Electric issued a follow-up document on this subject (Service Information Letter 483) which confirmed the previons failure assessment of RICSIL 019. The root cause of the Control Rod Drive cap screw cracking is attributed to inadequate design of the screws, The cap screws are made of material 

                            -susceptible to stress corrosion cracking in the under vessel environment.

Additional destructive testing is being performed to verify the cause and extent of the indications. A supplemental Licensee Event Report will be submitted when the results have been evaluated, mac ,eswie Jnaa 6Mei 

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C SMM 3asa i ntim t s. 4/20/91 i 1.!CENSEE EVENT REPORT (LER) 'S!".'J.',"jdC,3"c"'!?cti'd ,#*,fy Eff,y l TEXT CONTINUATION f,"".7,".",!"w'."$"ci 'i*',*,",'/*',','2 ["TW j l Mit'#."Jc'i""33 4'!:ZiT M M ,.I? , 08 wamactut hT Amo evCGET, naam*mGTON.CC ;cu3. i pacautv mawain DOctsTmuussaL2n tie avwett its P AGS 131 ) l n . t metig'+ c:,y Monticello Nucleer Gcnerating Plant o ls to lo lo { 2(6 l 3 9) 1 0l0l8 _. O{0 C [3 or 0; 4 ratsr m . - .m e ac m s .no ANALYSIS Service Information Letter 483 states Control Rod Drive loads can be supported by three uniformly distributed uncracked cap screws within the stress limits set forth in ASME codes and the probability of structurally significant cracks occurring in five or more screws on a single Control Rod Drive housing flange is extremely low. If, however, any such failure were to have occurred, the Control Rod Drive support structure would have allowed the Control Rod Drive to drop only one inch or less. An evaluation of the loss of one Control Rod Drive Sas been considered in the Updated Safety Analysis Re; art. At the time of discovery of the crack indications there was no danger of the cap screws on a housing flange breaking because the vessel was at atmospheric pressure and there was no driving force to rapidly eject a drive housing. Furthermore, there was no danger of a reactivity addition caused by an ejected rod since fuel had been removed from the vessel. Fuel was not reloaded until the safety impact of the cracked cap screws had been addressed by Safety Review ltem 91-016. Subsequent reactor operation is permissible and poses no health or safety risk to the public since four bolts on each Control Rod Drive housing flange have been replaced with new, uncracked cap screws. This ensures separation of the Control Rod Drive mounting flange from the vessel flange will not occur since Control Rod Drive loads can be supported within the stress limits set forth in ASME codes by as few as three uniformly distributed uncracked cap screws. Replacing four cap screws on each flange with original design equipment is acceptable because the General Electric investigation of the problem has shown that crack growth is very ilow or self arresting. This is further evidenced by the Monticello c.,. .c. 3, which have been in service since original plant startup with no failures, CORRECTIVE ACTION Completed Corrective Actions: , 1. Safety Review Item 91 016, Cracked Cap Screws, was prepared to address l the safety concerns associated with refueling, the cap screw change out, and operation with four possibly crackec cap screws. l I I 1 l l l Mac on a.sa us, 

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2. Four new, original equipment cap ' screws were installed on each Control Rod Drive.
3. A reactor vessel hydro was performed to ensure Control Rod Drive housing flange integrity prior to reactor start up.

Planned Corrective Actions:

1. Destructive testing on a sample of cracked screws will be performed to verify the mode of failure.
2. General Electric is developing a new design cap screw that is expected to be available in August of 1991. If available, new design cap screws will be installed at the next refueling outage as deemed necessary to ensure continued pressure boundary integrity.

ADDITIONAL INFOPS.ATION Failed Component Identification: . The cap screws are SA-193 B7, made of low alloy, high strength, AISI 4140 material. General Electric part No. Il7C4515P002 Previous Similar Events: None 

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