Information Notice 2002-09, Potential for Top Nozzle Separation and Dropping of a Certain Type of Westinghouse Fuel Assembly
| ML020440066 | |
| Person / Time | |
|---|---|
| Issue date: | 02/13/2002 |
| From: | Beckner W D NRC/NRR/DRIP/RORP |
| To: | |
| Jung I, NRR/DRIP, 415-1837 | |
| References | |
| TAC MB1622 IN-02-009 | |
| Download: ML020440066 (8) | |
UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001February 13, 2002NRC INFORMATION NOTICE 2002-09:POTENTIAL FOR TOP NOZZLE SEPARATIONAND DROPPING OF A CERTAIN TYPE OF
WESTINGHOUSE FUEL ASSEMBLY
Addressees
All holders of operating licenses for nuclear power reactors and non-power reactors and holdersof licenses for permanently shutdown facilities with fuel onsite.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alertaddressees to the recent nozzle separation and dropping of a Westinghouse fuel assembly
during movement. Even though the nozzle separation affects only fuel of a type last
manufactured almost 20 years ago, the fuel is perhaps being moved to dry storage or high- density racks and could drop during movement. It is expected that recipients will review the
information for applicability to their facilities and consider actions, as appropriate, to avoid
similar problems. However, suggestions contained in this information notice are not NRC
requirements; therefore, no specific action or written response is required. Description of CircumstanceOn March 24, 2001, operators at the North Anna Power Station of Virginia Electric and Power Company were inspecting older spent fuel assemblies in advance of transferring them to dry
cask storage. As assembly G45 was being returned to its spent fuel rack, the top nozzle
separated from the assembly and the assembly dropped about 12 feet into its storage cell. The
top nozzle, with the burnable poison rod assembly still attached, remained on the handling tool.
Since the assembly bottom nozzle was already in the cell, the falling assembly did not contact
any other fuel assemblies or the rack structure. There was no collateral damage. An initial
visual inspection of the top of the assembly within the cell using a TV camera revealed that the
bulge joints connecting the stainless steel sleeves to the Zircaloy 4 guide tubes had failed. No
fission gas activity was detected afterwards, indicating that none of the fuel rods in the
assembly had been fractured by the drop. The assembly was supplied by Westinghouse in 1984, spent a short time in the spent fuel poolduring 1984, and then was loaded into the Unit 1 reactor. It was burned during cycles 5 and 6 from September 1984 to April 1987 and then offloaded to the spent fuel pool. The assembly
was moved 10 times in all for the receipt inspection and refueling operations. Prior to the
recent event, it was last moved in 2000 for ultrasonic testing. DiscussionA historical review by Westinghouse revealed that similar events had occurred in the past. These included an event at Prairie Island in 1981 and several events at foreign plants in the
1980s. In all these events the fuel assembly had separated at the top bulge joint that connects
the stainless steel grid sleeves to the Zircaloy guide tube, just as at North Anna. Hot cell
metallography after the earlier events indicated that the likely root cause was intergranular
stress-corrosion cracking (IGSCC) accelerated by the presence of chlorides, fluorides, and
sulfates.The method of fabrication of the top grid assembly is believed to have been among majorfactors in these failures. The grid nozzle assembly was fabricated by brazing the Alloy 304 stainless steel grid sleeves to the upper nozzle assembly prior to age-hardening heat treatment.
It is believed that the heat sensitization of the Alloy 304 grid sleeves and the residual stresses
in the roll bulge used to connect the grid to the guide thimble make the area highly susceptible
to IGSCC when an appropriate contaminant like chloride, fluoride, or sulfate is present at a high
enough concentration. The grid sleeve was the part of the assembly that failed at North Anna.For this reason, the grid sleeve material was changed to Alloy 304L in 1984. This material wasnot subjected to heat sensitization during the age-hardening process, protecting against
IGSCC. Even though the design was changed in 1984, some Alloy 304 sleeves were shipped
up through 1987. The North Anna fuel assembly had Alloy 304 sleeves. In the late 80s, Westinghouse introduced removable top nozzles (RTNs). In this design, the guide tube is
bulged onto an Alloy 304L insert tube, which is held in place by an Alloy 304L lock tube. This
lock tube is then bulged into a groove in the top nozzle adapter plate. Neither the insert tube
nor the lock tube undergoes grid age-hardening heat treatment and neither is sensitive to
IGSCC.A review of the spent fuel pool chemistry records at North Anna indicated that the levels ofchlorides and fluorides in the spent fuel pool or reactor coolant system were not excessively
high while the assembly in question was exposed. Westinghouse did not specify a limit for
sulfates for spent fuels, nor did the Electric Power Research Institute (EPRI) guidelines for pool
chemistry. However, sulfates had been monitored in the North Anna spent fuel pool starting in
1989 and were below 150 ppb for most of the period. Sulfates exceeded 150 ppb for 469 days
of the 11 years measured, and never exceeded 630 ppb. Westinghouse therefore concluded
that acid sulfate contamination was unlikely to have caused the IGSCC of the joints.
Westinghouse speculated that a more aggressive reduced sulfur species caused the corrosion. According to Westinghouse, North Anna visually inspected 208 fuel assemblies; 54 hadindications of corrosion at the bulge joint and 10 had indications of cracking. North Anna, with
the help of Westinghouse, is procuring appropriate tooling to lift the assemblies without putting
load on the bulge joints. Westinghouse has informed all its fuel customers of the situation and
provided recommendations on inspecting or using special lifting methods. This information notice requires no specific action or written response. If you have anyquestions about the information in this notice, please contact the technical contacts listed below
or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager./RA/William D. Beckner, Program DirectorOperating Reactor Improvements Program
Division of Regulatory Improvement Programs
Office of Nuclear Reactor RegulationTechnical contacts:Michael J. Morgan, Region IIJames A. Canady, Region II540-894-5421540-894-5422 E-mail: mjm@nrc.gov E-mail: jac6@nrc.govIan C. Jung, NRR301-415-1837 E-mail: ixj@nrc.govAttachment: List of Recently Issued NRC Information Notices This information notice requires no specific action or written response. If you have anyquestions about the information in this notice, please contact the technical contacts listed below
or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager./RA/William D. Beckner, Program Director
Operating Reactor Improvements Program
Division of Regulatory Improvement Programs
Office of Nuclear Reactor RegulationTechnical contacts:Michael J. Morgan, Region IIJames A. Canady, Region II540-894-5421540-894-5422 E-mail: mjm@nrc.gov E-mail: jac6@nrc.govIan C. Jung, NRR301-415-1837 E-mail: ixj@nrc.govAttachment: List of Recently Issued NRC Information NoticesDISTRIBUTION:ADAMS
IN FileG:\REXB\IXJ\IN 2002-xx Fuel Assembly Drop.wpd
ADAMS ACCESSION NUMBER: ML020440066*See previous concurrenceOFFICERSE:RORP:DRIPTECH EDITORIMA:RORP:DRIPREXB:DSSAOD:SFPO:NMSS(A)SC:RORP:DRIPPD:RORP:DRIPNAMEICJung*PKleene*KAGray*JSWermiel*EWBrachTKoshyWDBecknerDATE02/07/200202/07/200202/07/2002 02 /08 /200202/12/200202/12/200202/13/2002OFFICIAL RECORD COPY
______________________________________________________________________________________OL = Operating License
CP = Construction PermitAttachment 1 LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICES_____________________________________________________________________________________InformationDate of
Notice No. SubjectIssuanceIssued to_____________________________________________________________________________________2002-08Pump Shaft Damage Due toExcessive Hardness of Shaft
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permanently ceased operations
and have certified that fuel has
been permanently removed from
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Supply Tanks01/28/2002All holders of operating licensesfor nuclear power except those
who have ceased operations and
have certified that fuel has been
permanently removed from the
reactor vessel.2002-06Design Vulnerability in BWRReactor Vessel Level
Instrumentation Backfill
Modification01/18/2002All holders of operating licensesor construction permits for boiling
water reactors (BWRs).2002-05Foreign Material in StandbyLiquid Control Storage Tanks01/17/2002All holders of licenses for nuclearpower reactors.2002-04Wire Degradation at BreakerCubicle Door Hinges 01/10/2002All holders of operating licensesfor nuclear power reactors.2002-03Highly Radioactive ParticleControl Problems During Spent
Fuel Pool Cleanout01/10/2002All holders of operating licensesfor nuclear power reactors, holders of licenses for
permanently shutdown facilities
with fuel onsite, and holders of
licenses for non-power reactors.2002-02Recent Experience withPlugged Steam Generator
Tubes01/08/2002All holders of operating licensesfor pressurized-water reactors
(PWRs), except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor.