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 NRC/NRR/DRIP/RORP |
| To: | |
| Jung I, NRR/DRIP, 415-1837 | |
| References | |
| TAC MB1622 IN-02-009 | |
| Download: ML020440066 (8) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001 February 13, 2002 NRC INFORMATION NOTICE 2002-09: POTENTIAL FOR TOP NOZZLE SEPARATION
AND DROPPING OF A CERTAIN TYPE OF
WESTINGHOUSE FUEL ASSEMBLY
Addressees
All holders of operating licenses for nuclear power reactors and non-power reactors and holders
of licenses for permanently shutdown facilities with fuel onsite.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert
addressees 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 Circumstance
On 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 pool
during 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.
Discussion
A 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 major
factors 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 was
not 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
A review of the spent fuel pool chemistry records at North Anna indicated that the levels of
chlorides 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 had
indications 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 any
questions 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 Regulation
Technical contacts: Michael J. Morgan, Region II James A. Canady, Region II
540-894-5421 540-894-5422 E-mail: mjm@nrc.gov E-mail: jac6@nrc.gov
Ian C. Jung, NRR
301-415-1837 E-mail: ixj@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
- See previous concurrence
OFFICE RSE:RORP:DRIP TECH EDITOR IMA:RORP:DRIP REXB:DSSA OD:SFPO:NMSS (A)SC:RORP:DRIP PD:RORP:DRIP
NAME ICJung* PKleene* KAGray* JSWermiel* EWBrach TKoshy WDBeckner
DATE 02/07/2002 02/07/2002 02/07/2002 02 /08 /2002 02/12/2002 02/12/2002 02/13/2002
Attachment 1 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
_____________________________________________________________________________________
2002-08 Pump Shaft Damage Due to 01/30/2002 All holders of operating licenses
Excessive Hardness of Shaft for nuclear power reactors, Sleeve except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor.
2002-07 Use of Sodium Hypochlorite for 01/28/2002 All holders of operating licenses
Cleaning Diesel Fuel Oil for nuclear power except those
Supply Tanks who have ceased operations and
have certified that fuel has been
permanently removed from the
reactor vessel.
2002-06 Design Vulnerability in BWR 01/18/2002 All holders of operating licenses
Reactor Vessel Level or construction permits for boiling
Instrumentation Backfill water reactors (BWRs).
Modification
2002-05 Foreign Material in Standby 01/17/2002 All holders of licenses for nuclear
Liquid Control Storage Tanks power reactors.
2002-04 Wire Degradation at Breaker 01/10/2002 All holders of operating licenses
Cubicle Door Hinges for nuclear power reactors.
2002-03 Highly Radioactive Particle 01/10/2002 All holders of operating licenses
Control Problems During Spent for nuclear power reactors, Fuel Pool Cleanout holders of licenses for
permanently shutdown facilities
with fuel onsite, and holders of
licenses for non-power reactors.
2002-02 Recent Experience with 01/08/2002 All holders of operating licenses
Plugged Steam Generator for pressurized-water reactors
Tubes (PWRs), except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor.
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit