Information Notice 2002-09, Potential for Top Nozzle Separation and Dropping of a Certain Type of Westinghouse Fuel Assembly

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

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

IGSCC.

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

ML020440066

• 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

______________________________________________________________________________________

OL = Operating License

CP = Construction Permit

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

Excessive Hardness of Shaft

Sleeve

01/30/2002

All holders of operating licenses

for nuclear power reactors, 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

Cleaning Diesel Fuel Oil

Supply Tanks

01/28/2002

All holders of operating licenses

for nuclear power except those

who have ceased operations and

have certified that fuel has been

permanently removed from the

reactor vessel.

2002-06

Design Vulnerability in BWR

Reactor Vessel Level

Instrumentation Backfill

Modification

01/18/2002

All holders of operating licenses

or construction permits for boiling

water reactors (BWRs).

2002-05

Foreign Material in Standby

Liquid Control Storage Tanks

01/17/2002

All holders of licenses for nuclear

power reactors.

2002-04

Wire Degradation at Breaker

Cubicle Door Hinges

01/10/2002

All holders of operating licenses

for nuclear power reactors.

2002-03

Highly Radioactive Particle

Control Problems During Spent

Fuel Pool Cleanout

01/10/2002

All holders of operating licenses

for nuclear power reactors, holders of licenses for

permanently shutdown facilities

with fuel onsite, and holders of

licenses for non-power reactors.

2002-02

Recent Experience with

Plugged Steam Generator

Tubes

01/08/2002

All holders of operating licenses

for pressurized-water reactors

(PWRs), except those who have

permanently ceased operations

and have certified that fuel has

been permanently removed from

the reactor.