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101

Information Notice No. 93­101: Jet Pump Hold­Down Beam Failure

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

December 17, 1993

NRC INFORMATION NOTICE 93‐101: JET PUMP HOLD‐DOWN BEAM FAILURE

Addressees

All holders of operating licenses or construction permits for boiling‐water

reactors (BWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to a recent jet pump hold‐down beam failure of a

type not previously described by vendor guidance or by generic communications.

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.

Background

On April 4, 1980, the NRC issued a bulletin, IEB 80‐07, "BWR Jet Pump Assembly

Failure," requesting that the owners of BWR‐3 and BWR‐4 plants perform visual

and ultrasonic examinations of jet pump hold‐down beams at the mid‐length

ligament sections bounding the beam bolt. This action resulted from a jet

pump hold‐down beam failure at Dresden Unit 3 and crack indications identified

at other BWR‐3 plants in early 1980. Observed cracks began at the bolt hole

in the center of the jet pump hold‐down beam. The bulletin also requested

that licensees conduct jet pump operability surveillance on a daily basis and

following unexpected changes in core flow indications, recirculation system

flow indications, or an established power‐core flow relationship. The nuclear

steam supply system vendor, General Electric (GE), issued service information

letters to owners of BWR plants on June 9, 1980, and in February 1981

providing guidance for recognizing jet pump problems using data gathered

during plant operations. During implementation of the inspections prescribed

in IEB 80‐07, several other BWR‐3 and BWR‐4 plants reported discovery of beam

cracking.

Description of Circumstances

On September 13, 1993, Grand Gulf power station (a BWR‐6 plant) experienced an

unplanned high‐pressure core spray system initiation, due to a reactor low‐

water‐level signal, that resulted in a reactor scram. Initially, the reasons

for the water level anomalies detected in the "C" and "G" channels of the
reactor water level instrumentation could not be determined. During restart

from the reactor scram, the plant operations personnel discovered jet pump

9312160095.

IN 93‐101

December 17, 1993

Page 2 of 3

flow differential pressure anomalies. During the investigation of the

problem, the plant experienced oscillating water level indications and jet

pump flow readings characteristic of a displaced jet pump mixer section.

These indications occurred at high recirculation flows in the reactor core

(77 percent core flow). Following reactor shutdown and disassembly, the

licensee found the mixer assembly for jet pump 10 had separated from the

diffuser and relocated between jet pump 8 and jet pump 9. The hold‐down beam

for jet pump 10 had cracked and failed.

Discussion

The jet pump hold‐down beam failure at the Grand Gulf power station in

September 1993 is unlike previous failures in that the hold‐down beam for jet

pump 10 failed in the transition area between the main body of the beam and

the beam end as shown in Attachment 1. There have not been any previous

reports of failures in this location. One beam end failed completely, causing

the beam to come out of the transition piece, removing the restraint on the

jet pump elbow and leading to the disassembly of jet pump 10. There is one

hold‐down beam for each jet pump assembly. Visual examination of the failed

beam showed a crack face covering more than 270� of the cross‐section of the

intact beam end; the other beam end had cracked in the same location and was

missing. The cracks began in an area where a radius machining cut had been

made in the forging and led to failure in a location of the beam with a cross‐

section smaller than the areas that had been affected in previous cases. GE

concluded that the probable cause of failure was an intergranular stress

corrosion crack that propagated over 80 percent of the fracture surface.

Fatigue striations covered the remaining 20 percent of the surface. GE stated

that the loss of preload as a result of the intergranular stress corrosion may

have induced the fatigue failure.

The licensee conducted ultrasonic examinations on the other inservice jet pump

beams and found indications on jet pump 8 and jet pump 21 at the bolt hole

area in the center of the hold‐down beams. This cracking was consistent with

that described in bulletin IEB 80‐07. The licensee replaced all the jet pump

beams with spare beams available on site.

In October 1993, ultrasonic test inspection of the hold‐down beams at the

Clinton power plant (a BWR‐6 plant) revealed that one of the beams had crack

indications around the center of the bolt hole region and the beam was

replaced. On November 22, 1993, Pennsylvania Power and Light Company notified

the NRC that it would be replacing all of the jet pump beams at Susquehanna

Unit 1 before restarting from their refueling outage. This action was being

taken as a precautionary measure given the new failure mode identified at the

Grand Gulf station. GE has made a recommendation to licensees that they

replace their jet pump hold‐down beams as soon as practical if (1) they have

beams of the same design as Grand Gulf and (2) the beams will have an

accumulated service of more than eight years at the next refueling outage.

The water level anomalies that occurred at Grand Gulf on September 13 and

September 28, 1993 were apparently caused by the turbulent flow conditions in

the vicinity of jet pump 10. The potential for level anomalies was not

discussed in bulletin IEB 80‐07..

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December 17, 1993
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The disassembly of a jet pump could result in an increased flow area through

the jet pump and a lower core flooding elevation. This could adversely affect

the water level in the core during the reflood phase of a LOCA as well as the

assumed blowdown flow during a LOCA. Although events to date have not

resulted in damage which affected the operability of safety‐related systems or

equipment, a loose jet pump assembly could potentially cause such damage.

The NRC staff will continue to evaluate information concerning the beam end

failure mechanism as it becomes available and will follow the progress of any

new inspection techniques and corrective actions that may be developed. The

NRC staff expects to meet with the BWR Owners Group and GE to discuss their

action plans as a part of the ongoing safety evaluation to determine the need

for further regulatory action. Areas of discussion will include evaluations

of damage potential due to a displaced jet pump, effects of a failed jet pump

on plant ATWS response, and the potential for LOCA loads to induce hold‐down

beam failures.

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.

/s/'d by BKGrimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contacts: Rudolph H. Bernhard, RII James A. Davis, NRR

(601) 437‐4620 (301) 504‐2713

Jonathan Witter, NRR

(301) 504‐2978

Attachments:

1. Figure ‐ Jet Pump Beam Failure Location

2. List of Recently Issued NRC Information Notices

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