United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-101: Jet Pump Hold-Down 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.

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                                                            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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                                                            Page 3 of 3


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
.
Page Last Reviewed/Updated Tuesday, November 12, 2013