United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-61: Failure of a Main Steam Safety Valve to Reseat Caused by an Improperly Installed Release Nut

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                          WASHINGTON, DC  20555-0001

                               November 20, 1996


NRC INFORMATION NOTICE 96-61:  FAILURE OF A MAIN STEAM SAFETY VALVE TO RESEAT  
                               CAUSED BY AN IMPROPERLY INSTALLED RELEASE NUT

Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the failure of a main steam safety valve (MSSV)
to reseat during a plant transient as a result of an improperly installed
release nut.  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 Circumstances

At Arkansas Nuclear One, Unit 1 (ANO-1), there are eight MSSVs on each of the
A and B steam generators.  On May 19, 1996, the plant experienced a feedwater
transient and a  reactor trip.  Six of the MSSVs on the B steam header opened
as designed, but one of these failed to close following the secondary
overpressure condition.  As a result of the stuck-open MSSV (PSV-2685), the
secondary side of the B steam generator was isolated in accordance with plant
emergency procedures and allowed to boil dry.  None of the A MSSVs opened
during this event, and the A steam generator remained operable.

The licensee determined that PSV-2685 failed to reseat because a release nut
on the valve spindle had been improperly installed.  The release nut is
installed to provide a surface against which a lifting lever can bear to
permit opening of the valve manually.  The nut is castellated to provide a
slot through which a cotter pin may be aligned with a hole in the spindle so
that, with the cotter pin engaged in the slot and extending through the hole
in the stem, the nut would be prevented from turning.  With the nut held in
its fixed position, there was to be a specified clearance between the bottom
of the nut and the top of the lifting lever beneath to allow for expansion and
contraction of the spindle during thermal cycles.  Following the ANO-1 event,
the licensee found that the release nut had been threaded onto the valve
spindle at a position too low to be properly staked by the cotter pin. 
Because the cotter pin could not prevent the rotation of the nut, vibration
occurring during the valve 
discharge caused the nut to turn and travel down the stem until it rested upon
the lifting lever mechanism.  Without any clearance between the bottom of the
nut and the top of the lever, the valve was held open. 

9611140131.                                                            IN 96-61
                                                            November 20, 1996
                                                            Page 2 of 3


As a result of its review of the event, the licensee concluded that a
contributing cause of the problem was an inadequate instruction related to
establishing the proper position of the release nut with respect to the cotter
pin hole in the stem.  The instruction, derived from vendor guidance,
specified the clearance between the release nut and the lifting lever as
1/16 inch to 1/8 inch.  The minimum value of 1/16 inch was necessary to allow
enough space to accommodate thermal effects on clearances, but the maximum
value of 1/8 inch was an unneeded limitation that distracted attention from
the need to ensure that the nut was staked sufficiently with the cotter pin to
preclude nut rotation.  The licensee changed the maintenance procedure to
eliminate the confusion about the nut-to-lever clearance and modified the
design of the release nut to increase the depth of the slot, thereby providing
greater assurance of proper engagement between the cotter pin and the nut.  In
subsequent action, the licensee determined that the manual lifting devices
were unnecessary from an operational standpoint and removed the lever and nut
assemblies from all the steam safety valves.

The licensee also found other MSSVs that had less-than-desirable engagement
between the release nut and the cotter pin.  These valves were on the main
steam headers from both the A and the B steam generators.  If a transient
caused valves on both headers to open and they failed to reseat, reactor decay
heat removal could have been accomplished using main or emergency feedwater in
a "trickle-feed" process specified in the existing emergency operating
procedures.                               

Discussion

Other operating reactor events have occurred that involved the failure of
MSSVs to reseat as a result of the rotation of release nuts.  In February
1984, at St. Lucie Unit 2, an MSSV stuck partially open following the
actuation of the valve.  An MSSV at the Davis-Besse Nuclear Power Station
stuck open in March 1984, permitting the steam generator to boil dry.  In both
of these events, the release nuts on the MSSVs rotated because the cotter pins
had corroded and failed.  The corrective actions consisted of either replacing
the cotter pins with stainless steel pins or removing those parts of the
lifting mechanism that the release nut would contact if the nut were to rotate
downward.  These events are addressed in NRC Information  Notice 84-33, "Main
Steam Safety Valve Failures Caused by Failed Cotter Pins."  More recently, in
1993, an MSSV failed to reseat at Crystal River Unit 3 because the release nut
was not engaged with the cotter pin, just as in the current ANO-1 event.  The
Crystal River event is described in Licensee Event Report 93-09, submitted to
the NRC by the licensee, Florida Power Corporation, by letter dated
October 13, 1993, Docket No. 50-302.

Licensees have also reported situations in which the cotter pins were missing
from the release nuts.  A main steam safety valve failed to reseat at ANO-1 in
May 1989 because the cotter pin was missing, as reported in Licensee Event
Report 89-018.  The Oconee licensee reported, under the requirements of 10 CFR
Part 50.72, that, during inspections of Units 2 and 3 on October 14, and
October 24, 1996, respectively, cotter pins were missing
from 4 of 16 main steam safety valves on Unit 2 and from 2 of 16 valves on
Unit 3.  On .                                                            IN 96-61
                                                            November 20, 1996
                                                            Page 3 of 3


October 31, 1996, the Millstone licensee reported, also under the requirements
of 10 CFR Part 50.72, that inspection of the main steam safety valves on
Unit 3 showed that the cotter pins were missing from 2 of the 20 valves.

The valves at ANO-1, Davis-Besse, Crystal River, and Millstone were
manufactured by Dresser Industries, Inc.  The valves at St. Lucie and Oconee
were manufactured by Crosby Valve and Gage company.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager. 


                                       signed by

                                    Thomas T. Martin, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Charles G. Hammer, NRR           
                   (301) 415-2791
                   Email:  cgh@nrc.gov

                   Terrence Reis, Region IV
                   (817) 860-8185
                   Email:  txr@nrc.gov

                   Robert A. Benedict, NRR
                   (301) 415-1157
                   Email:  rab1@nrc.gov

Attachments:  
1.  Steam Safety Valve
2.  List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Thursday, November 21, 2013