United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-02: Inoperability of Power-Operated Relief Valves Masked by Downstream Indications During Testing

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001
                                       
                                January 5, 1996


NRC INFORMATION NOTICE 96-02:  INOPERABILITY OF POWER-OPERATED RELIEF VALVES 
                               MASKED BY DOWNSTREAM INDICATIONS DURING TESTING


Addressees

All holders of operating licenses or construction permits for pressurized
water reactors.

Purpose

The United States Nuclear Regulatory Commission (NRC) is issuing this
information notice to alert addressees to a recent incident involving improper
installation of power-operated relief valve (PORV) internals which rendered
the PORVs inoperable, and inaccurate indications of the actuation of PORVs
based upon tailpipe acoustic monitoring data during testing.  The erroneous
data led a licensee to conclude that two inoperable PORVs were functioning
properly.  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 notice do not
constitute NRC requirements; therefore, no specific action or written response
is required.

Description of Circumstances

On August 9, 1995, surveillance testing at Saint Lucie Plant, Unit 1,
conducted in accordance with Section XI of the American Society of Mechanical
Engineers Boiler and Pressure Vessel Code, indicated that the PORVs were not
operating properly.  The licensee cooled and depressurized the unit and
removed and inspected the PORVs.  Inspection of the internals of the valves
revealed that a part required for proper operation of each of the two PORVs
was installed backwards, thus rendering each valve inoperable.  The valves had
been in service since being rebuilt during a refueling outage approximately 
10 months earlier.

In reviewing the methodology and acceptance criteria for PORV surveillances,
the licensee found that the valve stroke time had been based upon the time
lapse between the changing of a switch position and the receipt at the control
room panel of acoustic indication from the detectors on the tailpipes of the
subject valves.  After this event, the licensee determined that flow through
internal clearances within an inoperable valve could provide low-level
acoustic responses in the tailpipes. 

9512290129.                                                            IN 96-02
                                                            January 5, 1996
                                                            Page 2 of 4


Thus, without verifying that parameters were changing at the terminating point
of the tailpipe (in the case of Saint Lucie, a quench tank), valve operation
could not be determined based upon acoustic data alone.

Failure Mechanism

The PORVs used at Saint Lucie Unit 1 are Dresser Industries Model 31533VX-30
pilot-operated valves.  The principles of valve operation are as follows:

.    When PORV actuation is required, a signal is sent to an actuating
     solenoid within the pilot assembly, which strokes the pilot valve lever
     to open the pilot valve.  A vent path is thus established from the main
     valve, through the bleed-off port and the pilot valve, to a low-pressure
     area.

.    A differential pressure is established across the main disc when the
     valve pilot valve opens, thus venting a space inside the main disc (see
     Attachment 1) to a low-pressure area (in the case of Saint Lucie, the
     tailpipe) via the main valve bleed-off port.  The differential pressure
     exists because the area above the main disc is pressurized to reactor
     coolant system pressure through internal passages in the main valve (not
     shown in Attachment 1).

.    The main disc (responsible for actual reactor coolant system pressure
     relief) is opened by the force of water or steam acting on the main valve
     disc/seat interface.  The main disc moves within a guide cylinder, and
     its movement is governed by the differential pressure established across
     the disc and spring force, which tends to move the disc into a closed
     position.

.    When pressure relief is no longer required, the actuating solenoid closes
     the pilot valve and the space inside the main disc is pressurized by
     system pressure through the equalizing port.  Differential pressure
     across the main disc is thus reduced, and the main disc is returned to
     the closed position under force of spring pressure.

In the case of the subject PORV inoperability, the licensee found that the
main disc guide, a metal cylinder within which the main disc moves, was
incorrectly oriented in the valve.  One end of the guide contains a number of
holes, which provide a vent path for the pilot valve of the PORV from the
inner surfaces of the main disc.  With the main disc guide incorrectly
oriented upside down, the venting of the space within the main disc was
severely restricted.  This restriction resulted in insufficient venting of the
space and a subsequent failure to create the differential pressure across the
main disc necessary for the main disc to move and open the valve.

Although venting through the pilot valve was insufficient to result in a
change of state in the main valve, subsequent testing revealed that internal
clearances in the main valve are sufficient to pass water or steam through the
pilot valve despite a failure of the main valve to lift.  The tests the .                                                            IN 96-02
                                                            January 5, 1996
                                                            Page 3 of 4


licensee conducted in conjunction with Wyle Laboratories indicated that
significant pressures can be developed at the discharge of the pilot valve
(because of internal bypass flow paths) without movement of the main valve. 
Pilot valve discharge pressures ranging from 10.5 to 12.5 megapascals [1,500
to 1,800 pounds per square inch gage] were recorded when 16.7 megapascals  
[2,400 pounds per square inch gage] steam was applied to an incorrectly
assembled PORV of the same manufacture as those on Saint Lucie Unit 1.

Surveillance Testing

The licensee had performed surveillance testing twice on the subject valves
before they were identified as inoperable, once in November 1994 (after
installation in the system) and once in February 1995.  The November 1994 test
was performed at 1.69 megapascals [245 pounds per square inch absolute]
reactor coolant system pressure and the February 1995 test was conducted at
12.1 megapascals [1,750 pounds per square inch absolute].  In both cases,
stroke times, as measured using tailpipe acoustic data, were found to be
satisfactory.  At the time, reactor coolant system and surge tank parameters
were not used to verify that the main valves of the PORVs had changed state. 
On August 9, 1995, surveillance testing results were unsatisfactory because
threshold acoustic levels for control room indication were not achieved. 
Subsequent testing indicated that, although acoustic data were received,
changes in reactor coolant system pressure and in quench tank parameters had
not taken place.  These anomalous indications led the licensee to perform the
valve inspections previously described which, in turn, led to the subject
findings.

The acoustic monitors employed in the PORV tailpipes register 10 discrete
steps of 200 millivolts each.  The surveillance test methodology involved
timing of the interval by the operator from the moment a control switch was
cycled until an acoustic monitor annunciator, used to indicate when the
monitor exceeded the 400 millivolts output threshold, alarmed at the control
panel.  The acoustic level received in the erroneously accepted tests is not
known; however, in the in-situ tests of the repaired valves on August 16,
1995, at approximately 1.72 megapascals [250 pounds per square inch absolute]
reactor coolant system pressure, the valves reached the maximum design output
of the monitors on opening.  In addition, the licensee noted that the reactor
coolant system pressure dropped by approximately 0.04 megapascals [6 pounds
per square inch absolute] and the quench tank parameters indicated that energy
had been introduced into the quench tank when the valves opened.

Discussion

The licensee determined that the test methodology used to verify PORV main
valve actuation was inadequate.  The licensee changed its maintenance
procedure for the PORVs to verify that the main valve disc actuates when test
pressure is applied at the valve lift setpoint.  This verification will be
performed during bench testing prior to installation of the valve.  This
functional testing will confirm that the PORVs have been assembled correctly. 
The licensee also changed the procedure for conducting inservice testing on
the PORVs to provide more positive indication of PORV main valve actuation
.                                                            IN 96-02
                                                            January 5, 1996
                                                            Page 4 of 4


by using quench tank and pressurizer parameters for confirmation of PORV main
valve actuation during testing.

Acoustic monitors were added to the discharge piping of safety and relief
valves in response to Action Item II.D.3 of NUREG-0737, "Clarification of TMI
Action Plan Requirements."  Some licensees of pressurized water reactors have
attempted to use similar acoustic monitors for measuring stroke times of the
atmospheric dump valves.  However, the measurements have generally proven to
be ineffective for determining if the valves have fully opened.  Accordingly,
alternative means of monitoring the power-operated function of the valves have
been employed.  Other types of acoustic monitoring equipment are used for
testing the stroking of other types of valves, such as check valves, to verify
movement.

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 B. K. Grimes

                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Mark S. Miller, RII
                     (407) 464-7822
                     Internet:msm@nrc.gov
                    
                     Eric J. Benner, NRR
                     (301) 415-1171
                     Internet:ejb1@nrc.gov

Attachments:
1. Partial Section View of Power-Operated 
     Relief Valve
2. List of Recently Issued NRC Information Notices

(See File IN96002.WP1 for Att. 1.)
Page Last Reviewed/Updated Thursday, November 21, 2013