United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 83-65: Surveillance of Flow in RTD Bypass Loops Used in Westinghouse Plants

                                                           SSINS No.: 6835 
                                                           IN 83-65        

                               UNITED STATES  
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                               October 7, 1983

Information Notice No. 83-65:   SURVEILLANCE OF FLOW IN RTD BYPASS LOOPS 
                                   USED IN WESTINGHOUSE PLANTS 

Addressees: 

All Westinghouse nuclear power reactor facilities holding an operating 
license (OL) or a construction permit (CP). 

Purpose: 

The intent of this Information Notice is to alert Westinghouse licensees to 
the possibility of low flow through a reactor coolant loop resistance 
temperature detector (RTD) bypass line. Such low flow can degrade the 
response of the temperature measurements utilized in the reactor protection 
system. The presence of low flow in this line, should be revealed by a low 
flow alarm; however, no regulatory requirements or vendor recommendations 
are established regarding periodic surveillance of the flow sensor or alarm.

Description of Circumstances: 

On January 29, 1983, during routine shutdown operation at Salem Generating 
Station, Unit 1, a low flow alarm was received at the control room from the 
No. 14 reactor coolant loop RTD bypass line. Investigation of the problem, 
by alternately isolating the hot- and cold-leg sides of the flowpath, 
indicated that the hot-leg flowpath was obstructed. The No. 14 reactor 
coolant average temperature channel was therefore declared inoperable and a 
limiting condition for operation was entered, retroactive to the time of 
discovery of the problem. All bistables associated with the channel were 
immediately placed in the tripped condition. The redundant loop T avg 
channels were operable throughout the occurrence. 

In an attempt to dislodge the obstruction from the hot-leg bypass loop, its 
isolation valve was cycled, and then bypass flow was restored to normal. No 
other problems with the channels were evident, and the occurrence was 
assumed to involve an isolated instance of corrosion products restricting 
flow in the bypass line. 

Discussion: 

The reactor coolant loop temperature instruments are utilized in the reactor
protection system. Operability of the temperature instruments is required to
provide the overall reliability, redundancy, and diversity available in the 
facility design for the mitigation of accidents. The reactor coolant system 
hot- and cold-leg RTDs are located in reactor coolant bypass loops. A bypass 



8308300378 
.

                                                            IN 83-65       
                                                            October 7, 1983 
                                                            Page 2 of 2    

loop from upstream of the steam generator to the reactor coolant pump inlet 
is used for the hot-leg RTD, and a bypass loop from downstream of the 
reactor coolant pump to the pump inlet is used for the cold-leg RTD. 

The bypass loop flowrate affects the overall time response of the 
temperature signals provided for reactor protection. These response times 
are considered in performing FSAR accident analyses and are included in 
plant technical specification surveillance requirements. The RTD bypass 
loops have low flow alarms whose setpoint is 90% of the flow for clean 
piping. Bypass flow at or above this alarm setpoint is sufficient to 
maintain the proper RTD response time. Flow at or below the setpoint results 
in a control room alarm, which indicates to the operator that the overall 
RTD response time may have been degraded to a level inconsistent with plant 
technical specifications. 

As a result of the Salem experience the significance of the RTD bypass low 
flow alarm has been demonstrated. No regulatory requirements or vendor 
recommendations are established regarding periodic surveillance of the RTD 
bypass flow sensors; however, the Salem procedures require calibration of 
the flow detectors every three years. 

No action or written response to this notice is required; however, 
Westinghouse licensees may find it prudent to calibrate the flow sensors on 
a refueling outage basis and to verify the alarm setpoint on a monthly basis
to assure the operability of this monitoring function. If you have any 
questions regarding this notice, please contact the Regional Administrator 
of the appropriate NRC Regional Office, or a Technical Contact listed below.


                              Edward L. Jordan, Director 
                              Division of Emergency Preparedness 
                                and Engineering Response 
                              Office of Inspection & Enforcement 

Technical Contact:  I. Villalva, IE          M. Wigdor, NRR 
                    (301) 492-9635           (301) 492-7592 

Attachment:
List of Recently Issued IE Information Notices
.
Page Last Reviewed/Updated Tuesday, November 12, 2013