Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
.
