United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-70, Supplement 1: Reliance on Water Level Instrumentation with a Common Reference Leg

                                                            SSINS NO.:  6835
                                                            IN 84-70 SUPP. 1

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                               August 26, 1985

Information Notice No. 84-70 SUPPLEMENT 1:  RELIANCE ON WATER LEVEL
                                               INSTRUMENTATION WITH A COMMON
                                               REFERENCE LEG


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


This information notice is provided to alert licensees and applicants of the
potential for degradation of safety associated with operator reliance on level
instruments that share a common reference leg.  In this regard, this notice
supplements and reemphasizes the information contained in IE Information
Notice 84-70, Reliance On Water Level Instrumentation With a Common Reference
Leg.  This notice serves to alert licensees and applicants to the need for
operators to recognize normal and abnormal water level instrument behavior
under various plant conditions.  Recipients are expected to review the
information for applicability to their facilities and consider actions, if
appropriate, to preclude similar problems occurring at their facilities. 
However, suggestions contained in this information notice do not constitute
NRC requirements; therefore, no specific action or written response is

Description of Circumstances:

On February 13, 1985, while performing a reactor startup at TVA's Browns Ferry
Nuclear Plant, a half scram was received on low reactor water level.  A few
minutes before the half scram, the operators had noticed that two of the three
narrow-range General Electric measurement and control (GEMAC) water level
instruments were reading approximately 40 inches of reactor vessel water
level.  The other narrow-range instrument was indicating approximately 10
inches of water level.  Two wide-range YARWAY instruments were observed by the
operators to be indicating approximately 40 inches.  At the time of the half
scram, reactor pressure was approximately 40 psig and reactor coolant
temperature was approximately 286 F.  Although four of the instruments 
observed by the operators indicated nearly normal reactor water level (33 � 
5 inches), actual reactor water level was approximately 10 inches. 


                                                            IN 84-70 SUPP. 1
                                                            August 26, 1985
                                                            Page 2 of 3

The operators incorrectly concluded that the narrow-range instrument
indicating 10 inches was erroneous since four other level instruments were
indicating high.  The two GEMAC instruments that indicated 40 inches share a
common reference leg.  This reference leg had lost some of its water
inventory, causing all level instrumentation that tapped off that leg to
erroneously indicate high.  The GEMAC instrument that was reading 10 inches
tapped off a different reference column than the aforementioned instruments. 
The two wide-range YARWAY instruments each have separate reference columns not
shared by any of the narrow-range GEMAC instruments.  At approximately 40 psig
reactor pressure and with actual reactor water level at 33 � 5 inches, the
YARWAY instruments should indicate �60 inches, a normal YARWAY level
indication.  A YARWAY level indication of 40 inches should have alerted the
operators that water level was abnormally low.  The operators did not check
the shutdown vessel flooding range level indication which was available in the
control room.  This instrument would have confirmed actual low water level
conditions since it is calibrated for cold plant conditions.


The cause of this event was a partial loss of water inventory from a reference
leg that is common to several water level instruments, including instrument
channels required by the Technical Specifications.  From a reactor safety
perspective, this event highlights the need for operators to be cognizant of
level instruments that share a common reference leg and also to be aware of
level instrument behavior subject to various plant conditions.

A problem in a reference leg, such as that experienced at Browns Ferry, not
only affects level indication, but may also affect the operability of reactor
protection instrument channels required by the Technical Specifications. In
this event, the most critical technical specification instruments affected
were two level switches, one in each reactor protection trip system train. 
These switches were inoperable since they were common to the faulty reference
leg.  Had the operators realized earlier what instruments and switches were
affected by the faulty reference leg, proper corrective action may have been
taken to shut down the plant in a timely manner in accordance with the
Technical Specifications.

It is important that operators understand level instrument response to various
plant conditions.  One way to achieve this understanding is through training
to emphasize level instrument system design, temperature and/or pressure
compensation, instrument calibration, and the purpose of the instruments
(i.e., process monitor s. control).  Although the YARWAY instruments are
designed to provide wide-range accident level indication and are calibrated to
be most accurate at normal operating reactor pressure, the operators at Browns
Ferry could still have used these instruments as additional level response
indication.  Had the operators realized that a YARWAY level of 40 inches was
abnormally low for the existing low reactor pressure, they might have been
alerted earlier to the fact that actual vessel water level was low.

                                                            IN 84-70 SUPP. 1
                                                            August 26, 1985
                                                            Page 3 of 3

Licensees and applicants may wish to review their system descriptions,
operating procedures, and operator training programs to ensure that a common
reference leg shared by multiple level instruments is adequately addressed. 
Operator awareness of the effects a malfunction in a common reference leg can
have on the level instruments and recognition of proper water level indication
subject to various plant conditions can enhance plant safety.

No specific action or written response is required to this information notice. 
If you need additional information about this matter, please contact the
Regional Administrator of the appropriate NRC regional office or one of the
technical contacts listed below.

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

Technical Contacts:  Eric W. Weiss, IE
                     (301) 492-9005

                     P. D. Wagner, Region II
                     (404) 221-2688

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