United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 81-25: Open Equalizing Valve of Differential Pressure Transmitter Causes Reactor Scram and Loss of Redundant Safety Signals

                                                            SSINS No.: 6835 
                                                            Accession No.: 
                                                            8103300410     
                                                            IN 81-25       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                               August 24, 1981

Information Notice No. 81-25:   OPEN EQUALIZING VALVE OF DIFFERENTIAL 
                                   PRESSURE TRANSMITTER CAUSES REACTOR SCRAM
                                   AND LOSS OF REDUNDANT SAFETY SIGNALS 

Description of Circumstances: 

On March 13, 1981, the Tennessee Valley Authority reported to the NRC that a 
scram occurred at the Browns Ferry Unit 2 facility as a result of an open 
equalizing valve used with the lower wide-range Yarway water level 
transmitter. Equalizing valves, which are closed during normal plant 
operation, are used to conduct instrument calibrations and to prevent 
over-ranging the transmitter during instrument isolation valve 
manipulations. With the plant operating at normal full recirculation flow 
conditions, there is no clear indication that this equalizing valve is open. 
However, once recirculation flow is decreased, the reference leg will drain, 
causing erroneous delta pressure (dp) input signals to other transmitters 
connected to the same reference water column. This affects a large number of 
safety and control systems that use water level as an input. A review of the 
event showed that under reduced flow conditions the false high water level 
signals led to loss of redundancy in the logic for initiating safety 
functions, as well as initiation of a turbine trip which caused the reactor 
to scram. 

Safety system dp transmitters that could be affected by the drained 
reference leg provided water level input signals to the logic circuits for 
the following functions: primary containment isolation, low water level 
scram protection, automatic depressurization system confirmatory low water 
level, and high water level trip signal for high-pressure coolant injection 
and reactor core isolation cooling. In addition, a large number of control 
functions can be affected, either directly or indirectly, by the drained 
reference leg. 

Under normal operating conditions with full recirculation flow, the operator
would be unaware of the abnormal position of the equalizing valve because 
the lower wide-range Yarway transmitter, which was bypassed by the open 
equalizing valve and the affected safety system transmitters, would continue
to provide signals to the control room instrumentation that appear normal. 
The lower wide-range Yarway transmitter is designed to provide accurate 
water level signals only for accident conditions when there is no jet pump 
flow. With full recirculation flow, this transmitter reads full scale (high 
water level) whether or not the equalizer valve is open because the variable 
leg is connected near the high-pressure outlet section of the jet pump. 
Under these conditions, the common reference leg for the safety system dp 
transmitters identified above will not drain. Therefore, even if the 
equalizing valve is open, these transmitters also provide normal signals 
when there is full recirculation flow. 
.

                                                            IN 81-25       
                                                            August 24, 1981 
                                                            Page 2 of 2    

During the event at Browns Ferry Unit 2 on March 13, 1981, the abnormal 
condition became apparent to the operator when the recirculation flow rate 
wag being reduced as part of a planned reactor shutdown. Reactor power had 
been reduced from 100% to 54% over the previous 45 minutes. As the 
recirculation flow was reduced, the pressure across the jet pump diffuser 
changed so that at lower flow rates the driving force across the equalizing 
valve reversed, causing the reference leg to have a higher relatives 
pressure. Higher pressure on the reference leg, coupled with an open 
equalizing valve, caused the water in the reference leg to drain into the 
variable leg water column. This caused all level transmitters connected to 
the affected reference leg to indicate higher-than-actual water level. The 
false indication of high water level caused the feedwater control valve to 
close and caused a turbine trip. The turbine trip caused the reactor to 
scram. 

It is unknown when the lower wide-range transmitter equalizing valve was 
opened. However, thirty-six hours prior to the scram, a surveillance test 
was performed on the unit. 

Similar problems have occurred previously and, in July 1980, the nuclear 
steam system supplier (NSSS) prepared an information letter that delineates 
similar safety concerns with respect to the control of differential pressure
transmitter equalizing valves. This information letter provides 
recommendations to alleviate the chronic problem of open equalizing valves. 
An extract of the NSSS recommended actions is attached for information. 

This information notice is provided as notification of a possible 
significant matter. It is expected that recipients will review the 
information for applicability to their facilities. No specific action or 
response to this information is required at this time. If you have questions 
regarding this matter, please contact the Director of the appropriate NRC 
Regional Office. 

Attachments:
1.   Extract from NSSS Information Letter 
2.   Recently issued IE Information Notices

Page Last Reviewed/Updated Tuesday, November 12, 2013