Information Notice No. 84-86: Isolation Between Signals of the Protection System and Non-Safety-Related Equipment

                                                            SSINS No.:  6835
                                                            IN 84-86 

                               UNITED STATES 
                           WASHINGTON, D.C. 20655 

                              November 30, 1984

Information Notice No. 84-86:   ISOLATION BETWEEN SIGNALS OF THE 
                                   PROTECTION SYSTEM AND NON-SAFETY-RELATED 


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


This information notice is provided to alert recipients of potentially 
significant problems pertaining to isolation between signals of the 
protection system and non-safety-related equipment. It is expected that 
recipients will 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 and, therefore, no specific action
or written response is required. 

Description of Circumstances: 

On October 1, 1984, Louisiana Power and Light Company (LP&L) informed the 
NRC of a potentially significant construction deficiency at Waterford Unit 
3. The applicant indicated that the isolation between certain plant 
protection system (PPS) signals and the plant monitoring computer (PMC) did 
not meet the design criteria. 

During routine preoperational trouble shooting, it was determined that a 
failure in the mercury-wetted relays used in the non-safety-related remote 
multiplexer equipment of the PMC can cause significant changes in the 
instrument current loops for containment pressure, steam generator level, 
and steam generator pressure. The applicant's analysis indicates that relay 
failures would result in a nonconservative value for containment pressure 
input to the reactor protection system (RPS) trip. The analysis also 
indicates that similar failures would result in inaccurate (but probably 
conservative) values for steam generator pressure and level inputs to the 
RPS trip. A simulated failure mode in the field has supported the results of 
the analysis. The immediate corrective action taken by the applicant was to 
remove those PPS signals from the PMC by disconnecting the PPS analog set 
point signal cables from the PMC. Long-term corrective actions are being 


                                                           IN 84-86 
                                                           November 30, 1984
                                                           Page 2 of 2 

Discussion : 

The NRC staff has conducted a preliminary review of the Waterford Unit 3 
deficiency and has met with the applicant and Combustion Engineering, the 
nuclear steam supply system vendor, to discuss this matter. One conclusion 
of the review is that there is a fundamental shortcoming in the design of 
certain PPS circuits of Waterford Unit 3. The buffer devices in the circuits
for containment pressure, steam generator pressure, and steam generator 
level inputs to the PMC isolate only the "high" side of the electrical 
signal and not the "low" side (signal return). A simplified schematic of the 
steam generator level circuit, which is similar to the steam generator and 
containment pressure circuits, is shown in Enclosure 1 of this notice. The 
buffer in the circuits is used to transmit analog information regarding 
bistable set points to the PMC. As shown in the schematic, postulated faults
downstream of the buffer devices, such as those caused by failed 
mercury-wetted relays in the multiplexer equipment, would result in 
electrical interactions within the transmitter current loop. At Waterford 
Unit 3, the specific problem is limited to those transmitter current loops 
with multiple resistors used to provide signals to the PPS. The lack of 
adequate isolation on the "low" side of the circuits would thus result in 
adverse interactions and may lead to loss of protection from redundant 
channels monitoring the same plant variable and/or loss of protection from 
diverse plant variables. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 

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

CONTACT:  J. T. Beard, NRR 
          (301) 492-7465 

          R. N. Singh, IE 
          (301) 492-8985 

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