Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 84-86
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20655
November 30, 1984
Information Notice No. 84-86: ISOLATION BETWEEN SIGNALS OF THE
PROTECTION SYSTEM AND NON-SAFETY-RELATED
EQUIPMENT
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP).
Purpose:
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
evaluated.
8411280370
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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
Enclosures:
1. SG Level Signal
2. List of Recently Issued IE Information Notices
