Information Notice No. 81-05: Degraded DC System at Palisades

                                                            SSINS No.: 6835 
                                                            Accession No.: 
                                                            8011040271     
                                                            IN 81-05       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                               March 10, 1981

Information Notice No. 81-05: DEGRADED DC SYSTEM AT PALISADES 

Purpose: 

The intent of this notice is to enhance nuclear power plant safety by 
improving the reliability of the direct current (dc) distribution system in 
nuclear power plants. Toward this end, this information notice alerts 
holders of operating licenses and construction permits of an event that 
degraded the dc system at the Palisades facility and jeopardized plant 
safety. 

Description of Circumstances: 

On January 6, 1981, while performing monthly surveillance tests on both 
station batteries, maintenance personnel inadvertently opened the breakers 
from both station batteries to their 125 volt dc buses and left them open 
for approximately one hour. We view this personnel error as a common-mode 
failure that, if left uncorrected, would lead to a complete station blackout
(i.e., total loss of both alternating current (ac) and dc power]. 

Since the plant was in a normal mode of operation, dc power was being 
supplied by the ac system via the battery chargers; therefore, dc power was 
never interrupted during this period. Nevertheless, as described later, the 
safety of the plant was jeopardized. A loss of offsite power during this 
period would, in the absence of manual action, result in the loss of all 
control power, blocking the automatic transfer of power to the onsite diesel
generators. In many designs, such losses would also block the starting of 
the diesels. In contrast, the Palisades design would not block the starting 
of the diesels; however, the loss of control power would block the 
connecting of the generators to their emergency buses so that a complete 
station blackout would still result. In either case, the blackout would 
persist until the batter breakers were manually reclosed. During this time 
the ability of the plant to remove decay heat would be severely restricted. 
Since the tripping of the battery breakers is not annunciated in the 
Palisades control room, a subsequent loss of offsite power would lead to an 
undetected common-mode failure. Such a failure would be difficult to 
diagnose, thereby limiting the operator's ability to take timely corrective 
action. Consequently, an inordinate amount of time could be required to 
bring the plant to a normal mode of decay heat removal. 
.

                                                             IN 81-05      
                                                             March 10, 1981 
                                                             Page 2 of 2   

Plants designed to conform with Regulatory Guide 1.47, "Bypassed and 
Inoperable Status Indication for Nuclear Power Plant Safety," will 
automatically alert the operator whenever a battery is disconnected from its
bus, thereby precluding a Palisades type of event. As a corrective measure, 
Palisades is planning to install annunciators in the control room that will 
alert the operator whenever a station battery has been disconnected from its
bus. 

The information herein is being provided as an early notification of a 
possibly significant matter that is still under review by the NRC staff. 
Recipients should review the information for possible applicability to their
facilities. If NRC evaluations so indicate, further licensee actions may be 
requested. 

No written response to this Information Notice is required. If you desire 
additional information regarding this matter, contact the Director of the 
appropriate NRC Regional Office. 

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