United States Nuclear Regulatory Commission - Protecting People and the Environment

NRC: Information Notice No. 85-89: Potential Loss of Solid-State Instrumentation Following Failure of Control Room Cooling

                                                          SSINS No.: 6835 
                                                            IN 85-89       

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

                              November 19, 1985

Information Notice No. 85-89:   POTENTIAL LOSS OF SOLID-STATE 
                                   INSTRUMENTATION  FOLLOWING FAILURE OF 
                                   CONTROL ROOM COOLING 

Addressees: 

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

Purpose: 

This information notice is to alert recipients of a potentially significant 
problem involving the loss of solid-state instrumentation following the 
failure of control room cooling. Actions taken by the licensee to mitigate 
the event also are discussed. It is expected that recipients will review 
this information for applicability to their facilities and consider actions, 
if appropriate, to preclude a similar problem occurring at their facilities.
However, suggestions contained in this information notice do not constitute 
NRC requirements; therefore, no specific action or written response is 
required. 

Description of Circumstances: 

On June 4, 1984, both units of the McGuire Nuclear Station were operating at
100% power with one of the two main control room ventilation units out of 
service for maintenance. At 8:02 p.m., the remaining chiller tripped on low 
oil level resulting in a total loss of main control room cooling. At 
approximately 8:45 p.m., as the control room temperature increased, numerous
alarms on Unit 1 high reactor coolant loop C Tave were received, as well as 
alarms on Unit 1 pressurizer level. Attempts to restore the air conditioning
were unsuccessful and at 9:00 p.m. the air conditioning was declared 
inoperable. At 10:00 p.m., the operators opened the doors between the 
control room and the computer room, which still had cooling available. 
Operators also opened the doors of the Westinghouse PCS 7300 cabinets, which 
contain the solid-state circuit cards generating the alarms. The licensee 
then used portable fans with ducting to provide cooling from the computer 
room to the PCS 7300 cabinets. The required technical specification power 
reduction was started at 10:05 p.m. and terminated at 10:55 p.m. when one of 
the air conditioning units was returned to service. The solid-state 
instrumentation returned to normal following restoration of the air 
conditioning. 



8511150114 
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                                                         November 19, 1985 
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Discussion: 

Before June 4, 1984, the McGuire Nuclear Station had experienced numerous 
printed solid-state circuit card failures with the Westinghouse PCS 7300 
cabinets and associated solid-state protection system (SSPS). The card 
failures, which involved reactor trips and spurious instrument indications, 
were attributed, by the licensee, to overheating in the PCS 7300 cabinets. 
In some cases, the spurious instrumentation indications disappeared when 
adequate ventilation was provided to the cabinets; however, in other cases, 
continued erratic instrumentation indicated that the overheating had 
significantly shortened the life expectancy of the solid state components. 
The licensee also had previously reported that the air chillers develop oil 
level problems when loaded at less than full capacity. The heat load 
calculated during plant design was too large compared to the actual heat 
load resulting in oversized chillers. 

Following the event, the licensee took temperature measurements inside the 
PCS 7300 cabinets and determined that with an ambient temperature of about 
72F the cabinets had internal temperatures of up to 125F on the 
top rack. The McGuire operators estimate that the ambient temperature, 
during the event, reached 90F before alternate cooling was provided. 
The licensee has rebalanced the airflow in the control area ventilation 
system to provide additional cooling to the PCS cabinets. Though the 
licensee's remedial actions to provide better normal cooling appear to have 
increased the reliability of the solid-state cabinets under design operating 
conditions, the safety concern following loss of all control room HVAC units 
remains. 

The McGuire operators, alerted by prior experience, took prompt action to 
provide alternate cooling to the solid-state equipment during the event. 
Without such action, the possible loss of some instrumentation and erratic 
instrument readings may have made it difficult to bring the plant to a safe 
condition, such as hot shutdown. If no control room cooling is available to 
the solid-state cabinets, it may not be prudent to delay in going to a hot 
shutdown condition even though the plant technical specifications may allow 
appreciable time to achieve the shutdown. The failure rate of the 
instrumentation can be expected to increase as the control room temperature 
increases and the erratic instrumentation may cause a reactor trip at the 
same time that the instrumentation is unreliable or unavailable to assist 
the operators. 

Loss of all control room cooling may be more likely than previously thought.
In addition to the McGuire event, there has been recent identification of 
other reported possible common-mode HVAC failures at Browns Ferry and 
Limerick. Therefore, licensees should be alert for the possibility of the 
loss of control room cooling and the impact this may have on their 
solid-state instrumentation. 
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                                                         IN 85-89         
                                                         November 19, 1985 
                                                         Page 3 of 3      

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 

Technical Contact:  James Stewart, IE
                    (301) 492-9061

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