Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
.
IN 85-89
November 19, 1985
Page 2 of 3
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.
.
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
Attachment: List of Recently Issued IE Information Notices
