United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 80-44: Actuation of ECCS in the Recirculation Mode While in Hot Shutdown

                                                          SSINS No.: 6835  
                                                          Accession No.:   
                                                          8012160001       
                                                          IN 80-44         

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555 
                                     
                             December 16, 1980 

Information Notice No. 80-44:   ACTUATION OF ECCS IN THE RECIRCULATION 
                                   MODE WHILE IN HOT SHUTDOWN 

PURPOSE: 

The intent of this Information Notice is to alert PWR licensees and holders 
of construction permits of a potentially generic problem involving 
inadvertent actuation of the emergency core cooling system (ECCS) in the 
"Recirculation Mode". Such an event occurred at the Davis-Besse Nuclear 
Power Station (Docket No. 50-346) on December 5, 1980. The plant was in a 
hot shutdown mode of operation at the time of the event and was undergoing 
certain maintenance activities as described below. 

DISCUSSION: 

Just prior to the inadvertent actuation of the ECCS, the licensee was 
attempting to isolate electrical shorts and/or grounds in the Safety 
Features Actuation System (SFAS). Towards this end, A-C power had been 
removed from Channel 3 of the SFAS. Upon reenergizing Channel 3 it was noted 
that an indicating lamp was out; therefore, an attempt was made to replace 
the failed lamp with a spare unit. While removing a lamp from a spare output 
slot in a Channel 3 chassis, an arc was drawn between the lamp and the 
module chassis. This arcing was apparently due to a combination of shorts or 
grounds in the SFAS, which coupled with the "common" connection between 
Channel 1 and 3 resulted in the loss of a power supply in Channel 1. Since 
all the bistable trips in Channel 3 had not been completely reset, and since 
a power supply to Channel 1 was lost, SFAS Levels 1, 2, 3 and 5 were 
actuated by the two-out-of four actuation logic. Since SFAS Level 5 
indicates that the Borated Water Storage Tank (BWST) is at a low level, the 
ECCS was placed in a recirculation mode (i.e., the ECCS suction was aligned 
to the emergency containment sump.) 

In order to place the ECCS system in the recirculation mode, the supply 
valves leading to the Decay Heat Removal (DHR) pumps from the BWST start 
closing after those in the ECCS line and from the containment emergency sump
fully open. Thus, during this valve transition period, a flow path existed 
to the reactor coolant system (RCS) via the BWST and the ECCS pumping system
(i.e., the DHR pumps); however, since the RCS pressure was higher than that 
of the pumping system (2100 vs. 1600 psig) no BWST water was pumped into the
RCS. Rather, during the valve transition time of about 1-1/2 minutes, 
approximately 15,000 gallons of borated water was drained from the BWST to 
the containment emergency sump. 
.

                                                          IN 80-44         
                                                          December 16, 1980 
                                                          Page 2 of 2      

The Davis-Besse Nuclear Power Station has experienced several events during 
the past three and one-half years which have either degraded the DHR system 
or caused a temporary interruption of DHR operation. Of these, the one most 
closely paralleling the event described above occurred on April 19, 1980. 
(See Information Notice No. 80-20, "Loss of Decay Heat Removal Operability at
Davis Besse Unit 1 While in A Refueling Mode" dated May 8, 1980 and IE 
Bulletin No. 80-12, "Decay Heat Removal System Operability" dated May 9, 
1980.) During the December 5, 1980 event, the plant was in a hot shutdown 
mode with RCS pressure at approximately 2100 psig; therefore, the operator 
was able to promptly trip the DHR pumps, thereby precluding air entrapment 
in the system or pump damage. In contrast, the April 19, 1980 event occurred
while the plant was in a quasi refueling mode (i.e., the reactor head was in
place but detensioned, RCS water level was below the steam generator's 
bottom plate, the RCS was vented to the atmosphere by an open manhole at the 
top of the steam generator, and decay heat was being removed by one DHR 
system). Since the DHR pump was being used to remove decay heat during the 
April 19 event, it was left running for approximately two minutes after the 
pump had been aligned to the recirculation mode of operation, and tripped 
when water was discharged from a temporary tygon line. Since the DHR system 
was the only system available for decay heat removal, the DHR pump was 
restarted approximately forty minutes later; however, it was tripped shortly 
thereafter when it was determined that there was air in the suction line of 
the system. In all, the April 19, 1980 event required approximately two and 
one-half hours to vent the DHR system and reestablish DHR flow. As a 
consequence of the relatively long recovery time, the temperature limit 
specified in the Technical Specification for the refueling mode of operation 
was exceeded during the April 19, 1980 event. 

The above events illustrate means whereby the integrity of a major ECCS 
system can be jeopardized by inadvertently or prematurely aligning the ECCS 
to the recirculation mode of operation. The major concern in such cases is 
that the DHR pumps could become air bound if their suction lines are aligned
to a dry sump. At best, with the pumps air bound, the pump motor would trip 
automatically or could be tripped manually before any damage occurred, in 
which case flow could be established after the system is vented; at worst, 
the pump could be damaged and become inoperable, in which case the active 
portion of the ECCS would not be available, if needed at that time. 

This Information Notice is 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. No specific action or response is requested at this time; 
however, if NRC evaluations so indicate, further licensee actions may be 
requested or required. 

If you have any question regarding this matter, please contact the director 
of the appropriate NRC Regional Office. 

Page Last Reviewed/Updated Tuesday, November 12, 2013