United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 83-77: Air/Gas Entrainment Events Resulting in System Failures

                                                            SSINS No.: 6835 
                                                            IN 83-77       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                              November 14, 1983

Information Notice No. 83-77:   AIR/GAS ENTRAINMENT EVENTS RESULTING IN 
                                   SYSTEM FAILURES 

Addressees: 

All holders of a nuclear power reactor operating license (OL) or 
construction permit (CP). 

Purpose: 

This information notice is provided as notification of events that rendered 
redundant safety systems inoperable because air or gas entrainment caused 
pump cavitation. It is expected that recipients will review the information 
herein for applicability to their facilities. No specific action or response
is required. 

Description of Circumstances: 

Calvert Cliffs 

On May 20, 1980, Calvert Cliffs Unit 1 sustained a loss of both service 
water system (SWS) redundant trains. The SWS became air bound after a 
service water heat exchanger (SWHX) was returned to operation, following 
routine maintenance. 

During the time that the SWHX was not in operation, the heat exchanger 
outlet valve was closed and air accumulated on the shell side of the heat 
exchanger. The source of air was a failed tube in the instrument air 
compressor (IAC) aftercooler. When the SWHX was brought back on line, the 
trapped air was swept into the SWS. Shortly thereafter, high temperature 
alarms for components cooled by service water (SW) were observed. 
Simultaneously, low pressure was also observed in both SW headers and low 
amperage was being drawn by the two SW pumps that were running. These 
indications signaled that SW pumps were cavitating. The reactor was manually 
tripped because of increasing main turbine and feed pump turbine bearing 
temperatures. It was subsequently found that the air was swept into the 
common inlet header for the SW pumps, causing pump cavitation and disabling 
both safety-related subsystems. 

Although the SWS is provided with a number of constant vent valves, their 
relieving capacity was exceeded by the sudden influx of the large quantity 
of air that had accumulated in the SWHX, while it was out of service. After 



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                                                          IN 83-77         
                                                          November 14, 1983 
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the IAC aftercooler discharge valve was shut, open vents began discharging 
solid streams of water and the two SW subsystems were both operating soon 
thereafter. 

McGuire 

On February 12, 1982, McGuire Unit 1 experienced a loss of high head safety 
injection emergency boration and reactor coolant makeup capability. Hydrogen
from the positive displacement pump (PDP) suction dampener entered the 
common suction of the charging system, causing both centrifugal charging 
pumps and the PDP to be inoperable. The system was restored within 30 
minutes. The unit was in Mode 1, at 50% of full power at the time. This 
event was described in more detail in Information Notice No. 82-19. 

San Onofre 

During preoperational testing on March 14, 1982 at the San Onofre Nuclear 
Generating Station, Unit 2, the shutdown cooling system was inoperable for 
90 minutes when the low pressure safety injection (LPSI) pumps became 
nitrogen bound. The event resulted from an improper valve alignment during 
nitrogen backflushing of a purification filter in the chemical and volume 
control system. 

Backflushing consists of passing nitrogen gas at 350 psig through the 
isolated purification filter and discharging the gas and collected 
contaminants into the filter backflush storage tank. In this instance, as a 
result of either a system malfunction or operator error, the gaseous 
nitrogen passed through the purification line into the suction of the LPSI 
pumps which were being used for shutdown cooling. Flow from the operating 
LPSI pump fell from 4000 gpm to zero as the pump became gas bound and 
attempts to establish flow with this pump or the alternate LPSI pump were 
unsuccessful. The pumps and piping high points were vented and shutdown 
cooling flow was reestablished. 

St. Lucie 

On October 23, 1982, with St. Lucie Unit 1 in hot standby during recovery 
from a reactor trip, the three operating positive displacement charging 
pumps stopped injecting coolant to the reactor coolant system because the 
volume control tank (VCT) was pumped dry. The reactor had tripped on a low 
steam generator water level signal after a loss of feedwater flow to the 
steam generator.  

The VCT was empty although its two liquid level sensors indicated an 
acceptable liquid inventory and hence an apparently acceptable 
inflow/outflow balance from the VCT. The hydrogen cover-gas blanket of the 
VCT entered the suction of each pump. The false liquid level indication was 
caused by an empty reference leg that was shared by both liquid level 
sensors. The pumps were restored to operation by repeated venting after 
filling the VCT to a high level. 
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                                                          IN 83-77         
                                                          November 14, 1983 
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The aforementioned events are intended to be illustrative. Hence, licensees 
are cautioned that the types of system inoperability resulting from air or 
gas entrainment vary. Moreover, redundant safety-related trains or 
components can be affected as shown by the events cited above. The serious 
consequences that may result from such system or component impairment cannot 
be overemphasized. For example, although the actual consequences of the loss 
of shutdown cooling flow at San Onofre-2 were minimal because the event 
occurred prior to initial criticality, the event could have been more 
serious if the plant had been operating at full power for an extended period 
of time prior to the event. 

If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office, or this office. 


                                   Edward L. Jordan Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contacts: R. M. Young, IE
                    49-27275

                    E. V. Imbro, AEOD
                    49-24495

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