United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-85: Systems Interaction Event Resulting in Reactor System Safety Relief Valve Opening Following a Fire-protection Deluge System Malfunction

                                                            SSINS No.: 6835 
                                                            IN 85-85       

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

                              October 31, 1985

Information Notice No. 85-85:  SYSTEMS INTERACTION EVENT RESULTING IN REACTOR
                              SYSTEM SAFETY RELIEF VALVE OPENING FOLLOWING 
                              A FIRE-PROTECTION DELUGE SYSTEM MALFUNCTION 

Addressees: 

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

Purpose: 

This notice is provided to alert licensees of a serious systems interaction 
event involving the fire-protection deluge system located in the control 
room ventilation charcoal filter housing. Following inadvertent actuation of 
this system, an analog transient trip system (ATTS) panel was sprayed with 
water causing malfunctions in certain safety system components. 

It is expected that recipients will review this notice for applicability to 
their facilities and consider actions, if appropriate, to preclude a similar
problem occurring at their facilities. However, suggestions contained in 
this notice do not constitute requirements; therefore, no specific action or
written response is required. 

Description of Circumstances: 

On May 15, 1985, at Georgia Power Company's Hatch Unit 1, personnel manually
scrammed the reactor from 75% power because of a stuck open low-low-set 
safety relief valve (LLS-SRV). Shorting of one of the two redundant power 
supplies and/or possibly intermittent shorting of logic system contacts in 
the ATTS panel is believed to have caused the stuck open LLS-SRV. The panel 
is one of two redundant panels located in the control room. The cause of the
electrical shorts in the affected panel was water intrusion into the panel. 

The event began about 8:35 p.m. when an instrument water supply vent valve 
was damaged, apparently by dragging of a crane hook along the line. The 
instrument water supply line eventually depressurized causing a portion of 
the fire-protection deluge system to actuate. The water supply line is 
located above the control building and the deluge system is located in the 
control room charcoal filter housing. 

Following actuation of the deluge system, approximately 15 to 25 gal of 
water backed up into the ventilation header before the system could be 
secured. The 


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                                                          IN 85-85        
                                                          October 31, 1985 
                                                          Page 2 of 3     

backup was caused by plugged drains in the charcoal filter housing. Water 
eventually leaked through a hole in the ventilation piping that was located 
above the ATTS panel in the control room. When the water sprayed onto the 
panel, one of two redundant panel power supplies apparently shorted because 
of water intrusion into the panel. As a result, a LLS-SRV valve began to 
cycle open and closed. The SRV cycled three times and then opened and 
remained open. The operator manually scrammed the reactor from 75% power. A 
false turbine high exhaust pressure trip signal also was generated, 
temporarily disabling the high pressure core injection (HPCI) system. The 
reactor core isolation cooling (RCIC) system was inoperable at the time, so 
neither HPCI nor RCIC was immediately available for use. Fortunately, 
neither system was needed during the event. This is because the water level 
was restored and maintained by the reactor feedwater system until the MSIVs 
were, shut. Subsequent to MSIV closure, water level was maintained by the 
control rod drive (CRD) system with the excess water being dumped to the 
condenser via the reactor water cleanup system. The LLS-SRV closed without 
operator action at 9:52 pm. 

Discussion: 

The event is of considerable concern because of the potential for multiple 
safety system failures through unanalyzed systems interactions. In this 
event, the water from the fire-suppression deluge system in the control room
caused opening of a safety relief valve and loss of primary system 
inventory. The event could have been seriously aggravated by the spurious 
HPCI turbine high exhaust pressure trip that was received, also apparently 
as a result of the water intrusion. Because the RCIC system was inoperable 
at the time of the event, no safety-related high pressure injection system 
would have been immediately available to restore water level should that 
have been necessary..  The HPCI turbine trip signal was reset shortly after 
it occurred, however, and the system was returned to operability. 

Perhaps more serious is the potential effect the water could have had on 
numerous other safety systems. The ATTS panels have permissive and arming 
logic and trip logic for various safety systems, as well as water level 
inputs to the HPCI, RCIC, core spray (CS), automatic depressurizaion system 
(ADS), residual heat removal (RHR) system, and diesel activation logic. It 
is hard to predict the anomalous behavior that could occur if both power 
supplies had been lost, or if other portions of the logic had been shorted; 
but quite possibly, several safety systems could have malfunctioned, 
seriously handicapping the operators during their efforts to stabilized the 
unit. 

Prior to this event, no procedures were in place at Hatch Unit 1 for 
adequately cleaning the ventilation plenums or drains in the charcoal filter
units. Had these procedures been prepared and implemented, the drains would 
have functioned as designed with no serious adverse effects. In response to 
this event, the licensee cleaned and inspected drains in the remaining 
filter units and is preparing cleanout and inspection procedures to be added 
to the maintenance schedules. 
.

                                                          IN 85-85        
                                                          October 31, 1985 
                                                          Page 3 of 3     

Another example of a design feature which could cause potential adverse 
system interactions was recently found at Unit 1 of the South Texas Project.
A nonseismic, non-category I potable water line was found to pass through 
the control room envelope via a relay room next to the control room. This 
could subject the solid-state protection system cabinets and the 
Westinghouse 7300 process control system located nearby to water damage 
following a seismic event. Although this unit is under construction, it does 
point out that these problems can occur. Also, Information Notice No. 83-41, 
"Actuation of Fire Suppression System Causing Inoperability of Safety 
Related Equipment," was issued on June 22, 1983. That notice identified a 
number of instances in which automatic actuation of fire suppression systems 
degraded or jeopardized the operability of safety related equipment. 

No specific action or written response is required by this information 
notice. If you have any questions regarding this matter, please contact the 
Regional Administrator of the appropriate NRC regional office or the 
technical contact listed below. 


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

Technical Contact:  David R. Powell, IE
                    (301) 492-8373 

Attachment: List of Recently Issued IE Information Notices 
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Page Last Reviewed/Updated Tuesday, November 12, 2013