Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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.
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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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