United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-05: Inter-System Discharge of Reactor Coolant

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                              January 29, 1990


Information Notice No. 90-05:  INTER-SYSTEM DISCHARGE OF REACTOR COOLANT


Addressees: 

All holders of operating licenses or construction permits for nuclear power 
reactors. 

Purpose: 

This information notice is intended to alert addressees to a potentially 
significant problem in identifying and terminating reactor coolant system 
leakage in operating modes 4 and 5.  It is expected that licensees will 
review the information for applicability to their facilities and consider 
actions, as appropriate, to avoid similar problems.  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 December 1, 1989, Braidwood Unit 1 experienced the unplanned inter-system 
discharge of approximately 68,000 gallons of water.  The discharge was 
caused by the inadvertent opening of a residual heat removal (RHR) system 
suction relief valve.  The valve failed to reclose, allowing an open flow 
path from the reactor vessel, through the RHR system, into the unit's two 
recycle hold-up tanks (HUTs). 

The unit, which had been in a refueling outage since September 2, 1989, was 
heating up in operational mode 5, preparing to enter operational mode 4.  
The plant was solid and in the process of drawing a bubble in the 
pressurizer.  The RHR train "A" pump was in operation and, although the "B" 
pump was not running, the "B" train was unisolated and available.  The 
reactor coolant system (RCS) was at a pressure of 350 psig and a temperature 
of 175 F.  Charging flow to the vessel was being provided by the "A" 
charging pump.  Pressurizer heaters were on.  The "B" charging pump was 
isolated and tagged out of service.  (Technical Specifications governing 
cold overpressure protection require that only one charging pump be 
available.  The other charging pump and the safety injection pumps are 
required to be tagged out of service, with power supplies removed).  To 
protect against a pressure switch failure and the subsequent automatic 
isolation of the RHR system, the train "A" RHR suction isolation valve was 
open and tagged out of service. 






9001230126
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At 1:42 a.m., operators throttled the charging flow and maximized the 
letdown flow in preparation for drawing a bubble in the pressurizer.  The 
RCS pressure was 404 psig and the pressurizer level was off scale, high.  At 
1:44 a.m., a rapid reduction in the pressurizer level occurred, with the 
pressurizer level off scale, low, at 1:52 a.m.  Approximately 14,000 gallons 
of water drained from the pressurizer and the pressurizer surge line; 
however, the reactor vessel level instrumentation system indicated that the 
vessel level remained at 100 percent.  At 1:49 a.m., the charging flow was 
increased and the charging pump suction was switched from the volume control 
tank to the refueling water storage tank (RWST).  

About 30 to 50 gallons of water were observed on the floor of the auxiliary 
building in proximity to the RHR train "A" suction relief valve, leading 
plant personnel to believe that this valve had lifted.  At 1:53 a.m., the 
letdown flow was reduced to minimum and charging was maximized.  The RHR 
trains were switched from "A" to "B", the "A" pump was stopped, and the 
isolation of the "A" train was initiated.  At 1:59 a.m., one of the two 
running reactor coolant pumps (RCPs) was stopped because of low RCS 
pressure. 

A second charging pump, "B", was started following completion of the formal 
procedure for tagout removal.  At 2:35 a.m., the "A" RHR suction isolation 
valve was returned to service and closed, completing the isolation of the 
"A" train of the RHR system.  The pressurizer level began to recover and the 
RCS pressure increased slightly, giving operators the impression that the 
discharge had been isolated.  The "B" charging pump was therefore secured at 
2:45 a.m.  The pressurizer level, however, did not recover.  At 2:54 a.m., 
the "B" charging pump was restarted.  At 3:49 a.m., the inter-system 
discharge was terminated when the RHR train "A" pump was started, the "B" 
pump shut down, and the "B" train was isolated.  The level indication for 
the HUTs stabilized and the pressurizer level began to recover at 3:52 a.m. 

By 5:06 a.m., the pressurizer level had fully recovered and the unit was 
stabilized at 360 psi and 175 F.  Approximately 68,000 gallons of water had 
been discharged from the reactor vessel to the HUTs.  (The total amount of 
water was composed of 14,000 gallons of initial pressurizer inventory and 
54,000 gallons of makeup water). 

Following the event, it was determined that the RHR "B" train suction relief 
valve had lifted at 411 psi.  The lift setpoint for the valve should have 
been 450 psi.  The valve should have reclosed on reducing pressure but 
failed to do so.  The premature opening of the valve was attributed to the 
presence of foreign material lodged between the valve spindle and the 
spindle guide.  This foreign material either prohibited the correct 
adjustment of the valve or affected the valve's lift setpoint.  The valve's 
failure to reclose was attributed to improper nozzle ring adjustment.  The 
reset pressure is strongly influenced by the dynamic forces created by the 
nozzle ring.  If the ring is located too high on the nozzle, it may result 
in an inadequate ventilation area just above the nozzle.  Undesirable forces 
will develop which may cause a much lower reseat pressure. 

The water found near the RHR train "A" suction relief valve had leaked from 
a weep hole on a relief valve in a radwaste evaporator line connected to the 
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common discharge header of the train "A" and "B" suction relief valves.  
Contrary to original assumptions, there was no evidence that the "A" train 
suction relief valve had lifted.  The root cause of the problem with the 
relief valve on the evaporation line is under investigation but is thought 
to be unrelated to the failure of the "B" suction relief valve. 

Hampering operators' efforts throughout this event was the lack of an 
appropriate emergency operating procedure (EOP) to detect coolant leaks 
while in operating modes 4 and 5.  However, the operators were able to 
combine two related abnormal operating procedures for guidance during this 
event.  One of the procedures is designed to locate system leaks while in 
modes 3 and 4.  The other provides guidance for the restoration of the RHR 
system following its loss during conditions in which the reactor vessel 
inventory is at a reduced level. 

Discussion: 

The event at Braidwood 1 is significant because it underscores the need to 
have EOPs available for use in other than "at power" operating modes.  The 
fact that over 2 hours were required to locate the stuck-open valve, to 
terminate the discharge, and to begin refilling the pressurizer highlights 
the need to provide personnel with adequate tools to perform their tasks.  
Relying on ad hoc procedures during significant events places an unnecessary 
burden on operating personnel.  The lack of adequate EOPs could handicap the 
most competent operators in their efforts to address significant operational 
problems. 

Also illustrated by this event is the need for procedures to assure that 
adequate RCS makeup capability and cooling options are available in a timely 
fashion during shutdown.  The discharge through the stuck-open relief valve 
exceeded the capability of a single charging pump.  Starting a second 
charging pump required that formal procedures for tag removal be conducted.  
This effort necessitated a considerable amount of time, which may not be 
available should a similar event occur while the RCS is at a higher 
temperature. 

The severity of this event could have been increased if greater decay heat 
were present in the reactor vessel or if a gross failure of the relief valve 
discharge header had occurred.  Greater decay heat would have increased the 
potential for voiding in the core.  Also, because the header discharges to 
the HUTs which are located outside containment, a piping failure could have 
resulted in all or a portion of the RCS water being discharged to the 
building floor.  This event would have necessitated a major cleanup effort 
and increased the potential for personnel contamination. 

If this event had occurred at one of the nuclear plants that has a single 
suction line from the RCS to the RHR system, all shutdown cooling would have 
been lost as a result of isolating the failed suction relief valve.  An 
alternate heat sink would likely have been required; however, in mode 5, an 
alternate heat sink may not be readily available. 
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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate NRR project 
manager. 




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  Nick Fields, NRR 
                     (301) 492-1173

                     Julian Hinds, RIII 
                     (315) 388-5575

Attachment:  List of Recently Issued NRC Information Notices
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