Information Notice No. 90-06: Potential for Loss of Shutdown Cooling While at Low Reactor Coolant Levels

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

                              January 29, 1990


Information Notice No. 90-06: POTENTIAL FOR LOSS OF SHUTDOWN COOLING 
                                  WHILE AT LOW REACTOR COOLANT LEVELS 


Addressees: 

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

Purpose:

This information notice is intended to alert addressees to a potential 
problem affecting the operability of the residual heat removal (RHR) system 
during operating conditions involving mid-loop operation.  It is expected 
that recipients 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 July 18, 1989, at Comanche Peak Unit 1, train "A" of the residual heat 
removal system was in operation prior to initial fuel loading.  The air 
operated RHR heat exchanger flow control valve (FCV) was being used to 
maintain system flow below 2000 gpm.  The reactor vessel water level was 
being maintained slightly above the center line of the hot legs.

The inverter supplying power to the controller for the FCV failed.  The loss 
of power to the controller resulted in the FCV failing to its emergency core 
cooling system (ECCS) position of being fully open.  The open valve caused 
an increase in system flow to 4400 gpm.  This sudden, increased flow caused 
conditions where vortexing was expected at the suction of the RHR pipe 
connection to the RCS.

The power supply for the controllers was manually transferred to an 
alternate source and the FCV was throttled, thus terminating the event.

Discussion:

The FCV is manually throttled to maintain a desired rate of flow through the 
RHR heat exchanger.  The heat exchanger flow rate is adjusted to control the 
temperature of the water being returned to the vessel.  Total system flow is 
regulated by throttling the heat exchanger bypass valve coincident with the 
adjustment of the FCV.  




9001230175
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                                                            IN 90-06 
                                                            January 29, 1990
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During this event, there were no indications of pump air ingestion.  
However, the increased system flow corresponded to the rate at which 
vortexing had been calculated to occur based on the existing RCS level.  The 
licensee had previously determined maximum permitted flow rates for various 
reduced levels of the RCS to prevent the possibility of vortexing and air 
entrainment into the RHR pump. 

Although RHR pump operation was never actually impaired, the potential 
existed for air to be introduced to the pump's suction.  This concern is 
exacerbated by the fact that the heat exchanger FCVs for both trains of RHR 
are powered from the same inverter.  Since the FCVs open fully on a loss of 
power, both trains of RHR could become inoperable on a loss of power during 
mid-loop operation.  (A loss of instrument air would also cause the FCVs to 
fail to the full open position and could also result in the loss of both 
trains of the RHR system). 

During most modes of operation, having the FCVs go fully open on loss of 
power or instrument air is not a safety concern because having the valves 
fully open does not normally prevent the system from performing its safety 
function.  However, having the valves go fully open while at mid-loop could 
result in the loss of decay heat removal capability.  This loss could 
persist until electrical power or instrument air is restored and air is 
vented from the RHR system.

The licensee has revised procedures for mid-loop operation to require 
manually positioning the cold leg injection isolation valves to limit the 
maximum flow through the system, even with the FCVs fully open.  This lineup 
will be used whenever the unit is at mid-loop and protects against the 
consequences of failure of either electrical power or instrument air.

Several generic communications have been issued informing licensees of the 
consequences of the loss of residual (decay) heat removal and the operating 
conditions under which the probability of a loss increases; i.e., reduced 
inventory conditions.  The most recent publications include Generic Letter 
(GL) No. 87-12, "Loss of Decay Heat Removal While the Reactor Coolant System 
is Partially Filled;" GL No. 88-17, "Loss of Decay Heat Removal;" NRC 
Information Notice (IN) 87-23, "Loss of Decay Heat Removal During Low 
Reactor Coolant Level Operation;" IN 88-36, "Possible Sudden Loss of RCS 
Inventory During Low Coolant Level Operation;" and IN 89-67, "Loss of 
Residual Heat Removal Caused by Accumulator Nitrogen Injection."  The event 
described above introduces a subtle failure scenario which licensees may not 
have considered. 


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                                                            IN 90-06 
                                                            January 29, 1990
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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

                     Steve Bitter, RIV 
                     (817) 897-1500

Attachment:  List of Recently Issued NRC Information Notices

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