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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