United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-35: Degraded Ability of Steam Generators to Remove Decay Heat by Natural Circulation


                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                          WASHINGTON, D.C. 20555-0001

                                August 28, 1995


NRC INFORMATION NOTICE 95-35:  DEGRADED ABILITY OF STEAM GENERATORS
                               TO REMOVE DECAY HEAT BY NATURAL CIRCULATION


Addressees

All holders of operating licenses or construction permits for pressurized-
water reactors (PWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to conditions that may degrade the ability of the
steam generators (SGs) to remove decay heat by natural circulation of the
reactor coolant in Mode 5, cold shutdown.  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 are not NRC requirements; therefore, no
specific action or written response is required.

Description of Circumstances

During a refueling outage in September 1994 on Vogtle Unit 1, the licensee
found that the ability to remove decay heat by natural circulation through the
steam generators was degraded.  On September 11, 1994, Vogtle Unit 1 entered a
refueling outage.  On September 16, 1994, the unit was in Mode 5 with the
reactor coolant system (RCS) being drained down.  The reactor coolant pumps
were tagged out of service and residual heat removal (RHR) system Train B was
in service removing decay heat.  RHR Train A was operating intermittently but
had been administratively removed from service to adjust the suction valve
limit switch and allow stroking of local valves.  During this period, the SGs
were relied upon to meet the technical specification requirement for a second
source of decay heat removal.  While taking credit for the steam generators as
a heat sink, the RCS was vented to the containment atmosphere when the
pressurizer code safety valves were removed and a conoseal on the reactor
vessel head was disassembled.  The licensee later determined that the heat
removal capability by natural circulation through the SGs was degraded.  When
the RCS was vented, the inability to pressurize the RCS reduced the natural
circulation cooling capacity.

On February 13, 1995, the licensee for Turkey Point Units 3 and 4 reported
that during previous refueling outages it had relied upon the SGs for decay
heat removal when the SGs may not have been able to perform that function. 
Specifically, the licensee relied upon the SGs as one of the means of decay
heat removal while testing one of the RHR loops with the RCS vented.  The 


9508220031.                                                            IN 95-35
                                                            August 28, 1995
                                                            Page 2 of 3


licensee later concluded that the RCS cannot support subcooled natural
circulation decay heat removal through the SGs while the RCS is vented.

Discussion

These two examples illustrate plant conditions that were not adequate to fully
support natural circulation through the SGs as a method of decay heat removal
during operation in Mode 5 with loops filled and the reactor coolant pumps out
of service.

Technical specifications generally require two methods of decay heat removal
in Mode 5 with loops filled.  When this is the case, they generally go on to
indicate that this requirement can be satisfied by two loops of RHR or one
loop of RHR and a minimum water level in the SGs.  Decay heat can be removed
either through the RHR system or through the SGs by natural circulation after
the reactor coolant pumps are secured.  The heat removal mechanism with
residual heat removal is through forced circulation through the RHR heat
exchanger.  Heat removal with natural circulation of reactor coolant through
the SGs occurs because of the differential pressure created between the heated
water in the reactor core and the cooler water in the SG tubes.  This
differential pressure is created through temperature differences that in turn
create fluid density differences between these two locations.

When the RCS is being depressurized and cooled down, the reactor coolant pumps
are stopped, the RCS is depressurized and vented, and level is decreased in
preparation for Mode 6 (refueling) entry.  In Mode 6, both RHR trains must be
operable.  During the transition from Mode 5, with no reactor coolant pumps
running, to Mode 6, plant conditions may exist that are not adequate to
support natural circulation.  The second train of RHR may need to be operable
before proceeding with plant cooldown and depressurization to provide a second
method for RCS cooling.

During natural circulation, the SG secondary side water boils and steams off
through the atmospheric relief valves or other openings that may exist during
shutdown conditions.  The minimum temperature at which boiling will begin in
the SG is 100� C [212� F].  A minimum temperature differential of 28� C    
[50� F] between the RCS and the SG secondary water is routinely used for
evaluating conditions that would ensure sufficient natural circulation flow to
prevent boiling in the core.  The heat transfer rate across the steam
generator tubes is less for lower RCS-to-SG secondary temperature
differentials but still may be adequate to promote sufficient natural
circulation and prevent core boiling.  Adding the differential temperature of
28� C [50� F] to 100� C [212� F] results in a minimum RCS temperature of   
128� C [262� F] to maintain sufficient natural circulation flow.  The lowest
pressure point in the RCS, at the top of the SG tubes, should therefore be
maintained above the saturation pressure for 128� C [262� F].  If the RCS
pressure at the top of the SG tubes is allowed to fall below the primary fluid
saturation temperature, flashing and steam voiding may occur, interrupting or
degrading the natural circulation flow path.  Additionally, when system
pressure is dropped with elevated water temperatures, gases may come out of
solution.
.                                                            IN 95-35
                                                            August 28, 1995
                                                            Page 3 of 3


When relying on the ability of the SGs to remove decay heat by natural
circulation of reactor coolant in Mode 5, the following factors are worthy of
consideration:  (1) the ability to pressurize and control pressure in the RCS,
(2) secondary side water level in the SGs relied upon for decay heat removal,
(3) availability of a supply of feedwater, and (4) availability of an
auxiliary feedwater pump capable of injecting into the relied-upon SGs. 
Consideration should also be given to avoiding the potential for
pressurization of the SG secondary side.  It is also important to note that
during the decay heat removal scenario for the natural circulation process, a
mode change (Mode 5 to Mode 4) could occur due to heat up of the RCS.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.


                                          original signed by

                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contact:  Brian R. Bonser, RII
                    (706) 554-9901

Attachment:  
List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Monday, November 18, 2013