Information Notice No. 96-49: Thermally Induced Pressurization of Nuclear Power Facility Piping

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                         WASHINGTON, D.C.  20555-0001

                                August 20, 1996

                               POWER FACILITY PIPING  


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a number of scenarios reported by licensees that
involve thermal expansion of fluid in closed piping that could lead to
overpressurization of the piping and either degraded operability or loss of
function of safety systems.  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

Licensees have reported actual and postulated situations in which the thermal
expansion of water in piping systems within containment could produce
undesirable consequences.  Two situations are presented in this notice to
illustrate the range of scenarios, the postulated consequences, and the
corrective actions taken.  Each situation involves the heating of a section of
water-filled piping between two closed block or isolation valves.  Although
the examples involve piping inside containment, this condition could arise
elsewhere in the facility.  

Beaver Valley Unit 1

The first scenario was reported by the licensee for the Beaver Valley facility
in a notification dated July 3, 1996.  During surveillance testing of a motor-
operated butterfly valve located in the component cooling water inlet line to
the residual heat removal heat exchanger, it was observed that the valve which
was located inside containment would not open.  The licensee investigation
revealed that pressure in the piping section between this valve and a closed
manual butterfly valve located outside containment was measured as being
slightly higher than the system design pressure.  After the pressure was
relieved by the opening of a drain valve, the remotely operated valve was
opened.  The pressurization was caused by an increase in trapped water 

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temperature due to an increase in ambient temperatures from the conditions
that existed in the Spring, when the valves were closed, to those that existed
after plant heatup to mode 1 with a summer temperature of about 32 degrees C
[90 degrees F].  This could potentially jeopardize the structural integrity of
the containment penetration.  Immediate corrective actions for this and other
piping sections without thermal relief included draining fluid from piping
sections and opening the inboard valve to provide access to relief protection. 
The identified root cause was inadequate design reviews relating to contain-
ment penetration relief protection.

Maine Yankee

The second scenario was reported by the licensee for the Maine Yankee facility
in a notification dated July 19, 1996.  The primary component cooling water
system at Maine Yankee has a nonsafety-related subdivision that serves the
containment air coolers, and a safety-related subdivision that serves the
emergency core cooling system equipment.  The nonsafety subdivision of the
piping has a swing-check valve at the containment inlet (supply) penetration,
and an air-operated valve at the containment outlet (return) penetration.  In
a postulated accident scenario (a loss-of-coolant accident), the containment
isolation logic would initiate closure of the outlet valve, thereby causing
water flow to cease.  Heat from the containment environment would cause the
water in the fan coolers between the inlet check valve and closed outlet valve
to expand, potentially causing a rupture in the system.  The rupture would
depressurize the isolated portion of the system and reestablish water flow
from the operating safety-related portion located outside containment through
the inlet check valve.  The water would spill from the rupture and deplete the
surge tank, thereby causing failure from overheating of safety-related
equipment served by the remainder of the system.  Upon identifying this
postulated scenario, the licensee promptly shut down the facility.  Other
closed systems and containment penetrations were evaluated.  As a result, the
licensee took corrective action which included the installation of a pressure
relief valve on each of the six containment air cooler branch lines. 


Because of thermal expansion, water heated while trapped in closed piping is
capable of producing extremely high pressures.  This phenomenon is typically a
design consideration.  Piping design codes as far back as USAS B31.1 have
required consideration of fluid pressure caused by heating of fluid trapped
between two valves.  The potential for thermally induced expansion of fluid
trapped in valve bonnets was one reason for issuing Generic Letter 95-07,
"Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate
Valves."  In addition, several information notices have been issued discussing
pressurization of water trapped in valve bonnets.

The potential for failure of systems to perform their safety functions as a
result of thermally-induced overpressurization is dependent on many factors.
These factors include leak-tightness of valve seats, bonnets, packing glands
and flange gaskets; piping and component material properties, location and
geometry; ambient and post-accident temperature response; pipe fracture
mechanisms; fan coastdown characteristics and the effect of fan operation on  .                                                            
                                                            IN 96-49
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water in the associated fan cooling system; relief valves and their settings;
and system isolation logic and setpoints.  Engineering design and modification
evaluations, which include systematic evaluation of heat input to systems and
components with consideration of factors such as those above, can detect
conditions which may influence system operability under normal operations,
operational transients and accident conditions. 

Please also note that under the "single-failure concept," thermal over-
pressurization is a consequence of the event and is not considered to be an
independent active or passive failure.  Active or passive failures must also
be considered in the same and other systems in evaluating plant response to a
postulated accident.  If relief valves are installed to prevent overpressure
conditions, consideration must be given to the effects of a stuck-open relief
valve and consequent diversion of system flow, associated environmental
flooding and radiation hazards.

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

                                    Thomas T. Martin, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Jerry Carter, NRR
                     (301) 415-1153

                     Bill Long, NRR
                     (301) 415-3026


Page Last Reviewed/Updated Friday, May 22, 2015