United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-38: Thermal Stratification in Feedwater System Piping

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

                                June 13, 1991


Information Notice No. 91-38:  THERMAL STRATIFICATION IN FEEDWATER 
                                   SYSTEM PIPING 


Addressees:

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

Purpose:

This information notice is intended to alert addressees to feedwater system 
(FWS) piping that could be subjected to thermal stratification and cause 
unacceptable pipe movement.  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 December 12, 1990, the Duquesne Light Company (the licensee) discovered 
global thermal stratification over a long stretch of horizontal FWS piping 
inside containment at the Beaver Valley Power Station, Unit 1 (BV-1) during 
followup activities related to NRC Bulletin No. 79-13, "Cracking in 
Feedwater System Piping."  Global thermal stratification results in 
low-cycle fatigue, pipe movement, and stresses that might not have been 
considered in the design of the piping system (as opposed to cyclic thermal 
stratification and thermal striping which result in high-cycle fatigue and 
pipe cracks.)  The licensee detected global thermal stratification at BV-1 
using instrumentation installed to monitor the behavior of the feedwater 
line following unexpected movement of the feedwater piping in November 1989 
(see Figure 1).  This instrumentation detected feedwater temperatures at the 
top and bottom of the feedwater line that varied as much as 200�F.  The 
licensee attributed this thermal stratification to inadequate mixing of 
feedwater along a 90-foot section of horizontal piping inside the 
containment.  Although a vertical section of piping might be expected to 
provide sufficient mixing to prevent stratification, the horizontal section 
at BV-1 is preceded by a 20-foot vertical section that apparently did not 
provide adequate mixing to prevent stratification.  This global thermal 
stratification phenomenon was not previously considered in the design of the 
main feedwater piping system for BV-1.


9106060294
.

                                                            IN 91-38
                                                            June 13, 1991
                                                            Page 2 of 3


Discussion:

BV-1 is a 3-loop pressurized water reactor (PWR) designed by the 
Westinghouse Electric Corporation.  The three-loop design may be 
particularly susceptible to global thermal stratification in the FWS piping 
because it typically includes a long section of horizontal piping just 
inside containment.  However, other plant designs, including boiling water 
reactors (BWRs), may be susceptible to this phenomenon.  For instance, a 
similar FWS event occurred on August 22, 1984, at the Washington Nuclear 
Plant, Unit 2, (WNP-2) and is discussed in NRC Information Notice (IN) 
84-87, "Piping Thermal Deflection Induced by Stratified Flow."  In this 
event, the BWR was operating at about 1 percent power when the introduction 
of cold feedwater into hot feedwater piping, heated by the reactor water 
cleanup system, caused the pipe to deflect and damage the hangers and 
snubbers.  

Thermal stratification of the feedwater line may occur while a plant is 
starting up or cooling down, or when auxiliary feedwater (AFW) is being 
injected into the steam generator through the feedwater line.  While a BWR 
plant is starting up, stratified flow can be introduced when cold feedwater 
is added to hot FWS piping (as was the case at WNP-2).  While a BWR or PWR 
plant is cooling down, especially from full-power operation (Mode 1) to hot 
standby (Mode 3), the significant decrease in the FWS flow rate and 
temperature make the FWS piping susceptible to stratification.  For a PWR, 
AFW injection probably introduces the greatest potential for thermal 
stratification. When cold AFW is injected just after the full flow of hot 
feedwater, the temperature differential across the pipe can become as large 
as 200�F.  

In long, horizontal lengths of piping, 2 (or more) streams of fluid of 
different temperatures can flow in separate layers without appreciable 
mixing, making long sections of pipe susceptible to thermal stratification.  
This piping may be subjected to stresses that were not previously accounted 
for in the piping design.  The BV-1 event also demonstrates that a vertical 
section of pipe just upstream of a long section of horizontal pipe offers 
little (if any) protection from stratification.  

Another concern resulting from the BV-1 event is the adequacy of the design 
of supports for piping that may undergo thermal stratification.  Supports 
located along such piping may restrict pipe movement and contribute to pipe 
deformation or support damage.  A similar concern existed at Trojan during 
an event in 1988 (see NRC Bulletin 88-11, "Pressurizer Surge Line Thermal 
Stratification").  

These concerns are similar to those described in NRC Bulletin 79-13 
(Revision 2, October 16, 1979) and NRC IN 84-87.  However, this NRC 
information notice addresses primarily the effects of thermal stratification 
on feedwater system piping, whereas NRC Bulletin 79-13 addressed primarily 
the effects of thermal stratification on the steam generator nozzles.  IN 
84-87 addresses thermal stratification in the feedwater system at BWRs. 

.

                                                            IN 91-38
                                                            June 13, 1991
                                                            Page 3 of 3


The staff addressed thermal stratification in other systems important to 
safety in NRC Bulletins 88-08; 88-08, Supplement 1; and 88-08, Supplement 3; 
"Thermal Stress in Piping Connected to Reactor Coolant Systems," and in NRC 
INs 88-01, "Safety Injection Pipe Failure," and 88-80, "Unexpected Piping 
Movement Attributed to Thermal Stratification."  

Although thermal stratification is not a new problem, the most recent event 
demonstrates that mechanisms for thermal stratification continue to be 
identified.  This most recent finding indicates that stresses produced by 
stratification in systems with long sections of horizontal piping may result 
in an unanalyzed condition which can affect the integrity of piping and 
supports. 

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:  Angie P. Young, NRR        James E. Beall, RI   
                     (301) 492-1167             (412) 643-2000

                     Andrew J. Kugler, NRR      Shou-Nien Hou, NRR
                     (301) 492-0834             (301) 492-0793


Attachments:
1.  Figure 1.  Beaver Valley Unit 1 Main Feedwater 
      System Piping Configuration Inside Containment
2.  List of Recently Issued NRC Information Notices 
.
Page Last Reviewed/Updated Tuesday, November 12, 2013