Information Notice No. 84-87: Piping Thermal Deflection Induced by Stratified Flow

                                                           SSINS No. 6835  
                                                           IN 84-87        

                               UNITED STATES 
                          WASHINGTON, D. C. 20555 

                              December 3, 1984 

                              STRATIFIED FLOW 


All nuclear power reactor facilities holding an operating license (OL) or 
construction permit (CP). 


This notice is provided to inform licensees and applicants of a recent event
that demonstrates a previously unidentified mechanism for piping system and 
pipe support damage. Recipients are expected to review the information for 
applicability to their facilities and consider actions, if appropriate, to 
preclude similar problems occurring at their facilities. 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 August 22, 1984, WNP-2 experienced a thermal transient that damaged a 
portion of the feedwater system. Following an outage of about 5 days, the 
plant began to slowly admit feedwater to the reactor vessel with the reactor
at about 1% power. About 15 minutes after beginning flow, the licensee heard
a dull "thud" in the plant. The licensee found several feedwater pipe 
hangers and snubbers damaged and a flange loosened, allowing a small leak of
feedwater. The licensee reported the event to the NRC Operations Center as a 
"water hammer"; but after consulting with experts and considering other 
circumstances, the licensee determined that the event could be the result of
a thermal deflection induced by stratified flow. In this type of transient, 
the plant's configuration and the slow admission of cold feedwater to a pipe
filled with high temperature water causes stratified flow in the pipe with 
cold water cooling the bottom of the pipe and hot water remaining in the top
of the pipe. The difference in temperature between the top and bottom of the
pipe causes the pipe to bend and may pull hangers out of their supports. 

An unusual design feature of the WNP-2 plant allows the feedwater system to 
be heated by the reactor water cleanup system (RWCUS). The RWCUS return 
lines join two 24-inch feedwater lines upstream of two isolation check 
valves, but downstream of normally open motor-operated valves. In many 
boiling water reactors, the RWCUS enters the feedwater system between the 
inboard and outboard isolation check valves so that reverse flow of the 
RWCUS into the feedwater system is not possible. 


                                                          IN 84-87        
                                                          December 3, 1984 
                                                          Page 2 of 3     

When the RWCUS is operating at WNP-2, but feedwater is not, flow is 
insufficient to open both sets of check valves. Some of the RWCUS return 
flows in the reverse direction back into the feedwater system before 
returning to the vessel. Consequently, during hot standby, the RWCUS flow 
heats a long run of horizontal feedwater piping. When a low rate of 
feedwater flow is initiated, the cold feedwater (about 100 F) flows along 
the bottom of the pipe. The pipe bending phenomenon occurs because of the 
large temperature difference between the top of the pipe (previously heated 
to about 400 F by the RWCUS flow) and the bottom of the pipe cooled by the 
feedwater. Any system configuration and operating conditions where 
stratified flow can cause large temperature differences between the top and 
bottom of a pipe could produce the pipe bending phenomenon. 

Following the event on August 22, 1984, the licensee instrumented the 
feedwater line to detect and record pipe movement and differences in 
temperature between the top and bottom of the pipe. The additional 
instrumentation for detecting and recording pipe movement was installed both
in positions that were suspected of moving such as at a hanger that failed, 
and in positions that were suspected of not moving such as at a hanger that 
was believed to be acting as a fulcrum. Despite procedures designed to 
preclude recurrence of the event, the event occurred again following a scram
from 60% power on September 10, 1984. Because of the instrumentation on the 
feedwater lines the licensee can exclude other explanations for the 
phenomenon, such as water hammer. 


The licensee's investigation of this complex phenomenon is detailed in a 
report titled "Design Engineering Report, WNP-2 Feedwater Thermal Deflection
Events" and will not be repeated here. Copies of the report may be obtained 

          Mr. P. L. Powell, Manager 
          WNP-2 Licensing 
          Washington Public Power Supply System 
          P. O. Box 968 
          3000 George Washington Way 
          Richland, Washington 99352 

Although this report is useful as a description of the event, the NRC has 
not evaluated the report and does not necessarily endorse all of the 
conclusions of the report. 

Other licensees may wish to consider whether events that are apparently 
water hammer are caused by a pipe bending phenomenon similar to that which 
has occurred at WNP-2. Any time a licensee slowly feeds cold water into a 
hot filled pipe there may be the potential for this type of event. 

There are several reasons why licensees may wish to determine if some 
apparent water hammers are really a thermal-gradient-induced pipe bending 

1.   Piping systems can be designed to accommodate this phenomenon without 
     damage, thereby preventing further events. Repositioning, 
     strengthening, or modifying pipe supports or hangers and snubbers may 
     allow for pipe movement. 


                                                        IN 84-87          
                                                        December 3, 1984  
                                                        Page 3 of 3       

2.   The stresses on the pipes subject to thermal pipe bending may or may 
     not be within the design capacity of the pipes. 

3.   Changes to operating procedures may prevent pipe bending caused by 
     stratified flow. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 

                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  Eric Weiss, IE 
                    (301) 492-9005 

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