Information Notice No. 86-110: Anomalous Behavior of Recirculation Loop Flow in Jet Pump BWR Plants

                                                            SSINS No.:  6835
                                                            IN 86-110 

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                              December 31, 1986

Information Notice No. 86-110:  ANOMALOUS BEHAVIOR OF RECIRCULATION LOOP 
                                   FLOW IN JET PUMP BWR PLANTS 

Addressees: 

All jet pump boiling-water reactor (BWR) facilities holding an operating 
license or a construction permit. 

Purpose: 

This notice is to alert addressees to the potential for anomalous behavior 
of recirculation loop flow in jet pump BWRs. It is suggested that recipients
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: 

Following an outage to replace recirculation piping in early 1985, the 
Pilgrim nuclear power plant experienced unexpected fluctuations in indicated 
recirculation loop flow. 

In a report to Boston Edison, General Electric attributed this occurrence to
a bistable flow pattern at the header cross in the recirculation loop 
discharge piping. 

Following recirculation pipe replacement in August 1986, the Vermont Yankee 
nuclear power plant notified the resident inspector that the plant was 
experiencing loop flow oscillations. The phenomenon also has been observed 
at foreign reactors, including a BWR-3 following recirculation pipe 
replacement and at a new BWR-6. 

The magnitude and duration of the fluctuations in flow varied not only from 
plant to plant but between loops at the same plant. At Pilgrim, with reactor
gower and flow near rated conditions, the flow in the A loop periodically 
increased about 2 percent for a short time and then decreased again to its 
former value. The observed flow changes in loop A appeared to be random but 
averaged approximately three events per hour with a duration that varied 
between a few seconds and 10 minutes. The flow change in loop B was about 
the same magnitude and fluctuations occurred about three times every 8 
hours, 


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each lasting for 40 to 50 seconds. The 2 percent increase in the A loop flow
resulted in about a 1 percent power increase. For the low probability 
occurrence of simultaneous increase in both A and B loop flow rate, core 
power would increase about 2 percent. 

At Vermont Yankee, the flow in the A loop increased about 1 percent at 
random intervals about four times each hour. No flow anomalies were noted in 
the B loop. At the foreign BWR-3, fluctuations of about 3 percent occurred 
in the B loop. 

Discussion: 

General Electric performed a safety analysis specifically for the Pilgrim 
plant that concluded the phenomenon was not a safety concern at that plant. 
Whether this phenomenon will occur at other plants is not known. Current 
information available to the NRC indicates that the magnitude or duration of 
fluctuations are not predictable with existing analytic methods for a given 
piping system. 

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


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

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

                     C. Graves, NRR 
                     (301) 492-9461 

Attachment:    List of Recently Issued IE Information Notices