Information Notice No. 86-110:Anomalous Behavior of Recirculation Loop Flow in Jet Pump BWR Plants
SSINS No.: 6835
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
All jet pump boiling-water reactor (BWR) facilities holding an operating
license or a construction permit.
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
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
December 31, 1986
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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.
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
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
C. Graves, NRR
Attachment: List of Recently Issued IE Information Notices
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