Information Notice No. 88-80: Unexpected Piping Movement Attributed to Thermal Stratification
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 7, 1988
Information Notice No. 88-80: UNEXPECTED PIPING MOVEMENT ATTRIBUTED
TO THERMAL STRATIFICATION
Addressees:
All holders of operating licenses or construction permits for pressurized
water nuclear power reactors.
Purpose:
The purpose of this information notice is to alert addressees to unexpected
thermal movement of the pressurizer surge line attributed to thermal strati-
fication. 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:
The licensee for the Trojan plant has observed the results of unexpected move-
ment of the pressurizer surge line at every refueling outage since 1982 when
monitoring of the line movements began. The monitoring program was
implemented after removal of a thermal sleeve that was found to have cracked
attachment welds. During the last refueling outage, the licensee found that,
in addition to unexpected gap closures in the pipe whip restraints, one
restraint was in actual contact with the piping although the shims and gap
sizes had been adjusted on the basis of previous analysis. The recent
investigation indicated that the movement of piping was caused by thermal
stratification in the line. This phenomenon was not considered in the
original piping design.
Discussion:
Unexpected piping movements are highly undesirable because of potential high
piping stress that may exceed design limits on fatigue and stresses. The
problem can be more acute when the piping expansion is restricted, such as
through contact with pipe whip restraints. Plastic deformation could result,
which could lead to functional impairment of the line. The Trojan licensee's
report indicated that thermal stratification might have occurred in the pres-
surizer surge line during heatup, cooldown, and steady-state operation of the
plant.
8810030255
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IN 88-80
October 7, 1988
Page 2 of 2
During a typical plant heatup, water in the pressurizer is heated to about
440xF. A steam bubble is then formed in the pressurizer. Although the exact
phenomenon is not thoroughly understood, as the hot water flows (at a very low
flowrate) from the pressurizer through the surge line to the hot-leg piping,
the hot water rides on a layer of cooler water, causing the upper part of the
pipe to be heated to a higher temperature than the lower part. The
differential temperature could be as high as 300xF, based on limitations on
plant operation. Under this condition, analysis has shown that differential
thermal expansion of the pipe metal causes the pipe to deflect.
In the specific configuration of the pressurizer surge line in the Trojan
plant, the line deflected downward, contacted two pipe whip restraints, and
underwent plastic deformation, which resulted in the permanent deformation of
the pipe.
On the basis of the evaluation of the Trojan event, the staff concludes that
the thermal stratification hypothesis for the pressurizer surge line movement
is supportable by the observations. The licensee for Trojan is required by
the American Society of Mechanical Engineers Boiler and Pressure Vessel Code
to reconcile the pipe stresses and fatigue evaluation if any significant dif-
ferences are observed between the measured data and the analytical results for
the hypothesized conditions. The staff evaluation indicates that the thermal
stratification phenomenon could occur in all PWR surge lines. The staff's
concerns include unexpected bending, and thermal striping as they affect the
overall integrity of the surge line for its design life (e.g., the increase of
fatigue).
No specific action or written response is required by this information notice.
However, the staff may consider further generic communications on the subject.
If you have any questions about this matter, please contact the technical
contacts listed below or the Regional Administrator of the appropriate
regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: S. N. Hou, NRR
(301) 492-0904
N. P. Kadambi, NRR
(301) 492-1153
Attachment: List of Recently Issued NRC Information Notices
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