Bulletin 88-11: Pressurizer Surge Line Thermal Stratification
OMB No. 3150-0011
NRCB 88-11
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 20, 1988
NRC BULLETIN NO. 88-11: PRESSURIZER SURGE LINE THERMAL STRATIFICATION
Addressees:
All holders of operating licenses or construction permits for pressurized
water reactors (PWRs).
Purpose:
The purpose of this bulletin is to (1) request that addressees establish and
implement a program to confirm pressurizer surge line integrity in view of the
occurrence of thermal stratification and (2) require addressees to inform the
staff of the actions taken to resolve this issue.
Description of Circumstances:
The licensee for the Trojan plant has observed unexpected movement of the
pressurizer surge line during inspections performed at each refueling outage
since 1982, when monitoring of the line movements began. During the last
refueling outage, the licensee found that in addition to unexpected gap clo-
sures in the pipe whip restraints, the piping actually contacted two re-
straints. Although the licensee had repeatedly adjusted shims and gap sizes
based on analysis of various postulated conditions, the problem had not been
resolved. The most recent investigation by the licensee confirmed that the
movement of piping was caused by thermal stratification in the line. This
phenomenon was not considered in the original piping design. On October 7,
1988, the staff issued Information Notice 88-80, "Unexpected Piping Movement
Attributed to Thermal Stratification," regarding the Trojan experience and
indicated that further generic communication may be forthcoming. The licensee
for Beaver Valley 2 has also noticed unusual snubber movement and
significantly larger-than-expected surge line displacement during power
ascension.
The concerns raised by the above observations are similar to those described
in NRC Bulletins 79-13 (Revision 2, dated October 16, 1979), "Cracking in
Feedwater System Piping" and 88-08 (dated June 22, 1988), "Thermal Stresses in
Piping Connected to Reactor Coolant Systems."
8812150118
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December 20, 1988
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Discussion:
Unexpected piping movements are highly undesirable because of potential high
piping stress that may exceed design limits for 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 can result,
which can lead to high local stresses, low cycle fatigue and functional im-
pairment of the line. Analysis performed by the Trojan licensee indicated
that thermal stratification occurs in the pressurizer surge line during
heatup, cooldown, and steady-state operations of the plant.
During a typical plant heatup, water in the pressurizer is heated to about
440øF; 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 (see Figure 1).
The differential temperature could be as high as 300øF, based on expected
conditions during typical plant operations. Under this condition,
differential thermal expansion of the pipe metal can cause the pipe to deflect
significantly.
For the specific configuration of the pressurizer surge line in the Trojan
plant, the line deflected downward and when the surge line contacted two pipe
whip restraints, it underwent plastic deformation, resulting in permanent
deformation of the pipe.
The Trojan event demonstrates that thermal stratification in the pressurizer
surge line causes unexpected piping movement and potential plastic
deformation. The licensing basis according to 10 CFR 50.55a for all PWRs
requires that the licensee meet the American Society of Mechanical Engineers
Boiler and Pressure Vessel Code Sections III and XI and to reconcile the pipe
stresses and fatigue evaluation when any significant differences are observed
between measured data and the analytical results for the hypothesized
conditions. Staff evaluation indicates that the thermal stratification
phenomenon could occur in all PWR surge lines and may invalidate the analyses
supporting the integrity of the surge line. The staff's concerns include
unexpected bending and thermal striping (rapid oscillation of the thermal
boundary interface along the piping inside surface) as they affect the overall
integrity of the surge line for its design life (e.g., the increase of
fatigue).
Actions Requested:
Addressees are requested to take the following actions:
1. For all licensees of operating PWRs:
a. Licensees are requested to conduct a visual inspection (ASME,
Section XI, VT-3) of the pressurizer surge line at the first
available cold shutdown after receipt of this bulletin which exceeds
seven days.
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December 20, 1988
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This inspection should determine any gross discernable distress or
structural damage in the entire pressurizer surge line, including
piping, pipe supports, pipe whip restraints, and anchor bolts.
b. Within four months of receipt of this Bulletin, licensees of plants
in operation over 10 years (i.e., low power license prior to
January 1, 1979) are requested to demonstrate that the pressurizer
surge line meets the applicable design codes* and other FSAR and
regulatory commitments for the licensed life of the plant, consider-
ing the phenomenon of thermal stratification and thermal striping in
the fatigue and stress evaluations. This may be accomplished by
performing a plant specific or generic bounding analysis. If the
latter option is selected, licensees should demonstrate
applicability of the referenced generic bounding analysis.
Licensees of plants in operation less than ten years (i.e., low
power license after January 1, 1979), should complete the foregoing
analysis within one year of receipt of this bulletin. Since any
piping distress observed by addressees in performing action 1.a may
affect the analysis, the licensee should verify that the bounding
analysis remains valid. If the opportunity to perform the visual
inspection in 1.a does not occur within the periods specified in
this requested item, incorporation of the results of the visual
inspection into the analysis should be performed in a supplemental
analysis as appropriate.
Where the analysis shows that the surge line does not meet the
requirements and licensing commitments stated above for the duration
of the license, the licensee should submit a justification for
continued operation or bring the plant to cold shutdown, as
appropriate, and implement Items 1.c and 1.d below to develop a
detailed analysis of the surge line.
c. If the analysis in 1.b does not show compliance with the
requirements and licensing commitments stated therein for the
duration of the operating license, the licensee is requested to
obtain plant specific data on thermal stratification, thermal
striping, and line deflections. The licensee may choose, for
example, either to install instruments on the surge line to detect
temperature distribution and thermal movements or to obtain data
through collective efforts, such as from other plants with a similar
surge line design. If the latter option is selected, the licensee
should demonstrate similarity in geometry and operation.
d. Based on the applicable plant specific or referenced data, licensees
are requested to update their stress and fatigue analyses to ensure
compliance with applicable Code requirements, incorporating any
observations from 1.a above. The analysis should be completed no
later than two years after receipt of this bulletin. If a licensee
*Fatigue analysis should be performed in accordance with the latest ASME
Section III requirements incorporating high cycle fatigue.
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December 20, 1988
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is unable to show compliance with the applicable design codes and
other FSAR and regulatory commitments, the licensee is requested to
submit a justification for continued operation and a description of
the proposed corrective actions for effecting long term resolution.
2. For all applicants for PWR Operating Licenses:
a. Before issuance of the low power license, applicants are requested
to demonstrate that the pressurizer surge line meets the applicable
design codes and other FSAR and regulatory commitments for the
licensed life of the plant. This may be accomplished by performing
a plant-specific or generic bounding analysis. The analysis should
include consideration of thermal stratification and thermal striping
to ensure that fatigue and stresses are in compliance with
applicable code limits. The analysis and hot functional testing
should verify that piping thermal deflections result in no adverse
consequences, such as contacting the pipe whip restraints. If
analysis or test results show Code noncompliance, conduct of all
actions specified below is requested.
b. Applicants are requested to evaluate operational alternatives or
piping modifications needed to reduce fatigue and stresses to
acceptable levels.
c. Applicants are requested to either monitor the surge line for the
effects of thermal stratification, beginning with hot functional
testing, or obtain data through collective efforts to assess the
extent of thermal stratification, thermal striping and piping
deflections.
d. Applicants are requested to update stress and fatigue analyses, as
necessary, to ensure Code compliance.* The analyses should be
completed no later than one year after issuance of the low power
license.
3. Addressees are requested to generate records to document the development
and implementation of the program requested by Items 1 or 2, as well as
any subsequent corrective actions, and maintain these records in accor-
dance with 10 CFR Part 50, Appendix B and plant procedures.
Reporting Requirements:
1. Addressees shall report to the NRC any discernable distress and damage
observed in Action 1.a along with corrective actions taken or plans and
schedules for repair before restart of the unit.
*If compliance with the applicable codes is not demonstrated for the full
duration of an operating license, the staff may impose a license condition
such that normal operation is restricted to the duration that compliance is
actually demonstrated.
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December 20, 1988
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2. Addressees who cannot meet the schedule described in Items 1 or 2 of
Actions Requested are required to submit to the NRC within 60 days of
receipt of this bulletin an alternative schedule with justification for
the requested schedule.
3. Addressees shall submit a letter within 30 days after the completion of
these actions which notifies the NRC that the actions requested in Items
1b, ld or 2 of Actions Requested have been performed and that the results
are available for inspection. The letter shall include the justification
for continued operation, if appropriate, a description of the analytical
approaches used, and a summary of the results.
Although not requested by this bulletin, addressees are encouraged to work
collectively to address the technical concerns associated with this issue, as
well as to share pressurizer surge line data and operational experience. In
addition, addressees are encouraged to review piping in other systems which
may experience thermal stratification and thermal striping, especially in
light of the previously mentioned Bulletins 79-13 and 88-08. The NRC staff
intends to review operational experience giving appropriate recognition to
this phenomenon, so as to determine if further generic communications are in
order.
The letters required above shall be addressed to the U.S. Nuclear Regulatory
Commission, ATTN: Document Control Desk, Washington, D.C. 20555, under oath
or affirmation under the provisions of Section 182a, Atomic Energy Act of
1954, as amended. In addition, a copy shall be submitted to the appropriate
Regional Administrator.
This request is covered by Office of Management and Budget Clearance Number
3150-0011 which expires December 31, 1989. The estimated average burden hours
is approximately 3000 person-hours per licensee response, including assessment
of the new requirements, searching data sources, gathering and analyzing the
data, and preparing the required reports. These estimated average burden
hours pertain only to these identified response-related matters and do not
include the time for actual implementation of physical changes, such as test
equipment installation or component modification. The estimated average
radiation exposure is approximately 3.5 person-rems per licensee response.
Comments on the accuracy of this estimate and suggestions to reduce the burden
may be directed to the Office of Management and Budget, Room 3208, New Execu-
tive Office Building, Washington, D.C. 20503, and to the U.S. Nuclear Regula-
tory Commission, Records and Reports Management Branch, Office of
Administration and Resource Management, Washington, D.C. 20555.
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December 20, 1988
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If you have any questions about this matter, please contact one of the techni-
cal 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
S. S. Lee, NRR
(301) 492-0943
N. P. Kadambi, NRR
(301) 492-1153
Attachments:
1. Figure 1
2. List of Recently Issued NRC Bulletins
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