Bulletin 88-11: Pressurizer Surge Line Thermal Stratification

                                                            OMB No. 3150-0011 
                                                            NRCB 88-11 

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                December 20, 1988



All holders of operating licenses or construction permits for pressurized 
water reactors (PWRs). 


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 

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."  

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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 

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 

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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          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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          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 

     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 

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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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 

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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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

1.  Figure 1
2.  List of Recently Issued NRC Bulletins

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