United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 79-07: Seismic Stress Analysis of Safety-Related Piping

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

                               April 14, 1979

                                                      IE Bulletin No. 79-07 

SEISMIC STRESS ANALYSIS OF SAFETY-RELATED PIPING 

Description of Circumstances: 

In the course of evaluation of certain piping designs, significant 
discrepancies, were observed between the original piping analysis computer 
code used to analyze earthquake loads and a currently acceptable computer 
code developed for this purpose. This problem resulted in the Nuclear 
Regulatory Commission order to shutdown five power reactors whose design had
involved the use of the suspect computer codes (IE Information Notice No. 
79-06). The difference in predicted piping stresses between the two computer
codes is attributable to the fact that the piping analysis code used for a 
number of piping systems uses an algebraic summation of the loads predicted 
separately by the computer code for both the horizontal,  components and for
the vertical component of seismic events. This is an incorrect treatment of 
such loads and was not recognized as such at the time the original analyses 
were performed. Such codirectional loads should not be algebraically added 
(with predicted loads in the negative direction offsetting predicted loads 
in the positive direction) unless certain more complex time-history analyses
are performed. Rather, to properly account for the effects of earthquakes on
systems important to safety, as required by "Design Bases for Protection 
Against Natural Phenomena, " General Design Criterion 2 of Appendix A to 10 
CFR Part 50, such loads should be combined absolutely or, as is the case, in
the newer codes, using techniques such as the square root of the sum of the 
squares. These combinations of loads conform to current industry practice. 

The inappropriate analytical treatment of load combinations discussed above 
becomes significant for piping runs in which the horizontal seismic 
excitation can have both horizontal and vertical components of response on 
piping systems, and the vertical seismic excitation also has both horizontal
and vertical components of response. It is in these runs that the predicted 
earthquake loads may differ significantly. 
.

IE Bulletin Nol. 79-07                                      April 14, 1979 
                                                            Page 2 of 3  

Although the greatest differences in predicted loads would tend to be 
limited to localized stresses in pipe supports and restraints or in weld 
attachments to pipes, there could be a substantial number of areas of high 
stress in piping, as well as a number of areas in which there is potential 
for damage to adjacent restraints or supports. Any of these situations could 
have significant adverse effects on the ability of the piping system to 
withstand seismic events. 

The NRC staff has not yet determined that all of the piping systems 
important to safety that were designed using a piping analysis computer code 
which contains the algebraic summation error, have been identified. Certain 
information is needed in order to make this determination. 

Action To Be Taken By All Licensees and Permit Holders: 

For all power reactor facilities with an operating license or a construction
permit: 

     (1)  Identify which, if any, of the methods specified below were 
          employed or were used in computer codes for the seismic analysis 
          of safety related piping in your plant and provide a list of 
          safety systems (or portions thereof) affected: 

          Response Spectrum Model Analysis: 

          a.   Algebraic (considering signs) summation of the  codirectional
               spatial components (i.e., algebraic summation of the maximum 
               values of the codirectional responses caused by each of the 
               components of earthquake motion at a particular point in the 
               mathematical model). 

          b.   Algebraic (considering signs) summation of the codirectional 
               inter model responses (i.e., for the number of modes 
               considered, the maximum values of response for each mode 
               summed algebraically). 

          Time History Analysis: 

          a.   Algebraic summation of the codirectional maximum responses or
               the time dependent responses due to each of the components of
               earthquake motion acting simultaneously when the earthquake 
               directional motions are not statistically independent. 
.

IE Bulletin No. 79-07                                       April 14, 1979 
                                                            Page 3 of 3 

     (2)  Provide complete computer program listings for the dynamic 
          response analysis portions for the codes which employed the 
          techniques identified in Item 1, above. 

     (3)  Verify that all piping computer programs were checked against 
          either piping benchmark problems or compared to other piping 
          computer programs. You are requested to identify the benchmark 
          problems and/or the computer programs that were used for such 
          verifications or describe in detail how it was determined that 
          these programs yielded appropriate results (i.e., gave results 
          which corresponded to the correct performance of their intended 
          methodology). 

     (4)  If any of the methods listed in item 1 are identified, submit a 
          plan of action and an estimated schedule for the re-evaluation of 
          the safety related piping, supports, and equipment affected by 
          these analysis techniques. Also provide an estimate of the degree 
          to which the capability of the plant to safely withstand a seismic
          event in the interim is impacted. 

The responses for Items 1, 2 and 3 above, should include all subsequent 
piping system additions and modifications. Any re-evaluation required, in 
conformance with Item 4, should incorporate the "as built" conditions. 

Licensees of all operating power reactor facilities should submit the 
information identified in Items 1 through 4, above, within 10 days of the 
date of this letter. Holders of construction permits for power reactor 
facilities should submit this information within 45 days of the date of this
letter. 

Reports should be submitted to the Director of the appropriate NRC Regional 
Office and a copy should be forwarded to the NRC Office of Inspection and 
Enforcement, Division of Reactor Operations Inspection, Washington, D.C., 
20555. 

Approved by GAO, B180225 (R0072); clearance expires 7-31-80. Approval was 
given under a blanket clearance specifically for identified generic 
problems. 
Page Last Reviewed/Updated Tuesday, July 23, 2013