United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-02, Supplement 1: RELAP5/MOD3 Computer Code Error Associated with the Conservation of Energy Equation

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                              February 18, 1992

                                             ASSOCIATED WITH THE 
                                             CONSERVATION OF ENERGY EQUATION


All holders of operating licenses or construction permits for nuclear power 


The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplemental 
information notice to correct and clarify some of the information provided 
in Information Notice (IN) 92-02.  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 are not NRC requirements; therefore, no 
specific action or written response is required.  

Description of Circumstances

In IN 92-02, of January 3, 1992, the NRC staff described a Northeast 
Utilities (NU) containment response calculation performed using RELAP5/MOD3.  
NU noted that the calculation did not conserve energy in the containment 
volume, and this appeared to be associated with the energy conservation 
equation.  The analysis resulted in a nonconservative assessment of the 
environmental conditions in the Terry turbine room.  Therefore, NU treated 
this as a notification under Part 21 of Title 10 of the 
Code of Federal Regulations (10 CFR Part 21).  In IN 92-02, the staff 
described the problem as a computer code error and further noted that, 
"Appropriate corrections in the code are the responsibility of the Idaho 
National Engineering Laboratory (INEL)."

INEL developed the RELAP5 series of computer codes for NRC use in evaluating 
the response of the nuclear steam supply system (NSSS) to a wide variety of 
postulated transients and accidents.  The NRC designated INEL as the 
organization responsible for maintaining and modifying those versions of 
RELAP5 under NRC control at INEL.  RELAP5 and other NRC-developed software 
are publicly available and have been released to many organizations 
including nuclear utilities, vendors, and service organizations.  The NRC 
and its contractors are not responsible for modifying software released to 
these organizations.  All software release agreements state that the 
recipient organization is solely responsible for implementing the software 
for its purposes.


                                                  IN 92-02, Supplement 1
                                                  February 18, 1992
                                                  Page 2 of 2

The problem identified by NU is not new and has been communicated to RELAP5 
users previously.  Whenever the code is applied to situations in which the 
pressure drops significantly between cells, the energy in the downstream 
volume may be underestimated.  This is clearly the case for analyses of the 
containment's response to breaks in the primary or secondary system.  NSSS 
analyses involve relatively small cell-to-cell pressure drops, therefore the 
energy error is not significant for these cases.  Thus, NU found this 
problem because it applied the code to a situation in which it was not 

The problem is not an error in that no unintentional mistake was made in the 
RELAP5 energy equation formulation.  Rather, assumptions and simplifications 
are always made when equations are converted to difference form and 
numerical solutions are implemented.  Energy equation terms involving 
interconversion of mechanical and thermal energy are particularly difficult 
to implement.  These are the "work" and kinetic energy dissipation terms.  
Generally, simplifying assumptions regarding these terms are not important, 
except in the situations described above.  

Most advanced general purpose thermal hydraulic computer codes of this type 
use simplifying assumptions.  It is, therefore, important that all users and 
developers of these codes review their particular code applications in the 
context of the equation formulations to ensure they take measures to apply 
the codes within their limitations.  The NRC and its contractors will 
continue to inform users of appropriate code limitations for certain 
applications for NRC computer codes.

It is important to reiterate the central message of IN 92-02, which is 
RELAP5 was never intended as a containment analysis code.  In Generic Letter 
83-11, "Licensee Qualification for Performing Safety Analyses in Support of 
Licensing Actions," the staff stressed to licensees the importance of code 
verification and understanding code use and limitations if a code that was 
developed by others is used for computer analyses that support licensing 
actions.  This is particularly true when the application involves a novel 
use of the computer code.

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
the technical contact listed below or the appropriate Office of Nuclear 
Reactor Regulation (NRR) project manager. 

                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical contact:  Ralph Landry, NRR
                    (301) 504-2858

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