Information Notice No. 92-02, Supplement 1:RELAP5/MOD3 Computer Code Error Associated with the Conservation of Energy Equation
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 February 18, 1992 NRC INFORMATION NOTICE 92-02, SUPPLEMENT 1: RELAP5/MOD3 COMPUTER CODE ERROR ASSOCIATED WITH THE CONSERVATION OF ENERGY EQUATION Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose 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. 9202110056 . 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 intended. 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 Attachment: List of Recently Issued NRC Information Notices .
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021