Theory and implementation of nuclear safety system codes – Part II: System code closure relations, validation, and limitations

• The closure relations in the compared system codes are analyzed.
• The validation of the system codes is compared.
• The major limitations of the system codes are evaluated and clarified.
• The capabilities of the codes are evaluated for new nuclear power plant systems.
• This completes the series providing details of various nuclear system codes.

This is Part II of two articles describing the details of thermal-hydraulic system codes. In this second part of the article series, the system code closure relationships (used to model thermal and mechanical non-equilibrium and the coupling of the phases) for the governing equations are discussed and evaluated. These include several thermal and hydraulic models, such as heat transfer coefficients for various flow regimes, two phase pressure correlations, two phase friction correlations, drag coefficients and interfacial models between the fields. These models are often developed from experimental data. The experiment conditions should be understood to evaluate the efficacy of the closure models.Code verification and validation, including Separate Effects Tests (SETs) and Integral effects tests (IETs) is also assessed. It can be shown from the assessments that the test cases cover a significant section of the system code capabilities, but some of the more advanced reactor designs will push the limits of validation for the codes.Lastly, the limitations of the codes are discussed by considering next generation power plants, such as Small Modular Reactors (SMRs), analyzing not only existing nuclear power plants, but also next generation nuclear power plants. The nuclear industry is developing new, innovative reactor designs, such as Small Modular Reactors (SMRs), High-Temperature Gas-cooled Reactors (HTGRs) and others. Sub-types of these reactor designs utilize pebbles, prismatic graphite moderators, helical steam generators, innovative fuel types, liquid metal coolants, and many other design features that may not be fully analyzed by current system codes.This second part completes the series on the comparison and evaluation of the selected reactor system codes by discussing the closure relations, validation and limitations. These two articles indicate areas where the models can be improved to adequately address issues with new reactor design and development.