Optimization of the Nodalization of Nuclear System Thermal-Hydraulic Code Applied on Primary Loop Benchmark

In best estimate plus uncertainty approach for thermal-hydraulic simulation in nuclear engineering, a crucial step for the qualification of the scenario simulation is the discretization, i.e., the nodalization of nuclear power plants and related integral test facilities (ITFs). Since intermediate break loss-of-coolant accident (IBLOCA) simulation is getting more and more attention in this decade, we focused on the nodalization of an IBLOCA scenario—a primary loop (PKL) I2.2 benchmark delivered by the organization for economic cooperation and development PKL4-project—using the analyses of thermal-hydraulics for leaks and transients (ATHLET) code. This work followed mainly the nodalization methodology of Petruzzi and D'Auria, including both qualitative and quantitative criteria, being divided into three phases for component volume, steady-state, and transient, respectively. The authors used also some specific approaches: (1) for component volume qualification, a volume fractional parameter was introduced, considering not only the relative error of each component but also the volume fraction in the whole system (an 0.2% acceptability level was chosen for this parameter); (2) the experimental data were not used directly as a reference within the nodalization procedure but the calculated results delivered by the most refined nodalization. Based on the estimator of average amplitude in the fast Fourier transform-based method (FFTBM), the convergence, rationality, and an optimized result of nodalization in the simulation of an actual IBLOCA transient benchmark have been judged. After three phases of nodalization qualification, it has been proved that the final nodalization has the necessary degree of convergence for a good reproduction of the benchmark geometry, allowing the proper simulation of involved phenomena. Finally, a middle-refined nodalization was found as being optimal, fulfilling the convergence criteria with a reasonable central processing unit time consumption. The nodalization scheme in this work was not seen as being the single factor influencing the simulated results, but just as a prerequisite to allow further reliable improvements on the models used by ATHLET (aspects not referred to in this particular study). Therefore, the simulated results presented here will match the experimental ones only as general trends; improvements may be further achieved by using new and more precise models (e.g., for critical mass flow, heat transfer, countercurrent flow, etc.) in the system thermal-hydraulic code.

Development of AC2 for the simulation of advanced reactor design of Generation 3/3+ and light water cooled SMRs

The transition from Generation 2 to Generation 3/3+ and 4 reactors, as well as the development of small modular reactors (SMR), place new demands on computational programs designed to simulate conditions of normal operation, operational occurrences, design basis accidents and severe accidents. On the one hand, most passive safety systems of advanced and innovative plants operate at low pressures even down to vacuum conditions and the driving forces are low compared to active systems. On the other hand, the containment is no longer just a barrier to retain radioactive material in the event of leakage of the cooling system, but it is an important link in the passive cooling chain. This requires an expansion and improvement of the existing simulation programs for the cooling circuit and containment, as well as the realization of a coupling between these simulation programs. The new AC2 program package combines the proven simulation codes ATHLET/ATHLET-CD and COCOSYS in one software suite to hit this target. The individual components of the suite are continuously extended and validated for their application to novel safety systems. This makes it possible to simulate the entire spectrum of accidents for Generation 3/3+, 4 and light water cooled SMR systems with just one program package. This publication gives an overview of the current state of development of AC2 and its individual modules.