Status of Specialist Meeting in Thermal Hydraulics MADRID-2021

The topic of the present document is the summary of activities performed within the Specialists Meeting (SM) in Nuclear Thermal Hydraulics (NTH), SM-TH, organized by W-Gama of OECD/NEA/CSNI and hosted by CIEMAT/CSN in Madrid. The key goal of the SM-TH is to summarize the status in NTH and to formulate recommendations to CSNI. The confirmation of the multi-topics features for NTH, multi-scale and multi-physics analysis and modelling constitutes an outcome from the investigation. Namely, what was in the past the thermal hydraulics of experimentation, modelling and (system) code development, validation and application became part of, during the last two or three decades, a scientific and technology environment. This includes Computational Fluid Dynamics (CFD), Best Estimate Plus Uncertainty (BEPU), code coupling with several different areas, connection with Probabilistic Safety Assessment (PSA), Artificial Intelligence (AI) and Machine Learning (ML). All of this created a sort of virtual layers between NTH and nuclear reactors applications. Any statement about current status and development perspectives should consider this situation. The following items mention selected achievements of CSNI, mostly during the years 1970-2000, and give an idea of its role in the area of thermal hydraulics: A) Balance equations were an issue in the 70’s and progress from early SM brought to current understanding. B) Pioneering activities in the area of Validation started in early 80’s. C) Phenomena, derived from accident scenarios in ITF, were proposed in early 80’s (final SOAR, including ‘ranking’ was published in 1987) and constituted the basis of what became, later on, PIRT. D) Research focusing on accuracy and uncertainty started in early 80’s. E) The practice of ISP, established in the 70’s, constituted today a forum for progressing in the area. References [1] to [75] provide an overview of expertise in nuclear thermal hydraulics including specific pillars of knowledge inside NEA/CSNI. Before entering detailed topics, the authors wish to stress that there is the need to preserve and to update the CSNI knowledge in NTH area, including the organization of devoted training, like THICKET, and to keep alive the NEA database (not a topic for the present SM-TH). A complex picture resulted for nuclear thermal hydraulics from the Madrid2021 SM-TH. This is at the basis of conclusions and recommendations. General conclusions are: 1) Compensating, hidden errors stand over the on-going evolution activities in NTH, particularly involving constitutive laws, e.g. Transport of Interfacial Area, multi-field and phenomena like reflood and two-phase critical flow; those errors make difficult the advancement in knowledge or contribute to the nearly flat increase in code predictive capabilities, i.e. a motivation for the present Specialists Meeting. 2) CFD in nuclear applications is definitely a branch of NTH: however, related developments are somewhat independent from the main stream of activities e.g. validation and improvement of system thermal hydraulic codes. The 3D feature of system thermal hydraulic codes may constitute a bridge and an occasion to join research efforts in two-phase CFD and system codes. 3) Experiments, whatever ‘basic’, ‘separate’ and ‘integral‘ effects are necessary to keep the current competences and to contribute to safety evaluations of new reactors. Phenomena identification and characterization constitute a remarkable outcome and a way to achieve a common understanding. 4) Procedures within the BEPU approach stand in-between NTH and reactors applications: although those procedures were not key targets for the present specialists meeting, progress in the area is detected. This is specifically true for uncertainty and coupling of NTH, namely with 3D neutron physics: we regretfully observe the lack of or a marginal use of those procedures in the licensing process. Limited to the areas of uncertainty and scaling, the on-going attempts to characterize the ranges of variation of input uncertain parameters may fall into dead ends, and training is necessary, respectively. 5) Passive systems appear to form a fashion subject for industry and regulators: suitable methods, derived from NTH researches, are available for estimating the reliability of thermal hydraulic OECD/NEA/CSNI Specialists Meeting on Transient Thermal-hydraulics in Water Cooled Nuclear Reactors (SM-TH) – Dec. 13-17, 2021 - CIEMAT, Madrid (Spain) Final Summary Document (Draft02, Oct. 31, 2021) 9 performance of those systems. Again, regretfully, the application of those methods is not a primary concern in licensing processes for concerned nuclear reactors. 6) Prioritization of NTH research and proposition for precision targets of code calculations deserves attention in the near future. 7) Artificial Intelligence and connected techniques constitute research ways that need further investigations. 8) Innovative areas for moving the boundaries of knowledge in NTH are identified although difficulties to pursue related developments are apparent. Recommendations to CSNI involve consideration of the following broad research and development directions: A) Preserving the competences, expertise and database derived from research investments that are un-repeatable specifically in the experimental area. In this connection, on-going developments constituted the topic of discussions together with identification of limitations, namely in relation to reactor applications. B) Keeping alive and possibly strengthening the current CFD activities within the NTH area. C) Establishing the bases for an innovative NTH, including the attempt to overcome of above mentioned key limitations. D) Address other fluids than water and advanced reactors, i.e. not within the scope of the present SM. Other TH fields might benefit from the WCNR experience. The implementation of recommendations, mostly connected with items A) to C), needs establishing or maintaining Task Groups (TG) with proper combinations of senior and young experts. We call those task groups TG-1, TG-2 and TG-3; we roughly propose below activities and responsibilities to TG members or sub-groups of TG members. TG-1, or TG-CFD, is the existing CFD4NRS group without any key modification. The track of activities includes Direct Numerical Simulation (DNS) and the Two-Phase CFD. We envisage tight connection with newly formed TG. TG-2, or TG-SYSTH, to recover/maintain/improve current NTH technology: e.g. phenomena, system codes and procedures. It is expected that the TG members deal with the database of experiments and the connections between NTH and applications. TG-3, or TG-INNOVATION, to establish the bases for future NTH technology where key interests shall be modelling of turbulence, averaging process, possible expansion of current balance equation (angular momentum and not only), without commitment to deliverables following (at least) five working years. Within the above context, i.e. all TG, an activity to agree about parameters for establishing quantitative accuracy, i.e. predictive capabilities, for models and codes (e.g. adopting the Fast Fourier Transform Based Method [FFTBM] fully applied for International Standard Problem [ISP] 50 – ATLAS facility) is recommended. This might allow, among the other things: a) establishing a value for the prediction capabilities as a function of current year starting from 1970; b) a continuous check of eventual improvements by new models. Furthermore, we recommend pursuing common efforts to establish methods and procedures for research prioritization and to fix quantitative precision targets (frequently updated) for modelling. OECD/NEA/CSNI Specialists Meeting on Transient Thermal-hydraulics in Water Cooled Nuclear Reactors (SM-TH) – Dec. 13-17, 2021 - CIEMAT, Madrid (Spain) Final Summary Document (Draft02, Oct. 31, 2021) 10 1. INTRODUCTION Nuclear Thermal Hydraulics (NTH) constitutes one specialty of Nuclear Engineering (NE). This shall be associated with systems, components and procedures that are of interest for the deployment of nuclear reactors and, owing to their safety implications, of regulators’ concerns. NE constitutes by itself a broad field for R&D. Industrial systems include, for instance, mines to extract uranium, fission nuclear power plants, fusion reactors (demonstration systems), fuel enrichment, incineration of radioactive products and waste. Then, NE embeds almost all disciplines’ topics that have industrial relevance: chemistry, physics, mathematics, electronics, electricity, fluid dynamics, materials, civil, mechanics and even space applications. Engineers and researchers are necessary to design or to improve a generic ‘nuclear’ facility or system: they should have knowledge within any of the listed disciplines. When characterizing indispensable nuclear technology areas or disciplines, which need specific development to make possible the exploitation of fission energy, one may converge on the following shortlist (not in the order of importance): • Nuclear thermal hydraulics, • Radioprotection, • Neutron physics, • Structural mechanics. No progress could have been possible in NE area without appropriate development of quality assurance and reliability methods: these are cross cutting technological disciplines. A similar role belongs to materials technology; this deals with materials, which are (mostly) part of the core of nuclear reactors and need to withstand burn-up and radiation damage. NTH, or the topic for the present document shall not be isolated from the above context: so, advancement and applicability depend upon advancements in other technological areas. Still, the fate of NTH is associate