The future of nuclear thermal-hydraulics

Nuclear Engineering and Design 354 (2019) 110248

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Preface

The future of nuclear thermal-hydraulics

T

ABSTRACT

An attempt is made to identify perspectives in nuclear thermal-hydraulics based on devoted papers published during the last seven years. An understandable target is fixed which also appears consistent with the framework of this Special Issue (SI). Nevertheless, the target revealed ambitious and having multiple solutions: one might expect this situation considering the universe of topics which characterize nuclear thermal-hydraulics (e.g. according to a recently published book, see paper PI-2 of this SI). The key (obvious) finding is that developments follow different directions independent among each other. In order to arrive at a reasonable achievement, the Computational Fluid Dynamics (CFD) and the system thermal-hydraulics are distinguished first; then it is recognized that (a) developments in procedures, modeling and experiments are typically pursued by groups of scientists having little or no connections, and (b), different science and technology areas are connected with nuclear thermal-hydraulics, e.g. including, uncertainty, licensing, structural mechanics, numerics. Finally, technology sectors of Boiling Water Reactors (BWR), Pressurized Water Reactors (PWR) and non-water cooled reactors, advanced and Gen IV reactors typically pursue different directions in developments within nuclear thermal-hydraulics. Having this in mind, a few dozen topics or elements for reflections are selected and classified which hopefully are informative for understanding the current status and trends in nuclear thermal-hydraulics.

1. Introduction The paper has been thought and written to honor B. R. Sehgal, G. Yadigaroglu and G. Hewitt (the honorable scientists) who recently passed away during the short time frame of less than a year. The present one constitutes PI-4 within the introductory group of papers PI-2, PI-3 and PI-4 aiming at synthesizing a reference view point in relation to < what is nuclear thermal-hydraulics > , < where do we are now > and < what do we expect for the near future > , respectively. Noticeably, the first (PI-2) and the second topic (PI-3) are based upon the list of content of seven recently published books in the area and the content of the present Special Issue (SI). Ambitious and partly unrealistic tasks are concerned in all cases

(each of the three papers): in the case of PI-3 the small number of top level scientists who authored papers in the SI compared with a two orders magnitude larger number of researchers and technologists in the area should be noted. However, whatever the picture for Nuclear Thermal-Hydraulics (NTH) we may propose, it would have been severely evaluated by Bal Raj, George and Geoff; hopefully, they would indeed like the idea that we tried this in their memory. The history of NTH is neither the objective nor the interest for the present paper; see e.g. D’Auria (Ed.), 2017, for details. Nevertheless, a four statement history-summary, as given below, seems essential for any derived picture for the future: a) Pioneering NTH started at the time when water cooled reactors were

Abbreviations: ABWR, Advanced BWR; ANS, American Nuclear Society; AS, Accident Scenario; ASME, American Society of Mechanical Engineers; ATWS, Anticipated Transient Without Scram; BEMUSE, OECD/NEA/CSNI international project about Uncertainty; BEPU, Best Estimate Plus Uncertainty; BWR, Boiling Water Reactor; CASL, USDOE financed project in the area of NTH; CCFL, Counter Current Flow Limitation; CHF, Critical Heat Flux; CL, Col Leg; CSNI, Committee on the Safety of Nuclear Installations; CFD, Computational Fluid Dynamics; CFR, Code of Federal Regulation; Cont., Containment; DC, Downcomer; DNB, Departure from Nucleate Boiling; DNS, Direct Numerical Simulation; DWO, Density Wave Oscillations; ECC, Emergency Core Cooling; ECCS, ECC System; EOP, Emergency Operating Procedure; EPR, European Pressurized Reactor; Eqs., Equations; FA, Fuel Assembly; FAD, Fuel Assembly Distortion; Gen, Generation; GP, Generalized Parameter; HL, Hot Leg; HT, Heat Transfer; HTC, Heat Transfer Coefficient; IAEA, International Atomic Energy Agency; ICAPP, Series of ANS International Conferences; ICONE, Series of ASME International Conferences; IPA, Integral Parameters; LES, Large Eddy Simulation; LOCA, Loss of Coolant Accident; MSLB, Main Seam Line Break; Multi-D, Multi-Dimensional; NDP, Non Dimensional Parameter; NEA, Nuclear Energy Agency; NPP, Nuclear Power Plant; NRS, Nuclear Reactor Safety; NTH, Nuclear Thermal-Hydraulics; NURESAFE, EU supported Research Project in NTH; NURETH, Series of ANS International Conferences in NTH; NUTHOS, Series of ANS International Conferences in NTH; OECD, Organization for Economic Cooperation and Development; Ph.W, Phenomenological Window; PS, Primary System; QF, Quench Front; PCI, Pellet Clad Interaction; PCT, Peak Clad Temperature; PIRT, Phenomena Identification and Ranking Table; PSA, Probabilistic Safety Analysis; PTS, Pressurized Thermal Shock; PWR, Pressurized Water Reactor; RCS, Reactor Coolant System; RIA, Reactivity Initiated Accident; RPV, Reactor Pressure Vessel; RTA, Relevant Thermal-Hydraulic Aspect; SA, Severe Accident; SAMG, Severe Accident Management Guidelines; SBO, Station Blackout; SG, Steam Generator; SGIP, SG Inlet Plenum; SI, Special Issue; SMR, Small and Modular Reactor; SRV, Steam Relief Valve; SS, Secondary Side; Strat., Stratified; SVP, Single Valued Parameter; SYS, System; TH, Thermal-Hydraulics; TH-P, Thermal-Hydraulic Phenomena; TIA, Transport of Interfacial Area; TPCF, Two Phase Critical Flow; Turb., Turbulence; TSE, (parameter belonging to) Time Sequence of Events; U, Uncertainty; UP, Upper Plenum; USAEC, US Atomic Energy Commission; USDOE, US Department of Energy; V&V, Verification and Validation; V&V&C, V&V plus Consistency; WCNR, Water Cooled Nuclear Reactors; 3D, Three-Dimensional; 3D NK, 3D Neutron Physics https://doi.org/10.1016/j.nucengdes.2019.110248

Available online 14 August 2019 0029-5493/ © 2019 Published by Elsevier B.V.

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Preface

designed (e.g. Admiral Rickover endeavor) in the 50s or even the end of 40s in the previous century. Fundamental experiments and models were performed or developed, respectively. b) 1971 is the break through time: the Regulatory Commission (USAEC at the time) issue the Interim Acceptance Criteria for ECCS and promoted deep and large scale experimental and theoretical research: at the same time powerful computers became available and the era of system thermal-hydraulic codes (SYS TH) started. The results of huge financial investments allowed shaping NTH as it is now. Scaling, Validation, Data Base of Integral Experiments, Uncertainty and BEPU were the outcome. c) At the end of 90s continuous drop of investments (partly connected with the stagnation for nuclear technology in many Countries) and exponential growth of computer power oriented the research in NTH: Computational Fluid Dynamics (CFD) and code coupling (e.g. three-dimensional neutron physics and thermal-hydraulics) became popular. d) Around 2010, starting before and continuing nowadays, progress mainly occurred in the area of characterization of basic mechanisms in fluid flow and heat transfer, thus opening the way for a predictive-powerful NTH.

Parameters (NDP), as discussed by D’Auria et al. (1995). Then, General Parameters (GP, as proposed by D’Auria (Ed.) (2017)), right-central in the sketch, and basic models (and modeling) can be characterized. Next, V&V, scaling, uncertainty (evaluation), coupling (with different disciplines) constitute procedures, left-central in the sketch, which are needed to determine the applicability of models, and phenomena. The bottom end (bottom row in Fig. 1) is that all listed sectors are needed to progress in NTH, as already mentioned. Interconnections among identified sectors are more complex (and in larger number) than what shown in the sketch of Fig. 1. A as far as possible thorough consideration of the papers led to the definition of ‘elements for NTH development’ which include the discussed categories; furthermore, in relation to each paper:

• A statement at the beginning of each of Sections 2.1, 2.2, 2.3 and • •

Various ways can be pursued to answer the question < what do we expect for the near future > : consideration of the content of conferences in the area (noticeably NURETH, NUTHOS, ICONE, ICAPP), of topics of ongoing or recently completed research programs (CASL in the US, NURESAFE in EU, IAEA and OECD/NEA working groups activities) or of journals publishing in NTH constitute suitable and straightforward ways: huge efforts would be needed including the constitution of groups of scientists expected to work several months. This was not possible here; however, the benefit was taken from the publication of four perspective papers in the area of NTH during the last seven years. The selected papers reported in the order of their publication time are Aksan et al., 2018, Bestion, 2017, Saha et al., 2013, D’Auria, 2012. The objective of the paper is to present both a roadmap to identify trends in NTH and a necessarily not comprehensive list of ‘way-outs’ for current researches. Consideration has been given to the definition in paper PI-2: NTH implies a ’universe of knowledge’, D’Auria (Ed.), 2017, and is part of (general) thermal-hydraulics; additional attributes have been introduced like ‘system’, ‘basic’, ‘experiment’, ‘modeling’, ‘coupling’ (with other disciplines, as needed in nuclear technology application) and ‘procedures’. In this connection ‘basic’ is necessary to understand ‘system’ and ‘system’ knowledge is needed to evaluate the importance of ‘basic’; furthermore progress in ‘procedures’, as well as in any direction, is needed to progress in NTH.

•

2.4, respectively, provides a synthesis statement for the framework of the paper content; Sentences and statements are copied from each paper and reported in italics in the Sections 2.1–2.4: these constitute the ‘elements for NTH development’; Each sentence that implies an ‘Element for NTH development’ is classified as [XY] (z) where X can be 1 to N (depending upon the number of elements in the paper), Y can be ‘A’, ‘B’, ‘S’ or ‘D’ depending upon which paper is concerned (Aksan et al., 2018, Bestion, 2017, Saha et al., 2013, D’Auria, 2012, respectively) and ‘z’ can be (a), (b), (c) …, depending on sub-elements part of one element (if any). All the elements are gathered with homogenization in wording in chapter 3. (Table 1) and cross-linked among each other.

2.1. The paper by Aksan et al. (2018) The paper by Aksan et al. (2018), has not as an objective the derivation of priorities for future research in NTH; rather the objective is to gather NTH phenomena identified by international groups at IAEA and at OECD/NEA/CSNI as derived from researches performed basically to address item (b) in the Introduction. A list of 116 phenomena is developed, based upon a dozen internationally agreed reports. As part of the effort to prepare the list, the research priorities identified in 1997 making reference to a list of 67 phenomena have been considered, i.e. as defined by Aksan et al. (1997), and reported here. No surprise that almost all phenomena are considered by at least one of the remaining papers. The listing of internationally agreed 1997 NTH priorities is useful to evaluate progress in the last two decades: this also allows the connection of the status depicted under item (c) and the current status in NTH, i.e. jumping over important progresses made within the CFD area (basically, item (d) in Introduction). The 116 phenomena identified by Aksan et al. (2018), are individually considered by D’Auria (Ed.) (2017): their uses for future researches, or the areas in NTH where those phenomena may find an application, are presented in Fig. 2. The list of phenomena which were considered for future investigation in 1997 is as follows (comments are included in the list, if applicable):

2. An overview from four selected papers The overview of the four selected papers (Aksan et al., 2018, Bestion, 2017, Saha et al., 2013, D’Auria, 2012) showed that progress in several directions (or sectors) is on-going and is necessary to advance in NTH. The first issue to be addressed is to identify those sectors. To this aim the sketch in Fig. 1 is developed: starting from (nuclear) reactor design, construction, operation, safety and licensing (top in Fig. 1), experiments, legal environment (10 CFR 50.46 is selected as a reference) and reactor operational experience constitute the second row in Fig. 1. Then, procedures and computational tools (here computational tools cannot be utilized without suitable procedures) make the connection with the central elements in the diagram. Then water cooled or moderated reactors (WCNR) are at the origin of Accident Scenarios (AS) derived with the support of Probabilistic Safety Assessment (PSA). One may go from AS to ‘phenomena’ (possibly) passing through Phenomenological Windows (Ph.W.). Then, phenomena may be characterized by Relevant Thermal-Hydraulics Aspects (RTA), parameters belonging to the Time Sequence of Events (TSE), Single Value Parameters (SVP), Integrap Parameters (IPA) and Non-Dimensional

[1A] Boron mixing and transport. Progress has been made during the last two decades. [2A] CCFL in HL and CL. Progress has been made during the last two decades. [3A] Global multi-D fluid temperature, void and flow distribution-Core. Focus in the paper by Bestion (2017), Section 2.2. [4A] Global multi-D fluid temperature, void and flow distributionDowncomer. Focus in the paper by Bestion (2017), Section 2.2. [5A] Global multi-D fluid temperature, void and flow distribution-SG SS. Focus in the paper by Bestion (2017), Section 2.2. [6A] Global multi-D fluid temperature, void and flow distribution-UP. 2

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Fig. 1. Defining and cross linking key sectors of nuclear thermal-hydraulics.

different transient situations (noticeably bubble formation and droplets behavior including entrainment): the importance of three fields modeling and Transport of Interfacial Area (TIA) within a 3D numerical and physical framework is pointed out. The next topic for the paper is the uncertainty methods and, not specifically discussed, how the uncertainty evaluation may affect the identification of future researches in nuclear thermal-hydraulics. Needs in nuclear thermal-hydraulics or issues at the basis of needs are synthesized hereafter.

Focus in the paper by Bestion (2017), Section 2.2. [7A] Gravity driven reflood. [8A] Non condensable gas effect including impact on condensation in PS. [9A] Parallel channel effect and instability in BWR. Focus in the paper by Saha et al. (2013). [10A] Phase separation at branches when TPCF is concerned. [11A] Pressure drop at geometric discontinuities. Focus in the paper by D’Auria (2012). [12A] Pressure wave propagation. [13A] QF propagation in fuel rods. [14A] QF propagation in passive structures (fuel boxes in BWR and guide tubes in FA for PWR and BWR). [15A] Reflood (generic). Needs for further research in the area confirmed by the recently completed BEMUSE project, e.g. Glaeser et al., 2011. [15A] Steam line dynamics (including acoustic wave propagation). [16A] TPCF breaks. More details in the paper by D’Auria (2012). [17A] TPCF pipes. More details in the paper by D’Auria (2012). [18A] TPCF valves. More details in the paper by D’Auria (2012).

[1B] 1-Phase CFD (mostly dealing with ‘turbulent mixing’) issues to be addressed are: (a) erosion, corrosion and deposition; (b) boron dilution; (c) mixing: stratification/hot-leg heterogeneities; (d) heterogeneous flow distribution (e.g. in SG inlet plenum causing vibrations, etc.); (e) BWR/ABWR lower plenum flow; (f) Pressurized Thermal Shock; (g) Induced break (by loss of ductility); (h) thermal fatigue; (i) hydrogen distribution; (j) RPV mixing following MSLB; (k) influence in CHF of single-phase mixing in sub-channels. [2B] 2-Phase CFD issues dealing with bubbly flow, needed for: (a) the modelling of turbulence effects on momentum and heat transfers and on bubble dispersion; (b) the use of transport equation(s) to characterize the bubble size or bubble size distribution; (c) the modelling of forces acting on the bubbles such as drag and virtual mass forces, lift and turbulent diffusion forces; (d) Interfacial transfers of heat, mass and momentum; (e) boundary conditions which require wall functions for momentum and energy equations including bubble generation at the wall in case of boiling flows. [3B] 2-phase CFD issues dealing with PTS: three (sub-) categories of “flow processes” (I to III) are distinguished with “phenomena”, from (a) to (n), identified in each flow process. (I) The ECCS jet area: (a) instabilities of the jet from ECC injection; (b) condensation on the jet itself before mixing; (c) entrainment and migration of steam bubbles below the water level; (d) turbulence production below the jet. (II) The stratified flow in cold leg: (e) interfacial transfer of momentum at free surface; (f) interfacial transfer of heat & mass at free surface; (g) turbulence production in wall shear & in interfacial shear layers;

2.2. The paper by Bestion (2017) The framework and the focus for the paper by Bestion (2017), is the period (d) identified in the Introduction. The author leaded large international projects (EU funded) aimed at advancing in the area of CFD and 2-phase flow modeling. A broad picture of system thermal-hydraulics is provided by in the paper. An understandable concept is at the basis of the paper: “Experiments cannot reproduce at a reasonable cost the physical situation without any simplification or distortion and the numerical tools cannot simulate the problem by solving the exact equations.” Following a review of PIRT and scaling methods, examples of scaling distortions which unavoidably characterize experiments are given. Then the wording “distortion of physical processes in a system code” is introduced (possibly to create a parallel with experiment distortion): examples of modeling deficiencies are given making reference to

Fig. 2. The applicability and the exploitation of NTH phenomena for future developments. 3

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(h) heat transfers with cold leg and RPV walls; (i) effects of turbulent diffusion upon condensation; (j) interactions between interfacial waves, interfacial turbulence production and condensation; (k) effects of temperature stratification upon turbulent diffusion; (l) influence of non-condensable gases on condensation. (III) The mixing in the downcomer: (m) flow separation or not in downcomer at cold leg nozzle; (n) heat transfers with the walls. “The phenomena (d), (f), (j) and (m) are ranked as dominant phenomena”. [4B] 2-phase CFD issues for all flow regimes. The statement “(a) It was shown that a 4-field model has much better capabilities than a two-fluid approach to identify most complex regimes” may be taken as a suggestion for future improvements. Then, “(b) the choice of a space averaging seems more appropriate than the time averaging if a good accuracy is expected or if time fluctuations in intermittent flow have to be predicted”; (c) an important effort is required to model all interactions between sub-filter phenomena and the transfers from the sub-filter domain to the simulated domain, and (d) “a combination of field averaging for dispersed droplets and small bubbles with an interface tracking technique for large interface is being developed …” [5B] Pseudo-DNS or LES with simulated interfaces can also be used for the following processes: (a) Creation of droplets by film splitting at a quench front, with prediction of the drop size; (b) droplet splitting by a non-rewetted spacer grid; (c) droplet entrainment from liquid films along rewetted spacer grids, with prediction of the drop size. 38 items for consideration (32 specific items for thermal-hydraulics) are selected from the paper by Bestion (2017).

problems, primarily based on existing computational capabilities. [6S] EPR. Experimental and numerical research “is underway and continuing” for predicting “fluid flow in (a) lower plenum and (b) upper plenum”. [7S] EPR. Thermal fatigue namely in the proximity of branches is identified as an issue of interest: “averaged Navier–Stokes are worth investigating” to address the issue. 17 items for consideration (7 specific items for thermal-hydraulics) are selected from the paper by Saha et al., 2013. 2.4. The paper by D’Auria (2012) Personal views of the author are summarized in the paper by D’Auria (2012).The discussed concerns should be considered in addition to internationally established needs or as supporting for those needs. Stakeholders and brief history for nuclear thermal-hydraulics constitute the background in the paper; stakeholders include the authors and the editors of textbooks, journals and conferences, the industry the regulators, the research bodies and the universities. “A noncomprehensive and non-systematic review of perspectives and proposals for future activities in system thermal-hydraulics (SYS TH) … bringing personal views in relation to specific topics … has been performed and is applicable to NPP design and Nuclear Reactor Safety frameworks. Elements for reflection or issues (or Category / Target for improvement)”, in nuclear thermalhydraulics rather than topics are selected. Motivations and characterization of those elements, as well as tentative way-outs for addressing the related issues (not discussed here) are considered in the paper. Those elements are:

2.3. The paper by Saha et al. (2013)

[1D] The ‘local form loss’ coefficients (also reported as ‘K-factors’). The K-factor is the multiplicative term to the product ρw2 (where ‘ρ’ is the ‘reference’ density and ‘w’ is the ‘reference’ velocity of the concerned fluid) to calculate pressure drops at geometric discontinuities. The origin of the issue is connected with the derivation of the balance equations at the hearth of SYS TH codes. [2D] The multi-D HTC surface. The proposal … is to construct an as continuous as possible multi-dimensional (Multi-D) surface covering all the heat transfer regimes ( … e.g., nucleate boiling, saturated boiling, film boiling) and all the flow regimes (… e.g., bubbly, slug, churn, stratified and annular flow). Reflood and condensation should be included in the picture. [3D] Energy and Entropy balance following RCS blowdown and containment pressurization. In the case of application of system thermal-hydraulics codes, the issue is connected with the difficulty in estimating the energy flowing out of the break: TPCF model results can be qualified against experimental data and attaining the same qualification level is more complex in relation to the energy flow out of the break. Furthermore, (a), possible supersonic conditions downstream the break, phenomena connected with jet impingement in the containment, (b), the situation < TPCF on–off > (which occurs when the pressure difference between upstream or RPV side and downstream or containment side is not enough to induce TPCF), (c), TPCF jumps (the critical section may migrate or jump from one location to another, e.g. inside a valve during opening and closure cycles) affect energy and entropy exchanges. [4D] Precision Targets. The word precision is not commonly used in the system thermal-hydraulics area. Instead, we commonly use accuracy and uncertainty, i.e. the known error resulting from the comparison between measured and calculated data, and the unknown error in NPP predictions, respectively. The word precision may be used as a need to connect with the concepts of accuracy and uncertainty. [5D] The application of CFD-like approaches to NPP design and NRS technologies. Single- and two-phase CFD modeling are distinguished: more details are in the paper by Bestion (2017), and are

The framework and the focus for the paper by Saha et al. (2013), is the industrial vision for NTH. The history and the trends of nuclear technology and the description of moving from conservative to Best Estimate analysis for accident analyses, constitute the technical background. This is used to follow a top-down process to arrive at needs in nuclear thermal-hydraulics; Gen IV reactors receive the main attention and severe accident issues are discussed, too. Needs in nuclear thermalhydraulics or issues at the basis of needs are synthesized hereafter. [1S] BWR. After emphasizing that Direct Numerical Simulation (DNS) “is not a practical option at this time”, “the formulation of a transport equation for the interfacial area” is considered “an example of promising thermal-hydraulic R&D effort … that should be further explored”. [2S] BWR. The authors note “Increased flow and vapor quality resulting from the power increases”; then “This has increased the potential for fluid structure interactions and acoustic resonances”; then the need “to improve the understanding and characterization of fluid structure interaction”. [3S] BWR. “The increased power density coupled with the pressure drop characteristics and reduced thermal time constants of modern 10 × 10 fuels has reduced the margin to thermal-hydraulic instability”; then, “better characterization of the propagation and dissipation of density waves, including improved numerical methods, is encouraged.” [4S] BWR. “Improved characterization of post-dryout heat transfer for high void fraction dispersed-droplet flow at high temperatures, where half of the heat transfer is due to thermal radiation, would be very beneficial”. [5S] PWR. Operational and safety challenges are distinguished. “Operational: (a) CRUD-induced power shift; (b) CRUD-induced localized corrosion; (c) Grid-to-rod fretting failure; (d) Pellet-clad interaction (PCI); (e) Fuel assembly distortion (FAD). Safety: (f) Departure from nucleate boiling (DNB); (g) Cladding integrity during Loss of Coolant Accidents (LOCA); (h) Cladding integrity during reactivity insertion accidents (RIA); (i) Reactor vessel integrity; (j) Reactor internals integrity.” A process is outlined in the paper to address the challenge 4

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discussed above. [6D] The thermal-hydraulics of passive systems. Passive systems and related thermal-hydraulics issues are part of the history for NPP design and NRS: the configuration of the RCS for both BWR and PWR is determined considering the possibility to cool the core via Natural Circulation … accumulators are Passive System … and passive systems and related phenomena are part of SMR design (other than Gen IV reactors). Following pioneering studies performed at the beginning of current millennium in relation to the reliability of passive systems, those systems constitute the topic for an international project within the OECD/NEA/CSNI framework now running (and expected to be completed in 2020). So, this element of concern has been considered. [7D] The scaling issue and (connected) experiments. Notwithstanding the large number of papers and reports related to scaling, ‘scaling’ still constitutes a controversy in system thermalhydraulics and in applications. The scaling issue constituted the topic for an international project within the OECD/NEA/CSNI framework 2017, Bestion et al., 2017. So, this element of concern has been considered. [8D] The V & V for system thermal-hydraulics codes. V&V proved to be a powerful procedure to test the capabilities of computational tools. However, improvements are needed to make more effective the related procedures in the process of improving the modeling and the capabilities of those codes. Recently the proposal for V&V&C (where ‘C’ stands for consistency) has been formulated, D’Auria and Lanfredini, 2018. So, this element of concern has been considered, at least at the level of proposal. [9D] Uncertainty analysis. The concern (i.e. the element for reflection) is the continuous attention to be devoted in order to achieve an international consensus in relation to available capabilities and improvements in the qualification and robustness of adopted methods. This actually constitutes the topic for international project within the OECD/NEA/CSNI framework, Baccou et al., 2019, and Skorek et al., 2019. So, this element of concern has been considered. [10D] Coupling with system thermal-hydraulics (areas for coupling are identified in the paper by D’Auria et al. (2019), in this Special Issue, see also section 3.1 below). The coupling among codes implies addressing ‘numerical and informatics’ interfaces between the codes and the computer-compilers. Those interfaces are not necessarily homogeneous at the design level for different codes and generate issues connected with the transfer of information (data) among codes including the time step in transient calculations. The elements for reflection and the recommendations are: (a) To merge in different-unique system thermal-hydraulics code the capabilities of modeling RCS, core, sub-channel and containment: this brings the advantage of avoiding related code coupling; (b) To propose systematic V & V procedures for coupling also involving acceptance criteria for the coupled results. [11D] Modeling and structure for computational tools. The structure and the modeling of existing TH SYS codes (or computational tools) is based on the application of the ‘six-partial-derivative-balanceequations’ … (examples of) approximations or even inadequacies … needing further consideration are … (a) Two phase flow regimes are not accounted (in a mechanistic model) ; (b) Averaging is needed at the levels of cross section flow area, integration volume, and time; all of this is associated with the concept of Control Volume; impact of approximations upon predicted results is not quantified; (c) Consideration of two (or three) fluid fields as a solution domain; (d) … constitutive and (/or) closure equations which are unavoidably used outside their range of applicability / (validation); (e) the concept of Control Volume implies the need to reconstruct the continuous reality (i.e. the NPP configuration) by more or less large (discontinuous) pieces, thus introducing undue discontinuities … specific V&V effort needed; (f) quantification of errors from V&V (i.e. performing Validation is not < completing the job > :

quantification and judgement of errors are needed). [12D] Licensing needs. (a) the I & C has the potential to bring the NPP far from the initial condition following the occurrence of any (minor severity) postulated initiating event …; (b) conservatism not suited … safety cannot be demonstrated … (c) BEPU needs proper consideration (by regulators) in licensing; (d) need to use of 3D NK to perform the transient analyses … (e) (better) rules to consider the single failure, (f) (better) rules to consider the availability of systems and components; (g) BEPU analyses of transient involving PTS, ATWS and SBO; (h) … to establish procedures for determining the worst break position and, at a higher level of detail, (i) the break opening time which affects the calculation of mechanical load on internal and external components; (j) … to establish procedures to account for the impact of burnup upon the transient analysis (e.g. considering recently characterized nuclear fuel weaknesses: e.g. see the follow-up paper by D’Auria et al. (2019b)) (k) … to establish procedures to calculate the quantities affecting the acceptance criteria (of 10 CFR 50.46) other than the PCT, (l) the consideration of sump-recirculation; (m) … a procedure shall be established connecting accident analysis with the design and the applicability domain of system codes. [13D] PSA and system thermal-hydraulics. (a) The design of complex EOP and SAMG shall be connected with uncertainty analysis; (b) uncertainty evaluation should be considered for probability evaluation. [14D] Severe accident and system thermal-hydraulics. … The impact of severe accident analyses upon NPP design shall be avoided or minimized. Recommendations are (a) the analysis until the prediction of core degradation shall be performed by the best estimate system thermal–hydraulic codes (namely time of occurrence of core degradation); (b) severe accident code results should properly be supported by system thermal–hydraulic code applications e.g. for planning operator actions. [15D] Code user (analyst) effect, qualification and training. Several topics are connected with this issue. However, several training and qualification initiatives started in the last ten years by many national and international organizations (not listed here). So, this element of concern has been considered. 38 items for consideration (21 specific items for thermal-hydraulics) are selected from the paper by D’Auria (2012). 3. The elements to look at the future The contents of a paper focusing on international activities related to phenomena Aksan et al., 2018, a CFD research oriented paper, Bestion, 2017, a nuclear industry oriented paper, Saha et al., 2013, and personal opinions in support of recognized findings, D’Auria, 2012, have been used to identify elements for consideration for developments in NTH. As already mentioned, not available resources are needed for a comprehensive and systematic identification of current trends. Boundaries for defining the “universe of nuclear thermal-hydraulics” are considered (paper PI-2 in this SI) as derived from the analysis of seven recently issued books; in this connection, areas (or disciplines) for coupling with system thermal-hydraulics are identified. 3.1. Building-up the table A table cross-linking the elements in each paper and the characterization parameters defined below has been created.

• Numbering and alphabetic order of elements from the papers (in-

cluding editorial changes to minimize the number of characterizing words) constitute the 1st and the 2nd column of the table, respectively. In the 2nd column reference is made to the items in the papers as given in chapter 2. Notes (last column of the table) are used to support the definition in the 2nd column.

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Table 1 Summary of elements for consideration in future developments of nuclear thermal hydraulics. No

Element

Sector

Purpose

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

Availability of systems (licensing), [12D] (f) BEPU consideration (licensing), [12D] (c) BEPU: PTS, ATWS, SBO (licensing), [12D] (g) Break position (licensing), [12D] (h) Break opening time (licensing), [12D] (i) Boron dilution, [1B] (b) Boron dilution, [1A] Burn-up / fuel weaknesses, [12D], (j) BWR/ABWR lower plenum flow, [1B] (e) CCFL in HL and CL (need for 3D), [2A] CFD application/applicability to NRS, [5D] CHF mixing in sub-channel, [1B] (k) Cladding integrity-LOCA, [5S] (g) Cladding integrity-RIA, [5S] (h) Closure eqs. outside validation, [11D] (d) CL strat. heat & mass transfer, [3B] (f) CL strat. momentum transfer, [3B] (e) CL strat. heat transfers, [3B] (h) CL strat. non-condensable gases, [3B] (l) CL strat. turbulence production, [3B] (g) CL strat. turbulent diffusion, [3B] (i) CL strat. waves, turb., condensation, [3B] (j) CL temperature stratification, [3B] (k) Conservatism not suited (licensing), [12D] (b) Coupling with RCS-core, [10D] (a) CRUD-induced localized corrosion, [5S] (b) CRUD-induced power shift, [5S] (a) DC mixing: flow separation, [3B] (m) DC mixing: heat transfers, [3B] (n) DNB, [5S] (f) Droplets by film splitting, [5B] (a) Droplet entrainment from film, [5B] (c) Droplet split, [5B] (b) DWO, [3S] ECCS criterion (each, in licensing), [12D] (k) Entropy consideration, [3D] EOP & SAMG design, [13D] (a) Erosion, corrosion and deposition, [1B] (a) Field avg., disp. droplets & small bubbles, [4B] (d) Fluid-structure int. & acoustic vibration, [2S] Forces acting, [2B] (c) Fuel assembly distortion, [5S] (e) Global multi-D – core (need for 3D), [3A] Global multi-D – DC (need for 3D), [4A] Global multi-D – SG SS (need for 3D), [5A] Global multi-D – UP (need for 3D), [6A] Gravity driven reflood, [7A] Grid-to-rod fretting failure, [5S] (c) Heterogeneous flow distribution, [1B] (d) HTC multi-D surface, [2D] Hydrogen distribution, [1B] (j) Impact of averaging, [11D] (b) Impact of CV, [11D] (e) Induced break, [1B] (g) Interfacial area transport, [1S] Interfacial transfers, [2B] (d) I & C modeling need (licensing), [12D] (a) Jet condensation, [3B] (b) Jet entrainment and migration, [3B] (c) Jet instabilities, [3B] (a) Jet turbulence production, [3B] (d) Local pressure drop, [1D], [11A] (1997) LP flow, [6S] (a) Mixing & stratification / HL, [1B] (c) Multi-Field consideration, [11D] (c) Non condensable gas effect, [8A] Parallel channel effect and instability, [9A] Passive system, [6D] Pellet-clad interaction, [5S] (d) Phase separation at branches, [10A] Post-dryout HT & radiation, [4S]

SYS SYS SYS SYS SYS CFD-1 SYS SYS CFD-1 SYS CFD CFD-1 SYS SYS SYS CFD-2 PTS CFD-2 PTS CFD-2 PTS CFD-2 PTS CFD-2 PTS CFD-2 PTS CFD-2 PTS CFD-2 PTS SYS SYS SYS SYS CFD-2 PTS CFD-2 PTS SYS/CFD DNS-LES DNS-LES DNS-LES SYS SYS SYS (CFD) SYS CFD-1 CFD-2 All SYS CFD-2 BF SYS SYS SYS SYS SYS SYS SYS CFD-1 SYS/CFD CFD-1 SYS SYS CFD-1 SYS/CFD CFD-2 BF SYS CFD-2 PTS CFD-2 PTS CFD-2 PTS CFD-2 PTS CFD/SYS SYS/CFD CFD-1 SYS SYS SYS SYS SYS SYS SYS

P P P P P M E E M E M M E E E M M M M M M M M P P E E M M E M M M E P E P M P E M E E E E E E E M M M M M P M M P M M M M M M M M E E P E E E

C C C TH C TH TH C TH TH TH TH C C TH TH TH TH TH TH TH TH TH TH TH C C TH TH TH TH TH TH TH C TH TH C TH C TH C TH TH TH TH TH C TH TH TH N N C TH TH C TH TH TH TH TH TH TH TH TH TH TH C TH TH

TH or C

[A]

[B]

[S]

≈ ○ ○

○

○

≈

≈

≈

○ ○ ○

○

○

≈ ○

○

≈

○ ○

[D]

Notes = Originating ○ = Embedded ≈ = Marginal to become ‘robust’ to become mandatory to become mandatory to become ‘robust’ to become mandatory focus to mixing 1997, focus to system to be considered 1997 not specified PWR technology PWR technology specific activity needed at free surface at free surface CL & RPV walls Influence on condensation wall & interfacial shear effect upon condensation interactions at interface influence on turb. Diffusion Independent assessment sub-channel & containment PWR technology PWR technology at CL nozzle with walls PWR technology at QF, drop size prediction rewetted grid non rewetted grid BWR technology to become mandatory -BEPU PS-cont. (TPCF conditions) connected with PSA / BEPU interface track. Technique BWR technology drag, VM, lift and turb. diff. PWR technology 1997, temperature, void, flow 1997, temperature, void, flow 1997, temperature, void, flow 1997, temperature, void, flow 1997 PWR technology SGIP mixing and vibrations Including reflood & cond. in containment to be quantified see impact of averaging by loss of ductility BWR technology heat mass and momentum to become mandatory before mixing steam bubbles below level ECC injection below level at geometric discontinuities EPR technology two-phase flow 1997 condensation impact 1997 BWR TH phenomena reliability PWR technology 1997, TPCF conditions BWR technology

(continued on next page) 6

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Table 1 (continued) No

Element

Sector

Purpose

72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109

Precision targets, [4D] Pressure wave propagation, [12A] PTS, [1B] (f) QF propagation in fuel rods, [13A] QF propagation in passive structures, [14A] Qualification & applicability, [12D] (m) Reflood, [18A] RPV integrity, [5S] (i) RPV internals integrity, [5S] (j) RPV mixing following MSLB, [1B] (j) SA analysis up to core degradation, [14D] (a) SA application of codes, [14D] (b) Scaling, [7D] Single failure (licensing), [12D] (e) Space better than time averaging, [4B] (b) Steam line dynamics, [15A] Sub-filter phenomena and transfers, [4B] (c) Sump-recirculation, [12D] (l) Thermal fatigue, [1B] (h), [7S] TPCF breaks, [16A] TPCF downstream supersonic, [3D] (a) TPCF jumps, [3D] (c) TPCF on-off, [3D] (b) TPCF pipes, [17A] TPCF valves, [18A] Transport equations, [2B] (b) Turbulence effect, [2B] (a) Two phase flow regimes, [11D] (a) Uncertainty, [9D] Uncertainty in probability, [13D] (b) User training, qualification, [15D] UP flow, [6S] (b) V&V improvement, [8D] V&V of coupled codes, [10D] (b) V&V quantification of errors, [11D] (f) Wall functions, bubble generation, [2B] (e) 3D NK TH coupling (licensing), [12D] (d) 4-field model recommended, [4B] (a)

SYS/CFD SYS CFD-1 SYS SYS SYS SYS SYS SYS CFD-1 SYS SYS SYS/CFD SYS CFD-2 All SYS CFD-2 All SYS CFD-1 SYS SYS (CFD) SYS (CFD) SYS (CFD) SYS SYS CFD-2 BF CFD-2 BF SYS U SYS SYS/CFD SYS/CFD SYS (CFD) SYS (CFD) SYS (CFD) CFD-2 BF SYS CFD-2 All

P E P E E P E E E M P P P P P E P E E E E E E E E M M E P P P M P P P M P P

C TH C TH TH C TH C C TH C C TH C N TH N TH C TH TH TH TH TH TH TH TH TH TH C C TH TH TH TH TH C TH

• First • • •

characterization parameter, 3rd column in the table, is < sector > : it can be CFD (including additional terms), DNS-LES, SYS, U. Second characterization parameter, 4th column left side, is < purpose > : it can be ‘primarily modelling’ M, ‘procedure development or optimization’ P, ‘better understanding (maybe requiring experiments)’ E. Third characterizing parameter, 4th column, right side, is the answer to the question < is this a direct thermal-hydraulic issue or it has implication in coupling of thermal-hydraulics with different technological areas? > : the answer can be ‘thermal-hydraulics’ TH, ‘coupling’ C and numerics-oriented N. Columns 5 to 8 deal with the selected papers (labels are [A], [B], [S] an [D] for Aksan et al., 2018, Bestion, 2017, Saha et al., 2013, and D’Auria, 2012, respectively). Four marks are used, also reported under the ‘Notes’ heading, 9th column: = item originated in the marked paper; ○ = item embedded into the text of the marked paper; ≈ = item having marginal importance in the marked paper; blank (no sign) = item not considered in the concerned paper.

TH or C

[A]

[B]

[S]

[D]

for any model 1997

≈

1997 1997, e.g. BWR fuel box licensing relationship 1997, see also BEMUSE PWR technology PWR technology

○ ≈ ○

○

○

○

≈ ○ ≈

* ○

≈ ≈

○ ○

○ ○

Notes = Originating ○ = Embedded ≈ = Marginal

○

severe accident need support by TH codes TH code applicability to become ‘robust’ in Intermittent flow 1997, and acoustic waves need to be modeled to become mandatory –BEPU *EPR technology, branches 1997 e.g. SRV discharge pipes in space in time 1997 1997 size and distributions momentum & energy improved model needed consensus & qualification to be considered in PSA details in other papers EPR technology e.g. V&V&C momentum & energy to become mandatory

elements are shortly discussed below. Element 1 “Availability of systems (licensing)”. This is classified as a ‘system’ topic (SYS) which implies improvement of procedures (P) and is not directly connected with NTH (C). Actually PSA is the area connected with NTH where development should occur. The effort in the area of NTH consists in the capability of modeling ‘new’ systems eventually entering into the focus of attention in licensing. Element 2 “BEPU consideration (licensing)”. The same classification as element 1 applies (i.e. SYS, P and C). In this case the connection with NTH is straightforward: any NTH application to the analysis of accident in WCNR needs properly qualified BEPU process, tools and methods. Elements 6 and 7 “Boron dilution”. The concerned issue needs consideration at both the system (SYS) level, i.e. formation of boron deborated plug in loop seal, and the CFD level, i.e. mixing of deborated fluid with highly borated fluid in downcomer and lower plenum of PWR. In the former case modeling effort are proposed, while experimental data are requested in the latter case (an important research program has been completed at PKL in Germany considering this need). Elements 16–22, where element 16 is “CL stratification, heat & mass transfer”. Basic modeling is concerned (Fig. 1) for 2-phase flow by these elements; otherwise, PTS is also of interest for 1-phase flow (element 74) at a more general level (i.e. addressing several aspects related with 1-phase modeling). Element 68 “Passive systems”. NTH is essential to evaluate the reliability of passive systems: adequate computational capabilities are needed at the range of conditions (e.g. typically low velocities and small

3.2. The table and comments to selected elements The performed activity brought to the development of Table 1. A bit more than 100 elements considered in current researches or proposed for future developments are listed (and cross-linked). A full discussion of each topic is beyond the scope for the paper; rather, representative 7

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Preface

driving heads) which characterize passive systems. Large uncertainties from NTH calculation may imply low calculated values for reliability. Elements 82 and 83 connected with Severe Accident (SA). Severe Accident and NTH constitute different technological and scientific areas. Nevertheless any reactor event which evolves into SA shall be characterized by NTH models and methods. Scaling, V&V, Uncertainty and BEPU (items 84, 104-to-106, 100–101, and 2–3, respectively). What written for element 2 (BEPU) applies in these cases.

(neutron physics) needed to ensure quality of application for NTH. It is expected that a deeper, systematic and comprehensive effort (e.g. looking at ‘all’ recent conferences, journal papers, etc.), as well as considering the papers in this Special Issue, will provide a larger list of elements for future development in NTH without changing the main message from the present paper. Acknowledgements

4. Conclusions

Acknowledgments are due to all the authors of the four papers taken as reference in Table 1. Special thanks are due to D. Bestion who contributed not only with one the four papers but also with continuing discussions about the topics that require developments in nuclear thermal-hydraulics; however, not necessarily his ideas for priorities in the development are shared in the present paper.

The complex picture which constitutes nuclear thermal-hydraulics is confirmed by the performed review of the elements for future development. The picture has at least two levels of complexity: (a) a ‘horizontal level of complexity’ which originates the need to differentiate the areas for characterizing the development (in NTH); (b) a ‘vertical level of complexity’ which brings to distinguish between accident scenarios, phenomena and basic models (Fig. 1); this also originates the need to consider technological competence available in areas close to NTH.

References Aksan, N., D'Auria, F., Glaeser, H., Lillington, J., Pochard, R., Sjoberg, A., 1997. Systematic evaluation of the CSNI SET Validation Matrix data base and resulting needs. ASME/JSME Int. Conf. on Nuclear Engineering (ICONE5-2147). Aksan, N., D'Auria, F., Glaeser, H., 2018. Thermal-hydraulic phenomena for water cooled nuclear reactors. Nucl. Eng. Des. 330, 166–186. Baccou, J., Zhang, J., Fillion, P., Damblin, G., Petruzzi, A., Mendizábal, R., Reventós, F., Skorek, T., Couplet, M., Iooss, B., Oh, D.-Y., Takeda, T., 2019. Development of good practice guidance for quantification of thermal-hydraulic code model input uncertainty. Nucl. Eng. Des. 110173. https://doi.org/10.1016/j.nucengdes.2019. 110173. Bestion, D., 2017. System thermalhydraulics for design basis accident analysis and simulation: Status of tools and methods and direction for future R&D. Nucl. Eng. Des. 312, 12–29. Bestion, D., D’Auria, F., (Editor), Lien, P., Nakamura, H. (Lead Authors); Austregesilo, H., Skorek, T., Bae, B.U., Choi, K.Y., Kim, K.D., Moon, S.K., Martinez-Quiroga, V., Reventos, F., Mascari, F., Schollenberger, S., Umminger, K., Reyes, J.N., Rohatgi, U. S., Wang, W., Zaki, T. (Contributors), 2017. The OECD/NEA/CSNI Scaling State of the art Report – the S-SOAR, NEA/CSNI/R(2016)14, Paris (F), March. D'Auria, F., Debrecin, N., Galassi, G.M., 1995. Outline of the uncertainty methodology based on accuracy extrapolation (UMAE). J. Nucl. Technol. 109, 21–38. D’Auria, F., 2012. Perspectives in system thermal-hydraulics. Nucl. Eng. Technol. 44 (8), 855–870. D’Auria, F. (Editor) (Authors: Aksan, N., Bestion, D., D’Auria, F., Galassi, G.M., Glaeser, H., Hassan, Y., Jeong, J.J., Kirillov, P., Morel, C., Ninokata, H., Reventos, F., Rohatgi, U., Schultz, R.R., Umminger, K.), 2017. Thermal-Hydraulics in Water-Cooled Nuclear Reactors, Elsevier, Woodhead Publishing, pp. 1–1221. D’Auria, F., Lanfredini, M., 2018. Introducing V&V&C in nuclear thermal-hydraulics. ASME Verification and Validation Symposium (VVS2018-9321). D’Auria F. et al., 2019. Nuclear thermal-hydraulics: what is it? This Special Issue. D’Auria, F., Debrecin, N., Glaeser, H., 2019b. The need of adding a safety barrier to water cooled nuclear reactors. 11th ISTC “Safety Assurance of NPP with VVER, OKBGIDROPRESS, Podolsk (Ru), 21–24 May. Glaeser, H., D’Auria, F., De Crecy, A., Reventos, F., 2011. Main results of the OECD best estimate methods. Uncertainty and Sensitivity Evaluation (BEMUSE) Programme, NURETH-14, Toronto, CA, Sept. 25–30. Saha, P., Aksan, N., Andersen, J., Yan, J., Simoneau, J.P., Leung, L., Bertrand, F., Aoto, K., Kamide, H., 2013. Issues and future direction of thermal-hydraulics research and development in nuclear power reactors. Nucl. Eng. Des. 264, 3–23. Skorek, T., de Crécy, A., Kovtonyuk, A., Petruzzi, A., Mendizábal, R., de Alfonso, E., Reventós, F., Sarrette, C., Kyncl, M., Pernica, R., Baccou, J., Fouet, F., Probst, P., Chung, B.-D., Tram, T.T., Oh, D.-Y., Gusev, A., Falkov, A., Shvestov, Y., Li, D., Liu, X., Zhang, J., Alku, T., Kurki, J., Jäger, W., Sánchez, V., Wicaksono, D., Zerkak, O., Pautz, A., 2019. Quantification of the uncertainty of the physical models in the system thermal-hydraulic codes – PREMIUM benchmark. Nucl. Eng. Des. 110199. https://doi.org/10.1016/j.nucengdes.2019.110199.

The complexity picture shall be complemented (not discussed in the paper) by the continuously detected (sometimes large) discrepancies between experimental data and models (or codes) predicted results. Then, the following additional situations shall be noted: (c) the interaction occurs among the areas for characterizing the development: an improvement in one area not supported by improvements in other areas may not bring to an improvement in application of NTH; (d) improvements in basic modeling, or at small scale, may cause worsening of prediction capabilities at a large scale; in other words, the unavoidable application of averaging procedures at large scale may be at the origin of compensating errors and the correction of one error at small scale may eventually negatively affect the overall predictive capability. A multi-face situation for developments in NTH constitutes the key result and the main message from the present overview: more than one hundred elements for future consideration have been identified and partly (as far as possible) characterized. An idea of the derived elements can be drawn from the sample synthesis below:

• For • • • •

system (nuclear) thermalhydraulics, development needs for pressure drop at geometric discontinuities (line 62 in Table 1) are identified. Condensation around a jet of subcooled liquid into a steam region (line 58 in Table 1) is taken as an example for 2-phase flow CFD. Mixing and stratification in HL (line 64 in Table 1) is an example for 1-phase flow CFD. BEPU application in licensing (line 2 in Table 1) is an example of procedures needed for NTH. Coupling between nuclear thermal-hydraulics and neutron physics (line 108 in Table 1) is an example of development in an area

F. D'Auria, N. Aksan, Y. Hassan

8