Dottorato di Ricerca in Ingegneria Industriale
Curriculum in Ingegneria Nucleare e Sicurezza Industriale
Ciclo XXX
Coupling between System and CFD codes
for the analysis of thermal-hydraulic phenomena
relevant for LMFR
Author
Morena ANGELUCCI
Supervisors
Prof. Ing. Nicola FORGIONE
Dott. Ing. Daniele MARTELLI
Dott. Ing. Ivan DI PIAZZA
1 General framework of the activity
The present PhD work set in a dynamic national and international context, within a collaborative network among several universities and research centres. The synergetic endeavour is focused on the investigation of physical and technical key aspects for the development of Generation IV nuclear systems cooled by liquid metals, i.e. Sodium Fast Reactors (SFRs) and Lead-cooled Fast Reactors (LFRs). In particular, the University of Pisa is involved in two HORIZON2020 projects, devoted to the investigation of the thermal-hydraulics challenging issues for liquid metal cooled reactors, from both experimental and numerical point of view. Among the main objectives of the above-mentioned EU projects, there is the assessment of the modelling capabilities of existing and new calculation tools for the analysis of these systems. One of the modelling strategies under development concerns the use of multi-scale approaches, due to their potentiality to simulate integral system with different levels of detail. Nevertheless, the extensive use of such tools need qualification in order to be used for the safety assessment of nuclear reactors.
2 Objectives of the research activity
The main aim of this work was the improvement and optimization of a coupling tool, already under development at the University of Pisa, between a modified version of the thermal-hydraulic system code RELAP5/Mod3.3 and the CFD code ANSYS Fluent. Coupled calculations are devoted to the analyses of thermal-hydraulic phenomena inherent to the Generation IV nuclear reactors cooled by Heavy Liquid Metals (HLMs). Moreover, experimental campaigns were performed in collaboration with R.C. ENEA Brasimone, in order to collect integral system and local data for the validation of the developed tool and to carry out heat transfer experimental analysis on a wire-spaced core configuration cooled by HLMs.
The first part of the work was devoted to improving a semi-implicit numerical scheme for the coupling procedure to enhance the stability of the calculations. The methodology was also extended for the coupling at the thermal boundary to adapt this technique for heat transfer analysis. A Matlab script, used as coupling supervisor, was optimized to reduce the global computational time required to manage the exchange of data between the two codes and to handle the storage of the main numerical results.
The experimental part of this research was conducted in parallel with the numerical activities, and it was mainly devoted to gather suitable data for the qualification process of the coupling strategy under development. The experiments were performed on the NACIE-UP loop-type facility, designed to work in natural and forced circulation flow regime thanks to a gas-injection system used to enhance the circulation of the main coolant. A prototypical 19-pins wire-spaced electrically heated fuel bundle simulator was installed in the facility as main test section. Steady state and transient tests were performed at several flow conditions, investigating natural and mixed circulation flow regimes characterized by a sub-channel Reynolds number in the range of 1500–15000 and a wall heat flux in the range of 50–500 kW/m2. Heat transfer analysis in the wire-spaced bundle were performed by computing the local Nusselt number (in the single sub-channels) and the section-averaged Nusselt number. The obtained results were graphically plotted as a function of the Péclet number and
compared with correlations existing in the available literature for rod bundles cooled by liquid metals. Instrumentation installed in the loop and in the bundle provides local temperature measurements, helpful for the verification of the numerical simulations performed with the RELAP5-CFD coupling technique.
The coupling methodology was first applied to a loop-type system, i.e., NACIE-UP facility. After a simple test case considered to verify the correctness of the setup models, blind pre-test calculations of NACIE-UP tests were accomplished for a benchmark activity intended for STH and STH-CFD coupled codes. In a second phase, the available experimental data were used to evaluate the numerical results and individuate the main limits of the NACIE-UP setup modelling. Some of these constraints were overcome to achieve post-test calculations, where a more satisfactory comparison between numerical results and experimental outcomes was obtained.
Eventually, the coupled codes calculations were used to perform thermal-hydraulic analysis of a pool-type systems, i.e. the CIRCE facility designed and built at ENEA Brasimone R.C.. At first, a STH-CFD coupled model of the CIRCE-ICE configuration was created and tested by performing the simulation of isothermal tests in gas-induced circulation. After, a PLOHS+LOF - like experimental test was simulated to analyse the occurring complex and inter-connected thermal-hydraulic phenomena: i.e. the transition from forced to natural circulation, the loss of the main heat exchanger cooling capabilities with activation of the DHR system. In particular, the use of coupled calculation allowed predicting the thermal stratification in the CIRCE pool, which was modelled with the CFD code, both at the steady-state in normal operation and during the accidental scenario. Due to the obtained satisfactory results, the STH-CFD coupled models were employed also to perform first analysis of the new test section (HERO) which will be installed in the CIRCE pool. In particular, coupled simulations of the CIRCE-HERO configuration and the test section alone (HERO) were accomplished.
A schematic flowchart of the PhD activity, summarizing the key sections of the work, is illustrated in Figure 1.
3 Achieved results
The present research activity dealt with thermal-hydraulic phenomena relevant for the safety aspects of Generation IV nuclear reactors cooled by heavy liquid metals. Fundamental topics, such as the core coolability during normal and accidental scenarios, as well as heat transfer mechanisms during the transition from forced to natural circulation flow regime, were addressed with experimental and numerical approaches. The assessment of the modelling capabilities of a multi-scale approach, concerning the coupling procedure between a STH code and a CFD code, was analysed and discussed. The main achieved results are summarized in the following paragraphs, which are related to the main sections of the work.
NACIE-UP experiments
• A first experimental campaign was conducted, where steady-state tests at different combinations of FPS power and LBE mass flow rate were achieved.
• Local sub-channel and section-averaged heat transfer coefficients were obtained from the temperature measurements and scaled in a non-dimensional form (Nusselt number). The resulting Nu were graphically plotted as function of the Péclet number and compared with correlations found in the literature for fuel bundle cooled by liquid metals, i.e., Kazimi & Carelli, Ushakov and Mikityuk correlations.
• Local Nu data in the inner sub-channels were well predicted by Ushakov and Mikityuk correlations, with some noticeable asymmetries between the two monitored sub-channels (S2 and S5) due to the different relative position between the pin and the wire.
• Nu in the external sub-channels was lower since the temperature distributions in this area is affected by the presence of the external wrapper.
• The Nusselt number averaged over the total flow area resulted lower than the Ushakov and Mikityuk correlations and slightly above the Kazimi &Carelli one, which is more conservative. • The error analysis, performed on the obtained data, highlighted as results were largely affected by the uncertainty on the LBE physical properties. The maximum errors on Re and Pr were close to 20%, maximum errors on Nu could reach 36%.
• Following the first campaign, the facility underwent a slight upgrade and two preliminary tests were performed to characterize the flow and the thermal losses. A simplified analytical approach was also followed to describe the relationship between gas-injected and induced LBE mass flow rate and to individuate the total thermal resistance between the loop and the external environment.
• A second experimental campaign, focused on the thermal-hydraulic behaviour of the entire loop and the fuel assembly during power or/and mass flow transients, was performed.
• Heat transfer analyses on the fuel bundle, in term of Nusselt number, were performed by comparing the results with the same correlations and with the results from the first campaign. The obtained section-averaged Nu set close the Kazimi-Carelli correlation, resulting slightly lower than the data from the previous campaign. Moreover, the more recent data were affected by lower uncertainties.
• Again, the heat transfer coefficient in the inner sub-channels was well predicted by Ushakov and Mikityuk correlations, which seem to be more suitable to estimate the convective heat transfer coefficient for an infinite lattice.
• The overall local and integral system transient data provided a good reference to be used for the qualification of system code, CFD codes and STH/CFD coupled methodologies for HLM-cooled systems.
NACIE-UP coupled simulations
• A coupling model of the NACIE-UP facility was setup, where the CFD code was employed to simulate the test section (FPS) and the RELAP5/Mod3.3 was used to model the remaining part of the loop and the water secondary system of the heat exchanger.
• At first, RELAP5 standalone simulations of the two characterization tests emphasized some limitations of the code in predicting the gas-enhanced circulation of the main coolant.
• Then, pre-test blind simulations of the NACIE-UP benchmark tests were performed with coupled codes. This tool was helpful in predicting with quite good precision the total pressure loss inside the bundle and to investigate the 3D flow distribution and the heat transfer in the wire-spaced bundle.
• In a following phase, the comparison with experimental data allowed to identify the main constrains of the NACIE-UP setup model: the gas-induced flow, the HX modelling and the power distribution in the bundle. These issues were partially overcome in the post-test calculation.
• The comparison between numerical results and experimental data was much more satisfactory in the post-test simulation, with good prediction of the loop temperatures and much of the local temperatures in the bundle.
• Some discrepancies in the evaluation of local temperatures in the hottest section of the bundle (close to the outlet) seem to be caused by the inadequacy of the available turbulence models to well capture secondary flow and turbulent heat transfer at low Prandtl numbers.
CIRCE coupled simulations
• A coupled model of the CIRCE facility was likewise developed, by using the RELAP5 code to model the ICE test section installed in the CIRCE pool, which instead was modelled with the CFD code. This geometrical domain was later modified and adapted to the new configuration of the facility, where the new heat exchanger HERO will be installed.
• The setup models were verified by simulating isothermal tests in gas-induced circulation. The good results obtained comparing the numerical results with the experimental data of the induced LBE mass flow rate proved that the coupled calculations were capable of reproducing the main flow path in the pool facility.
• The results from 3D CFD model of the pool were more accurate than the ones from the 2D geometry; the 3D domain also allowed individuating the secondary flows and recirculation zones in the pool.
• The coupled model was then used to simulate a PLOF+LOHS transient, which involves inter-connected thermal-hydraulic phenomena. For this purpose, the 2D geometrical model was preferred to reduce the computational time required.
• A first simulation considered only the hydraulic coupling at the test section boundaries (FPS inlet and HX outlet). The coupled code calculation was able to reproduce the main phenomena involved in the transient, such as the transition from forced to natural circulation, the LBE flow
path in the pool and in the DHR shell and the DHR cooling function after the loss of the main heat exchanger.
• The main limit of the first model was individuated in the lack of conjugate heat transfer between the test section and the pool, which was implemented in a second phase to improve the calculation.
• The improved calculation could better predict the thermal stratification in the pool, including the temperature gradient at the HX outlet and the linear trend along the active length of the HX. • A coupled model of the new heat exchanger (HERO) alone was also created to analyse the performance of this component. For this purpose, the CFD code was used to simulate the primary side (LBE) and the heat structure of the HX, whereas the RELAP5 code was employed to model the water/vapour secondary side, with the coupling at the thermal interfaces. The coupling tool allowed a detailed characterization of the LBE flow in the shell side of HERO and permitted to analyse the different flow characteristics of the water/vapour mixture in the central and the lateral pipes. Thanks to the use of a CFD model in the LBE side, it was possible to evaluate the convective heat transfer coefficient directly from the calculation, avoiding the use of an empirical correlation.
4 Future perspectives
Three different aspects can be individuated for the future developments of this research activity: • Methodology: The current coupling strategy requires a case-dependent and often prolonged
pre-processing phase. The MATLAB script is customized to the single test case, referring to its geometry and specific sections of the domains. This procedure could be generalized using a pre-defined scheme to specify the number and type of hydraulic and thermal boundaries and to associate the appropriate geometrical boundary from the respective domains.
• Applications: The applications presented in this work can be further improved and extended to the analysis of new test-sections. Since the coupling technique is in principle general, it can be further adapted to various physical applications, such as for the thermal-hydraulic analysis in fusion technology systems.
• Guidelines: Among the priorities of the research community, there is the need to create a set of reference data to be used as validation base for the recent multi-scale strategies. In addition, best-practise guidelines should be identified and collected in reference documents to help the user in setting their modelling as well as possible. This point should be developed with the mutual endeavour of the main experts of this sector and in the framework of collaborative activities like the ongoing EU projects.
Publications achieved during the PhD activity
Journal papers
• S. Bassini, I. Di Piazza, A. Antonelli, M. Angelucci, V. Sermenghi, G. Polazzi, M. Tarantino, “In-loop oxygen reduction in HLM thermal-hydraulic facility NACIE-UP”, Progress in Nuclear Energy, 105 (2018) 137-145. • M. Angelucci, D. Martelli, G. Barone, I. Di Piazza, N. Forgione, “STH-CFD Codes Coupled Calculations Applied to HLM Loop and Pool Systems”, Science and Technology of Nuclear Installations, Article
ID.1936894, Volume 2017, October 2017.
• I. Di Piazza, M. Angelucci, N. Forgione, R. Marinari, M. Tarantino, “Heat transfer on HLM cooled wire-spaced fuel pin bundle simulator in the NACIE-UP facility”, Nuclear Engineering and Design, 300 (2016)
256–267.
Journal papers under review
• M. Angelucci, I. Di Piazza D. Martelli, “Experimental campaign on the HLM loop NACIE-UP with instrumented wire-spaced fuel pin simulator”, send to Nuclear Engineering and Design.
• R. Marinari, I. Di Piazza, M. Tarantino, M.Angelucci, D. Martelli, “Experimental tests and post-test analysis of non-uniformly heated 19-pins fuel bundle cooled by Heavy Liquid MEtal”, send to Nuclear Engineering
and Design
Conference papers
• M. Angelucci, D.Martelli, N. Forgione, “RELAP5 STH AND FLUENT CFD COUPLED CALCULATIONS OF A PLOHS + LOF TRANSIENT IN THE HLM EXPERIMENTAL FACILITY CIRCE”, Proceedings of
25th International Conference On Nuclear Engineering (ICONE 25), 02-0h July 2017, Shanghai, China.
• M. Angelucci, I. Di Piazza, N. Forgione, M. Tarantino, G. Polazzi, V. Sermenghi, “NACIE-UP: a HLM loop facility for natural circulation experiments”, Abstract for International Conference on Fast Reactors and
Related Fuel Cycles: Next Generation Nuclear Systems for Sustainable Development (FR17), Yekaterinburg, Russian Federation, 26–29 June 2017.
• I. Di Piazza, M. Angelucci, N. Forgione, R. Marinari, G. Polazzi, V. Sermenghi, L. Laffi, D. Giannotti, M. Tarantino, “Experimental Fuel Pin Bundle characterization in the NACIE-UP HLM Facility”, Proceedings of
the 16th International Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-16), Chicago, IL, August 30-September 4, 2015.
International Technical Reports
• N. Forgione, M. Angelucci, G. Barone, et al., “NACIE-UP blind simulations”, SESAME-GA 654935 Deliverable D5.18, October 2017.
• I. Di Piazza, M. Angelucci, G. Polazzi, V. Sermenghi, “NACIE-UP data for PLOFA experiment”, SESAME-GA 654935 Deliverable D4.10, October 2017.
• I. Di Piazza, M. Angelucci, G. Polazzi, V. Sermenghi, “NACIE-UP experimental setup and test matrix for PLOFA experiment”, SESAME -GA 654935 Deliverable D4.9, January 2016.
• I. Di Piazza, M. Angelucci, G. Polazzi, M. Tarantino, “Experimental results on free convection in heavy liquid metals using the NACIE facility”, SEARCH-CN 295736 Deliverable D2.4, May 2015.
• I. Di Piazza, M. Angelucci, G. Polazzi, M. Tarantino, “Experimental results on forced convection in heavy liquid metals using the NACIE facility”, SEARCH-CN 295736 Deliverable D2.6, May 2015.
• I. Di Piazza, M. Angelucci, G. Polazzi, M. Tarantino, “Post-test analysis on free and forced convection measurements in HLM using NACIE facility”, SEARCH-CN 295736 Deliverable D2.7, May 2015.
Other Technical Reports
• C. Ulissi, M. Angelucci, G. Barone, R. Lo Frano, N. Forgione, “Application of RELAP5/mod3.3 – Fluent coupling codes to CIRCE-HERO”, CERSE-UNIPI 511/2017, September 2017.
• I. Di Piazza, M. Angelucci, V. Sermenghi, G. Polazzi, “Experimental Tests in the HLM facility NACIE-UP with non-uniformly heated 19-pins fuel bundle”, Adp MSE-ENEA LP2.C1, September 2016.
• M. Angelucci, D. Martelli, D. Rozzia, N. Forgione, A. Giovinazzi, F. D’Auria, W. Ambrosini, “Verification and Validation of RELAP5 STH and FLUENT CFD Coupled Codes Applied to Pool Systems”, Adp MSE-ENEA LP2.C1, September 2016.
