Experimental and Numerical Thermal-hydraulic Analyses in support of GEN-IV Lead-cooled Fast Reactor Design

L’attività di ricerca oggetto del presente lavoro di tesi di dottorato è stata svolta presso il Dipartimento di Ingegneria Civile e Industriale dell’università di Pisa in collaborazione con l’ Unità Tecnica di Ingegneria Sperimentale presso il centro di ricerche ENEA Brasimone. Lo scopo dell’attività è stato quello di indagare fenomeni di termoidraulica di interesse per il supporto allo sviluppo dei reattori di quarta generazione refrigerati con piombo e piombo bismuto. Il candidato ha sviluppato simulazioni numeriche e ha condotto tre campagne sperimentali sulle facility sperimentali CIRCE (Circolazione Eutettico) e NACIE (Natural Circulation) in supporto alla analisi numeriche effettuate. Lo studio ha avuto inizio a partire da Gennaio 2012. Inizialmente è stata effettuata un’analisi preliminare con dati sperimentali reperiti in letteratura per verificare limiti e capacità del codice di calcolo CFD utilizzato (Ansys Fluent) nel prevedere fenomeni di stratificazione termica in sistemi a piscina (pool type reactor). Una volta verificate le capacità e i limiti del codice, lo si è applicato allo studio della facility sperimentale CIRCE (progettata per lo studio di fenomeni di termoidraulica in reattori di tipo a piscina composta da un vessel principale contenente circa 70 tonnellate di piombo bismuto liquido riscaldato da un bundle elettrico costituito da 37 pins per una potenza complessiva di circa 1 MW). E’ stata sviluppata un’analisi di pre-test simulando un transitorio incidentale di interesse. Al fine di simulare correttamente il transitorio studiato, è stata sviluppata una metodologia di calcolo accoppiato monodirezionale tra il codice di sistema RELAP5 e il codice di CFD Fluent in cui i dati ottenuti in una simulazione RELAP5 precedentemente condotta, sono stati utilizzati come condizioni al contorno per il codice CFD. Tale analisi è stata di supporto al posizionamento della strumentazione di controllo all’interno della sezione di prova. L’analisi di pre-test è stata succeduta dalla campagna sperimentale e da simulazioni di post-test, mostrando un buon accordo trai dati ottenuti sperimentalmente e quelli derivati dalle simulazioni numeriche. Fenomeni di stratificazione termica sono stati evidenziati all’interno dell’apparecchiatura sperimentale e la transizione da circolazione forzata a naturale è stata investigata. Per migliorare l’accuratezza delle simulazioni si è sviluppato un metodo di accoppiamento tra codici di sistema e codici CFD (RELAP5- Ansys Fluent). In particolare, in tale metodologia il dominio geometrico viene suddiviso in regioni opportunamente simulate attraverso il codice di sistema (più idoneo a simulare flussi bifase) e regioni simulate attraverso il codice CFD (più idoneo a simulare regioni caratterizzate da flussi turbolenti e fortemente tridimensionali). Questa suddivisione identifica delle interfacce ben definite attraverso le quali i due codici si scambiano le informazioni necessarie a procedere nel transitorio simulato secondo i vari schemi numerici implementati e gestiti tramite l’apposita interfaccia creata. La metodologia sviluppata è stata applicata alla facility sperimentale NACIE (loop rettangolare in cui circolano circa 1000 kg di piombo bismuto liquido ad una temperatura massima di 550°C e accoppiato ad un circuito secondario ad acqua tramite uno scambiatore di calore). La parte del dominio simulata con il codice di CFD è stata quella relativa al fuel bundle mentre la restante porzione del sistema è stata simulata con il codice di termoidraulica RELAP5. E’ stata condotta una validazione preliminare del metodo di calcolo sviluppato mediante il confronto dei risultati numerici con quelli ottenuti dalla campagna sperimentale mostrando un buon accordo sia per i tests in circolazione naturale che in circolazione forzata. Infine, lo scambio termico all’interno di un fuel bundle refrigerato a piombo bismuto è stato investigato sperimentalmente nella facility CIRCE. La quasi totalità dei dati reperibili nella letteratura scientifica tratta scambio termico in bundle refrigerati con sodio-potassio o mercurio e le correlazioni disponibili sono state sviluppate per tali fluidi. I dati sperimentali forniti nel presente lavoro rappresentano quindi il primo set di dati ottenuto in una facility a piscina di dimensioni rilevanti e costituiscono un supporto di dati sperimentale per la progettazione di reattori di quarta generazione refrigerati con piombo liquido

Reasoning about Actions with Temporal Answer Sets

In this paper we combine Answer Set Programming (ASP) with Dynamic Linear Time Temporal Logic (DLTL) to define a temporal logic programming language for reasoning about complex actions and infinite computations. DLTL extends propositional temporal logic of linear time with regular programs of propositional dynamic logic, which are used for indexing temporal modalities. The action language allows general DLTL formulas to be included in domain descriptions to constrain the space of possible extensions. We introduce a notion of Temporal Answer Set for domain descriptions, based on the usual notion of Answer Set. Also, we provide a translation of domain descriptions into standard ASP and we use Bounded Model Checking techniques for the verification of DLTL constraints.Comment: To appear in Theory and Practice of Logic Programmin

STH/CFD coupled simulation of the protected loss of flow accident in the CIRCE-HERO facility

The paper presents the application of a coupling methodology between Computational Fluid Dynamics (CFD) and System Thermal Hydraulic (STH) codes developed at the University of Pisa. The methodology was applied to the CIRCE-HERO facility in order to reproduce the recently performed experimental conditions simulating a Protected Loss Of Flow Accident (PLOFA). The facility consists of an internal loop, equipped with a fuel pin simulator and a steam generator, and an external pool. In this coupling application, the System code RELAP5 is adopted for the simulation of the internal loop while the CFD code ANSYS Fluent is used for the sake of simulating the pool. The connection between the two addressed domains is provided at the inlet and outlet section of the internal loop; a thermal coupling is also performed in order to reproduce the observed thermal stratification phenomenon. The obtained results are promising and a good agreement was obtained for both the mass flow rates and temperature measurements. Capabilities and limitations of the adopted coupling technique are discussed in the present paper also providing suggestions for improvements and developments to be achieved in the frame of future applications

Numerical analysis of the CIRCE-HERO PLOFA scenarios

The present work deals with simulations carried out at the University of Pisa by using the System Thermal Hydraulics code RELAP5/Mod3.3 to support the experimental campaign conducted at the ENEA (Energia Nucleare ed Energie Alternative) Brasimone Research Centre on the CIRColazione Eutettico—Heavy liquid mEtal pRessurized water cOoled tubes (CIRCE-HERO) facility. CIRCE is an integral effect pool type facility dedicated to the study of innovative nuclear systems and cooled by heavy liquid metal, while HERO is a heat exchanger heavy liquid metal/ pressurized cooling water system hosted inside the CIRCE facility. Beside the H2020 project Multi-Purpose Hybrid Research Reactor for High-Tech Applications (MYRRHA) Research and Transmutation Endeavour (MYRTE), a series of experiments were performed with the CIRCE-HERO facility, for both nominal steady-state settings and accidental scenarios. In this framework, the RELAP5/Mod3.3 code was used to simulate the experimental tests assessing the heat losses of the facility and analyzing the thermal hydraulics phenomena occurring during the postulated Protected Loss Of Flow Accident (PLOFA). The modified version Mod. 3.3 of the source code RELAP5 was developed by the University of Pisa to include the updated thermo–physical properties and convective heat transfer correlations suitable for heavy liquid metals. After reproducing the facility through an accurate nodalization, boundary conditions were applied according to the experiments. Then, the PLOFA scenarios were reproduced by implementing the information obtained by the experimental campaign. Sensitivity analyses of the main parameters affecting the thermofluidynamics of the Lead-Bismuth Eutectic (LBE) were carried out. In the simulated scenario, the LBE mass flow rate strongly depends on the injected argon flow time trend. The numerical results are in agreement with the experimental data, however further investigations are planned to analyze the complex phenomena involved

Coupled simulations of the NACIE facility using RELAP5 and ANSYS FLUENT codes

This work deals with the development and preliminarily assessment of a coupling methodology between a modified version of RELAP5/Mod3.3 STH code and FLUENT commercial CFD code, applied to the NACIE (natural circulation experiment) LBE (lead bismuth eutectic) experimental loop (built and located at the ENEA Brasimone research centre). The coupling tool is used to simulate experiments representative of both natural circulation conditions and isothermal gas enhanced (assisted) circulation. Furthermore, an accidental test reproducing an Unprotected Loss of Flow (ULOF) scenario is also simulated and the outcomes are presented. A preliminary sensitivity analysis has shown that, to guarantee a suitable numerical convergence, the assisted circulation tests require a time step one order of magnitude lower compared to natural circulation ones. The comparison between the RELAP5 stand-alone simulations and RELAP5/FLUENT coupled simulations proved the capability to simulate the thermal-hydraulic behaviour of a loop experimental facility for all the examined conditions

Experimental campaign on the upgraded He-FUS3 facility

An extensive thermal-hydraulic experimental campaign was conducted on He-FUS3 helium loop facility to support the conceptual design of HCLL and HCPB Test Blanket System. The experiments were divided into three distinct phases. The first one was dedicated to the evaluation of the new ATEKO Turbo Circulator (TC) performances, identifying its operating limits in terms of supplied helium mass flow as a function of rotational speed, cold by-pass opening and loop pressure. The outcomes were compared with the manufacturer theoretical performance map and with a RELAP5-3D pre-test computation. In the second phase, experiments were carried out to analyze the facility dynamic response in hot conditions and to characterize its main components (TC, heaters, economizer, cooling system and valves). The wide amount of collected data will serve for the development and validation of a numerical model of the facility at TBS conditions. For the third phase, the tests were designed to investigate He-FUS3 behavior in accidental conditions representative of LOFAs and LOCAs scenarios

STH-CFD codes coupled calculations applied to HLM loop and pool systems

This work describes the coupling methodology between a modified version of RELAP5/Mod3.3 and ANSYS Fluent CFD code developed at the University of Pisa. The described coupling procedure can be classified as "two-way," nonoverlapping, "online" coupling. In this work, a semi-implicit numerical scheme has been implemented, giving greater stability to the simulations. A MATLAB script manages both the codes, oversees the reading and writing of the boundary conditions at the interfaces, and handles the exchange of data. A new tool was used to control the Fluent session, allowing a reduction of the time required for the exchange of data. The coupling tool was used to simulate a loop system (NACIE facility) and a pool system (CIRCE facility), both working with Lead Bismuth Eutectic and located at ENEA Brasimone Research Centre. Some modifications in the coupling procedure turned out to be necessary to apply the methodology in the pool system. In this paper, the comparison between the obtained coupled numerical results and the experimental data is presented. The good agreement between experiments and calculations evinces the capability of the coupled calculation to model correctly the involved phenomena

Validation of SIMMER-III code for in-box LOCA of WCLL BB: Pre-test numerical analysis of Test D1.1 in LIFUS5/Mod3 facility

Abstract One of the four breeding blanket concepts for European DEMO nuclear fusion reactor is the Water-Cooled Lithium Lead Breeding Blanket (WCLL BB). The WCLL in-box LOCA (Loss Of Coolant Accident) is a major safety concern of this component, therefore transient behavior shall be investigated to support the design, to evaluate the consequences and to adopt mitigating countermeasures. To fulfill this objective, at first, SIMMER-III code was improved by implementing the chemical reaction model between PbLi and water. Then, SIMMER-III Verification and Validation (V&V) procedures have been established and conducted to obtain a qualified code for deterministic safety analysis. The verification activity was successfully completed, while the validation activity requires further effort according to the R&D plan set up in the framework of the EUROfusion Project. In view of this, an experimental campaign and a test matrix has been designed in LIFUS5/Mod3 facility performing pre-test analyses of Test D1.1. The preliminarily-defined test matrix will be used for the validation SIMMER-III according to a standard procedure. At the present stage, a pre-test numerical analysis was performed to support future experimental tests. The presented work aims to support the upcoming experimental activity in terms of setting up Boundary & Initial condition, specifying the most important parameters to be measured during tests and calculated by SIMMER-III code during transient and obtaining the best nodalization for the post-testing simulation. In particular, a qualitative analysis of obtained results was performed according to the available data time trends and based on engineering considerations. It aims to interpret the resulting sequence of main events and the identification of phenomenological windows and aspects, relevant to pressure transient and hydrogen production due to the chemical reaction between heavy liquid metal and water

Effect of Cold Rolling on Microstructural and Mechanical Properties of a Dual-Phase Steel for Automotive Field

A new advanced dual-phase (DP) steel characterized by ferrite and bainite presence in equal fractions has been studied within this paper. The anisotropy change of this steel was assessed as a progressively more severe cold rolling process was introduced. Specifically, tensile tests were used to build a strain-hardening curve, which describes the evolution of this DP steel's mechanical properties as the thinning level increases from 20 to 70% with 10% step increments. As expected, the cold rolling process increases mechanical properties, profoundly altering the material's microstructure, which was assessed in depth using Electron Backscatter Diffraction (EBSD) analysis coupled with the Kernel Average Misorientation (KAM) maps. At the same time, the process strongly modifies the material planar anisotropy. Microstructural and mechanical assessment and the Kocks-Mecking model applied to this steel evidenced that a 50% strain hardening makes the DP steel isotropic. The material retains or resumes anisotropic behavior for a lower or higher degree of deformation. Furthermore, the paper evaluated the forming limit of this DP steel and introduced geometric limitations to testing the thin steel plates' mechanical properties