Thermal Hydraulic Analysis of Postulated Accidents in a HLM Cooled Fast Reactor

This thesis work, carried out at the Dipartimento di Ingegneria Civile ed Industriale (DICI) of the University of Pisa, concerns the thermo-hydraulic analysis of postulated accidents in a HLM cooled fast reactor, i.e. the MYRRHA-FASTEF reactor. The first part of the work describes the general features and the historical background of the SIMMER-III code, such as the code assessment, and also a state of the art concerning the applications of the code to both separate effect facilities and full scale reactors. In this context, the SIMMER-III code can be adopted in analyses of sodium-cooled fast reactor, lead-cooled and LBE-cooled fast reactors; with some limitations and integration by additional models, it can be also applied to molten salt reactors and light water reactors. The second part focuses on the description of the MYRRHA-FASTEF system and on its modelling by SIMMER-III, highlighting the adopted modelling of the different components. The reactor was simulated by a 2-Dimensional axial-symmetric geometry. The results of steady-state and transient calculations are then reported. The steady-state analysis was performed in order to assess the correctness of the code and of the adopted model; so, the obtained results in relation to the major variables were compared with the design values. In particular, the most relevant results obtained for temperature trends and profile, both in the core and in the PHX, and the velocity and mass flow rate trends are reported. Significant thermal stratification is predicted by SIMMER-III in the upper plenum of the vessel which is responsible for temperature oscillation at the PHX inlet. Finally, transient analyses were performed. Selected design basis condition transients and design extended condition transients were addressed in order to assure a sufficient safety level of the reactor, following postulated accidents or in unprotected transients. The sensitivity to the MOX density on fuel redistribution in the primary circuit has been also investigated. After fuel release, a certain amount of fuel particles is transported by the LBE coolant and, depending on the fuel porosity and the type of circulation, it tends to settle down or to float at the LBE free surface

Implementation of the chemical PbLi/water reaction in the SIMMER code

The availability of a qualified system code for the deterministic safety analysis of the in-box LOCA postulated accident is of primary importance. Considering the renewed interest for the WCLL breeding blanket, such code shall be multi-phase, shall manage the thermodynamic interaction among the fluids, and shall include the exothermic chemical reaction between lithium-lead and water, generating oxides and hydrogen. The paper presents the implementation of the chemical correlations in SIMMER-III code, the verification of the code model in simple geometries and the first validation activity based on BLAST Test N°5 experimental data

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

Development of a SIMMER\RELAP5 coupling tool

Abstract The In-Box Loss Of Coolant (LOCA) postulated accident is considered a major concern for the safety connected with the development of EU-DEMO fusion reactor. Relating to the renewed interest in the Water-Cooled Lithium Led blanket concept, an innovative experimental campaign is under development at ENEA Brasimone laboratories aiming at investigating the consequences related to the In-Box LOCA applied to the WCLL breeding blanket. In this frame, a new coupling tool between the SIMMER-III (modified version to implement the PbLi/water chemical interaction) and the RELAP5/Mod3.3 codes (modified version to implement PbLi thermo-physical properties) has been developed together with its preliminary application to simple test cases with water as working fluid. The coupling procedure can be defined as a "two-way", "non-overlapping", "online" technique aiming at investigating multi-physics and multi-scales phenomena in support of the development of fusion reactor technologies

Conceptual Design of the Steam Generators for the EU DEMO WCLL Reactor

In the framework of the EUROfusion Horizon Europe Programme, ENEA and its linked third parties are in charge of the conceptual design of the steam generators belonging to EU DEMO WCLL Breeding Blanket Primary Heat Transfer Systems (BB PHTSs). In particular, in 2021, design activities and supporting numerical simulations were carried out in order to achieve a feasible and robust preliminary concept design of the Once Through Steam Generators (OTSGs), selected as reference technology for the DEMO Balance of Plant at the end of the Horizon 2020 Programme. The design of these components is very challenging. In fact, the steam generators have to deliver the thermal power removed from the two principal blanket subsystems, i.e., the First Wall (FW) and the Breeding Zone (BZ), to the Power Conversion System (PCS) for its conversion into electricity, operating under plasma pulsed regime and staying in dwell period at a very low power level (decay power). Consequently, the OTSG stability and control represent a key point for these systems' operability and the success of a DEMO BoP configuration with direct coupling between the BB PHTS and the PCS. In this paper, the authors reported and critically discussed the FW and BZ steam generators' thermal-hydraulic and mechanical design, the developed 3D CAD models, as well as the main results of the stability analyses and the control strategy to be adopted

Safety Investigation of in-box LOCA for DEMO Reactor: Experiments and Analyses

The aim of this Ph.D. thesis work is the safety investigation of in-box Loss Of Coolant Accident for the Water Cooled Lithium Lead breeding blanket concept in DEMO reactor. The research work, conducted at ENEA C.R. Brasimone, is a crosscutting activity carried out under the international framework of the EUROfusion Consortium Breeding Blanket Design and Safety Projects. The research activity starts with a comprehensive literature review. The interaction between heavy liquid metal and water is investigated with main focus on the phenomena relevant to the safety of the WCLL breeding blanket. Past experiments and numerical activities have been identified and reviewed. The study highlights the lack of qualified and reliable numerical codes able to predict the phenomena involved in such postulated accidental scenario. Starting from the outcomes of the literature review, main and innovative goal of the Ph.D. research activity is twofold: 1) setting-up a qualified computer code for deterministic safety analysis of the WCLL BB in-box LOCA, also in support to the design of the breeding blanket and its connected systems, 2) validating the code against experimental data available in literature or provided by the new LIFUS5/Mod3 campaign specifically designed for code validation purposes. The first objective is fulfilled by the PbLi/water chemical reaction model implementation in SIMMER-III and SIMMER-IV codes and the successfully verification and validation processes. A methodology for code validation is established based on a three-step procedure: 1) the initial condition results, 2) the reference calculation results, and 3) the results from sensitivity analyses. The methodology is applied to all available LIFUS5 tests. The post-test analyses highlight open issues of test execution and of experimental data, as well as code limitations and capabilities. The qualitative accuracy evaluation is performed through a systematic comparison between experimental and calculated time trends based on the engineering analysis, the resulting sequence of main events, and the identification of phenomenological windows and of relevant thermo-hydraulic aspects. Finally, the accuracy of the code prediction is evaluated from quantitative point of view by means of selected, widely used, figures of merit. Second key point of the research activity is the design and the follow up of a new experimental campaign in LIFUS5/Mod3. The experimental campaign is unique and innovative, focused on chemical reaction code model validation, thanks to the generation of meaningful, qualified and reliable data with well-known initial and boundary conditions. Supporting and pre-tests analyses by SIMMER-III and RELAP5/Mod3.3 codes are executed to provide useful data (e.g. injection pressure, water mass flow rate, volume of cover gas, temperature map, hydrogen measurement line) for the final design of the facility configuration and instrumentation choice. Finally, the validated SIMMER code is applied to the WCLL BB in-box LOCA. Preliminary safety investigations are carried out to evaluate the consequences of the double-ended water pipe break in the breeding zone and to investigate possible countermeasures to mitigate the transient

Fuel Dispersion and Flow Blockage Analyses for the Myrrha-Fastef Reactor by Simmer Code

This work illustrates the 3D set up model and the results concerning the recent analysis of fuel dispersion in the MYRRHA-FASTEF reactor performed with SIMMER code within the EU-FP7 SEARCH Project. Under severe accidental conditions, the release of fuel in the primary system can occur in case of fuel rod clad failure and degradation. Two cases were therefore taken into account, an imposed fuel release to study key parameters which influence the dispersion phenomenon and a coolant flow blockage in a fuel assembly. The reactor was simulated by a 3D Cartesian geometry with 65x63x42 cell mesh. Steady-state and transient analyses were performed by SIMMER-IV. Steady-state analysis was performed in order to assess the correct operability of the code and of the model. The results were compared with the design values. The most significant results obtained for temperature trends and profiles, velocity and mass flow rate trends are reported. Transient results were also analysed, i.e. fuel dispersion transients were simulated, comparing the effect of fuel porosity on the fuel dispersion inside the pool. In addition, the effects of the release position and the fuel particle dimension on the dispersion phenomenon were also investigated. The final section of the paper describes the effects of a flow blockage on the core degradation and dispersion of fuel particles in the primary circuit of the MYRRHA reactor. This simulation, with fuel porosity equal to 5%, started after a preliminary steady state condition. The mass flow rate in one of the inner fuel assemblies was then reduced to about 10% of the initial value. The results show that the SIMMER-IV code is capable of predicting steady-state results in good agreement with the nominal values, also confirming the correctness of the set up model

Validation Methodology applied to SIMMER code for Fusion Applications

The V&V activity of the modified version of SIMMER-III code for fusion applications has been carried out applying a standard methodology for code validation. The methodology is based on a three-steps procedure and through qualitative and quantitative evaluations: 1) the initial condition results, 2) the reference calculation results, and 3) the results from sensitivity analyses. The qualitative accuracy evaluation is performed through a systematic comparison between experimental and calculated time trends based on the engineering analysis, the resulting sequence of main events, the identification of phenomenological windows and of relevant thermo-hydraulic aspects. Finally, the accuracy of the code prediction is evaluated from quantitative point of view by means of selected, widely used, figures of merit. The methodology was applied to the LIFUS5 campaign, available in literature. Post-test analyses highlighted open issues of test execution and of experimental data, as well as code limitations and capabilities

Implementation of the chemical PbLi/Water reaction in the SIMMER code

The availability of a qualified system code for the deterministic safety analysis of the in-box LOCA postulated accident is of primary importance. Considering the renewed interest for the WCLL breeding blanket, such code shall be multi-fluid multi-phase, shall manage the thermodynamic interaction among the fluids, and shall include the exothermic chemical reaction between lithium-lead and water, generating oxides and hydrogen. The paper presents the implementation of the chemical correlations in SIMMER-III code, the verification of the code model in simple geometries and the first validation activity based on BLAST Test N°5 experimental data. The validation activity will be based on a validation matrix, which will be based on the new experimental infrastructure LIFUS5/Mod3 installed at ENEA CR Brasimone

SIMMER-III/ANSYS coupling (Preliminary development and validation activities of an integral tool for the design and analysis of WCLL components)

In the framework of the development of fusion energy, one of the most prominent technologies arising to address the issues of tritium breeding and power conversion is the Water-Cooled Lithium-Lead (WCLL). This technology utilizes a molten eutectic alloy of Lithium and Lead which circulates inside the Breeding Blanket (BB) and is irradiated with neutrons to produce tritium. Water is then circulated inside the system to cool the components. The simultaneous presence inside critical areas of the reactor of molten metal alloy and water, at high temperature and pressure, poses significant safety concerns. For this reason, adequate design and analysis techniques are required to ensure the ability of the system to survive and mitigate any possible damage in case of the in-box Loss of Coolant Accident (LOCA), the most critical postulated accidental scenario. With this aim in mind, a novel approach was implemented with the aim of coupling the SIMMER-III code and the ANSYS Mechanical code for the modelling of both the chemical and thermodynamical interactions between water and the alloy, and the resulting effects on the structures. This work presents the status of the coupling technique development and the results of the preliminary validation activities performed against experimental data provided by the LIFUS5 facility operating at ENEA Brasimone Research Centre. The resulting comparison between these data and the codes' predictions allows a careful evaluation of the errors introduced in each step of the chain. Moreover, it provides confidence in the capacity of the methodology to correctly predict the ability of the structures to withstand incidental loads without suffering extensive damage.This work aims at providing engineers with a usable and powerful tool that allows for the safety analysis of WCLL-based components during the early stages of the design phase. This would help save time, and effort and reduce the economic cost that might arise from any undetected issue propagating downstream the design process