Experimental and numerical activities for the thermal hydraulic analysis of PbLi eutectic in breeding-blanket concepts of nuclear fusion reactors

This PhD thesis aims to contribute to the thermal hydraulic analyses in support of the development of a Breeding Blanket for nuclear fusion reactors. To achieve this goal, the thesis focused on Lead-Lithium Eutectic (LLE) technologies and leant on some pillars: experiments, numerical simulations and, to a lesser extent, design of experimental facilities. This research activity was performed at the Department of Civil and Industrial Engineering of the University of Pisa, for the numerical activities, at ENEA Brasimone RC, to perform experiments in the existing facilities, and in the Fusion Laboratory at UCLA, to install and commission an upgraded experimental facility. The first chapter of the thesis constitutes an attempt to delineate the several open points on the research involving LLE, reviewing the facilities currently in operation in the world, briefly discussing on the exact eutectic point of the lead-lithium alloys and summarizing the many different correlations proposed for the thermophysical properties of this alloy. If the first two points were required to understand the breadth of the research on LLE and the originality of the thesis, the review of thermophysical properties was needed also to select the correlations to be used during the activities and to be implemented in RELAP5/mod3.3. The first pillar is represented by the experimental activities. Three experimental campaigns were carried out in the facilities IELLLO and THALLIUM at ENEA Brasimone RC. The campaign in IELLLO aimed to qualify components and instrumentation for the use in LLE and, in particular, for the following activities in THALLIUM. The campaigns in THALLIUM, instead, simulated the release of high pressure helium in stagnant LLE, an accidental transient known as In-box LOCA for the HCLL TBS. Eleven injections were performed with different parameters in order to achieve a good comprehension of the involved phenomena. The post-test analyses described how the pressure wave would likely propagate in the LLE loop of the HCLL TBS and highlighted which are the key components to design an effective mitigation strategy. The experimental data from the three campaigns were used to perform numerical simulations, which represent the second pillar. The experiments on LLE circulation and heat transfer carried out in IELLLO were simulated with the system code RELAP5-3D, validating the nodalization of the facility and contributing to test the code with LLE. The eleven injections of THALLIUM were simulated with RELAP5-3D, using Lead-Bismuth Eutectic for lack of LLE properties above 40 bar, and, for injections 6-11, with RELAP5/mod3.3, after the implementation of LLE properties at the University of Pisa. The use of the system code RELAP5 greatly contributed to delve into the dynamics of the In-box LOCA. The simulations showed a good agreement with the experimental data, with two main discrepancies: the delayed effect of the relief valve and the absence of one of the pressure peaks, likely originated by the elasticity of the structure. The third pillar is represented by the design of experimental facilities. In this thesis, the contribution to the upgrade of MaPLE facility is described. The activity, carried out within the framework of the agreement between EUROfusion and UCLA, implied the realization of the P&ID, the design and installation of two main components, the air cooler and the permanent magnet pump, and the fulfillment of the commissioning tests for the whole facility