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.