Setting-up and Qualification of an Approach for the Safety Analyses of the In-Box LOCA for DEMO Reactor

The main focus of the present research activity was on the numerical and experimental investigation of lead-lithium eutectic alloy (PbLi) and water interaction during an In-box Loss of Coolant Accident (LOCA) scenario. The thermodynamic characteristics of the Water-Cooled Lithium-Lead Breeding Blanket (WCLL-BB) for the demonstration thermonuclear fusion reactor (so-called DEMO reactor) were applied for the study. The conceptual design of the DEMO is developed and continuously improved under the supervision and coordination of the EUROfusion Consortium Agreement. The two activities were carried out in parallel during the three-year research period and performed mainly at the Experimental Engineering Division of the Department of Fusion and Technologies for Nuclear Safety and Security (FSN-ING-SIS) of ENEA Brasimone R.C., in the framework of a bilateral collaboration between ENEA and the Department of Civil and Industrial Engineering (DICI) of the University of Pisa. The main investigated objectives were the pressure and temperature transients, Hydrogen and heat generation and release due to the chemical reaction during water injection transient. These factors are among the most relevant parameters for the safety objectives of the DEMO reactor. The experimental activity was split into two separate campaigns, so-called Series D and Series E, and aimed at capturing and measuring the water injection transient by means of some high precision instrumentation, which were employed to acquire the real-time experimental signals during the experiments. The Series D tests were related to the experiments with a specific amount of pressurized water to be injected into the liquid PbLi, while the Series E tests were linked to continuous injection of pressurized water into the liquid PbLi. For this purpose, the LIFUS5/Mod3 test facility was constructed at ENEA research centre, Brasimone. The instrumentation was optimized and installed on all sections of the test facility. Furthermore, a new test section module was designed to support the in-vessel thermocouples. The acquired experimental results enrich the set of data available in the literature, which are essential for the characterization of such kind of chemical interaction and for the assessment of multiphase multicomponent safety codes. The numerical activities concerned pre-test and post-test studies for Series D and E, adopting the two-dimensional SIMMER-III (SIII) code on LIFUS5/Mod3 facility. This version of SIII code had been previously improved as a part of research activity and identified as “SIMMER-III Ver. 3F Mod. 0.1” at the University of Pisa by implementing the chemical reaction between PbLi eutectic alloy and water. Pre-test SIII simulations were developed for the experimental campaign (Series D and Series E) to support the experimental activities by specifying the proper initial and boundary conditions and criteria for the relevant transient parameters. Post-test SIII analyses were developed for Series D with an identical nodalization and facility configuration, based on a standard code assessment procedure. The applied methodology for the code validation was based on a standard three-step procedure including qualitative analysis, quantitative analysis, and the results from sensitivity analyses. Then the qualitative accuracy evaluation was performed for all performed tests through a systematic comparison between experimental and calculated time trends based on the engineering analysis. The analyses were followed by applying the Fast Fourier Transform Method (FFTM) to the experimental signals and all the sensitivity calculations. Finally, the accuracy of the numerical model was evaluated from a quantitative point of view and the outcomes of the whole code assessment procedure were reported as the code qualification.