Thermal-hydraulics of passive safety systems for advanced Nuclear Reactors

The present thesis addresses several topics of thermal-hydraulics applied to the heat transfer in the nuclear field, both in normal operating and accidental conditions. The problem is to demonstrate that innovative heat exchanger geometries and safety systems are able to effectively remove thermal power from a primary system of a nuclear reactor. Normal operating condition was addressed by looking at several heat exchanger and steam generator geometries for heat removal in advanced nuclear reactors, such as Small Modular Reactors and generation IV reactors. Helical coil, microchannel and bayonet tube geometries have been studied. Concerning helical coil heat exchangers, a numerical model has been developed to study the thermal performance of a unit on Matlab Environment. The model requires input data such as the geometry, the type of fluids and boundary conditions, and is able to compute the spatial behaviour of the most important thermal-hydraulic parameters of the flow such as temperature, pressure, void fraction, quality, heat transfer coefficients, etc. The model adopts an iterative solving scheme of solution for the different control volumes of the component and semi-empirical correlations to compute heat transfer coefficients and pressure drops. The model has been validated against reference data of the steam generator of IRIS reactor and BREST fast reactor, and demonstrated to be able to reproduce the reference data with a very high accuracy. The microchannel configuration has been studied for the large modular reactor I2S by means of the system code RELAP5-3D. The bayonet configuration has been widely studied for ALFRED reactor in LEADER configuration by means of the system code RELAP5-3D. Sensitivity studies have revealed the impact of some important geometrical parameters and the role of regenerative heat transfer on heat exchanger performance. Results obtained during the PhD period have been used for optimization purposes of ALFRED steam generator. Some heat transfer devices connecting safety systems to the environment have been studied in the light of accidental sequences. In particular, air heat exchangers and pool heat exchangers have been investigated through several RELAP5-3D simulations in the light of a Station Black Out (SBO) event for the large integral reactor I2S. The most important topic of the thesis is the decay heat removal system of ALFRED reactor. The safety system is both able to remove power due to decay heat and to delay primary coolant freezing in the long term of operation by means of noncondensable gases. Both safety functions are performed in a passive manner. The safety system makes use of the bayonet steam generator for normal operation and connects it to a ultimate heat sink, which is an isolation condenser. A tank is connected to the bottom of the isolation condenser and allows the housing of noncondensable gases. Following an accident, noncondensable gases are firstly collected in the tank and then released during time to reduce the heat transfer in the isolation condenser, so to avoid primary system overcooling. In this framework, the present PhD thesis presents a set of sensitivity studies performed on the safety system by means of the system code RELAP5-3D. Initial gas pressure and tank volume have been addressed. The study have highlighted the most important parameters for design optimization and revealed some important physical phenomena. In a second part, the safety system has been used as reference to design an experimental facility able to reproduce the most important physical phenomena on which the safety system relies. The scaled facility will be built in SIET laboratories in Piacenza thanks to the SIRIO project, partially funded by the Italian Ministry of Economic Development. The present thesis reports the development of the conceptual design of the facility and some preliminary simulations to ensure that the facility can reproduce the most important phenomena of the safety system