Design evaluation of a scaled experiment to investigate natural circulation in MYRRHA

MYRRHA (Multi-purpose HYbrid Research Reactor for High-tech Applications) is a multifunctional research nuclear reactor, conceived as an AcceleratorDriven System (ADS), able to operate in sub-critical mode, and with the removal of the spallation target and the insertion of control and safety rods, is also able to operate in critical mode. It consists of a 600 MeV proton accelerator, a spallation neutron source and a nuclear core with MOX fuel, cooled by liquid Lead-Bismuth Eutectic (LBE), which is composed by 44.5% of lead and 55.5% of bismuth. MYRRHA will be a flexible fast spectrum irradiation facility used for: material development, study of transmutation of high level nuclear waste, and production of radioisotopes for medical and industrial applications, for example the production of Mo99. This thesis focuses on the safety aspects of the reactor system during postulated incidents in which the coolant pumps fail or are not available. In these particular conditions the cooling of the reactor must be assured by natural circulation. Natural circulation is an important and intrinsic safety feature in nuclear reactors. The purpose of this thesis is to analyze if the natural circulation of the LBE could remove the decay heat after the shutdown as a passive cooling system. In order to simulate the natural circulation of the LBE it is important to scale the reactor, to achieve the same behavior. Therefore, as the main goal of this thesis I will try to scale a simplified model of the reactor, in order to simulate the passive decay heat removal. Generation IV reactors, to which the MYRRHA project belongs, must follow some criteria, such as a lower likelihood of accidents and a lower influence on the environment. These criteria are given with the core damage frequency (CDF) and large early release frequency (LERF), which need to be ten times lower than the previous generation. Therefore, passive safety systems are used in order to cool down the reactor without relying on external mechanical or electrical systems, but rather relying only on processes such as natural convection heat transfer, vapor condensation, liquid evaporation. This last generation of reactors, which are still being developed, is founded on 4 main principles, which are: sustainability, safety and reliability, economic viability and no proliferation. It is necessary to explain these principles one by one: • Sustainability concerns two different aspects. The first one is to produce clean energy and guarantee long term durability of the system. The second one regards minimizing nuclear waste as much as possible, improving protection for the public safety and for the environment; • Safety and reliability means that this kind of reactor will have low likelihood and degree of reactor core damage, in order to avoid accidents like the one in Chernobyl; • Economic viability means that the energy and the products from nuclear power plants will have cost advantage over other energy sources, and a level of financial risk comparable to other energy projects; • Proliferation resistance and physical protection means that the materials from the nuclear energy cannot be used to produce nuclear weapons and it is important to improve the physical protection against acts of terrorism. The purpose of the MYRRHA project is to reach all these principles and trying to achieve natural circulation as a passive safety system works in this direction. Natural circulation happens in presence of temperature and density gradients in a force field. There are a heat sink and a heat source, with the latter placed below the former. As a consequence of the heat fluxes, the heated part of the fluid becomes lighter and rises, because its temperature increases, while the cooled part becomes denser and is dropped down by gravity. There are many problems to achieve the natural circulation in a reactor: • Low driving forces: to increase those it is necessary to increase the vertical distance between the heat sink and the heat source, but that is uneconomic and the height has to be less than 10 meters due to seismic concerns; • Low system pressure losses: with low driving force, the only way to obtain reasonably large flow rates is to design for low pressure losses; • Low mass flux: because of the first two points, the flux may be low if the driving forces are not able to win the pressure losses in a loop; • Instability effects: any change in the driving force affects the flow, which in turn affects the driving force. This can cause an oscillatory behavior. In MYRRHA there are several heat sources competing each other: the decay heat from the spent fuel stored into the vessel, the decay heat of the assemblies in the core, and the distributed heat of the Polonium created into the LBE. There are also several possible heat sinks: the four primary heat exchangers (PHX), and the RVACS (Reactor Vault Cooling System), which is the space between the 2 layers of the vessel, which in case of an accident can be filled with water which by boiling removes heat. The first chapter of this thesis will look at the structure of MYRRHA, the properties of the LBE as a coolant, and how natural circulation works. The second chapter will be about the simulations of a cavity, filled by different fluids, as a reference case study of natural convection heat transfer. The third chapter will estimate the temperature difference inside a simplified model of the reactor to remove a representative power of the decay heat (1MW), after that the behavior inside a specific part of the reactor will be simulated in order to validate the calculations. The fourth chapter will focus on the scaling and a 3D simulation of the reactor, with two heat sources and two heat sinks. Finally, Appendix A will explain what a turbulent flow is and what turbulence models have been used for these simulations, the equation of the turbulence models, and also what changes when you simulate a low Pr flow.