Thermodynamics applied to Severe Accidents in Nuclear Reactors

To understand and model all the physical phenomena that can occur during a severe accident in a nuclear reactor, a good understanding of the properties of reactor core materials, and in particular their thermodynamic properties, is essential. During such an accident, very high temperatures can be reached (potentially in excess of 3120 K, which is the melting temperature of UO ₂ fuel) so that the materials in the various core components (control rod, cladding, fuel...) can melt and interact to form complex mixtures (a mixture of materials commonly known as corium). Corium is generally characterized by the presence within it of a large number of chemical elements and may present a multiphase appearance (for example, a mixture of a liquid phase and solid phases, a mixture of two immiscible liquid phases...).

Thermodynamics tells us what the physical state is at equilibrium, how this state changes with state variables such as composition and temperature, and therefore the conditions under which a transformation can take place in a given direction. Of course, it says nothing about the mechanisms of transformation, or how long they take, and therefore nothing about the kinetics of reaching equilibrium. But to be able to predict the evolution of core degradation in an accident situation, it is important to be able to distinguish possible evolutions from those that are not, and this is what thermodynamics enables us to do with certainty. In particular, it enables us to predict the material's state of order (in other words, the phases at thermodynamic equilibrium) as a function of the state variables. Knowledge of this state of order is a prerequisite for the implementation of many of the models and approaches used in severe accident simulation codes.

The difficulty of understanding the thermodynamic behavior of materials, both experimentally and in terms of modeling, lies in the fact that it must cover not only the materials of the core components taken individually, but also the mixtures resulting from the interaction of these materials with each other, over a temperature range extending from the nominal operating temperature of the reactor to temperatures that can reach fuel melting (3,120 K for UO ₂ ). The difficulty of the task is easy to appreciate.

Traditionally, and for a long time now, knowledge of the thermodynamics of a material is gained by drawing up a phase diagram, which is a graphical representation of the material's state of order as a function, generally, of composition and temperature. The phase diagram is determined experimentally, based on measurements of various properties (phase change temperatures, phase compositions after quenching,...