Thermal Hydraulics for Pressurized Water Reactors

Thermal-hydraulics is the study of heat transfer coupled with flow hydraulics, taking into account variable density resulting, for example, from fluid heating or cooling. However, the most complex aspect remains the phase change and treatment of two-phase water-steam flows. In a pressurized water reactor, the water in the primary circuit has a dual role as coolant and moderator. In addition to the challenges of controlling energy transfer and therefore temperatures in the systems and components of a pressurized water reactor, its temperature or the quantity of steam present has a primordial impact on the reactivity and therefore the neutronics of the core.

Thermal-hydraulics is concerned with the state of the fluid, its mode of circulation and heat exchanges in the reactor, in normal operating situations, incidents and accidents, including the most serious ones, to ensure that safety criteria are met in all circumstances.

The methods and models presented in this article constitute the basic principles for understanding fluid mechanics problems involving heat exchange or two-phase flow (liquid and steam). We adopt an empirical approach, since theoretical models and equations are the domain of specialized literature. These empirical models are mainly based on load hydraulics and the thermal properties of circular pipes. This paper is not intended to cover the whole range of knowledge, and in particular does not cover phenomena relating to the analysis of severe accidents. This article provides an introduction to physical phenomena, their interpretation and modeling.