ABSTRACT
The fuel matrix itself and the hermetically sealed cladding of the fuel pin are the first two barriers to the release of radioactivity in a nuclear reactor (the third and fourth being the reactor coolant system boundary and the containment structure, respectively). Consequently, an accurate description of the temperature distributions in fuel elements and reactor structures is essential to the prediction of the lifetime behavior of these components. The temperature gradients, which control the thermal stress levels in the materials, together with the mechanical loads contribute to the determination of the potential for plastic deformation at high temperatures or cracking at low temperatures. The temperature level at coolant-solid surfaces controls chemical reactions and diffusion processes, thus profoundly affecting the corrosion process. Furthermore, the impact of the fuel and coolant temperatures on neutron reaction rates provides an incentive for accurate modeling of the temperature behavior under transient as well as steady-state operating conditions. In this chapter, the focus is on the steady-state temperature field in the fuel elements. Many of the principles presented are also useful for describing the temperature field in the structural components.
