Material requirements for a thorium based nuclear fuel

Abstract

The increase in the energy consumption and the expected growth in the nuclear capacity make it necessary to look for alternative fuels to replace uranium. The fuel chosen, which was also considered in the early stages of nuclear energy, is thorium. Thorium has some characteristics that make it valuable as a fuel, like its abundance, the low radio-toxicity of the waste generated, the higher economy regarding its larger absorption cross-section and higher burn-ups and the proliferation resistance as compared to uranium. Despite these benefits it also raises some questions relating its safe operation in the reactor.

The aim of this work is to offer an overview about the use of thorium as a fuel element in a power reactor and the critical issues that the cladding faces. The programs run in different countries to use thorium, the benefits and challenges that presents and the physical configurations inside the reactor are explained. This work focuses in the configuration proposed by A.Radkowsky which is to have thorium (blanket) and enriched uranium (seed) in different assemblies. The physical schemes in the reactor core are the seed-blanket unit and the whole-assembly seed and blanket core.

The increased power density, higher burn-up and longer residence time in the reactor of thorium fuel enhance some potential failure mechanisms which are presented in this work. This thesis also seeks to give a general idea about the materials used in the reactor, focusing on the cladding that is the first barrier and the element subjected to toughest operating conditions.

A modeling program called FEMAXI is used to simulate the interaction between the fuel element and the cladding in the high burn-up region. Two physical phenomena are modeled, inner pressure and cladding corrosion, showing that the limiting factor would be corrosion due to the long residence time in the reactor.

In order to understand the difficulties to reach the operating conditions of thorium fuel, an overlook at the licensing process is done. It shows the strict safety conditions which have to be accomplished, especially with postulated accidents.