Activity assessment of structure materials in the research reactor Thetis before its dismantling

Thetis is a pool-type research reactor, located at the nuclear science institute of Gent University in Belgium. Its nominal power is 250 kW. Thetis is fuelled With UO2 (5% enriched in U-235), cooled and moderated by light water. The definitive shutdown of Thetis is planned for the end of 2003. Before its dismantling, it is necessary to evaluate the types and the quantities of radioactive waste produced during its operating life and their contribution to the total activity. In this framework, this paper provides the methodology (computer codes and nuclear data) followed for the activity assessment in the structure materials, especially graphite (reflector) and stainless steel and concrete zones (biological shielding), and the calculation results concerning the level of activities reached and their characterization.