[en] The Halden Boiling Water Reactor (HBWR) is used for fuel irradiations in support of safe and reliable operation of nuclear power plants. One of the strengths of the HBWR is the capabilities that have been developed on-site for performing fuel irradiations with in-core instrumentation for on-line monitoring of key parameters. For basic fuel studies, instrumentation for monitoring fuel thermal and mechanical behaviour as well as fission gas release has been developed. In addition, instrumentation for monitoring the fuel pellet interaction with the cladding or for separately investigating the mechanical behaviour of the cladding can be utilized. The different in-core instruments developed for use in the HBWR can be used with either un-irradiated or irradiated fuel rods and under steady-state or transient operating conditions. Purpose-built equipment has been designed and produced in order to attach instruments to re-fabricated segments taken from commercially irradiated fuel rods. Such instruments include fuel centreline thermocouples and in-rod pressure transducers, with all re-fabrication and instrumentation operations being carried out in hot-cell. This paper describes the instrumentation that has been developed at Halden and how it is implemented for characterising fuel performance and behaviour under different testing conditions. (author)

[en] Highlights: • Three different thorium containing fuel types are test irradiated. • Fuel temperatures, cladding elongation and rod pressures are monitored online. • Addition of 7% thorium to the uranium matrix lowers fuel temperatures. • Measurements also indicate lowered temperatures of thorium–plutonium MOX fuel. - Abstract: Thorium based fuels are being tested in the Halden Research Reactor in Norway with the aim of producing the data necessary for licensing of these fuels in today’s light water reactors. The fuel types currently under irradiation are thorium oxide fuel with plutonium as the fissile component, and uranium fuel with thorium as an additive for enhancement of thermo-mechanical and neutronic fuel properties. Fuel temperatures, rod pressures and dimensional changes are monitored on-line for quantification of thermo-mechanical behavior and fission gas release. Preliminary irradiation results show benefits in terms of lower fuel temperatures, mainly caused by improved thermal conductivity of the thorium fuels. In parallel with the irradiation, a manufacturing procedure for thorium–plutonium mixed oxide fuel is developed with the aim to manufacture industrially relevant high-quality fuel pellets for the next phase of the irradiation campaign