Several in-pile experiments in the Halden Boiling Water Reactor with Zircaloy-clad UO2fuel provided a comparison between vibrocompacted and pelleted fuel operating at power densities producing permanent central melting and density effect on oxide thermal conductivity. In addition to the experiments measuring the effect of cold diameter gap on gap heat conductance, fission gas release and the axial expansion of fuel and cladding were measured. The data were obtained from fully instrumented test assemblies using thermocouples, pressure transducers, and elongation differential transformers. Power was measured byγthermometers andβ-current neutron detectors.