In the framework of improving nuclear power plant safety during accidents, the current study is focused on developing and applying a new technique for the high temperature determination of thermal conductivity, specific heat, thermal diffusivity, spectral and total hemispherical emissivity, as well as the melting points of nuclear materials. The new inverse method is based on the laser flash approach. The novel use of a finite element analysis (FEA) model (as part of the inverse method) allows accurate description of complex boundary conditions and has led to an improved accuracy of the evaluated properties. The inverse method has been tested for use with two devices – a pyrometer and an infrared camera. All experimental results are complemented by novel and existing solid state theory. The results obtained on isotropic, isostatically pressed, graphite (neutron moderator) are in good agreement with literature values (available to around 1500 K) and extend the available data up to 2800 K. Values are reported of specific heat, thermal conductivity and thermal diffusivity of UO2 from 1500 K to 2900 K. The specific heat model and measurements show, for the first time that a gradual pre-melting transition is consistent with high temperature literature values – enthalpy increment measurements and independently measured high temperature oxygen defect concentrations. Experimental results are also presented for the thermal conductivity, specific heat, thermal diffusivity, spectral and total hemispherical emissivity of ThO2 from 2000 K to 3050 K. It is the first time direct measurements of thermal conductivity and total hemispherical emissivity have been carried out on ThO2 at such high temperatures. All new results commit to a better understanding of the properties of nuclear materials, in particular nuclear fuel, under extreme conditions. Hence, this work will contribute towards improved predictions of nuclear core behaviour during an accident (using fuel performance or severe accident codes).