New Insights into the Thermal Expansion of Neptunium Dioxide up to 2000 K

We report for the first time the thermal expansion of NpO2 up to 2000 K and compare it to the one of UO2 and PuO2. Lattice parameters were measured as a function of temperature by in situ X-ray diffraction with neutral, reducing and oxidizing atmospheres. Data for NpO2 under neutral and reducing conditions perfectly superpose. Up to 1550 K, thermal expansion varies linearly with increasing temperature. Above, thermal expansion presents a curvature towards higher lattice parameters. No bibliographic data were available for this temperature range for comparison. As for the thermal expansion of UO2, we propose this curvature is certainly inferred by the formation of lattice defects in the oxygen sub-lattice. Expression of thermal expansion and coefficients of thermal expansion at given temperatures for NpO2, are presented.