In situX-ray diffraction study of point defects in neptunium dioxide at elevated temperature

Abstract

High-temperature X-ray diffraction measurements have been performed on neptunium dioxide up to 2000 K for the first time under He, He/5%H2and air atmospheres. Up to 1643 K, NpO2remains stoichiometric under all the considered atmospheres, and the coefficients of thermal expansion have been evaluated. Above 1643 K, the lattice parameter departs from linearity towards higher values. The atomic displacement parameters of the O and Np atoms were determined from Rietveld refinement and the Debye temperature subsequently obtained. This was used to study the contribution of point defects to the evolution of the lattice parameter at elevated temperature by estimating the energy of formation of vacancies. It is shown that only the chemical reduction of NpO2to NpO2−xis responsible for the departure from linearity below 1750 K. Above this temperature, the evolution is due to the simultaneous effect of reduction and the formation of oxygen Frenkel pairs.

Melting behavior of (Th,U)O2 and (Th,Pu)O2 mixed oxides

“…On the contrary, previous experimental studies reported abrupt increase of specific heat due to formation of Frenkel pairs and order-disorder pre-melting transitions [51][52][53][54][55].…”

“…Fink et al [25] estimated the specific heat Cp(liquid) to be 61.76 J K the formation of oxygen defects, and in particular of Frenkel pairs, leads to an abrupt increase of the heat capacity as temperature approaches the melting point [51,52]. In the case of UO2 and ThO2, even a pre-melting order-disorder ''lambda'' transition has been proposed by certain authors [53][54][55].…”

Lattice thermal expansion of Pu1−y Am y O2−x plutonium–americium mixed oxides

“…Consequently, as the drift from linearity observed as a function of temperature tends towards greater values of the lattice parameter of the fluorite structure, the materials studied are progressively reduced, i.e. their O/M ratio decreases (Chollet et al, 2014;Epifano et al, 2016Epifano et al, , 2017Vauchy, Belin, Robisson & Hodaj, 2016). As this drift increases with the americium content, one can expect that, at a given temperature, the greater the americium content, the lower the O/M ratio.…”

New sample stage for characterizing radioactive materials by X-ray powder diffraction: application on five actinide dioxides ThO2, UO2, NpO2, PuO2 and AmO2

“…In some materials, the structural damage generated by decay can lead to a complete amorphization of the material (Walter et al, 2007;Vigier et al, 2019). Furthermore, the self-irradiation generally induces a significant lattice swelling over time (Turcotte, 1976;Weber, 1984;Baclet et al, 2007;Thiebaut et al, 2007;Ravat et al, 2007;Nishi et al, 2013;Lebreton et al, 2014, (Baena et al, 2015) 5.47127 ( 8) (Leinders et al, 2015) 5.434 (1) (Chollet et al, 2014(Chollet et al, , 2012 5.39819 (1) (Belin et al, 2004) 5.3755 ( 5) (Vauchy et al, 2017) 5.593 (3) (Horlait et al, 2012) 5.4702 (4) (Yamashita et al, 1997) 5.4338 (3) (Yamashita et al, 1997) 5.3954 (4) (Yamashita et al, 1997) 5.375 (1) (Epifano et al, 2017(Epifano et al, , 2016 5.5974 (6) (Yamashita et al, 1997) 5.470 (Markin & Street, 1967) 5.4328 (2) (Fahey et al, 1974) 5.3953 (4) (Noe & Fuger, 1974) 5.3733 (Fahey et al, 1974) 5.5972 (5) (Zachariasen, 1948) 5.4704 (1) (Baldock et al, 1966) 5.434 (Sudakov et al, 1973) 5.3960 (3) (Ellinger, 1961) 5.377 (3) (Zachariasen, 1949) Figure 12 Experimental lattice parameters of the fluorite structure of ThO 2 , UO 2 , NpO 2 , PuO 2 and AmO 2 obtained in this study compared with the values in the literature from Table 2. 2016; Vigier et al, 2018).…”