Highlights of experimental thermodynamics in the field of nuclear fuel development

Results of basic thermodynamic studies are presented in the fields of oxide fuel pins and their alloys with fission products and of the neutron multiplier beryllium. Gibbs energy of formation measurements of solid phases using galvanic cells with CaF2 electrolyte are exemplarily described for chromium carbides, URu3 and URh3. The calculation of the corresponding value of UPd3 is demonstrated using the phase diagram of the U–Pd–Sn system. The phase behaviour of (U, Pu)O2 in a steady-state temperature gradient between the solidus and the liquidus is explained. The O/(U + Pu) redistribution in a temperature gradient is measured which allows the calculation of the heat of transport of the oxygen vacancies. The obstacles for measurements of the enthalpies of transformation and melting of beryllium are commented, the temperatures are only 14 degrees apart from each other below melting at 1283 °C. The high enthalpy of transformation is interpreted by the compressed c/a-axis ratio of the hcp modification.