Mass and gamma spectrometric measurements of fission products released from overheated, fresh irradiated, uranium dioxide

Abstract Small, sol/gel-fabricated, UO 2 spheres were completely evaporated in a tungsten crucible shortly after irradiation. The crucible content was monitored with a germanium gamma spectrometer and the particle bundle emerging from the crucible was analysed with a quadrupole mass spectrometer. 3-D gamma tomography was applied on some samples but will not be discussed here. The intensities of the uranium peaks in the mass spectra which are a function of temperature were used in an evaporation model to calculate the initial stoichiometry of the sample. There appeared to be no clear relation between the burnup and the initial stoichiometry. Hypostoichiometric samples showed a considerable retention of the fission gasses, Cs, Te and I, even at temperatures as high as 2500 K. This could indicate the existence of a retention mechanism involving oxygen. In one case Ba was more volatile then I. Evaporation of fuel matrix material was the dominant release mechanism above 2400 K. The fission products Cs, Te, I, Ba and Sr were released as atoms, Ba as BaO in oxygen-rich fuel. Sr did not behave like Ba. The metals Zr, Mo, Pd and Tc were not released before complete evaporation of the sample. Nb can be as volatile as Ba if the UO 2 has a high oxygen potential. The lanthanides and Y appeared to be nonvolatile and were released upon evaporation of the matrix. Lanthanum itself was released somewhat earlier in two tests. In some tests Ce was released (as CeO 2 ) before evaporation of the matrix became dominant.