EXAFS STUDY OF THE AGING OF LANI5 AND LANI4.5MN0.5 COMPOUNDS DURING TRITIUM STORAGE

The modification of the thermodynamic and structural properties of LaN i5 and LaNi4.5Mn0.5 tritides, due to the presence of helium coming fro m the radioactive decay of tritium is studied by pressure-composition- temperature (P-C-T) isotherms determination, X-ray diffraction (XRD) a nd X-ray absorption spectroscopy (XAS) experiments. For high He conten t a loss of long range order is observed and Extended X-ray Absorption Fine Structure (EXAFS) study show a small increase of the static diso rder around Ni and Mn atoms and a larger one around La atoms. The assu mption that a great part of the helium is located in the largest inter stitial sites of the LaNi5 or LaNi5-xMx cell, leading to a cell distor tion for high He content, allows to explain both thermodynamic and str uctural results.