Process for manufacturing a pellet of at least one metal oxide

A process for sintering a compacted powder of at least one oxide of a metal M1 selected from an actinide and a lanthanide, this process comprising the following successive steps (a) to (c), carried out in a furnace and under an atmosphere comprising an inert gas, dihydrogen and water, of:

• (a) increasing the temperature from an initial temperature TI to a hold temperature TP, TP being between 1 400° C. and 1 800° C.,
• (b) maintaining the temperature at the hold temperature TP for a duration between 1 h and 10 h, and
• (c) decreasing the temperature from the hold temperature TP to a final temperature TF,
• wherein the ratio of the partial pressures P(H2)/P(H2O) in said atmosphere is such that:
• 500<P(H2)/P(H2O)≤50 000, during step (a), from TI until a first intermediate temperature Ti1 between 1 000° C. and TP is reached, and
• P(H2)/P(H2O)≤500, at least during step (c), from a second intermediate temperature Ti2 between TP and 1 000° C., until TF is reached.