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MOX deposition tests were continued in 2002 at RIAR, to increase the Pu concentration in the MOX deposit. The objective was achieved but a significant amount of PuO2precipitates was found. This phenomenon was investigated by evaluating a hypothetical quantity named “Quasi Equilibrium (Q.E.) concentration” of plutonyls in the melt and their solubility. It was found that the oxygen pressure was too high in the 2002 tests, which made the Q.E. concentration too high and the chlorine pressure was too low, which reduced the solubility of plutonyls in the melt. It may be necessary to increase the oxygen pressure to obtain MOX deposit with Pu concentration higher than 30%, but the pressure must be limited depending on the chlorine pressure to avoid the Pu precipitation.