One of the keystones for MOX manufacturing technologies is the ability of re-using plutonium oxide with an evolving isotopy to manufacture MOX ((U,Pu)O2) fuel pellets. This evolution of isotopy (higher proportions of 238Pu, 241Pu and 241Am) can originate from increased irradiation time of uranium oxide fuel pellets (UO2, UOX) or multi-recycling of MOX fuels. In both cases, the organic additives used in the manufacturing process are facing an increased radiolysis and thermolysis effects.
Two kinds of organic additives are added to the uranium and plutonium oxides after grinding during manufacturing of MOX fuel pellets. A lubricant helps minimising the friction between the powder and the press die, and maximising powder compaction during pressing. A porogen compound is also introduced to regulate densification during sintering, allowing enough closed porosity to remain in order to accommodate fission gases during irradiation and ensure dimensional stability to the pellets.
The lubricant (zinc stearate, StZn) and the porogen (azodicarbonamide, ADCA) currently used may be limiting regarding radiolysis and thermolysis. Therefore, new ones, less sensitive to the demanding conditions of MOX manufacturing, are under study. For example, calcium stearate (lubricant, StCa) has a higher melting point than StZn, and oxamide (pore former) is a potential decomposition product of ADCA.
One criterion for those additives is that they must be removed from the pressed pellets undergoing sintering before the beginning of densification (around 1000°C) with the lowest level of impurities left inside the sintered pellet. To ensure the thermal degradation up to 1000°C meets these requirements, thermogravimetric analyses were conducted on the different organic compounds studied. By using different constant heating rates, it is possible to obtain kinetic laws describing the thermal decomposition of the organic additives of interest.
Zinc and calcium stearates begin their thermal degradation after their fusion point and show around 10wt% of residues. The degradation of zinc stearate starts after 200°C, with a main mass loss starting before 400°C. Three competitive mass losses are observed on the TGA curves. Calcium stearate starts to degrade around 300°C, with the main TGA event taking place after 400°C. The TGA curve also shows three apparent reactions, with independent mass losses.
The thermal degradation of porogens takes place in solid state, and does not yield any residue at 1000°C. ADCA loses over half its mass around 200°C, followed by two further degradation steps, which all appear competitive under the analytic conditions. Oxamide exhibits an ideal behaviour for removal during the sintering process: one apparent degradation step taking place over a restricted temperature range (150-300°C).
The thermal degradation of the organic additives studied has been successfully described using predictive models based on the kinetic parameters determined using different methods and numerical optimisation, if necessary.