Abstract
In the preparation process of a MOX powder for a MOX fuel fabrication facility, zinc stearate will be added into the MOX powder as an additional material to adjust the density. If too much of the material is added by mistake, the criticality characteristics of the MOX fuel will be influenced because of its neutron moderation effect. If a criticality condition should be induced by the excess zinc stearate, melting and pyrolysis of the material could be caused by the fission energy, and, therefore, the dynamic characteristics of the MOX fuel would be affected in a feedback mechanism. To provide a quantitative evaluation of such dynamics, thermal properties such as exothermic and endothermic calorific values, reaction rates, etc. were measured and correlated with the respective physical variations and release behavior of pyrolysis gas with the pyrolysis reaction. It was found that the exo/endothermic behavior of the material with rising temperature could be divided into six regions and that a rapid rise in pressure occurred over about 400°C caused by the pyrolysis reaction. Furthermore, on the basis of these results, an evaluation model for the thermal properties under criticality conditions was also investigated.
