Application of pore former for density control of TRU nitride pellets

Takaki, Seiya  ; Otobe, Haruyoshi  ; Takano, Masahide 

Nitride fuel with ZrN inert matrix has been studied as a candidate for the transmutation fuel of minor actinides (MA) in an accelerator-driven system (ADS). The pellet densities were designed to be about 85 % of the theoretical density to ensure a margin against swelling, but a dense microstructure is required. One of promising method of controlling the sintering density is by using a pore former (PF). This study aims to investigate the appropriate PF using (Dy,Zr)N as a surrogate nitride fuel with MA and evaluate its technical feasibility through sintering tests of (TRU,Zr)N (TRU: Np, Pu). Three types of polymer particles (A: stearic acid, B: polyester, C: polyethylene) as PF were mixed with (Dy,Zr)N powder respectively, and then the mixed powder was compacted and sintered to obtain the pellets. The dependence of oxygen and carbon impurity concentrations on PF type and concentration was not clear, whereas the pellets mixed with A had numerous cracks formed perpendicular to the direction of compaction. From the above, B and C were used as candidate PFs in the demonstration tests of (TRU,Zr)N. The sintered pellets were prepared using (TRU,Zr)N powder mixed with selected PFs. The sintered density of the pellets decreased linearly with PFs addition concentration as in the (Dy,Zr)N tests, and the almost no clacks occurred in the pellets. We can therefore conclude that B and C are fully applicable to control the sintering density of MA-containing pellets for ADS.