Sol-Gel Microsphere Pelletisation Process for Fabrication of (U,Pu)O2, (U,Pu)C and (U,Pu)N Fuel Pellets for the Prototype Fast Breeder Reactor in India

A "dust-free" sol-gel microsphere pelletisation (SGMP) process has been developed for fabrication of (U,Pu)O2, (U,Pu)C and (U,Pu)N fuel pellets containing around 15% plutonium for the forthcoming prototype fast breeder reactor (PFBR) in India. The objective was to produce homogeneous sintered pellets of ∼85% T.D. with a predominantly open-pore structure. Hydrated gel-microspheres of UO3 + PuO2 and UO3 + PuO2 + C have been prepared from nitrate solutions of uranium and plutonium by the "ammonia internal gelation" process, using hexamethylene tetramine (HMTA) as an ammonia generator and silicone oil at 90 ± 1°C as gelation bath. For oxide fuel pellets, the hydrated UO3 + PuO2 gel-microspheres were calcined at around 700°C in Ar + 8% H2 atmosphere to produce "non-porous", "free-flowing" and coarse (around 400 micron) microspheres which could be directly pelletised at 550 MPa to green pellets. The mixed oxide pellets were subjected either to low temperature (∼1100°C) oxidative sintering (LTS) in N2 + air containing ∼1500 ppm O2 or to high temperature (1650°C) sintering (HTS) in Ar + 8% H2. For monocarbide and mononitride pellets, hydrated gel-microspheres of UO3 + PuO2 + C were subjected to carbothermic synthesis in vacuum (∼1 Pa) and flowing nitrogen (flow rate: 1.2 m3/h) in the temperature range of 1450-1550°C respectively. The monocarbide and mononitride microspheres thus produced were relatively hard and required higher compaction pressure (∼1200 MPa) for making green pellets which could be sintered to 85% T.D in Ar + 8% H2 at 1700°C. The sintered oxide, monocarbide and mononitride pellets had a "blackberry" "open" pore microstructure with fine grain size. The microspheres retained their individual identity in the sintered pellets because during sintering densification took place mainly within and not between the microspheres.