Use of dopants for thoria sintering temperature reduction: characterization of ThO₂

Thorium is nearly three times more abundant than uranium in the Earth's crust. Some papers evaluate the thorium resources in Brazil over 1.200.000 metric t. These figures mean that the country is probably the biggest thorium resource in the world, with only part of the territory prospected. Nevertheless, Brazil has not a research program for use of thorium in nuclear reactors, even having dedicated special attention to the subject in the beginning of its nuclear activities, in the fifties and sixties. From 1985 until 2003 IPEN operated a pilot plant for thorium nitrate production and purification, used by Brazilian industry for production of gas mantles. This facility produced over 170 metric t of thorium nitrate. Despite the non-nuclear application, the pilot plant was unique in the southern hemisphere. On the other hand, Brazil has the biggest world niobium resources. The Brazilian thorium and niobium resources added to the predictable future importance of alternative fissile materials have motivated this research, since uranium is a finite resource if used in the present thermal nuclear reactors. Besides this, thorium oxide is an important nuclear reactor material. It is a refractory oxide and its ceramic fabrication process involves a very high temperature sintering treatment considering that thoria melting point is very high (3650 K). Cations of elements of the group VB (V, Nb and Ta) have a known effect in the reduction of thoria sintering temperature. IPEN has initiated an investigation about the use of niobium as a dopant for thoria sintering temperature reduction. The thoria used in the research was produced in the IPEN's pilot plant and different amounts of niobium oxide (Nb₂O₅) will be added to thoria by different routes. The powders will be compressed and the compacted pellets will be sintered at different temperatures. The influence of the different parameters in the density of sintered pellets is being investigated. This paper presents the chemical and physical characterization for the thoria used in the investigation. (author)