GB1074426A - Improvements in or relating to nuclear fuels - Google Patents

Abstract

A nuclear fuel particle comprises a spherule of normally solid fissile material, such as uranium, uranium oxide, uranium carbide, uranium - thorium carbides, thorium carbide and plutonium carbide, encased in a self-supporting shell of a substantially impermeable refractory material, such as pyrolytic carbon or a refractory metal carbide, e.g. zirconium, tungsten, or tantalum carbide, the volume enclosed by the shell being greater than the volume of the non-gaseous portions of the fissile material at all temperatures below the m.p. of the refractory material, the refractory material having a higher m.p. than the fissile material. Such a fuel particle may be made by first forming a spherule which contains numerous voids throughout its volume. This spherule is coated with an impermeable shell of refractory material at a temperature below the m.p. of the fissile material and the coated particle then heated to above this m.p. whereupon the fissile material melts and decreases in volume owing to the elimination of the voids. A space is thus left between the fissile material and the shell which provides room for expansion as well as accommodating gaseous fission products. Alternatively, a spherical particle of uranium metal is coated with a resin which is carbonized in an inert atmosphere, the composite coated particle then being coated with pyrolytic carbon. Thereafter, the particle is heated to a temperature above the m.p. of uranium, whereupon the uranium reacts with the carbon to form uranium carbide, a space being left between the shell and the spherical particle due to the consolidation of the uranium and the carbon on reaction. This space contains at the most, only a small amount of gas and is therefore at a pressure below atmospheric. This is advantageous in that a greater build up of pressure due to fission product accumulation is possible without rupture of the shell. In another method, a spherule substantially free from voids is first coated with a resin, which may be a foamed resin, and this is carbonized either before application of the encasing shell or during its formation. The coating which is produced is one which yields when pressure is applied and thus provides space for expansion of the fissile material and accommodation of gaseous fission products.ALSO:Nuclear fuel particles are made by coating spherules of fissile material, e.g. uranium dicarbide, with pyrolytic carbon by maintaining the spherules in a fluidized bed while at the same time spraying a solution of carboxymethyl cellulose in acetone on to the suspended spherules, a sufficient amount of solution being employed to produce a coating on the spherules approximately 30 microns thick. The coated spherules are then transferred to a graphite crucible and placed in an induction heated furnace. A stream of argon containing 5% by volume of methane is passed through the furnace to displace the air and, after thorough flushing, the crucible is rotated while heating the furnace to 1300 DEG to 1400 DEG C. The heating and rotation is continued for approximately 1 hour while a shell of pyrolytic carbon forms as a uniform coating over each spherule.