Multi-pyrocarbon coated nuclear fuel and poison particles and method of preparing same

Multi-coated articles, in particular nuclear fuel and neutron poison particles, comprise a core coated with an inner, low density, spongy, shock absorbing pyrolytic carbon coating and at least two distinct outer coatings of dense, thermally conductive pyrolytic carbon, the interface between the two outer coatings forming a barrier which prevents propagation of cracks through these coatings. In the case of nuclear fuel particles, the core is formed from a fissile material such as uranium carbide or a uranium carbide-thorium carbide mixture, whereas for a poison it may be boron carbide or gadolinium carbide. The fuel or poison particles are dispersed as a fluid bed in a stream of helium containing acetylene at a relatively high partial pressure, e.g. 0.65 to 0.90 atmosphere, and heated to between 800 and 1400 DEG C., whereupon the acetylene gas decomposes and forms a low density, spongy, pyrolytic carbon coating on the particles. When the desired thickness, e.g. 5 to 50 microns, has been deposited, the flow of acetylene gas is terminated. In the case of fuel particles, the thickness is preferably one to two fission "recoil ranges", i.e. 12 to 50 microns, so as to absorb the fission recoils and prevent them from striking and rupturing the brittle, dense fission products retentive pyrolytic carbon coatings. This inner coating also prevents thermal and irradiation stresses from being transmitted to the outer coatings. Two distinct outer coatings are obtained by the thermal decomposition of methane gas in such a manner that they are not cohesively bonded to one another. This may be achieved by reducing the temperature of the particles after each coating so that the methane does not decompose, by interrupting the flow of methane after each coating, or by both cooling the particles and interrupting the flow of methane. Preferably, however, the coatings are applied in such manner as to have different crystallite structures. For example, the particles may be dispersed as a fluid bed in a stream of helium containing methane and heated to between 1200 and 2200 DEG C. If the partial pressure of the methane is between 0.08 and 0.80 atmosphere at a total gas flow of 1000 to 10,000 cc/min. through a 1 in. dia. reaction tube, the pyrolytic carbon coating obtained will be laminar whereas if the partial pressure is reduced to between 0.001 and 0.08 atmosphere, the coating will be columnar.