Pyrolytic carbon coating of fuel clads

Fuel bundles of the Candu-37 elements type are used to fuel Candu power reactors, with burnup of 8000-12000 Mwd/T. This burnup limitation has been attributed to the susceptibility of the Zircaloy clad to fail under fission gas attack at burnups, even with existence of the inner coating of the clad (CANLUB) . New fuel designs which could allow for substantially higher burnups would call for improved coating of the fuel clad. In this work, a new technique has been carried out to coat the inner surface of Zircaloy-4 tubes with pyrolytic carbon at a relatively low temperatures. The coating gas was a mixture of commercial butane gas as a source for carbon, and argon gas as a carrier gas. Some factors affecting the coating process has been investigated. These factors were: a) coating temperature (250-450 degree C) b) coating time (30-90 min.) c) flow rate of gases: i- commercial butane gas (50 and 90 cm3/min.) i i- argon gas (60 cm3/min.) A stainless steel tubular reactor (115 cm length, 6 cm inner diameter, 0.5 cm thickness) was used in this work. Heat was provided by a tubular furnace. A photo-cell-reflection system has been used to determine the low coating weight gain. Tensile testing and hardness measurements for bare and coated tubes have been used to identify the possible mechanical changes due to coating process. The results from this investigation indicated that the coating weight gain was increased by increasing the coating temperatures, coating time and flow rate of commercial butane gas in the studied ranges. The average deposition rate of pyrolytic carbon coatings was increased by increasing the coating temperature and the flow rate of commercial butane gas. It is observed also, that the average deposition rate was very slow. The coating efficiency of pyrolytic carbon coatings was increased by increasing the coating temperature and decreasing the flow rate of commercial butane gas. Hardness and tensile tests measurements on coated and bare samples are also reported