Irradiation of uranium-metal and uranium-oxide fuel pins to high burn-up at high temperature

Abstract

Fuel pins have been tested in Dounreay Materials Testing Reactor under onerous fast-reactor type irradiation conditions and have achieved a high burn-up at higher temperature and rating than heretofore. The fuel pins are representative of a power fast reactor but scaled down proportionally to reduce the diameter to 0·122 in. o.d., which enables a high power density of 550 W/g to be used, so reducing irradiation time to 17 days per 1 per cent burn-up. Uranium-metal was irradiated to 4·75 per cent burn-up at 700°C centre-temperature, and uranium-oxide to 5·3 per cent burn-up at 1600°C centre-temperature, in 0011 in. thick cans of Nimonic 80 at 500°C fuel surface-temperature approximately. The pins survived with no measurable distortion. The results appear to confirm the author's theory of burn-up and his proposals to achieve high burn-up (BLAKE, 1961) by introducing about 30 per cent void space in the fuel and containing it in a strong can. Theory suggests that the pins are capable of three times this burn-up, and this is partly confirmed by post-irradiation, high-temperature tests. Achieving high burn-up at high temperature is the key development problem of the fast reactor: 3 per cent burn-up under similar temperature conditions, should enable economic electrical power to be generated in the U.K. After one abortive attempt, a rig was developed to test the fuel pins in D.M.T.R. at high power density under controlled temperature conditions. It should now be straightforward to develop this rig further to test four or five 1 ft long fuel pins per rig at up to 1 kW/g rating, with temperatures and specific power indicated to within ± 5 per cent error; can failure would also be indicated. A technique has also been established of X-raying the complete rig and returning it to the reactor, so enabling fuel swelling and distortion to be measured and plotted during the irradiation.