Test pieces used in the development of CANLUB fuel. During the irradiation of UO2 fuel in a nuclear reactor, the ceramic-like fuel pellets tend to crack and swell. This swelling leads to expansion and increased stress and strain on the zirconium fuel sheet. The increased strain can lead to failure of the sheath. Decreasing the friction between the fuel pellets and the sheath at the crack area, in particular, can reduce the problems. CANLUB uses a layer of graphite on the inside of the sheath to reduce this friction.
These test pieces of fuel sheath, some of which were irradiated to simulate reactor usage, were used in the experiments. A ceramic ring was used to simulate the fuel while an aluminum core, which could be externally compressed with a mechanical plunger, simulated the forces produced in the fuel during irradiation. Some of the samples were compressed to test the bursting limits. The tests, carried out at Chalk River in 1973, showed that the graphite coating substantially reduced the friction and resultant stress on the sheath and CANLUB fuel, which uses the coating, is now in use in many CANDU power stations.
