The advanced gas-cooled reactors are designed to be refueled while in opera­tion. Initial on-load refueling operations with the first two AGRs at Hunterston and Hinkley Point were confined to the charging of fuel into channels in which dummy fuel assemblies had been loaded when the reactor was first charged with fuel. By November 1978, some 15 fuel assemblies at Hinkley and 20 at Hunterston had been charged on-load into these so-called vacancy channels.

On November 19, 1978, a fuel assembly was being withdrawn from channel 4K05 on Hinkley Point B reactor R4. The assembly was raised about 10 ft and then snagged, and the charge machine hoist tripped out on overload. Subse­quently, it was successfully raised into the charge machine. Visual examination of the connected string of fuel elements withdrawn from this reactor channel (the stringer) showed the graphite sleeves surrounding the third, fourth, and fifth elements to be severely damaged. Damage to the graphite sleeve resulted in the fuel elements above the damaged sleeve being starved of coolant and thus overheating, resulting in failure of some of the fuel “pins” that made up the element. Subsequently, a large portion of graphite sleeve from element 4 was recovered from the reactor during a statutory in-reactor inspection. The level of radiation from the sleeve suggested that it was never in the reactor core and that the damage occurred during the loading process. The damaged assembly had been loaded into a vacancy channel at 82% power earlier in the year. The incident caused doubts about the safety of refueling AGRs at power, and an em­bargo was placed on on-load refueling. A program of investigations was begun to establish the cause of the problem.

When the fuel is being lowered into the reactor, it receives considerable buf­feting from the very high gas flow through the empty channel. It is believed that small cracks may have been present in a number of fuel element sleeves and that the sleeve of element 4 cracked further due to the pressure differential across the sleeve during on-load refueling. Techniques have been developed to detect cracks in sleeves, and these and other improvements have been incor­porated into the AGRs. On-load refueling has been resumed at low power.