The magnox fuel elements consisted of refined uranium enclosed in a loose-fitting magnox shell and then pressurized with helium. The outside of the shell was typically finned in order to improve heat exchange with the CO2. Magnox alloy is reactive with water, which means it cannot be left in a cooling pond after extraction from the reactor for extended periods. In contrast to the Windscale layout, the Magnox design used vertical fuel channels. This required the fuel shells to lock together end-to-end, or to sit one on top the other to allow them to be pulled out of the channels from the top.