The creep ductility of Magnox AL80 (Mg/0.8 wt per cent A1/0.005 wt per cent Be) fuel element cans is known to decrease with increasing irradiation. A possible cause of this embrittlement is void nucleation at inert gas bubbles formed on grain boundaries. The necessary size and spacing of these bubbles are calculated together with the total number of gas atoms which they must contain. The numbers of inert gas atoms arising from the following processes are also calculated: (a) the lithium (n, α) reaction, (b) the injection of fission fragments into the can, (c) fission of plutonium which has diffused into the can. Each mechanism can produce more than the theoretical minimum quantity of gas to account for the observed embrittlement. However, the appropriate grain boundary distribution of gas is unlikely to arise from (b) and (c) and the quantity from (a) exceeds the minimum only slightly, even if all the gas is in bubble nuclei. The need for continued control over the lithium content of the canning stock material is emphasized. It is demonstrated that greater plutonium penetration could cause extensive embrittlement of a Magnox ZR55 (Mg/0.55 wt per cent Zr) alloy can.