sixth embodiment referring to fig2 there is shown a fuel rod 6 comprising an outer cladding 8 and a fuel pellet stack 10 a container 2 as described above is provided at the bottom of and retained in place by bottom end cap 12 and the fuel stack 10 containing metal oxide fig2 illustrates the situation where the container is at the bottom of the fuel stack however a similar container may also be placed at the top of the fuel stack in the usual arrangement a container is placed at the bottom of the fuel stack and a further container may optionally be present at the top of the stack when a container is at the top of the fuel stack there is a plenum and a retainer spring not shown which presses down on the container to hold it in place the specific metal oxide to be used for secondary hydriding mitigation may be selected from the oxides of ni fe sn bi cu co cr and mn the metal oxide is typically present in each fuel rod in an amount of up to about 15 grams more usually up to about 12 grams for example 2 to 10 grams the specific metal oxide to be chosen is to be based on whether the metal oxide reacts with hydrogen rapidly enough the rapidity of this reaction must be such that the rate is.