a reactor core and it could accordingly be given alternative positions as burn up of a core causes reactivity to change the vehicle isalso adaptable in that the fuel to absorber ratio in any one unit can be preselected for example it is possible to have any whole number ratio in a six compartment unit between 1 5 and 5 1 it is also important to note that the bellows 14 operates unstressed and hence is not subjected to stress recycling risks as the reactor changes temperature in fig7 there is shown a bellows 70 having a sodium filling 71 the bellows has a top fixed end closure 72 and a bottom free end closure 73 the closure 72 is held in one part of a structural frame 74 by a screw75 and the closure 73 is constrained laterally but free to move axially in a cup 76 the cup is supported on a spring trigger 77 fig8 and it has a stem 78 movable in a guide aperture 79 in another part of the structuralframe 74 in fig8 the spring trigger 77 is shown having two divergent curved wall parts 80 and a curved base 81 with a hole 82 to accommodate the stem 78 ofthe cup 76 the upper edges of the trigger 77 can support control elements 83 by engaging under lips 84 on the rods this is illustrated in fig1 in fig9 the cup 76 is shown in perspective view the cup has flats 85 to accommodate movement of the wall parts 80 of the trigger in fig1 another sub assembly vehicle 90 is shown the vehicle 90 consists of a hollow body part 91 in which there are located six units 92 these may be either control units like elements 83 or fuel units.