The fuel rod on the right above is a Magnox fuel rod. This is quite a distinctive design.
It seems that they used unenriched uranium metal in this type of British fuel rod. The name comes from the casing which is made of a magnesium oxide and aluminium alloy because it would not oxidise in the extreme conditions in the reactor. The nuclear power station at Wylfa on Anglesey thatI saw last year is the last remaining Magnox station. Fission of the uranium-235 nucleus results in smaller nuclei called fission fragments. A uranium nucleus is only stable because it has far more neutrons than it should to help hold the nucleus together. Small nuclei like carbon or oxygen have equal numbers of protons and neutrons. The neutrons provide extra strong nuclear force to overcome the electrostatic repulsion of the protons. The electrostatic repulsion works over a much larger range so in a big nucleus more neutrons are needed because the effect of their strong nuclear force is only local. These extra neutrons are not needed in the smaller fission fragments. The only way to lose them is by beta-minus decay. The neutron decays into a proton, releasing an electron (from the nucleus!) and an anti-neutrino. The rate of decay is high. Each decay releases energy as heat. Hence the spent rods need to be put into cooling ponds until this initial fast decay has spent its energy. One problem with Magnox was that the fuel casing reacted with water. Magnesium does react with hot water! One final thing to note is the large surface area of the fins used to help the rods to lose thermal energy more efficiently.
