Probably in an effort to keep the problem of nuclear waste hidden from the public, these plants feature huge pools of water up in the higher level of the containment building above the reactors, which hold and store the spent fuel rods from the reactor. These rods are still "hot" but besides the uranium fuel pellets, they also contain the highly radioactive and potentially biologically active decay products of the fission process--particularly radioactive Cesium 137, Iodine 131 and Strontium 90. (Some of GE's plants in the US feature this same design. The two GE Peach Bottom reactors near me, for example, each have two spent fuel tanks sitting above their reactors.)
Source: CounterPunch