Select from below the three statements that are true. Question $18$Answer The energy released in a thermal reactor is due mainly to the fission of nuclei of U$-235$ and plutonium. Spent fuel from a nuclear power station is much less radioactive than new fuel. Nuclear fuel is removed from the reactor once all the U$-235$ has decayed. Nuclear fuel is removed from the reactor when the increasing amounts of nuclear poisons significantly disrupt the chain reaction. Due to the wide range of different ways a fissioning nucleus can split and the complicated nature of the subsequent decay chains, spent fuel from a nuclear reactor contains a range of different elements and isotopes.