In fact, I think a little nuclear physics knowledge is needed.
According to the specific binding energy curve (below), when an element with a lower specific binding energy is converted into an element which has bigger specific binding energy, the nuclear reaction will release energy, means it can be used to produce energy.The bigger the difference from the specific binding energy, the more energy is released in reaction.
In this picture, you will find, when very heavy elements like U-235 are converted into light elements like Ba-138 and Kr-95 (fission), the specific binding energy have increased, so they are available.
Also, very light elements like H can have fusion reaction.(D + T --> He + n) They are available too.
This hydrogen fusion reaction changes more specific binding energy than the uranium fission reaction, so the energy released is naturally more.
But if you want to combine two heavy element into one, you will decrease its specific binding energy (look at the part which after Fe) , means costing energy, you should use an particle accelerator to give them kinetic energy.
the wandering earth is a better translation