I am currently studying A level Physics. I understand that, in a nuclear fusion reaction, the mass of the nucleus we end up with is smaller than that of its constituent nuclei, so Δm is negative and so energy is lost in huge amounts. But what about a nuclear fission reaction? The mass of the products is bigger than that of the nucleus we started with, so there is no mass defect, but a mass gain instead. Keeping this in mind, how is energy released in nuclear fission? Where have I misunderstood?