The nucleus in the centre of an atom has loads of positively charged particles.
These things normally repel one another and there is a huge amount of energy involved in holding them together within the nucleus.
When the nucleus is broken and these particles form smaller nuclei this energy is released.
It’s a huge amount of energy because you need a huge amount of energy to hold all the positively charged things together.
thanks for this amazing question
the nucleus (proton and neutrons) are very powerful so can you imagine that neutrons, electrons, and alpha particles that shoot through space at very high speeds, something like 100,000 miles per second.