When a heavy atomic nucleus undergoes fission, a significant mass loss occurs, so a large amount of energy is released according to the mass energy equation (E=mc^2). Since the speed of light on the right side of the equation is a relatively large number, a small mass loss Both will release huge amounts of energy.
For example, the mass loss rate of fission materials used in atomic bombs is about 0.09%. In the Hiroshima atomic bomb, the quality loss is less than one gram, but it is enough to destroy a city.
The manufacturing principle of the atomic bomb is not difficult. As long as the purity of the fissile material is continuously increased, and then the overall mass exceeds the critical mass of the fissile material, a chain reaction can occur, but it is more difficult to obtain a high-purity nuclear fission material than to go to heaven.