It is based on an uncontrolled chain reaction. The two pieces of U235 or Pu239 each of mass somewhat greater than half the critical size are brought together, by somewhat mechanical operation. The total mass exceeds the critical size and a fast neutron chain reaction builds up which explodes the whole mass producing tremendous energy in a couple of microseconds. No external agency or mechanism is required to start the process as there are always some stray neutrons present in any mass of metal. The neutrons are produced by cosmic ray radiations.
The preparation of the bomb requires a sufficient amount of pure U-235 or Pu-239. The critical size depends upon the shape of the bomb and the nature of the material in which the bomb is enclosed. The two pieces to be detonated are enveloped in a cover of a highly dense substance called the temper which serves two purposes. Firstly, it reflects back into the bomb neutrons which might otherwise escape into the air and secondly, it delays the explosion until the temperature and pressure of high order build up. The arrangement is so made that the mechanical jerk produced on dropping the bomb, brings the two perfectly safe and stable masses (each less than the critical size) together and the fission reaction starts. The explosion produces a temperature of the order of 107 K and pressure of several million atmospheres as a result of the blast and visible, ultra-violet, X-rays and γ-rays are produced. A large quantity of radioactive matter is also released by fission products. The radioactive substances formed are carried away by air currents and get deposited on surrounding objects. The time for which the explosion lasts is very small and depends on the rate of expansion of the active material.