There are three types of nuclear weapons.

The first one is assembled by putting together a few kilograms of fissionable or fissile material within as short a period as possible -- (within some microseconds if possible). This can be achieved by using chemical devices, i.e., imploding several small amounts (subcritical amounts) into a small volume inside the bomb. Several subcritical amounts when fired into a small volume, exceed critcal mass and thus fissile material (U-235 or Plutonium-239) fissions with neutrons. Each fission of either U-235 or Pu-235 nucleus induced by a neutron, produces two radioactive fission products and on the average 2.5 neutrons with the evolution of a large amount of energy. It is an uncontrolled chain reaction and thus a fraction of fissile material is fissioned. Fission products that are produed along with enormous amount of energy, disperse in the environment. This type of bomb was dropped n Hiroshima and on Nagasaki.

The second one (commonly known as hydrogen bomb) consists of an atom bomb with deuterium and tritium compounds like lithium deuteride etc. Fission bomb is first triggered so that temperature in the assembly is raised to several million degrees (between 10 -100 million degrees). At these high temperatures deuterium and tritium start reacting with each other to form a helium atom and 14.6- MeV neutron. As you can see that it produces a stable atom of helium and a fast-moving neutron. This neutron can react with other atoms in the environment. For example, it can react with nitrogen-14 to give carbon-14.(a radioactive isotope of carbon). One can produce additional number of radioisotopes as well. In short, in addition to fission products we also have neutron-induced radioisotopes. These are also dispersed along with enormous amount of energy in the environment.

This types of bomb has been tested by many nations in South Pacific and in Siberia from 1952 to 1963. Generally, a hydrogen bomb is 100 to 1000 times more distructive than a fission bomb.

The third kind (neutron bomb) is similar to the hydrogen bomb. The difference is that it is detonated at high altitudes so that neutrons can travel to ground level and destroy life. Of course, some neutrons do react with other material and produce radioisotopes. The fission bomb is kept as small as one can assemble and the amount of tritium and deuterium is kept large. Once the fission bomb raises the temperature so as to initiate tritium-deuterium (D-T) reaction, the fusion energy evolved in the D-T reaction keeps the temperature high for a longer duration and thus keeps the reaction going for relatively a longer time. 14.6-MeV neutrons shoot out in all direction. They can be deflected to some extent toward the earth. Human life is destroyed by neutrons over a certain area under the bomb. As the distance becomes longer between the spot where the bomb is detonated and the ground, the neutron flux also reduces. 14.6-Mev neutrons fly to all directions. The ones that are directed toward the sky and are not deflected, do not harm humans or cause property damage. It is not as distructive as the hydrogen bomb but it is false notion that there is very little radioactivity associated with it. It is descrbed as 'not a dirty bomb'. However, it is also a dirty bomb.