Introduction To Nuclear Fission

Fission means ‘cleavage’ or ‘splitting’, hence in simple words nuclear fission is a nuclear reaction or a decay process (radioactive in nature) in which the nucleus of an atom splits into smaller and lighter nuclei, often producing sub-atomic particles which may be free neutrons and photons (gamma rays), and leading to a release of a large amount of energy.

It is an exothermic reaction which can release energy both as electromagnetic energy and as kinetic energy of the fragments. In order for fission to produce energy, the total binding energy of the resulting elements must be greater than that of the starting element. Heavy elements, such as uranium, thorium, and plutonium, undergo both spontaneous fission (a form of radioactive decay) and induced fission, a form of nuclear reaction.

The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very dense source of energy.

Basic types of fission reactions:

Binary Fission: Those producing two charged fragments.

Ternary Fission: those producing three charged fragments (happens just 2 to 4 times per 1000 events!!).

For the nuclides, "Fissile" is distinct from "fissionable"

Fissile isotopes: Isotopes that undergo fission when struck by a thermal, slow moving neutron are also called fissile.

Fissionable isotopes: Elemental isotopes that undergo induced fission when struck by a free neutron are called fissionable.

According to the fissile rule, heavy isotopes with 90 ≤ Z ≤ 100 and 2 × Z – N = 43 ± 2, with few exceptions, are fissile (where N = number of neutrons and Z = number of protons).

²³⁵U and ²³⁹Pu are generally used as nuclear fuels as they can sustain chain reactions and can be obtained in large enough quantities to be useful. ²³⁸U, the most abundant form of uranium, is fissionable but not fissile: it undergoes induced fission when impacted by an energetic neutron with over 1 MeV of kinetic energy. So no chain reaction is possible with this isotope.

 





Nuclear Fission



Uranium Fission

The fission of U-235 in reactors is triggered by the absorption of a low energy neutron, often termed a "slow neutron" or a "thermal neutron".

Why choosing a so called ‘neutron bullet’ for fission?

Now, two particles emitted by radioactive elements are the Alpha particle and the neutron. The Alpha particle is essentially a Helium (2+) nucleus & carries an overall positive charge. So, due to repulsion from the nucleus, the Alpha particle won’t be able to get there .And because neutron is electrically neutral and thus would not get repelled from a positive nucleus, it can reach the nucleus to trigger reaction.

When a neutron strikes a 235U nucleus, it is at first absorbed into it. This creates 236U. 236U is unstable and this causes the atom to fission. The fissioning of 236U can produce over twenty different products like 92Kr , 142Ba, 92Sr,  140Xe.