• A nuclear unit becoming critical means when a chain reaction that causes the automatic splitting of the fuel nuclei has been established.
• The reactor core must be designed so that the mass of fuel will be just sufficient to ensure one neutron from each fission causing fission in another atom.
• A mass and configuration of fissionable material in which this occurs are termed the ‘critical mass’, or reactor in which this condition is achieved is said to have ‘gone critical’.
• Criticality is the normal operating condition of a nuclear reactor, in which nuclear fuel sustains a fission chain reaction.
• A reactor achieves criticality (and is said to be critical) when each fission releases a sufficient number of neutrons to sustain an ongoing series of nuclear reactions.

U₂₃₈ will undergo fission by high-energy (fast) neutrons alone.

U ₂₃₈ is a fissionable isotope, meaning that it can undergo nuclear fission, but the neutrons fired at it would need much more energy in order for fission to take place.

U₂₃₈ has an even mass, and odd nuclei are more fissile because the extra neutron adds energy - more than what is required to fission the resulting nucleus.

Because of the large amount of energy needed, U₂₃₈ will not normally undergo fission in a nuclear reactor. however, U- 238 can also form Pu ₂₃₉, which can undergo fission in a nuclear reactor.

U₂₃₅ will undergo fission by either fast or slow neutrons.

When a nucleus of uranium-235 undergoes fission, it splits into two smaller atoms and, at the same time, releases neutrons ( n) and energy. Some of these neutrons are absorbed by other atoms of uranium-235. In turn, these atoms split apart, releasing more energy and more neutrons.

A crucial difference between U-235 and U-238 is that a nucleus of U-235 will undergo fission when any neutron, even one moving very slowly, collides with it.

Artificial radioactive isotopes are formed when an atom is bombed with an accelerator or exposing it to slow-moving neutrons in a nuclear reactor.

Artificial radioactive isotopes find application in the medical field.

Artificially radioactive isotopes can be used in the treatment of disease as well as in its diagnosis. For example, an artificially radioactive isotope of cobalt is used to treat some types of cancer patients. The radiation from the cobalt damages cancerous cells and may prevent the spread of the disease.

• An electromagnetic-type pump is used in a liquid metal-cooled reactor for the circulation of liquid metal.
• A liquid metal-cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal.
• Liquid metals, being electrically highly conductive, can be moved by electromagnetic pumps.
• An electromagnetic pump is a pump that moves liquid metal (or any electrically conductive liquid) using electromagnetism.
• A magnetic field is set at right angles to the direction the liquid moves in, and a current is passed through it.
• This causes an electromagnetic force that moves the liquid. Applications include pumping liquid metal through a cooling system.

Neutron emission:- When the nucleus of the radioactive material (isotope) is excited to the extent that the binding energy per nucleon is of the order of 7.8 MeV, then the neutron is emitted and the mass number is reduced by unity.

• In nuclear fission original elements change into a completely different element.
• Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei.
• The fission process often produces gamma photons and releases a very large amount of energy even by the energetic standards of radioactive decay.
• For the fission of uranium-235, the predominant radioactive fission products include isotopes of iodine, cesium, strontium, xenon, and barium.

• Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei.
• Nuclear reactions generate a large amount of energy compared to chemical reactions.
• One fission event results in the release of about 200 MeV of energy for U-235 or 3.2 x 10^-11 joule. This is about 82 TJ/kg.

• A fission chain reaction in uranium can be developed by slowing down fast neutrons so that Uz fission continues by slow-motion neutrons.
• When a free neutron hits the nucleus of a fissile atom-like uranium-235 (235U), the uranium splits into two smaller atoms called fission fragments, plus more neutrons.
• Fission can be self-sustaining because it produces more neutrons with the speed required to cause new fissions. This creates a chain reaction.
• The fast neutrons are absorbed or slowed by the nuclei of a graphite moderator, which allows just enough slow neutrons to continue the fission chain reaction at a constant rate.

Where reactor operation is designed with fast neutrons such as in reactors using highly enriched fuel then no moderator is needed.

• A fast-neutron reactor is a category of a nuclear reactor in which the fission chain reaction is sustained by fast neutrons.
• Nuclear reactors can be either thermal or fast.
• Thermal nuclear require a moderator to slow down fast neutrons to the thermal level so that nuclear fission can continue.
• However, in fast reactors, a moderator is not needed, and the neutrons within it move much more quickly.

Fast reactors are beneficial as they enhance the sustainability of nuclear power. This is because they have the ability to get more neutrons out of their fuel, can transform nuclear waste into products that decay more quickly, and they respond better to potentially catastrophic equipment failures.