Breeder reactor, fast breeder reactor, pressurized water reactor, pressurized heavy water reactor, boiling water reactor, water – cooled reactor, gas – cooled reactor, fusion reactor and thermal reactor are some types of nuclear reactors, which are used in different places worldwide.
The process in which two light nuclei combine to form a heavier nucleus is termed as 'Nuclear fusion'. $ \text { E.g. }{ }_1 H ^2+{ }_1 H ^2 \rightarrow{ }_2 He ^4+ Q (\text { Energy }) $
The process of breaking (splitting) up of a heavier nucleus into two smaller nuclei with the release of a large amount of energy and a few neutrons are called 'nuclear fission'. E.g. Nuclear fission of a uranium nucleus $\left( U ^{235}\right)$ $ 92^{ U ^{235}}+{ }_0 n ^1 \rightarrow_{56} Ba ^{141}+{ }_{36} Kr ^{92}+{ }_{30} n ^1+ Q (\text { energy }) $
A nuclear reaction, in which an unstable parent nucleus emits a beta particle and forms a stable daughter nucleus, is called 'beta decay'. E.g. Beta decay of phosphorous. $ { }_{15} P ^{32} \rightarrow{ }_{16} S ^{32}+{ }_{-1} e ^0(\beta-\text { decay }) $
A nuclear reaction in which an unstable parent nucleus emits an alpha particle and forms a stable daughter nucleus is called 'alpha decay'. E.g. Decay of uranium $\left( U ^{238}\right)$ to thorium $\left( Th ^{234}\right)$ with the emission of an alpha particle. ${ }_{92} U ^{238} \rightarrow{ }_{90} Th ^{234}+{ }_2 He ^4(\alpha-$ decay $)$.
It is defined as the quantity of a radioactive substance which undergoes $3.7 \times 10^{10}$ disintegrations in one second. This is actually close to the activity of 1 g of radium 226.
Curie $=3.7 \times 10^{10}$ disintegrations per second.
The phenomenon by which even light elements are made radioactive, by artificial or induced methods, is called ‘Artificial radioactivity’ or ‘Man – made radioactivity’.