A Nuclear reactor is a device in which the nuclear fission reaction takes place in a self-sustained and controlled manner to produce electricity.
(i) Fuel:
a) A fissile material is used as fuel.
b) The commonly used fuel material is uranium
(ii) Moderator:
a) A moderator is used to slow down the high-energy neutrons to get slow neutrons.
b) Graphite and heavy water are the commonly used moderators.
(iii) Control rod:
a) Control rods are used to control the number of neutrons in order to have a sustained chain reaction.
b) Mostly boron or cadmium rods are used as control rods.
(iv) Coolant:
a)A coolant is used to remove the heat produced in the reactor core to produce steam.
b) This steam is used to run a turbine to produce electricity.
c) Water, air and helium are some of the coolants.
(v) Protection wall:
 A thick concrete lead wall is built around the nuclear reactor in order to prevent the harmful radiations from escaping into the environment.

(a) Controlled chain reaction:
(i) In the controlled chain reaction the number of neutrons released is maintained to be one.
(ii) This is achieved by absorbing the extra neutrons with a neutron absorber leaving only one neutron to produce further fission.
(iii) Thus, the reaction is sustained in a controlled manner.
(iv) The energy released due to controlled chain reaction can be utilized for constructive purpose.
(v) Controlled chain reaction is used in a nuclear reactor to produce energy in a sustained and controlled manner.
(b) Uncontrolled chain reaction:
(i) In the uncontrolled chain reaction the number of neutrons multiplies indefinitely and cause fission in a large amount of fissile material.
(ii) This results in the release of huge amount of energy within a fraction of second.
(iii) This kind of chain reaction is used in atom bomb to produce explosion.

(i) The intensity of radiation used in X-ray machine is small, so the chance that X-ray will cause severe problems is very low.
(ii) X-ray posses almost equal energy like gamma radiation.
(iii) So, if we take often, it may destructive living cell present in our body.
(iv) It may leads to cancer.

Let X and Y be the resulting nucleus after the emission of the alpha and beta particles respectively.
₉₂U²³⁵  \(\overset { \alpha \ decay }{ \longrightarrow } \) ₉₀X²³¹ + ₂He⁴
₉₀X²³¹ \(\overset { \beta \ decay }{ \longrightarrow } \) ₉₁Y²³¹ + _-1e⁰
Number of neutrons = Mass number – Atomic number
= 231 – 91
= 140

(i) ₁₃AI²⁷ + ₂He⁴  \(\longrightarrow \) ₁₅P³⁰ + ₀n¹
(ii) ₁₂Mg²⁴ + ₀n¹  \(\longrightarrow \) ₁₁Na²⁴ + ₁H¹
(iii) ₉₂U²³⁸ + ₀n¹ \(\longrightarrow \) ₉₃Np²³⁹ + _-1e⁰
A is alpha particle, B is neutron, C is proton, and D is electron.