The fuel bundles which consist of tiny pellets of uranium oxide are placed in calandria reactor vessel. The part of the reactor vessel which contains the fuel rod is known as reactor core.

Heavy nuclei, for example, \(_{ 92 }^{ }{ { U }^{ 235 } }\) contain 92 protons and 146 neutrons. The wide difference between the number of protons and neutrons makes the heavy nuclei unstable.

\(_{ 86 }^{ 220 }{ Rn }\rightarrow _{ 84 }^{ 216 }{ Po }+_{ 2 }^{ 4 }{ He }\)
The mass number of Po = 216
The atomic number of Po = 84
The mass number of He=  4
The atomic number of He = 2

Neutrons are classified according to their kinetic energy as
(i) slow neutrons (0 to 1000 eV)
(ii) fast neutrons (0.5 MeV to 10 MeV).

Neutrons are stable inside the nucleus. But outside the nucleus they are unstable. If the neutron comes out of the nucleus (free neutron), it decays with the emission of proton, electron, and antineutrino with a half-life of 13 minutes.

It is defined as the quantity of radiation which produces 1.6 x 10¹² pair of ions in 1 gram of air.

In nuclear fusion reaction, the energy liberated per unit mass of the nuclei taking part in the reaction is many times larger than the energy liberated in a fission reaction.

According to quark model,
(i) Proton is made up of two up quarks and one down quark.
(ii) Neutron is made up of one up quark and two down quarks.

One curie was defined as number of decays per second in 1 gram of radium. 1 1 curie = 3.7 X 10¹⁰ decays/s.

The mean life time of the nucleus is the ratio of sum or integration of life times of all nuclei to the total number nuclei present initially.
\(\tau =\frac { 1 }{ \lambda } \)

Binding energy per nucleon is defined as minimum energy required to take out either a proton or a neutron from nucleus.
\(\overline {BE}=\frac{BE}{A}\)

The impact parameter is defined as the perpendicular distance between the centre of the gold nucleus and the direction of velocity vector of alpha particle when it is at a large distance in the Rutherford's alpha particles scattering experiment.

The minimum distance between the centre of the nucleus and the alpha particle just before it gets reflected back through 180⁰ is defined as the distance of closest approach r_o (also known as contact distance).

The energy required to excite an electron from lower energy state to any higher energy state is known as excitation energy.

According to Einstein mass - energy equivalence, E = mc²
Here \(m=l u=1.66 \times 10^{-27} \mathrm{~kg} \)
\(E =1 \mathrm{u} \times \mathrm{c}^{2}=1.66 \times 10^{-27} \times\left(3 \times 10^{8}\right)^{2} \mathrm{~J} \)
\(E =\frac{1.66 \times 10^{-27} \times\left(3 \times 10^{8}\right)^{2}}{1.6 \times 10^{-19}} \mathrm{eV}=931 \times 10^{6} \mathrm{eV}=931 \mathrm{MeV} \)