Nuclear Fission And Fusion Difference

Understanding the difference between nuclear fission and fusion is essential for students, researchers, and anyone interested in modern energy technologies. Both processes release enormous amounts of energy, but they operate in fundamentally different ways. Nuclear fission involves splitting heavy atomic nuclei, while nuclear fusion combines light nuclei to form heavier ones. These reactions power everything from today’s nuclear reactors to the sun and future clean-energy technologies.

1.0Nuclear Fission and Chain Reaction

Nuclear Fission: Splitting of a heavy nucleus (A > 230) into two or more lighter nuclei when struck by a neutron.

In this process certain mass disappears which is obtained in the form of energy (enormous amount)

$A+n\to \text{excited nucleus}\to B+C+Q$

Hahn and Strassmann did the first fission (fission of nucleus of U²³⁵).

When U²³⁵ is bombarded by a neutron it splits into two fragments and 2 or 3 secondary neutrons and releases about 200 MeV energy per fission (or from single nucleus)

Fragments are uncertain but each time energy released is almost the same.

Possible reactions are

U²³⁵ + ₀n¹ → Ba + Kr + 3 ₀n¹ + 200 MeV
U²³⁵ + ₀n¹ → Xe + Sr + 2 ₀n¹ + 200 MeV

and many other reactions are possible.

(i) The average number of secondary neutrons is 2.5.
(ii) Nuclear fission can be explained by using the "liquid drop model" also.
(iii) The mass defect Δm is about 0.1% of mass of fissioned nucleus
(iv) About 93% of released energy (Q) is appear in the form of kinetic energies of products and about 7% part in the form of γ–rays.

Nuclear Chain Reaction

The equation of fission of U²³⁵ is
U²³⁵ + ₀n¹ → ₅₆Ba¹⁴⁴ + ₃₆Kr⁸⁹ + 3 ₀n¹ + Q

These three secondary neutrons produced in the reaction may cause of fission of three more U²³⁵ and give 9 neutrons, which in turn, may cause of nine more fissions of U²³⁵ and so on. Thus a continuous 'Nuclear Chain reaction' would start.

If there is no control on chain reaction then in a short time (≈10⁻⁶ sec.) a huge amount of energy will be released. (This is the principle of 'Atom bomb')
If chain reaction is controlled then produced energy can be used for peaceful purposes.
For example, a nuclear reactor (Based on fission) generates electricity.

Natural Uranium

It is mixture of U²³⁵ (0.7%) and U²³⁸ (99.3%). U²³⁵ is easily fissionable, by slow neutrons (or thermal neutrons) having K.E. of the order of 0.03 eV.

To improve the quality, percentage of U²³⁵ is increased to 3%. The improved uranium is called 'Enriched Uranium' (97% U²³⁸ and 3% U²³⁵).

Losses of Secondary Neutrons

Leakage of neutrons from the system: Due to their high K.E. some neutrons escape from the system.

Absorption of neutrons by U²³⁸: U²³⁸ is not fissionable by these secondary fast neutrons. But U²³⁸ absorbs some fast neutrons.

Critical Size (or mass)

In order to sustain chain reaction in a sample of enriched uranium, it is required that the number of lost neutrons should be much smaller than the number of neutrons produced in a fission process. For it the size of a uranium block should be equal or greater than a certain size called critical size.

Reproduction factor (K) = $\frac{\text{rate of production of neutrons}}{\text{rate of loss of neutrons}}$

(1) If the size of Uranium used is 'Critical' then K = 1 and the chain reaction will be steady or sustained (As in nuclear reaction).

(2) If the size of Uranium used is 'Super critical' then K > 1 and chain reaction will accelerate resulting in an explosion (As in atom bomb).

(3) If the size of Uranium used is 'Sub Critical' then K < 1 and chain reaction will retard and ultimately stop.

2.0Nuclear Reactor and Fast Breeder Reactors

Nuclear Reactor (K = 1)

Its main constituents are –

(1) Nuclear Fuel: Commonly used are U²³⁵, Pu²³⁹. Pu²³⁹ is the best. But Pu²³⁹ is not naturally available and U²³⁵ is used in most of the reactors.

(2) Moderator: Its function is to slow down the fast-secondary neutrons. Because only slow neutrons are capable of the fission of U²³⁵. The moderator should be light and it should not absorb neutrons. Commonly, Heavy water (D₂O, molecular weight 20 gm.) are used.

(3) Control rods: They have the ability to capture the slow neutrons and can control the rate of chain reaction at any stage. Boron and cadmium are the best absorbers of neutrons.

(4) Coolant: A substance which absorbs the produced heat and transfers it to water for further use. Generally coolant is water at high pressure.

Fast breeder reactors

The atomic reactor in which fresh fissionable fuel Pu²³⁹ is produced along with energy.
Fuel : Natural Uranium.

During fission of U²³⁵, energy and secondary neutrons are produced. These secondary neutrons are absorbed by U²³⁸ and U²³⁹ is formed. This U²³⁹ converts into Pu²³⁹ after two beta decay. This Pu²³⁹ can be separated, its half life is 2400 years.

${}_{92}{U}^{238}{+}_0{n}^1{\to }_{92}{U}^{239}{\to }_{94}{U}^{239}\text{(best fuel of fission)}$

This Pu²³⁹ can be used in nuclear weapons because of its small critical size than U²³⁵.

3.0Nuclear Fusion

It is the phenomenon of fusing two or more lighter nuclei to form a single heavy nucleus.

$$\begin{gathered} A+B\to C+Q(\text{Fusion}) \\ \text{The product (C)is more stable than reactants (A and B).} \\ And{m}_c<(m_a+m_b)\text{and mass defect}\Delta m=[(m_a+m_b)-m_c]\text{amu} \\ \text{Energy released is}E=m\times 931\text{MeV/amu} \\ \text{The total binding energy and binding energy per nucleon of C both are more than of A and B.} \\ \Delta E=E_c-(E_a+E_b) \\ \text{Fusion of four hydrogen nuclei into helium nucleus-} \\ 4\left({}_1^1{\mathrm{H}}^{+}\right)\to {}_2^4\mathrm{He}+2{}_{+1}^0\beta +2\nu +26\text{MeV} \\ \text{(1) Energy released per fission >> Energy released per fusion} \\ \text{(2) Energy per nucleon in fission}\left[\frac{200}{235}\simeq 0.85\text{MeV}\right]\text{<< energy per nucleon in fusion}\left[\frac{26.4}{4}=6.5\text{MeV}\right] \end{gathered}$$

4.0Pair Production and Pair Annihilation

Pair Production: A γ-photon of energy more than ≥ 1.02 MeV, when interact with a nucleus produces pair of electron (e⁻) and positron (e⁺).

The energy equivalent to rest mass of e⁻ (or e⁺) = 0.51 MeV. The energy equivalent to rest mass of pair
(e⁻ + e⁺) = 1.02 MeV.

For pair production Energy of photon 1.02 MeV.

If energy of photon is more than 1.02 MeV, the extra energy (E − 1.02) MeV divides approximately in equal amounts to each particle as the kinetic energy.

$(K.E{)}_{e^{-}or{e}^{+}}=[\frac{E_{ph}-1.02}{2}]MeV$

If E < 1.02 MeV, the pair will not produce.

Pair Annihilation: When electron and positron combine they annihilates each other and only energy is released in the form of two γ-photons.

5.0Nuclear Fission And Fusion Difference