What is Nuclear Fission - Definition, Difference, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:49 PM IST

In this article, we explain nuclear fission basics, fission and fusion definition, the energy released in fission, nuclear fission and fusion example, spontaneous fission, and the application of nuclear fission.
Note: Nuclear fission in Hindi is विखण्डन

Nuclear fission basics:

Atomic structure: An atom consists of electrons revolving around the nucleus which is present at the center. The nucleus of an atom consists of a proton, neutron. Electrically positive charges are protons, neutrons are electrically neutral. Protons and neutrons are together called nucleons.

The atomic number of an element is the number of protons present in the nucleus of an atom and it differentiates one element from another. The atomic number is denoted by Z. Elements can have the same number of protons but not the same number of neutrons. Elements having the same number of protons and a different number of neutrons are called isotopes. Isotopes have the same atomic number.

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Nuclear fission basics:
What is nuclear fission reaction?/write nuclear fission meaning.
Application or uses of nuclear fission:
Difference between nuclear fission and nuclear fusion:

What is Nuclear Fission - Definition, Difference, FAQs

The sum of protons and neutrons gives the mass number of an atom. The mass number is independent of electrons as its mass is negligible.

Representation of atomic number, chemical symbol, and mass number of isotopes of Carbon

Figure 1 Representation of atomic number, chemical symbol, and mass number of isotopes of Carbon

As shown in the above figure, Carbon C has two stable isotopes having mass numbers 12 and 13. The atomic number is 6.

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Binding energy:

A strong nuclear force binds the nuclei of an atom. The minimum energy by which the nucleus of an atom is divided into constituent protons and neutrons is called the binding energy. Binding energy is specific for different nuclei. The binding energy of the nucleon is much greater than the electrical forces present in between the electrons. The difference in the binding energy between the initial nucleus and the end product of nuclear reaction constitutes nuclear energy. Binding energy is also stated as the amount of energy released when a nucleon is disassembled from the nucleus.

What is nuclear fission reaction?/write nuclear fission meaning.

According to Einstein’s mass-energy relation,

Energy, E=mc²where m is the mass and c (c=3x10⁸ m/s²) is the speed of light.

Consider for m= 1 kilogram, energy E=1kg×(3×10⁸)²

=1×3×10⁸×3×10⁸Joules

=9×10¹⁶ Joules

For 1kg of mass, a huge amount of energy is released. Nuclei with higher binding energy per nucleon have a lower atomic weight per nucleon since mass and energy are comparable. The bombardment of heavy atoms with their neutrons is required in splitting those atoms. Uranium and plutonium are some of the heavy atoms.

The reaction in which the nucleus of an atom splits into two or more lighter nuclei with the release of energy is called nuclear fission.

The mass and energy of nuclear fission are represented as follows:

Consider the binding energy of the heavy nucleus be B_h and two fission products be B₁and B₂.

Then the amount of energy released per fission, E=(B₁+B₂)-B_h

Amount of mass transferred to energy, E/c² =[(B₁+B₂)-B_h]/c²

Nuclear fission reaction

Figure 2 Nuclear fission reaction in U-235 atom

The above nuclear fission diagram shows the nuclear fission in Uranium-235. Here 235 is the mass number of uranium. In the first step, uranium-235 splits into uranium-236 by absorbing a neutron. Uranium-236 then undergoes fission and splits into two fragments. Nuclear fission results in further fragments.

To have a controlled chain reaction, the end product should trigger further fission. The chain reaction is the one in which additional fission occurs at least one further nucleus by the neutrons released in fission. Controlled nuclear fission is possible only when one nucleus hits the other uranium nucleus.

Nuclear reactors are the device used to control a nuclear fission reaction.

The uranium nuclear fission equation is given by,

23592U+n⟹23692U

23692U⟹9038Sr+14454Xe+2n+E

E is the fission energy released by the nucleus and n is the neutron.

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Radioactive decay:

By radiation, if an unstable nucleus loses energy in a fission process then it is called radioactive decay.

Spontaneous fission:

If the unstable nuclei of heavier elements divide into two identical fragments then that radioactive decay is called spontaneous fission.

Application or uses of nuclear fission:

Nuclear fission is useful as follows:

Nuclear fission reaction is widely applied in nuclear power plants.
Nuclear fission reactions in Uranium and Plutonium are commonly used in nuclear power reactors.
Nuclear fission is a principal source of heat and electricity.
It supplies power for propelling submarines.
Nuclear fission produces rare radioisotopes which have application in photographic devices.

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Difference between nuclear fission and nuclear fusion:

Nuclear fission and nuclear fusion are differed by the following factors:

Nuclear fission is a reaction in which the nucleus of an atom breaks into lighter nuclei. Whereas nuclear fusion is a reaction in which two or more lighter nuclei bind together to form a heavier nucleus.
Nuclear fission requires less energy to break an atom while nuclear fusion requires more energy to bind the nuclei.
At the end of the reaction, an enormous amount of energy is released in nuclear fusion whereas nuclear fission releases less energy.
Fission reaction is not visible naturally but fusion is observed in stars and the sun.
Nuclear fission is used in nuclear power plants and nuclear fusion is applied in small-scale fusion reactors.

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NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. What is the meaning of fission?/ Write fission meaning.

Splitting into two or more divisions is called fission.

2. What is the meaning of nuclear?

Nuclear means the concept relating to the nucleus of an atom.

3. Define nuclear fission and fusion for example.

Nuclear fission means the splitting of atomic nuclei into smaller units whereas nuclear fusion is the binding of nuclei together.

Example for nuclear fission: Uranium atom splits into xenon, iodine, barium, etc.

Example for nuclear fusion: A Hydrogen nucleus fuses to form helium.

4. Nuclear fission can be explained by which model?

Nuclear fission is explained by the liquid drop model.

5. Who discovered nuclear fission?

Dr. Otto Hahn, Dr. Lise Meitner, and DR. Fritz Strassman are the scientist who discovered nuclear fission. Dr. Hahn is also called the “Father of nuclear chemistry”.

6. What is the energy released in nuclear fission?

The energy released in nuclear fission averages about 200MeV for U-235.

7. What is chain reaction fission of uranium 235?

In the chain reaction fission of Uranium-235, the nucleus splits into two by absorbing the neutron.

8. What is nuclear fission class 10?

Splitting up a heavy nucleus into smaller nuclei when bombarded with neutrons having low energy is called nuclear fission.

9. Define nuclear fission class for 12.

Through a nuclear reaction, when the nucleus of an atom splits into lighter nuclei it is called nuclear fission.

10. How do nuclear power plants harness fission energy?

Nuclear power plants use the heat generated by controlled fission reactions to boil water into steam. This steam then drives turbines connected to generators, which produce electricity.

11. How is energy released during nuclear fission?

Energy is released in nuclear fission primarily as kinetic energy of the fission fragments and neutrons, as well as gamma radiation. This energy is then converted to heat, which can be used to generate electricity in nuclear power plants.

12. How does nuclear fission contribute to radioactive waste?

Nuclear fission produces radioactive fission products and activation products. These materials remain radioactive for varying periods, from days to thousands of years, and require careful handling and storage.

13. What is the role of control rods in nuclear fission reactors?

Control rods are used to regulate the rate of fission reactions in a nuclear reactor. They are made of neutron-absorbing materials and can be inserted or withdrawn to control the number of neutrons available for fission, thus managing the reactor's power output.

14. What are the main safety concerns associated with nuclear fission reactors?

The primary safety concerns include potential radiation leaks, nuclear meltdowns, and the long-term storage of radioactive waste. Proper containment, cooling systems, and waste management are crucial for safe operation.

15. How does nuclear fission differ from nuclear fusion?

Nuclear fission involves splitting heavy nuclei into lighter ones, while nuclear fusion combines light nuclei to form heavier ones. Fission releases energy by breaking apart large atoms, whereas fusion releases energy by joining small atoms together.

16. How does the binding energy per nucleon relate to nuclear fission?

The binding energy per nucleon is lower for very heavy elements compared to medium-weight elements. This difference allows energy to be released when heavy nuclei split into medium-weight nuclei during fission.

17. How does nuclear fission compare to chemical reactions in terms of energy release?

Nuclear fission releases millions of times more energy per atom than chemical reactions. This is because fission involves changes to the nucleus, while chemical reactions only involve electron interactions.

18. What is the difference between fissile and fertile materials?

Fissile materials, like uranium-235, can undergo fission when bombarded with low-energy neutrons. Fertile materials, like uranium-238, cannot directly undergo fission with low-energy neutrons but can be converted into fissile materials through neutron capture.

19. What is nuclear transmutation, and how does it relate to fission?

Nuclear transmutation is the conversion of one element or isotope into another. In fission reactors, this occurs when neutrons are captured by non-fissile isotopes, potentially creating new fissile materials or radioactive waste products.

20. What is nuclear fission?

Nuclear fission is the process where a heavy atomic nucleus splits into two or more lighter nuclei. This process releases energy in the form of kinetic energy of the fragments, neutrons, and gamma radiation. It's the principle behind nuclear power plants and atomic bombs.

21. How does nuclear fission affect the atomic number and mass number of the original nucleus?

During fission, the original nucleus splits into two lighter nuclei with lower atomic numbers. The sum of the mass numbers of the products is slightly less than the original nucleus, with the mass difference converted to energy.

22. How does the concept of binding energy explain why fission releases energy?

The total binding energy of the fission products is greater than that of the original nucleus. This difference in binding energy is released as kinetic energy and radiation during the fission process.

23. How do fast neutrons differ from thermal neutrons in nuclear fission?

Fast neutrons have high kinetic energy and are produced directly from fission. Thermal neutrons have much lower energy and are more likely to cause fission in certain isotopes. Moderation converts fast neutrons to thermal neutrons.

24. What is the importance of neutron multiplication factor in nuclear reactors?

The neutron multiplication factor (k) determines whether a fission reaction will sustain itself, grow, or die out. When k = 1, the reaction is self-sustaining; k > 1 leads to increasing power, while k < 1 causes the reaction to die out.

25. What triggers a nuclear fission reaction?

A nuclear fission reaction is typically triggered when a nucleus absorbs a neutron, becoming unstable and splitting apart. This process can be initiated by bombarding certain isotopes with neutrons.

26. Which elements are commonly used in nuclear fission reactions?

The most common elements used in nuclear fission are uranium-235 and plutonium-239. These isotopes are fissile, meaning they can sustain a fission chain reaction when bombarded with neutrons.

27. What is a chain reaction in nuclear fission?

A chain reaction occurs when the neutrons released by one fission event trigger subsequent fission events in nearby nuclei. This self-sustaining process can lead to a rapid release of energy if not controlled.

28. What is the significance of critical mass in nuclear fission?

Critical mass is the minimum amount of fissile material needed to sustain a nuclear chain reaction. When the mass of fissile material exceeds the critical mass, the chain reaction becomes self-sustaining and can lead to a rapid energy release.

29. What is the role of moderators in nuclear fission reactors?

Moderators slow down fast neutrons produced during fission, increasing the likelihood of them causing further fission events. Common moderators include water, heavy water, and graphite.

30. What is the significance of delayed neutrons in nuclear fission reactors?

Delayed neutrons, emitted by fission products some time after the initial fission event, play a crucial role in controlling nuclear reactors. They allow for slower and more manageable control of the chain reaction.

31. How does the fission of uranium-235 differ from that of plutonium-239?

While both undergo fission with thermal neutrons, plutonium-239 produces more neutrons per fission on average. This affects reactor design and fuel cycle considerations.

32. What is the role of neutron reflectors in nuclear reactors?

Neutron reflectors surround the reactor core and reflect escaping neutrons back into the fuel, increasing efficiency and reducing the amount of fuel needed for a sustained reaction.

33. How does nuclear fission contribute to the production of radioisotopes for medical use?

Fission reactors can produce various radioisotopes as byproducts or through neutron activation of target materials. These isotopes are used in medical imaging, cancer treatment, and other applications.

34. What is the difference between controlled and uncontrolled fission reactions?

Controlled fission reactions, as in nuclear reactors, maintain a steady rate of energy release through the use of control rods and moderators. Uncontrolled reactions, as in nuclear weapons, result in a rapid, explosive release of energy.

35. How does the concept of cross-section apply to nuclear fission?

Cross-section in nuclear physics represents the probability of an interaction between a neutron and a nucleus. Fissile materials have large fission cross-sections for thermal neutrons, making them more likely to undergo fission.

36. What is the significance of the neutron-to-proton ratio in fission products?

Fission products often have a higher neutron-to-proton ratio than stable isotopes, making them neutron-rich and radioactive. This excess of neutrons leads to beta decay as the products move towards stability.

37. How does the fission barrier affect the likelihood of nuclear fission?

The fission barrier is the energy threshold that must be overcome for a nucleus to undergo fission. Nuclei with lower fission barriers are more likely to fission when bombarded with neutrons.

38. What is the role of breeder reactors in nuclear fission technology?

Breeder reactors are designed to produce more fissile material than they consume by converting fertile materials like uranium-238 into fissile plutonium-239, potentially extending fuel supplies.

39. How does the mass defect relate to energy release in nuclear fission?

The mass defect is the difference between the mass of the original nucleus and the sum of the masses of its constituent protons and neutrons. In fission, this mass difference is converted to energy according to E=mc².

40. What is the significance of prompt critical and delayed critical states in nuclear reactors?

A reactor is prompt critical when it can sustain a chain reaction using only prompt neutrons, leading to rapid power increase. Delayed critical relies on both prompt and delayed neutrons, allowing for controlled operation.

41. How does the neutron energy spectrum affect fission reactor design?

The neutron energy spectrum influences reactor design choices, such as fuel composition, moderator type, and control systems. Fast reactors operate with high-energy neutrons, while thermal reactors use slowed-down neutrons.

42. What is the role of neutron poisons in nuclear fission reactors?

Neutron poisons are materials with high neutron absorption cross-sections that can reduce reactor reactivity. Some fission products act as neutron poisons, affecting long-term reactor operation.

43. How does the concept of reactivity feedback contribute to reactor safety?

Reactivity feedback mechanisms, such as temperature coefficients, cause the reactor to naturally counteract changes in power level, enhancing stability and safety.

44. What is the difference between thermal and fast fission?

Thermal fission occurs with low-energy neutrons and is common in most current reactors. Fast fission uses high-energy neutrons and is employed in fast breeder reactors, offering different fuel cycle possibilities.

45. How does the fission yield curve relate to the production of different isotopes?

The fission yield curve shows the probability distribution of different fission products. It typically has two peaks, corresponding to asymmetric fission, which influences the types and quantities of isotopes produced.

46. What is the significance of the neutron economy in nuclear reactors?

Neutron economy refers to the balance between neutron production and loss in a reactor. Optimizing neutron economy is crucial for efficient fuel utilization and sustained chain reactions.

47. How does nuclear fission contribute to the production of transuranic elements?

Transuranic elements can be produced in reactors when uranium or plutonium nuclei capture neutrons instead of undergoing fission, leading to the formation of heavier elements.

48. What is the role of burnable poisons in nuclear fuel design?

Burnable poisons are materials added to nuclear fuel that have high neutron absorption cross-sections but are gradually depleted. They help control reactivity over the fuel's lifetime, improving fuel efficiency.

49. How does the concept of breeding ratio apply to nuclear fission reactors?

The breeding ratio is the number of fissile nuclei produced per fissile nucleus consumed. A ratio greater than one indicates that the reactor is producing more fuel than it consumes.

50. What is the significance of the neutron flux distribution in a nuclear reactor?

The neutron flux distribution determines the power distribution within the reactor core. Understanding and managing this distribution is crucial for efficient and safe reactor operation.

51. How does the concept of burnup relate to nuclear fuel efficiency?

Burnup measures the amount of energy extracted from nuclear fuel. Higher burnup indicates more efficient fuel utilization but also affects fuel integrity and waste composition.

52. What is the role of neutron leakage in reactor design and operation?

Neutron leakage refers to neutrons escaping the reactor core. Minimizing leakage is important for efficiency, but some leakage is necessary for instrumentation and safety systems.

53. How does the fission process contribute to nuclear waste characteristics?

Fission produces a variety of radioactive isotopes with different half-lives and decay modes. Understanding these characteristics is crucial for waste management and storage strategies.

54. What is the significance of the neutron life cycle in nuclear reactors?

The neutron life cycle describes the stages a neutron goes through from its birth in fission to causing another fission event or being lost. Understanding this cycle is fundamental to reactor physics and control.

55. How does the concept of void coefficient affect reactor safety?

The void coefficient describes how reactor reactivity changes with the formation of voids (e.g., steam bubbles) in the coolant. A positive void coefficient can lead to instability and is a crucial safety consideration.

56. What is the role of neutron absorbers in spent fuel storage?

Neutron absorbers are used in spent fuel storage to prevent criticality by absorbing excess neutrons. This is crucial for safe long-term storage of highly radioactive spent fuel.

57. How does the concept of criticality safety apply to nuclear fuel processing and storage?

Criticality safety involves preventing accidental nuclear chain reactions in facilities handling fissile materials. It's crucial in fuel fabrication, reprocessing, and waste storage to prevent uncontrolled fission events.

58. What is the significance of the neutron generation time in reactor kinetics?

The neutron generation time is the average time between a neutron's birth and its inducing fission. This parameter is crucial for understanding reactor dynamics and control system design.

59. How does the isotopic composition of nuclear fuel change during burnup?

As nuclear fuel is used, its isotopic composition changes due to fission, neutron capture, and decay processes. This affects reactor performance and waste characteristics over time.