Chain Reaction — Revision Notes

⚡ 30-Second Revision

Fission: — Heavy nucleus splits, releases energy + neutrons.
Chain Reaction: — Self-sustaining sequence of fissions.
Fissile Materials: — $^{235}\text{U}$, $^{239}\text{Pu}$ (fission by thermal neutrons).
Neutron Multiplication Factor (k):

- $k < 1$: Subcritical, dies out. - $k = 1$: Critical, steady rate (reactor). - $k > 1$: Supercritical, exponential growth (bomb).

Critical Mass: — Minimum mass for sustained reaction.
Moderator: — Slows fast neutrons to thermal (e.g., heavy water, graphite).
Control Rods: — Absorb excess neutrons (e.g., cadmium, boron).
Energy per Fission: — $approx 200,\text{MeV}$.

2-Minute Revision

A nuclear chain reaction is a self-propagating series of nuclear fission events. It begins when a neutron strikes a fissile nucleus, like Uranium-235, causing it to split, release significant energy (around 200 MeV), and emit 2-3 new neutrons.

These new neutrons can then induce further fissions, continuing the chain. The behavior of this reaction is governed by the neutron multiplication factor, 'k'. For a steady, controlled reaction, as in nuclear power plants, 'k' must be maintained at exactly 1 (critical state).

If 'k' falls below 1 (subcritical), the reaction dies out. If 'k' rises above 1 (supercritical), the reaction accelerates exponentially, leading to an uncontrolled energy release, as in an atomic bomb.

To sustain a chain reaction, a minimum amount of fissile material, known as the critical mass, is required to prevent excessive neutron leakage. In reactors, moderators (like heavy water or graphite) slow down fast neutrons to make them more effective at causing fission, while control rods (made of neutron absorbers like cadmium or boron) regulate the reaction rate by soaking up excess neutrons.

5-Minute Revision

The core of nuclear technology, the chain reaction, relies on nuclear fission. When a slow neutron hits a fissile nucleus (e.g., Uranium-235), it causes the nucleus to split, releasing about 200 MeV of energy and an average of 2.

5 fast neutrons. These fast neutrons are the key to the chain. If they can induce further fissions, the reaction propagates. The neutron multiplication factor (k) quantifies this: $k = \frac{\text{neutrons in current generation}}{\text{neutrons in previous generation}}$.

If $k < 1$, the reaction is subcritical and fades. If $k = 1$, it's critical and self-sustaining at a constant rate, ideal for power reactors. If $k > 1$, it's supercritical, leading to exponential growth and an explosion.

To achieve and maintain criticality, the critical mass is essential – the minimum amount of fissile material needed to ensure enough neutrons cause fission rather than escaping. In nuclear reactors, moderators (like heavy water or graphite) are used to slow down the fast fission neutrons to thermal energies, as thermal neutrons are far more effective at causing fission in $^{235}\text{U}$.

Control rods (made of neutron-absorbing materials like cadmium or boron) are inserted or withdrawn to precisely adjust 'k' to 1, thus regulating the reactor's power output. Coolants remove the immense heat generated.

Understanding these components and their roles is vital for NEET, as questions often test the distinction between controlled (reactor) and uncontrolled (bomb) chain reactions.

Prelims Revision Notes

Chain Reaction: Key Facts for NEET

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Definition: — A self-sustaining series of nuclear fission reactions where neutrons released from one fission induce further fissions.
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Nuclear Fission: — Splitting of a heavy nucleus (e.g., $^{235}\text{U}$) into lighter nuclei, releasing $approx 200,\text{MeV}$ energy and 2-3 neutrons, initiated by a neutron.
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Fissile Materials: — Substances that readily undergo fission by thermal neutrons. Examples: Uranium-235 ($^{235}\text{U}$), Plutonium-239 ($^{239}\text{Pu}$). Natural uranium is mostly $^{238}\text{U}$ (non-fissile by thermal neutrons).
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Neutron Multiplication Factor (k):

* $k < 1$: Subcritical. Reaction dies out. (Reactor shutdown) * $k = 1$: Critical. Reaction sustained at a constant rate. (Nuclear power reactor operation) * $k > 1$: Supercritical. Reaction grows exponentially. (Atomic bomb, runaway reactor)

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Critical Mass: — The minimum mass of fissile material required for a self-sustaining chain reaction. Below critical mass, too many neutrons escape.
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Components of a Nuclear Reactor:

* Fuel: Fissile material (e.g., enriched uranium) in fuel rods. * Moderator: Slows down fast neutrons to thermal energies (e.g., heavy water ($D_2O$), graphite). Thermal neutrons are more effective for $^{235}\text{U}$ fission.

* Control Rods: Absorb excess neutrons to regulate the reaction rate (e.g., Cadmium, Boron). Inserted to decrease 'k', withdrawn to increase 'k'. * Coolant: Transfers heat from the reactor core (e.

g., water, heavy water, liquid sodium). * Shielding: Protects personnel from radiation (e.g., concrete, lead).

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Controlled vs. Uncontrolled Chain Reaction:

* Controlled: $k=1$, steady energy release, used in nuclear reactors for power. * Uncontrolled: $k>1$, rapid exponential energy release, used in nuclear weapons.

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Energy Release: — Approximately $200,\text{MeV}$ per fission of $^{235}\text{U}$. This is significantly higher than chemical reactions.

Vyyuha Quick Recall

To remember reactor components and their roles: For My Cool Core, Safety is Paramount.

Fuel: Fissile material
Moderator: Slows neutrons
Control rods: Control reaction (absorb neutrons)
Coolant: Cools reactor
Shielding: Safety from radiation
Power: Purpose (electricity)