Historic: PM on Kalpakkam Nuclear Reactor Attaining Criticality

Relevance: : GS Paper III – Science & Technology | Energy | Nuclear Technology | Infrastructure

Important Keywords for Prelims and Mains

For Prelims:

Fast Breeder Reactor (FBR), Prototype Fast Breeder Reactor (PFBR), Criticality, Kalpakkam, BHAVINI, Three-Stage Nuclear Power Programme, Plutonium-239, Uranium-238, Thorium-232, MOX Fuel, Sodium Coolant, Atomic Energy Regulatory Board (AERB)

For Mains:

energy security, indigenous technology, strategic autonomy, clean energy transition, nuclear self-reliance, thorium economy, base-load power, sustainable development, Viksit Bharat, low-carbon growth

Why in News?

The Prototype Fast Breeder Reactor (PFBR) at Kalpakkam in Tamil Nadu has achieved criticality, marking a major milestone in India’s nuclear energy journey.

Prime Minister Narendra Modi described it as a historic achievement because the reactor has been built entirely using indigenous technology and represents India’s formal entry into the second stage of its Three-Stage Nuclear Power Programme.
This development strengthens India’s long-term energy security strategy and supports the goal of reducing dependence on imported fossil fuels and uranium.
The achievement is also important because India becomes one of the very few countries in the world to operate a commercial-scale Fast Breeder Reactor.

What Happened at Kalpakkam?

The reactor at Kalpakkam is India’s Prototype Fast Breeder Reactor (PFBR), located near the Madras Atomic Power Station in Tamil Nadu.
It is a 500 MW sodium-cooled Fast Breeder Reactor developed by Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI).
The reactor has now attained criticality, which means it has successfully started a self-sustaining nuclear chain reaction for the first time.
This marks the transition from the construction phase to the operational phase and is one of the most important technical milestones before commercial power generation begins.
The PFBR is expected to play a central role in India’s future nuclear expansion and thorium utilization strategy.

What is Criticality in a Nuclear Reactor?

Criticality refers to the state in which a nuclear reactor’s chain reaction becomes self-sustaining.

It occurs when each fission of a nucleus (usually Uranium-235 or Plutonium-239) releases enough neutrons to cause, on average, exactly one more fission reaction.

This maintains a steady and continuous release of energy without increasing or decreasing.

Types of Criticality

1. Subcritical State (k < 1)
Each fission causes less than one further fission on average.
The chain reaction gradually dies out.

2. Critical State (k = 1)
Each fission causes exactly one more fission.
The reaction remains stable and controlled.
This is the normal operating condition of a power reactor.

3. Supercritical State (k > 1)
Each fission causes more than one additional fission.
The reaction increases rapidly, releasing more energy.
Used carefully during reactor startup; uncontrolled supercriticality can be dangerous.

Importance of Achieving Criticality

When a reactor “attains criticality,” it means:

The reactor has successfully started functioning
Controlled nuclear fission is taking place
It can begin producing heat for electricity generation

Thus, achieving criticality is considered a major milestone in a nuclear power project.

What is a Fast Breeder Reactor (FBR)?

A Fast Breeder Reactor (FBR) is a type of nuclear reactor that produces more fissile fuel than it consumes.
It uses fast neutrons (high-energy neutrons) to sustain the chain reaction and converts fertile material like Uranium-238 or Thorium-232 into fissile material such as Plutonium-239 or Uranium-233.
This process is called breeding, hence the name “Breeder Reactor.”

How it Works?

Fuel used: Usually Plutonium-239 + Uranium-238
No moderator is used because fast neutrons are needed
Coolant used: Commonly liquid sodium
A surrounding blanket of fertile material absorbs neutrons and converts into new fuel

Example:

Uranium-238 → absorbs neutron → converts into → Plutonium-239

Thus, it generates energy and also creates new fuel.

Key Features

Produces more fuel than it consumes
Improves fuel efficiency
Reduces dependence on natural uranium
Supports long-term nuclear energy security
Important for countries with limited uranium reserves

How an FBR Works?

The core of the reactor uses Mixed Oxide Fuel (MOX), which is a combination of plutonium and uranium.
Around the fuel core is a breeding blanket made of Uranium-238.
When fast neutrons from the core strike Uranium-238, it gets converted into Plutonium-239, which is a fissile fuel. This process is called breeding.
Liquid sodium is used as a coolant because it transfers heat efficiently and does not slow down neutrons, which is necessary for fast neutron reactions.
This design allows the reactor to generate electricity while also producing more nuclear fuel for future use.

India’s Three-Stage Nuclear Power Programme

India’s nuclear programme was designed by Dr. Homi J. Bhabha to achieve long-term energy independence using India’s vast thorium reserves.

Stage I – Pressurized Heavy Water Reactors (PHWRs)

These reactors use natural uranium as fuel and produce Plutonium-239 as a by-product.
This plutonium becomes the fuel for the second stage.

Stage II – Fast Breeder Reactors (FBRs)

These reactors use plutonium-based MOX fuel and produce more plutonium while generating electricity.
This is the stage India has now entered with the PFBR at Kalpakkam.

Stage III – Thorium-Based Reactors

These reactors will use Thorium-232 to produce Uranium-233, leading to long-term sustainable nuclear power generation.
Since India has one of the world’s largest thorium reserves, this stage is crucial for energy security.

Role of PFBR in India’s Nuclear Strategy

The PFBR is the bridge between India’s uranium-based first stage and thorium-based third stage.
Without successful Fast Breeder Reactors, India cannot effectively transition to thorium-based nuclear energy.
The PFBR helps India multiply its fissile material resources and reduces long-term dependence on imported uranium.
It also supports India’s clean energy goals by providing reliable base-load power without carbon emissions associated with fossil fuels.
This makes PFBR not just a reactor, but the foundation of India’s long-term nuclear sovereignty.

BHAVINI and Indigenous Development

The PFBR has been designed and developed by Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI), a public sector enterprise under the Department of Atomic Energy.
It represents one of India’s most advanced indigenous technology achievements in the strategic energy sector.
More than 200 Indian industries contributed to the manufacturing of reactor components, reflecting strong domestic industrial capability.
This supports the goals of Aatmanirbhar Bharat and Viksit Bharat by reducing dependence on foreign nuclear technology and creating high-end scientific capacity within India.

Significance of the Achievement

The PFBR strengthens India’s energy security by enabling efficient use of limited uranium resources.
It supports climate goals by expanding low-carbon electricity generation and reducing fossil fuel dependence.
It marks India’s entry into a highly advanced group of nations with commercial Fast Breeder Reactor capability, with only Russia having similar large-scale operational success.
The achievement also promotes scientific innovation, advanced metallurgy, reactor engineering, and strategic industrial development.
In the long term, it improves India’s preparedness for a thorium-driven energy future.

Challenges Associated with Fast Breeder Reactors

Fast Breeder Reactors are technologically complex and expensive to build and operate.
Liquid sodium coolant, though efficient, is highly reactive with air and water, creating safety challenges.
Construction delays and cost overruns have historically affected breeder reactor projects globally.
Many countries such as the United States, France, and Japan reduced or closed breeder reactor programmes because of safety and economic concerns.
Public concerns regarding nuclear safety and radioactive waste management also remain major policy challenges.
Therefore, strong regulation and long-term technical planning are essential.

Way Forward

India must ensure the safe commissioning and stable operation of the PFBR before expanding breeder reactor capacity further.
Investment in Small Modular Reactors (SMRs) and advanced nuclear technologies should continue alongside breeder reactor development.
The Atomic Energy Regulatory Board must maintain strong safety oversight and public trust through transparent regulation.
Research on thorium fuel cycles must be accelerated so that the transition to Stage III becomes practically achievable.
Private sector participation in manufacturing and nuclear supply chains should be expanded carefully while retaining strategic control.
A strong domestic ecosystem of nuclear science, engineering, and skilled manpower is essential for sustained growth.

Conclusion

The criticality of the Kalpakkam Fast Breeder Reactor is not merely a scientific milestone—it is a strategic turning point in India’s energy future.It confirms India’s progress toward self-reliant nuclear technology and brings the country closer to realizing its long-term thorium-based energy vision.

As India moves toward cleaner growth and greater strategic autonomy, the success of the PFBR will remain one of the defining pillars of its civil nuclear programme.This achievement represents both technological confidence and national energy security.

CARE MCQ

Q. Consider the following statements about Critical Mass in the context of nuclear physics:

Critical mass is the minimum amount of fissile material required to sustain a nuclear chain reaction.
Critical mass is independent of the shape, size, and density of the fissile material.
The concept of critical mass applies only to nuclear weapons and not to nuclear reactors.
Uranium-235 and Plutonium-239 are examples of materials capable of achieving critical mass.

Which of the statements given above are correct?

(a) 1 and 4 only
(b) 2 and 3 only
(c) 1, 2 and 4 only
(d) All of the above

Ans: (a)

Explanation:

Statement 1 is correct: Critical mass refers to the minimum quantity of a fissile material needed to maintain a self-sustaining nuclear chain reaction. If the amount of fissile material is below this level, too many neutrons escape and the chain reaction stops. Once critical mass is reached, enough neutrons remain available to continue fission reactions continuously.

Statement 2 is incorrect: Critical mass is not fixed only by quantity; it also depends on the shape, size, density, purity, and presence of neutron reflectors around the material. For example, a spherical shape requires less critical mass because it minimizes neutron leakage compared to irregular shapes. Therefore, this statement is incorrect.

Statement 3 is incorrect: The concept of critical mass applies to both nuclear weapons and nuclear reactors. In nuclear weapons, an uncontrolled chain reaction is produced rapidly after achieving supercritical mass. In nuclear reactors, the system is carefully maintained at criticality so that energy is released in a controlled manner for electricity generation. Hence, this statement is incorrect.

Statement 4 is correct: Uranium-235 (U-235) and Plutonium-239 (Pu-239) are important fissile isotopes capable of achieving critical mass. They are widely used in nuclear reactors and nuclear weapons because they can sustain chain reactions through neutron-induced fission. Therefore, this statement is correct.

Q. Which of the following statements accurately describes the corrosion resistance of liquid sodium when used as a coolant in Fast Breeder Reactors (FBRs)?

(a) Liquid sodium, due to its inherent inertness, does not react with reactor materials irrespective of its purity.

(b) The excellent corrosion resistance of liquid sodium towards stainless steel is maintained by ensuring extremely low levels of dissolved oxygen.

(c) Liquid sodium forms a stable, self-healing oxide layer on reactor components which protects them from further corrosion at high temperatures.

(d) High operating temperatures in FBRs significantly reduce the corrosive nature of liquid sodium, making it compatible with most metals.

Ans: (b)

Explanation:

Option (b) is correct:

In Fast Breeder Reactors (FBRs), liquid sodium is widely used as a coolant because of its excellent heat transfer properties, high boiling point, and ability to operate at low pressure. Although sodium is highly reactive with air and water, it shows good compatibility with structural materials such as stainless steel when its purity is carefully maintained.

The key factor for corrosion resistance is keeping the level of dissolved oxygen extremely low. If oxygen is present in higher amounts, sodium reacts with metals and forms sodium-metal oxides, which increase corrosion and damage reactor components. Therefore, strict control of oxygen impurities is essential for maintaining the integrity and long life of the reactor system

Q. Consider the following statements about a nuclear chain reaction:

A nuclear chain reaction occurs when one nuclear reaction causes one or more subsequent nuclear reactions.
A chain reaction becomes self-sustaining only when the average number of neutrons causing further fission is less than one.
Nuclear chain reactions are the fundamental principle behind the working of nuclear reactors.
The concept of a nuclear chain reaction was first theorized by Hungarian physicist Leo Szilard in 1933.

Which of the statements given above are correct?

(a) 1 and 3 only
(b) 2 and 4 only
(c) 1, 3 and 4 only
(d) All of the above

Ans: (c)

Explanation:

Statement 1 is correct: A nuclear chain reaction happens when neutrons released from one fission event trigger additional fission events in nearby fissile atoms. This creates a sequence where one reaction leads to one or more further reactions, making the process continuous. This is the basic principle behind nuclear energy production.

Statement 2 is incorrect: For a chain reaction to be self-sustaining, the multiplication factor (k) must be equal to or greater than 1. This means that on average, at least one neutron from each fission must cause another fission. If the number is less than one, the reaction becomes subcritical and gradually stops. Therefore, the statement is incorrect.

Statement 3 is correct: Nuclear reactors operate on the principle of a controlled nuclear chain reaction. The heat released during continuous fission is used to produce steam, which drives turbines and generates electricity. Without chain reactions, nuclear power generation would not be possible. Hence, this statement is correct.

Statement 4 is correct: The idea of a nuclear chain reaction was first proposed by Leo Szilard, a Hungarian physicist, in 1933. He realized that if neutrons released from one reaction could trigger further reactions, enormous energy could be produced. This idea later became the foundation for both nuclear reactors and atomic bombs. Therefore, this statement is correct.