Nuclear Criticality in Nuclear Fission and Nuclear Energy

• Context (TH | TP): India’s second indigenous 700 MWe nuclear reactor at Kakrapar Atomic Power Station (KAPS) in Gujarat moves closer to commercial operation after achieving first criticality.

700 MWe units are the largest indigenous nuclear power reactors built by the Nuclear Power Corporation of India Limited (NPCIL), a public sector undertaking of the Department of Atomic Energy.

• These reactors are pressurised heavy water reactors (PHWRs), which use natural uranium as fuel and heavy water as coolant and moderator.
• The reactor achieved criticality after meeting the specified conditions of the Atomic Energy Regulatory Board (AERB), India’s nuclear safety regulator.

Nuclear Fission

• Nuclear fission splits a heavy atomic nucleus into two smaller nuclei, releasing energy through heat, light, and radiation.
• Radioactive atoms (or isotopes) are used in nuclear fission.
• Uranium-235 (U-235) and plutonium-239 (Pu-239) are commonly used isotopes for fission.
• U-235, a rare isotope of the heavy metal uranium, is the most commonly used nuclear fuel.

Radioactive Atoms (or Isotopes) Radioactive atoms have an unstable nucleus and undergo spontaneous nuclear (radioactive) decay. An unstable atom has a composition of protons and neutrons that prevents the nucleus from holding itself together. Radioactive decay is when an unstable atomic nucleus loses energy by emitting radiation. Isotopes are elements with identical proton numbers but varying neutron numbers.

How does Nuclear Fission take place?

• The nuclear fission is initiated by subjecting a U-235 or Pu-239 nucleus to neutrons.
• The nucleus absorbs an extra neutron, becomes unstable, and splits into two lighter atoms and additional neutrons. This process releases what is known as atomic energy.
• The fission of a U-235 or Pu-239 atom produces about 2 to 3 new neutrons.
• If other U-235 or Pu-239 atoms absorb these new neutrons, it creates an exponentially growing chain reaction. Such a chain reaction releases large amounts of energy.

Controlled Nuclear Fission

• If controlled in a nuclear reactor, such a chain reaction from nuclear creation can generate power.
• If uncontrolled, it can lead to an enormous explosion (atomic bomb).
• For a self-sustained, controlled nuclear reaction, only one neutron should strike another uranium nucleus for every 2 or 3 neutrons released.
• If this ratio is less than one, the reaction will die out; if it is greater than one, it will grow uncontrolled.

Nuclear Criticality

• In nuclear reactor operation, criticality is the self-sustaining state of a nuclear chain reaction.
• When there is a perfect balance between neutron production and loss rates, the nuclear system is considered critical.
• During reactor startup, neutron population is gradually increased in a controlled manner, ensuring more neutrons are produced than lost.
• When the desired power level is achieved, the nuclear reactor is placed into a critical configuration.

Subcritical describes a nuclear system where neutron loss exceeds neutron production. Supercritical describes a nuclear system where neutron production exceeds neutron loss.

India and the Importance of Commercial Nuclear Energy

• India is facing the dual challenge of development and environmental sustainability.
• Amidst escalating climate change concerns, the global community advocates phasing out fossil fuels, India’s most important energy source.
• In the recent UNFCCC COP 28 (UAE), the major push was in favour of phasing out fossil fuels.
• But India is pitching for phasing down rather than phasing out fossil fuels.
• India contends that, as a developing nation experiencing rapid economic growth, phasing out fossil fuels would impede its economic progress.
• Moreover, India asserts that, as a non-historical emitter, it should not be disproportionately burdened as it goes against the Common But Differentiated Responsibilities and Respective Capabilities (CBDR-RC) principle, enshrined in the Earth Summit 1992.
• At COP 28, 22 countries have pledged to triple global nuclear capacity by 2050 for a net-zero emissions goal. India has abstained from this commitment.
• None of these means India is against adopting alternative energy sources for protecting the environment. It simply means India wants the process to be gradual rather than sudden and rushed.
• India has set its target to reach net-zero emissions by 2070.
• Nuclear energy is a renewable energy source that does not emit greenhouse gases, so India wants to increase its share to meet its climate goals.
• It aims to generate 50% of its total electricity from non-fossil fuel sources by 2030, based on its commitment at the COP 26 in Glasgow in 2021.
• As of 2023, India’s total generation capacity is 417 GW, with 43% coming from renewable sources.
• India stands alone among developing nations in generating electricity using domestically developed, demonstrated, and deployed nuclear reactors.
• Nuclear energy is India’s fifth-largest source of electricity.
• India aims to boost its nuclear power contribution from 3.2% to 5% by 2031.

Challenges for the Nuclear Sector to Meet Net-Zero Emission Target

• High initial investment: Complex technology makes building new nuclear power plants expensive.
• Decommissioning costs: Dismantling nuclear plants at the end of their lifespan is complex and costly.
• Safety concerns: E.g., the Chernobyl and Fukushima disasters.
• Nuclear waste disposal: Safe, long-term storage for radioactive waste is vital for the sustainability of nuclear energy.
• Thermal pollution: Nuclear plants release heated water, impacting aquatic ecosystems.
• Public perception: Public concerns about safety, waste disposal, and nuclear weapons proliferation can limit public support for nuclear energy.
• Regulatory framework: Complex regulations can increase costs and delays in nuclear projects.
• Fuel availability: While uranium resources are relatively abundant, supply chain risks and geopolitical factors necessitate careful planning and source diversification.

Way Forward

• Scaling up capacity: The government aims to triple nuclear power capacity by 2032, with plans to increase it from 6,780 MW to 22,480 MW by 2031.
• Embracing advanced technologies: Moving beyond Pressurized Heavy Water Reactors (PHWRs), India is exploring Light Water Reactors (LWRs) and Small Modular Reactors (SMRs). These offer faster deployment, potentially lower costs, and enhanced safety features.
• Leveraging the Thorium advantage: India boasts the world’s largest thorium reserves, a cleaner and more abundant fuel than uranium. Developing Thorium-based Advanced Reactors (TARs) could revolutionise India’s energy sector, reducing imports and minimising waste.
• Addressing public concern
• Ensuring safety and security
• Regulatory measures
• Waste management solutions