Nuclear Fission

- Overview

Nuclear fission is a reaction where a heavy atomic nucleus, such as 𝑈²³⁵, absorbs a neutron and splits into smaller nuclei (e.g., barium and krypton), releasing intense heat, radiation, and additional neutrons. 

These new neutrons trigger a self-sustaining chain reaction, releasing millions of times more energy than chemical reactions.

This process is fundamentally different from nuclear fusion, which combines lighter atoms rather than splitting heavy ones.

Key Aspects of Nuclear Fission: 

• Process: A neutron strikes a heavy nucleus (𝑈²³⁵ or 𝑃𝑢²³⁹), causing it to become unstable and split.
• Energy Release: The mass of the resulting fragments is less than the original nucleus, with this "missing mass" converting into significant kinetic energy and heat.
• Chain Reaction: The 2–3 neutrons released per fission event hit other nuclei, causing a rapid, multiplying, and continuous reaction.
• Applications: The heat generated is used to boil water, create steam, and drive turbines to produce electricity in power plants.
• Byproducts: Fission produces radioactive waste requiring specialized storage and cooling.

- A Short History of Nuclear Fission

Nuclear fission is the process where the nucleus of an atom splits into two or more smaller nuclei, releasing a significant amount of energy. 

1. Key Historical Milestones:

• 1789: German chemist Martin Klaproth discovered uranium, naming it after the planet Uranus.
• 1934: Italian physicist Enrico Fermi achieved the first man-made nuclear fission by bombarding uranium with neutrons, though he did not initially realize the atom had split.
• December 19, 1938: German chemists Otto Hahn and Fritz Strassmann experimentally proved nuclear fission had occurred after finding barium in the products of neutron-bombarded uranium.
• January 1939: Physicist Lise Meitner and her nephew Otto Robert Frisch provided the theoretical explanation for Hahn's results. Frisch coined the term "fission" based on the biological process of cell division.
• December 2, 1942: The first controlled, self-sustaining nuclear chain reaction was achieved by Enrico Fermi and his team at the University of Chicago (Chicago Pile-1).

2. Scientific Principles:

• Energy Release: For heavy nuclides, fission is an exothermic reaction. Energy is released as electromagnetic radiation (gamma photons) and kinetic energy of the fragments.
• Binding Energy: To produce energy, the total binding energy of the resulting elements must be greater than that of the original starting element.

[Wikipedia: Simple diagram of nuclear fission. In the first frame, a neutron is about to be captured by the nucleus of a U-235 atom. In the second frame, the neutron has been absorbed and briefly turned the nucleus into a highly excited U-236 atom. In the third frame, the U-236 atom has fissioned, resulting in two fission fragments (Ba-141 and Kr-92) and three neutrons, all with very large amounts of kinetic energy.]

- Nuclear Transmutation

Nuclear transmutation changes an atom's identity by altering its nuclear proton count, occurring naturally (radioactive decay) or artificially. Fission is a form of this, breaking heavy nuclei into lighter fragments, often producing two (binary) or rarely three (ternary) pieces. Spontaneous fission is a natural, non-induced, and significant decay form.

(A) Key Aspects of Nuclear Transmutation: 

1. Definition: The process of transforming one element or isotope into another by changing the number of protons or neutrons in the nucleus. 

2. Types:

• Natural: Occurs via spontaneous radioactive decay.
• Artificial: Achieved by bombarding nuclei with particles (neutrons, alpha particles) in reactors or accelerators.

3. Fission as Transmutation: A heavy nucleus splits into smaller daughter atoms, often with a mass ratio of roughly 3∶2. 

4. Binary vs. Ternary: Most fissions produce two fragments, but about 0.2% to 0.4% are ternary, producing three charged fragments. 

5. Spontaneous Fission: Discovered in 1940 by Flyorov, Petrzhak, and Kurchatov, this non-neutron induced decay occurs in very high-mass-number isotopes.

(B) Historical Context:

• Discovery: Spontaneous fission was identified when researchers found that uranium fission rates were not negligible, contradicting earlier assumptions that neutron bombardment was always required.
• Concept: While related to the alchemical goal of changing base metals, modern nuclear science achieves this through rearranging subatomic particles, not chemical reactions.

- Radioactive Decay

Radioactive decay is the spontaneous, random process by which unstable atomic nuclei (radionuclides) lose energy and transform into more stable atoms by emitting ionizing radiation, such as alpha particles, beta particles, or gamma rays. This natural, unpredictable process (at the single-atom level) changes the nucleus to a different element, a process known as transmutation.

Common examples include Uranium-238 decaying into Thorium-234, and Phosphorus-32 decaying into Sulfur-32. 

Key Aspects of Radioactive Decay: 

1. Causes: Unstable atomic structures with excess energy or improper nucleon balance.
Process: A "parent" nucleus emits radiation and transforms into a stable or, frequently, another radioactive "daughter" nucleus in a cascade called a decay chain. 

2. Types:

• Alpha Decay: Emission of an alpha particle (2 protons, 2 neutrons), reducing the atomic number by 2 and mass by 4.
• Beta Decay: Emission of a beta particle (electron or positron) or electron capture, transforming a neutron to a proton or vice versa.
• Gamma Decay: Emission of high-energy electromagnetic radiation to reduce nuclear excitement without changing the element.

3. Rate & Half-Life: While random, decay occurs at a characteristic, predictable rate for a large number of atoms, defined by its "half-life"—the time required for half the radioactive atoms to decay. 

4. Formula: N(t)=N₀e^−λt  where N(t) is the remaining quantity, 𝑁₀ is the initial quantity, and 𝜆 is the decay constant.
 

- Nuclear Chain Reaction

A nuclear chain reaction occurs when a single nuclear fission event releases neutrons that strike other fissile nuclei (e.g., ²³⁵𝑈 or ²³⁹𝑃𝑢), causing further fissions. 

This self-sustaining process releases immense amounts of energy and heat. It is controlled in nuclear reactors for power or uncontrolled in weapons.

The first man-made, self-sustaining nuclear chain reaction was achieved by Enrico Fermi on December 2, 1942, in Chicago. Today, this process provides roughly 20% of U.S. electricity.

1. How a Nuclear Chain Reaction Works: 

• Initiation: A neutron strikes a fissionable nucleus, making it unstable.
• Fission & Release: The nucleus splits into smaller elements, releasing massive energy, radiation, and two or three additional neutrons.
• Propagation: These new neutrons strike surrounding nuclei, causing more fissions in an exponential, self-sustaining loop.
• Critical Mass: For this to continue, the fissile material must reach a minimum mass, known as critical mass.

2. Key Aspects and Types:

• Controlled Chain Reaction: Used in power plants (e.g., Chicago Pile-1 in 1942). Control rods absorb excess neutrons to maintain a steady, safe rate of energy production.
• Uncontrolled Chain Reaction: Occurs in nuclear weapons, where exponential, rapid fission causes an explosion.
• Neutron Multiplication Factor (k): If 𝑘=1, the reaction is critical (stable); if 𝑘>1, it is supercritical (uncontrolled/increasing).

- Nuclear Waste

Nuclear waste is radioactive byproduct from reactors, medicine, and industry, categorized mainly into low-level (contaminated tools) and high-level (spent fuel) waste. It remains hazardous for thousands of years, requiring secure, long-term storage in shielded pools or dry casks. 

Management involves strict federal regulation, reprocessing for recycling, and planned deep geological disposal. 

1. Types of Radioactive Waste: 

• High-Level Waste (HLW): Primarily spent fuel from nuclear reactors; it is highly radioactive, hot, and contains plutonium and fission products.
• Intermediate-Level Waste (ILW): Requires shielding but not cooling, consisting of resins, chemical sludges, and reactor components.
• Low-Level Waste (LLW): Makes up ~90% of total volume but low radioactivity; includes contaminated protective clothing, tools, and rags.
• Uranium Mine/Mill Waste: Waste from mining and processing ore, often containing naturally occurring radioactive materials.

2. Sources of Radioactive Waste:

• Nuclear Power Generation: The largest source, producing spent fuel rods and operational waste.
• Defense Programs: Remnants from nuclear weapons manufacturing.
• Medical & Industrial Use: Hospitals and research facilities generate waste through radioisotope use.

3. Management and Disposal:

• Storage: Initially, spent fuel is stored in water pools to cool for several years, then moved to concrete-and-steel dry casks on-site.
• Disposal: Long-term solutions involve deep geological repositories designed to isolate waste for thousands of years.
• Reprocessing/Recycling: ~96% of spent fuel is recyclable, allowing it to be reused as fuel, which reduces the waste volume and long-term radioactivity.
• Transmutation: Advanced techniques like accelerator-driven systems (ADS) can "burn" long-lived isotopes, reducing the hazardous life of waste from 100,000 years to 300 years.

4. Radioactive Lifespan:

• Low-level waste may be safe after decades.
• High-level waste (unprocessed) can remain dangerous for approximately 100,000 years or more.

5. Regulatory Oversight: 

In the US, the Department of Energy (DOE) manages disposal, the Environmental Protection Agency (EPA) sets safety standards, and the Nuclear Regulatory Commission (NRC) enforces security.