Source: Hindu

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Context

• Researchers have reported groundbreaking results in the study of plutonium-240 (Pu-240) fission.
• This marks the first attempt to measure the prompt fission neutron spectrum (PFNS) of induced fission in Pu-240 with neutrons of energy greater than 0.85 MeV.
• The findings reveal significant deviations from model predictions, impacting reactor design, radiation shielding, and nuclear medicine.
  

Details

• A part of the fission energy carried away by neutrons is called the prompt fission neutron spectrum.
• ‘Prompt’ stands for neutrons Pu-240 might emit right after it has captured a neutron with the energy to destabilise it
• Pu-240 undergoes spontaneous fission, and emits alpha particles.
• The isotope is considered a contaminant of weapons-grade plutonium, where its composition by weight is restricted to under 7%

Experimental Setup

• Location: Los Alamos Neutron Science Centre (LANSCE)
• Setup: A tungsten disc bombarded by proton pulses produces neutrons (0.01-800 MeV).
• Detector: Liquid scintillators arranged around a 20-milligram, 99.875% pure Pu-240 sample detect the emitted neutrons and other fission products.
• Neutron Energy: 1-20 MeV
• Data Extraction: Careful subtraction of contributions from spontaneous fission and alpha particles to isolate PFNS data.

Findings

• PFNS Differences: Observed PFNS deviated notably from model predictions.
• Second-Chance Fission: Higher-than-expected rates of second-chance fission in Pu-240, indicating a nucleus becomes fissionable after losing a neutron.
• Third-Chance Fission: Signs of a smaller contribution from third-chance fission, though difficult to observe directly.

Relevance to Nuclear Technology and Research

Reactor Design and Safety

• Plutonium Utilization: Information crucial for reactors using MOX fuel and fast breeder reactors.
• PFBR: Prototype Fast Breeder Reactor at Madras Atomic Power Station, India, using plutonium from CANDU spent fuel.

Radiation Shielding

• Improved PFNS data enhances accuracy in designing radiation shielding.

Nuclear Medicine

• More precise neutron emission data aids in calculating radiation doses for medical applications.

Nuclear Fission

• Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into two or more smaller nuclei, along with the release of a significant amount of energy.
• This process is fundamental to both nuclear power generation and nuclear weapons.
• Energy Release: The process releases a tremendous amount of energy due to the conversion of mass into energy, as described by Einstein's equation E=mc2E=mc2.

Key Elements Involved

• Fissile Materials: Substances that can undergo fission, such as Uranium-235 and Plutonium-239.
• Neutrons: Neutrons play a crucial role in initiating and sustaining the fission process.

Mechanism of Nuclear Fission

• Neutron Absorption: A heavy nucleus absorbs a neutron.
• Nucleus Becomes Unstable: The nucleus becomes highly unstable and deforms.
• Splitting of the Nucleus: The nucleus splits into two or more smaller nuclei (fission fragments), releasing additional neutrons and energy.
• Sustained Chain Reaction: Each fission event releases more neutrons, which can induce further fission events.
• Critical Mass: The minimum amount of fissile material needed to maintain a self-sustaining chain reaction.

Applications of Nuclear Fission

• Nuclear Reactors: Devices used to control nuclear fission to produce energy.
  • Components: Core, fuel rods, control rods, coolant, and moderator.
  • Types of Reactors: Pressurized Water Reactors (PWR), Boiling Water Reactors (BWR), Fast Breeder Reactors (FBR).
• Energy Production: Fission heats water to create steam, which drives turbines to generate electricity.
• Atomic Bombs: Weapons that use uncontrolled fission reactions to release enormous destructive energy.
  • Designs: Gun-type (e.g., Hiroshima's "Little Boy") and implosion-type (e.g., Nagasaki's "Fat Man").

Plutonium

• Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94.
• It is a key material in nuclear chemistry, with significant applications in nuclear power generation and nuclear weapons.
• Discovery: Plutonium was discovered in 1940 by a team led by Glenn T. Seaborg at the University of California, Berkeley.
• Naming: Named after the dwarf planet Pluto, following the tradition of naming elements after celestial bodies.
• Atomic Weight: Approximately 244
• Appearance: Silvery-gray metal that tarnishes in air
• Reactor-Grade vs. Weapons-Grade: Reactor-grade plutonium contains more than 19% Pu-240, while weapons-grade plutonium contains less.

Chemical and Physical Characteristics

Physical Properties

• Phases: Exhibits six different allotropes at various temperatures.
• Density: Approximately 19.86 g/cm³
• Melting Point: 639.4 °C
• Boiling Point: 3,228 °C

Chemical Reactivity

• Oxidation States: +3, +4, +5, and +6; most stable in the +4 oxidation state.
• Compounds: Forms oxides (PuO2), halides, and other compounds.

Isotopes of Plutonium

• Plutonium-239 (Pu-239): Most important isotope, used in nuclear reactors and weapons.
• Plutonium-238 (Pu-238): Used as a heat source in radioisotope thermoelectric generators (RTGs).
• Pu-239: Half-life of 24,100 years, fissile material.
• Pu-238: Half-life of 87.7 years, emits alpha particles, not fissile.

Production of Plutonium

• Plutonium is present in trace amounts in uranium ores but is predominantly a man-made element.
• Nuclear Reactors: Produced by neutron capture in uranium-238, followed by beta decay.
• Chemical Separation: Plutonium is chemically separated from spent nuclear fuel through processes like PUREX (Plutonium Uranium Redox EXtraction).

Applications of Plutonium

• Mixed Oxide (MOX) Fuel: Plutonium oxide mixed with uranium oxide, used in nuclear reactors to generate electricity.
• Implosion-Type Bombs: Plutonium-239 is used in the core of nuclear weapons, e.g., the "Fat Man" bomb dropped on Nagasaki.
• RTGs: Plutonium-238 used to provide power for spacecraft and rovers (e.g., Curiosity rover, Voyager probes).

Sources:

Hindu

PRACTICE QUESTION Q. Plutonium is a critical element in nuclear science with diverse applications in energy, defense, and space exploration. Discuss. (10 marks)