Uranium-235

Overview

Uranium-235 (²³⁵U) is an isotope of uranium that is of significant importance due to its fissile properties, which allow it to sustain a nuclear chain reaction. It is the only naturally occurring fissile isotope. This characteristic makes it a key component in both nuclear power generation and the production of nuclear weapons.

Properties

Atomic and Physical Properties

• Atomic Symbol: ²³⁵U
• Protons: 92
• Neutrons: 143
• Appearance: Silvery-white metal when refined and enriched.
• Density: Uranium is an extremely dense metal, approximately 65% denser than lead.

Nuclear Properties

• Fissile Material: Uranium-235 is unique as the only naturally occurring fissile material. This means its nucleus can be split by slow-moving neutrons, a process known as nuclear fission.
• Chain Reaction: When a ²³⁵U nucleus absorbs a neutron, it becomes unstable and splits, releasing a significant amount of energy, gamma rays, and two to three additional neutrons. These newly released neutrons can then go on to cause other ²³⁵U atoms to fission, creating a self-sustaining chain reaction.
• Half-life: Uranium-235 has a half-life of approximately 704 million years.
• Decay: It decays via alpha radiation, emitting an alpha particle (two protons and two neutrons). The decay chain of Uranium-235 ultimately ends with the stable isotope lead-207.
• Natural Abundance: Uranium-235 constitutes about 0.72% of natural uranium, with the vast majority being uranium-238.

Uses

The primary applications of Uranium-235 are derived from its ability to undergo nuclear fission.

Nuclear Power

• Fuel for Nuclear Reactors: Enriched uranium, with a higher concentration of ²³⁵U, is used as fuel in most nuclear power plants. The heat generated from the controlled fission of ²³⁵U is used to create steam, which drives turbines to produce electricity. One kilogram of uranium-235 can theoretically produce as much energy as 1,500 tonnes of coal.
• Naval Propulsion: Highly enriched uranium is also used to power nuclear reactors in naval ships and submarines.

Nuclear Weapons

• Fissile Material for Bombs: Highly enriched uranium (typically 90% ²³⁵U or more) is a key fissile material used in the construction of nuclear weapons. The atomic bomb detonated over Hiroshima, named "Little Boy," utilized uranium fission. As little as 15 pounds (6.8 kg) of uranium-235 can be used to create an atomic bomb.

History

• Discovery: The phenomenon of nuclear fission in Uranium-235 was discovered in 1938 by Otto Hahn and Fritz Strassmann. Lise Meitner and Otto Frisch provided the theoretical explanation for this process in 1939, identifying ²³⁵U as the first known fissile isotope. Arthur Jeffrey Dempster is credited with the discovery of Uranium-235 in 1935.

Enrichment

Due to its low natural abundance, uranium must undergo a process called enrichment to increase the concentration of ²³⁵U for most applications. This process separates ²³⁵U from the more abundant ²³⁸U. Methods like gaseous diffusion and gas centrifugation are used for enrichment. Low-enriched uranium (2-3% ²³⁵U) is used for nuclear reactors, while highly enriched uranium (90% or more ²³⁵U) is used for weapons.

Safety

• Radioactivity: Uranium-235 is radioactive and decays by emitting alpha particles. External exposure to uranium is less dangerous than other radioactive elements because alpha particles are blocked by the skin.
• Toxicity: Ingesting or inhaling significant concentrations of uranium can lead to severe health effects, including cancer of the bone or liver and lung cancer from alpha particle exposure. Uranium is also a toxic heavy metal, and its chemical properties can cause kidney damage more rapidly than its radioactive properties.