uranium-235

uranium-235 is a fissile isotope of uranium that constitutes approximately 0.72% of natural uranium. It is the only naturally occurring fissile nuclide, a property that makes it critically important for both nuclear power and nuclear weapons. Its discovery and subsequent utilization fundamentally shaped the 20th century, most notably through the Manhattan Project and the development of the first atomic bomb.

Properties

Uranium-235 is characterized by its ability to sustain a nuclear chain reaction. It undergoes alpha decay to thorium-231 with a half-life of about 704 million years. The nucleus has 92 protons and 143 neutrons, making it an odd-numbered, odd-mass nuclide, which contributes to its fissility. When bombarded with thermal neutrons, it has a high probability of undergoing nuclear fission, releasing a significant amount of energy, on the order of 200 megaelectronvolts per fission event. Its specific nuclear cross section for thermal fission is notably large, a key parameter in reactor design.

Occurrence and production

Naturally, uranium-235 is found in all uranium ores, such as pitchblende and uraninite, but at the low concentration of 0.72%. The majority of the remaining natural uranium is the fertile isotope uranium-238. Because most nuclear applications require a higher concentration, the isotope must be enriched through processes that separate it from uranium-238. The primary methods for this isotope separation are gaseous diffusion, as used historically at the Oak Ridge National Laboratory, and gas centrifuge technology. Other techniques include aerodynamic separation and laser isotope separation.

Nuclear fission

The fission of a uranium-235 nucleus, typically induced by a low-energy neutron, splits the nucleus into two lighter fission products, such as barium and krypton, and releases additional neutrons and gamma rays. This process releases a tremendous amount of energy from the conversion of mass as described by Albert Einstein's mass–energy equivalence. The emitted neutrons can then induce fission in other uranium-235 nuclei, creating a self-sustaining chain reaction. This reaction is the fundamental principle behind both nuclear reactors, like those at Chernobyl and Fukushima Daiichi, and nuclear explosives, such as the Little Boy bomb dropped on Hiroshima.

Applications

The primary application of uranium-235 is as fuel in nuclear power plants, where controlled fission generates heat to produce steam for turbines and electrical generators. It is also the key fissile material in nuclear weapons; the gun-type fission weapon design used in Little Boy relied on it. Furthermore, highly enriched uranium-235 is used in some research reactors, such as those operated by the International Atomic Energy Agency, and as fuel for naval propulsion in vessels like the USS Nautilus (SSN-571). Depleted uranium, primarily uranium-238, is a byproduct of the enrichment process and has uses in armor-piercing ammunition and radiation shielding.

Safety and handling

Uranium-235, like all radioactive materials, requires careful handling due to its alpha particle emissions and the potential for criticality accidents if sufficient mass is assembled. Operations involving its enrichment or use in fuel fabrication are strictly regulated by bodies like the Nuclear Regulatory Commission in the United States. The International Atomic Energy Agency also monitors its production and trade to prevent nuclear proliferation. Long-term storage of spent nuclear fuel, which contains fission products like plutonium-239 and cesium-137, presents significant challenges, with proposed solutions including deep geological repositories like the Yucca Mountain nuclear waste repository.

Category:Isotopes of uranium Category:Nuclear materials Category:Fissile materials