Uranium-235

Uranium-235 is a fissile isotope of uranium with 92 protons and 143 neutrons. It can sustain a fission chain reaction and is used in nuclear-reactor fuel and some nuclear weapons.

Nuclear fission: The splitting of an atomic nucleus into lighter nuclei, often releasing neutrons and energy. Uranium-235 releases energy when its nucleus splits after absorbing a neutron.
Isotope: One of two or more atoms of the same element with different numbers of neutrons. Uranium-235 differs from other uranium isotopes by its neutron count.
Nuclear fuel: Material used in a nuclear reactor to release energy through fission or fusion. Uranium-235 is a principal fissile component of many reactor fuels.
Arthur Jeffrey Dempster: A Canadian-American physicist who developed mass-spectrograph techniques and identified uranium-235 in 1935. Dempster’s mass-spectrographic work established uranium-235 as a distinct uranium isotope.
Fission chain reaction: A process in which neutrons from one nuclear fission induce further fissions. Each fission can release neutrons that continue the reaction in uranium-235.
Uranium: A naturally occurring radioactive chemical element with atomic number 92. Uranium-235 is one isotope within this element.
Low-enriched uranium: Uranium in which uranium-235 makes up less than 20 percent of the total uranium. Most commercial power reactors use fuel enriched within this range.
Mass spectrometry: A technique that identifies and measures ions by their mass-to-charge ratios. Mass spectrometry made it possible to distinguish uranium-235 from uranium-238.
Neutron cross section: A measure of the probability that a nucleus will interact with an incident neutron. Its large thermal-neutron fission cross section helps uranium-235 sustain reactor reactions.
Uranium-238: The most abundant uranium isotope, with 92 protons and 146 neutrons; it is not fissile with thermal neutrons. Its greater abundance and different neutron behavior contrast with uranium-235.

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