plutonium-238

plutonium-238 is a radioactive isotope of the element plutonium with a half-life of 87.7 years. It is a powerful alpha emitter, generating significant heat as it decays, which makes it uniquely valuable as a long-lived power source. This property has led to its primary use in fueling radioisotope thermoelectric generators (RTGs) for deep-space exploration and specialized medical devices.

Properties

Plutonium-238 decays primarily by alpha emission to uranium-234, a process that releases considerable thermal energy—approximately 0.57 watts per gram. Its specific activity is about 17 curies per gram. Unlike the fissile isotope plutonium-239 used in nuclear weapons, it has a very low rate of spontaneous fission and a negligible critical mass, making it unsuitable for nuclear fission weapons. The alpha particles it emits are easily shielded, but the material requires handling to prevent the inhalation of particles, which poses a significant radiological hazard. The decay chain eventually leads to stable lead-206.

Production

Historically, plutonium-238 was produced by irradiating neptunium-237 with neutrons in high-flux nuclear reactors, such as those at the Savannah River Site in the United States. The neptunium-237 captures a neutron to become neptunium-238, which beta decays to plutonium-238. Since the closure of major production facilities, new campaigns have been initiated by the U.S. Department of Energy in partnership with Oak Ridge National Laboratory to produce the isotope by irradiating neptunium-237 targets in the High Flux Isotope Reactor. Alternative methods have included neutron irradiation of americium-241 in reactors like the BN-350 in the former Soviet Union.

Applications

The dominant application of plutonium-238 is in radioisotope thermoelectric generators (RTGs) and smaller radioisotope heater units (RHUs) for NASA spacecraft. Notable missions powered by it include the Voyager probes, the Cassini–Huygens mission to Saturn, and the Mars Science Laboratory rover, Curiosity. It has also been used in terrestrial applications, such as powering pacemakers in the 1970s, though this practice was discontinued. The New Horizons mission to Pluto and the Perseverance rover on Mars also rely on RTGs fueled by this isotope.

Safety and handling

Handling plutonium-238 requires stringent containment protocols due to its high radiotoxicity if ingested or inhaled. The primary hazard is from alpha radiation, which necessitates operations within sealed gloveboxes or hot cells. Its intense heat generation also requires specialized design in fuel forms, such as the ceramic plutonium(IV) oxide used in RTGs, to prevent rupture and release. Major incidents involving the isotope include the 1964 Thule Air Base B-52 crash and the failure of the Transit 5BN-3 navigation satellite, which resulted in the dispersal of material over the Indian Ocean and Madagascar respectively. International transport is governed by the International Atomic Energy Agency regulations.

History

Plutonium-238 was first identified in 1940 by a team led by Glenn T. Seaborg at the University of California, Berkeley during research on transuranium elements. Large-scale production began in the 1960s at the Savannah River Site to support the SNAP program for space nuclear power. The Apollo program used RTGs on Apollo lunar surface experiments. Production in the U.S. ceased in the late 1980s, leading to a shortage that threatened future planetary science missions. This prompted the U.S. Congress to authorize new production funding in 2012, with ongoing efforts led by the Department of Energy and managed by the Idaho National Laboratory. Category:Plutonium Category:Isotopes Category:Radioisotopes