Plutonium-240

Plutonium-240. It is a significant isotope of the transuranic element Plutonium, produced as a byproduct in nuclear reactors during the irradiation of Uranium-238. With a half-life of approximately 6,561 years, it undergoes alpha decay to Uranium-236. Its presence is a critical parameter in determining the grade of plutonium used in nuclear weapons and influences the design of both reactor fuel and explosive devices.

Properties

Plutonium-240 is a radioactive actinide metal with a characteristic silvery appearance that tarnishes in air. It crystallizes in several allotropic forms at different temperatures, similar to other isotopes of Plutonium. Its atomic mass is 240.0538135 u, and it has a moderately high spontaneous fission rate, emitting an average of 415,000 neutrons per second per kilogram. This property is a key differentiator from its sibling isotope Plutonium-239 and has profound implications for its usability in nuclear explosives. The isotope's decay energy is 5.25583 MeV, released primarily as alpha particles.

Production

Plutonium-240 is not found in nature and is artificially produced in nuclear reactors. It is generated primarily through the neutron capture of Plutonium-239, a process that occurs when Plutonium-239 atoms in a reactor core absorb an additional neutron without undergoing fission. The production rate depends on the neutron flux and the duration of irradiation, known as burnup, within reactors like the Magnox or PWR. It can also be produced in minute quantities from the alpha decay of Curium-244, which is itself a product of intense neutron irradiation. The Hanford Site and the Sellafield facility are historically significant locations for its production.

Decay

The primary decay mode of Plutonium-240 is alpha decay, with a half-life of 6,561 years, transforming into Uranium-236. The decay chain proceeds as Uranium-236 decays to Thorium-232, which is the start of the thorium decay series. A defining characteristic is its significant rate of spontaneous fission, which is about 40,000 times higher than that of Plutonium-239. This process yields neutrons and fission products like krypton and barium. The isotope also has a very small probability of decaying by cluster emission, but this mode is negligible compared to alpha decay.

Applications

Due to its high spontaneous fission rate, Plutonium-240 is generally considered an undesirable contaminant in weapons-grade plutonium, where it can cause pre-initiation in a nuclear weapon design. However, it is a major component of reactor-grade plutonium used in MOX fuel for civilian power reactors like those operated by EDF. In this context, its presence is managed within fuel assembly designs. The isotope also serves as a fertile material, capable of being transmuted into Plutonium-241 upon neutron capture, which is a fissile isotope. Its properties are critical in studies conducted at facilities like the Los Alamos National Laboratory and the JINR.

Safety and handling

Handling Plutonium-240 requires stringent radiological controls due to its alpha radioactivity and being a pyrophoric material. It poses a significant internal hazard if ingested or inhaled, primarily as a potent alpha emitter that can cause damage to lung tissue and increase the risk of lung cancer. Operations are conducted within gloveboxes or hot cells under an inert atmosphere like argon to prevent ignition. Major incidents involving plutonium isotopes, including this one, have occurred at places like the Rocky Flats Plant. Its storage and transport are governed by regulations from the IAEA and the U.S. Department of Energy, with criticality safety being a paramount concern due to its potential for sustaining a neutron chain reaction. Category:Plutonium isotopes Category:Actinide isotopes Category:Nuclear materials