Plutonium-239 is a radioactive isotope of plutonium, a heavy metal that is used primarily in nuclear reactors and weapons. It has a half-life of about 24,100 years, meaning it takes that long for half of a sample to decay. This isotope is produced in nuclear reactors from uranium-238 and is known for its ability to sustain a nuclear chain reaction. Due to its properties, plutonium-239 is a key material in the development of nuclear weapons. It can be used in both fission and fusion reactions, making it highly valuable for military applications. However, its radioactivity poses significant health and environmental risks, necessitating careful handling and disposal.