Plutonium-239

Plutonium-239, often called Pu-239, is a specific isotope of the element plutonium. It is a fissile material, which means its atoms can be split to release massive amounts of energy. This property makes it a vital resource for two very different uses. It is a primary material used to build nuclear weapons. It also serves as a key fuel for thermal spectrum nuclear reactors. Because it can be produced relatively cheaply, it is often preferred over highly enriched uranium-235.

The creation of Pu-239 is a process called transmutation. It begins with uranium-238, which is the most abundant isotope of uranium. When a uranium-238 nucleus is exposed to neutron radiation, it captures a neutron. This change transforms the atom into uranium-239. This new atom is unstable and undergoes beta-minus decay, which means it emits an electron and an antineutrino. This first decay turns the uranium-239 into neptunium-239. After about 2.356 days, a second beta-minus decay occurs. This final step transforms the neptunium-239 into the stable plutonium-239 we are studying.

Plutonium-239 is exceptionally efficient at releasing energy through fission. When a single atom of Pu-239 splits, it generates 207.1 MeV of energy. This is equivalent to about 23 gigawatt hours per kilogram. One of its most significant features is its small critical mass. Critical mass is the minimum amount of fissile material needed to maintain a nuclear chain reaction. For an untampered sphere of Pu-239, this mass is approximately 11 kg, or 24.2 lbs. This sphere would be only 10.2 cm in diameter. By using neutron reflectors, tampers, and specific implosion geometry, engineers can reduce this mass to less than half of that amount.

Not all plutonium is the same, so scientists classify it into different grades. These grades are determined by the amount of plutonium-240 present as a contaminant. Pu-240 is created when Pu-239 absorbs an extra neutron during production. This contaminant is problematic because it has a very high rate of spontaneous fission. In a weapon, too much Pu-240 can cause a "fizzle." A fizzle is a small explosion that destroys the device but fails to trigger a full fission reaction. To prevent this, weapons-grade plutonium must contain no more than 7% Pu-240. Other categories include supergrade (2–3%), fuel grade (7–18%), and reactor grade (18% or more).

Different types of nuclear reactors manage plutonium production in various ways. Most commercial power reactors operate at a high "burnup," meaning they stay active for years. This causes a significant amount of Pu-239 to build up in the fuel. In these reactors, the Pu-239 actually provides more than one-third of the total energy produced. Some specialized designs, called breeder reactors, are even more efficient. They are designed to produce more plutonium than they consume in fuel. Other designs, like the Canadian CANDU reactor, can refuel while they are still running. This makes them different from many commercial reactors that must shut down for weeks to change fuel elements.

While plutonium is useful, it presents serious biological hazards. Pu-239 is radioactive and acts as an alpha emitter. This means it releases alpha particles as it decays into uranium-235. If the material is outside the body, it is not a particularly high risk for external radiation. However, if the dust is inhaled, it becomes extremely dangerous and carcinogenic. Some estimates suggest that inhaling just one pound of plutonium oxide nanoparticles could cause cancer in two million people. It is also a heavy metal, which makes it chemically toxic to humans. Interestingly, ingesting it is much less dangerous because the body absorbs very little of it through the digestive tract.

Understanding plutonium-239 helps us connect the physics of the atom to global energy and security. The ability to control its fission allows for steady electricity in power plants. It also allows for the creation of powerful energy releases in weapons. The way we manage its production, from breeder reactors to the use of MOX fuel, shapes how we use natural uranium. MOX fuel is a mixture of uranium dioxide and plutonium dioxide. Using MOX can actually reduce the need to enrich uranium. By studying these isotopes, scientists continue to learn how to manage the incredible energy stored within the nucleus.