Plutonium-239

Some tiny bits can make power. They can also make big booms. One of these bits is called plutonium. It is made inside big machines. These machines use a different bit called uranium. When uranium bits get hit, they change. They turn into plutonium.

This new bit can make much heat. This heat makes power for us. It is also used for weapons. It is very strong.

We must be very careful. If you breathe in its dust, it can make you sick. It stays around for a very long time. It lasts for many thousands of years. This is a very special bit of our world.

Plutonium-239 is a special type of matter. Scientists call it an isotope. It is used to make nuclear weapons. It can also be used as fuel in nuclear reactors. This fuel helps make power for us.

This material is made from uranium-238. This is the most common kind of uranium. Inside a reactor, uranium-238 can catch a tiny particle called a neutron. This change turns the uranium into plutonium-239. This set of steps takes a little time. First, it becomes uranium-239. Then, it quickly changes into neptunium-239. Finally, it turns into plutonium-239.

Plutonium-239 is very powerful. It has a small critical mass. This means only a small amount is needed to start a reaction. An untampered sphere of it weighs about 11 kg. This is about 24.2 lbs. It has a half-life of 24,110 years. This means it stays around for a very long time. We must be careful with it. If a person breathes in its dust, it can cause cancer. It is also a heavy metal that is toxic to the body.

Plutonium-239 is a very special kind of matter. Scientists call it an isotope of plutonium. It is a fissile material, which means it can split apart to release energy. This makes it very important for two main reasons. It is used to make nuclear weapons. It is also used as fuel in many nuclear reactors.

Making this material is a step-by-step process called transmutation. It starts with uranium-238, which is the most common kind of uranium. Inside a reactor, an atom of uranium-238 catches a tiny particle called a neutron. This changes the atom into uranium-239. Then, it quickly changes into neptunium-239. Finally, it becomes plutonium-239.

This material is very powerful because it has a small critical mass. Critical mass is the smallest amount of material needed to start a nuclear reaction. An untampered sphere of plutonium-239 weighs about 11 kg. That is about 24.2 lbs and is only 10.2 cm wide. With special tools like triggers and reflectors, the mass can be even smaller.

People sort plutonium into different grades based on its purity. Weapons-grade plutonium must have no more than 7% of a contaminant called plutonium-240. If there is too much plutonium-240, it can cause a "fizzle" during an explosion. This is a small explosion that destroys the weapon without using all the fuel. Other grades include fuel grade and reactor grade.

We must handle plutonium-239 with great care. It is radioactive and emits alpha particles. If a person breathes in its dust, it can be very dangerous and cause cancer. It is also a heavy metal, which means it is chemically toxic to the body. However, it is less dangerous if it is swallowed. This is because very little of it is absorbed by the body.

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.