Uranium-238

Uranium-238, written as ²³⁸U or U-238, is the most common form of isotope of uranium found in nature, making up over 99% of all uranium. Unlike another form called uranium-235, U-238 cannot keep a chain reaction going in regular reactors that use slow-moving neutrons. However, it can be used in special ways when hit by fast-moving neutrons and can change into another useful material called plutonium-239.

This isotope takes an extremely long time to change — about 4.463 billion years — which is why it is still found in large amounts today. Because of how long it lasts and how much of it exists, U-238 helps create about 40% of the heat inside our Earth. As it changes over time, it gives off tiny particles that scientists can detect, helping them learn more about our planet.

When uranium is changed and processed for use, like in depleted uranium or even in fuel that has a little more uranium-235, most of it is still U-238. This form is very important for scientists who study very old rocks and materials using a method called radiometric dating.

Nuclear energy applications

In a nuclear reactor, uranium-238 can help create plutonium-239, which can be used for energy or in special powerful tools. In normal reactors, a lot of the power comes from splitting plutonium-239, which is made from uranium-238 when it meets special particles called neutrons.

Uranium-238 can also create energy by splitting when hit by fast-moving neutrons. Special reactors called breeder reactors can turn uranium-238 into plutonium-239, giving us more fuel for power. There is a lot of uranium-238 available for this purpose.

Uranium-238 is also used to protect against harmful invisible rays, as it can block many types of radiation very well. It is especially good at stopping certain rays, making it useful in safety materials.

Nuclear weapons

Most modern nuclear weapons use ²³⁸U as a material called a "tamper." This material surrounds the core of the weapon and helps make it work better by reflecting neutrons and adding stability. In thermonuclear weapons, ²³⁸U can also surround the fusion fuel. When the fusion reaction happens, it creates very energetic neutrons that cause the ²³⁸U to split, adding more energy to the explosion. These are called fission-fusion-fission weapons.

A large part of the explosion in this design comes from the final splitting stage fueled by ²³⁸U, which creates a lot of radioactive materials. For example, about 77% of the explosion from the Ivy Mike test in 1952 came from this splitting of uranium. Because natural uranium does not have a critical mass, it can be used in very large amounts in these weapons. The Soviet Union tested the Tsar Bomba in 1961, which produced less explosion than it could have because they used lead instead of ²³⁸U for the final stage. If they had used ²³⁸U, the explosion could have been much larger and created a lot of radioactive fallout.

Decay

Uranium-238 changes over time into other elements. It starts by giving off a type of energy called an alpha particle, turning into thorium-234. Thorium-234 then changes by giving off beta particles, and this process continues through several steps. This series of changes is known as the radium series or uranium series, and it includes many different elements like astatine, bismuth, lead, polonium, protactinium, radium, radon, thallium, thorium, and uranium.

The process of changing from one element to another happens over very long periods. For example, uranium-238 takes about 4.5 billion years to change into thorium-234. Each step in this process has its own time it takes to happen, and eventually, the series ends with a stable form of lead. While uranium-238 itself is only slightly radioactive, the elements it changes into can be more active, giving off energy in different ways as they change as well.

Radioactive dating

²³⁸U and its changes over time are used in special ways to figure out how old things are. One common way is called uranium-lead dating, which helps scientists find the age of very old rocks, even ones that are more than a million years old. This method has helped figure out that some of the oldest rocks on Earth are about 4.4 billion years old.

We can also learn the age of certain sediments and seawater by looking at the relationship between ²³⁸U and another type called ²³⁴U. This works for ages between 100,000 and 1,200,000 years. The end product of ²³⁸U, called ²⁰⁶Pb, is important in another dating method known as lead-lead dating, which helped find the age of the Earth. Even the Voyager spacecraft carry a tiny bit of pure ²³⁸U to help scientists date things far into the future.

Health concerns

Uranium sends out a type of radiation called alpha radiation. If you are outside the uranium, it does not harm you much. But if tiny bits of uranium or its breakdown products, like thorium-230, radium-226, and radon-222, get inside your body, they can make you very sick and might cause health problems such as cancer in the bone or liver.

Uranium can also harm your body because of its chemical properties. If you swallow uranium, it can damage your kidneys faster than the radiation could cause cancer.