Uranium-238 is a common radioactive isotope of Uranium. It is not a fissile substance thus cannot sustain nuclear fission. However this isotope is a fertile material, which means other fissile materials are generated from it.

Identification of Uranium-238

CAS Number: 7440-61-1

Uranium-238 Source

Almost all Uranium in nature is found in the form of Uranium-238. Other isotopes like Uranium 234, Uranium 235 and Uranium 236 are found in smaller quantities in natural Uranium.

Uranium-238 Chemical Formula

The formula for this radioactive isotope is ²³⁸U. It is also denoted with U-238.

Picture 1 – Uranium-238
Source – unitednuclear.com

Uranium-238 Properties

The radioactive and physical properties of this substance include:

• Appearance: It is a hard, silver white metal.
• Molecular Weight: The molecular weight of this radioactive metal is 236.03 gm/mol.
• Atomic Number: The atomic number of Uranium-238 is 92.
• Mass Number: Its mass number is 238.02891(3) u (unified atomic mass unit).
• Density: The density of this material is 18.95 gm/cm³. It has 65% more density compared to Lead.
• Solubility: It is soluble in Nitric Acid (HNO₃) and Hydrochloric Acid (HCl).
• Melting Point: It has the melting point of 1,132 °C.
• Boiling Point: The boiling point of this radioactive metal is 3,818 °C.
• Specific Gravity: Its specific gravity is 9.1 at 25 °C temperature.

Uranium-238 Radioactive Decay

The unstable atomic nucleus of Uranium-238 emits ionizing particles and loses energy in order to achieve a stable state. This process is called the Radioactive decay. This isotope undergoes Alpha decay by emitting Alpha rays. It has a decay energy of 4.267 MeV.

Uranium-238 Decay Equation

Following is the equation for the Alpha (α) radiation of this isotope:

²³⁸U → ²³⁴₉₀Th + ⁴₂He²⁺

The above equation can also be denoted as

²³⁸U → ²³⁴Th + α

In the first equation the ⁴₂He (Helium) is similar to an Alpha particle having mass number of 4 and atomic number 2. Due to this reason it has been denoted by an Alpha particle in the second reaction.

Uranium-238 Decay Chain

Thorium-234 is the next radioactive substance in the decay process of Uranium-238. It means Thorium-234 is the daughter nuclide of this isotope. Lead (stable) is the final element of this Alpha decay process. Following is the complete decay series:

Uranium-238 → Thorium-234→ Protactinium-234m→ Uranium-234→ Thorium-230→ Radium-226→ Rodon-222→ Polonium-218→ Lead-214→ Bismuth-214→ Polonium-214→ Lead-210→ Bismuth-210→ Polonium-210→ Lead-205 (stable)

This series is also known as “Radium Series”. All the above elements are present (even if for a short time) in any sample containing Uranium be it metal, mineral or compound.

Uranium-238 nucleus

One atom of this substance contains 92 protons and 145 neutrons.

Uranium-238 Half Lif

Half life is the time period taken by a radioactive substance to decay and reduce to the half of its original amount. Uranium-238 has a very long half life of 4.468 billon years.

Uranium-238 Fission Reaction

This material does not undergo fission unless struck by a high energy neutron. It collides with a neutron and turns into Uranium 239, which undergoes decay and produces Plutonium-239. This final radioactive isotope is highly useful in power plants.

Uranium-238 uses

This radioactive metal has a very long half life. The Depleted Uranium (DU) is very heavy having a high density level. These properties make this substance useful in various industries.

As Breeder

Fertile uranium-238 isotope is used in Breeder Reactors for its neutron capture ability. It produces fissile products like Plutonium- 239, which is used as a nuclear fuel to produce high amounts of energy. This technology is used in many experimental nuclear reactors.

As Radiation Shield

It is used as a shield against harmful radiation in the form of Depleted Uranium Dioxide and Depleted Uranium. The non-radioactive casing of the Shield can easily stop its Alpha radiation from causing any harm. The high atomic weight and electron numbers of this material can efficiently absorb Gamma Rays and X Rays. However, it cannot stop fast neutrons having a speed of 14,000 km/s, as effectively as ordinary water.

Researchers are trying to find out whether Uranium Dioxide concretes can be used as a Cask Storage material for storing radioactive waste.

In Radioactive Dating

The radioactive property of a material is applied to determine the age of objects like rocks and fossils. Uranium-238 is used in this dating process. The decay chain of this isotope is well documented with Lead being the final stable element. The whole decay series happens at a constant rate which helps to correctly date rocks and minerals.

In this process, the age of an object is determined by adding the amount of the daughter product (e.g. Thorium) in the object with the amount of the parent isotope (e.g. Uranium).

In Nuclear Weapons

Uranium-238 is used as a “tamper” material in nuclear weapons. Its function is to reduce the required critical mass and make the weapon work more efficiently. It is used in thermonuclear weapons for the purpose of encasing the fusion fuel which helps to make the weapon more powerful.

Downblending

Downnblending is the opposite process of enriching. Enriched Uranium is downblended with the help of depleted Uranium for using it as commercial nuclear fuel. It is used to produce Mixed Oxide Fuel (MOX) along with Plutonium- 239.

Is Uranium-238 Harmful for Human Health?

Radioactive Uranium can be found in nature which can cause health problems for humans. It can enter the body through inhalation, ingestion and sometimes through open wounds. However, it cannot harm an organism from the outside as its Alpha radiation does not penetrate the skin.

Most of the Uranium-238 ingested or inhaled usually leaves the body. But a very small amount gets accumulated in the bones. It remains there undergoing radioactive decay for a very long time. This radiation can cause adverse health effects like kidney damage and cancer.

Uranium-238 Price

There are many companies who supply this material in different parts of the world. The price for one pound of this metal ranges from $ 100 to $ 110.

Uranium-238 is the main radioactive isotope of natural Uranium. It produces many other useful radioactive elements while undergoing decay. This makes this radioactive metal quite useful and valuable.

References
1. https://encyclopedia2.thefreedictionary.com/uranium+238
2. https://www.epa.gov/radiation/radionuclide-basics-uranium
3. http://www.nucleartourist.com/basics/basic.htm