Uranium 238 and 235

A radioactive and strategic element

The uranium atom is the heaviest atom present in the natural environment. Its radioactivity is very low. Its very long life of several billion years has allowed uranium to be still present. It is a rare chemical element found in the Earth’s crust with an average of 3 grams per tonne.

The uranium image has suffered from its association with the first atomic bombs. Its reputation as a malevolent radioisotope, however, is undeserved: in fact, the decay rate of uranium is among the slowest known to man. The activity of a sample of uranium could be compared to the water flow escaping from a pond through a pinprick.

These reassuring features not prevent this unfortunate element to be regularly presented by TV channels as a dangerous radioactive substance. Is our complacence born of ignorance? Contrary to the widespread fears, uranium presents low risks owing to its very low radioactivity. Its radioactive toxicity, according to experts from the CEA, is a hundred times weaker than its chemical toxicity, which itself is no different from the chemical danger posed by common heavy elements such as lead.

All  isotopes of uranium are unstable and radioactive, but uranium 238 and uranium 235 have half-lives sufficiently long to have allowed them to still be present in the Solar System and on Earth. The half-life of uranium 238 is of 4.5 billion years, while uranium 235 has a half-life of ‘only’ 700 million years.

While both isotopes were at the time of Earth formation equally abundant, natural uranium today consists today of 99.3% uranium 238 and only 0.70% uranium 235. The nuclei of uranium 235 and 238 are, along with those of thorium 232, the heaviest present in nature. They were all formed billions of years ago by the explosion of heavy stars (supernovae).

From a chemical point of view, uranium belongs to the actinide family, a group of atoms whose properties are similar to those of actinium, the 89th element in Mendeleyev’s periodic table. Uranium can be found in the Earth crust at 3 parts per million, particularly in granite and volcanic rocks. Certain uranium compounds (hexavalent ones) are highly soluble whereas others (tetravalent) are not.

As a particularly heavy element, uranium isotopes are primarily alpha emitters, though these radiations are sometimes accompanied by gamma rays.

Uranium 235 is the only natural nucleus that can easily undergo fission. Highly sought-after, it can be used as a fuel in nuclear reactors and as an explosive in atomic bombs.

The more abundant uranium 238 is sometimes called fertile. Fission occurs comparatively rarely, and even under bombardment with energetic neutrons the probability of fission remains very low. What happens more frequently is that a neutron capture causes the nucleus to become unstable. After a few days, uranium 239 has transformed into plutonium 239, a radioisotope with a half-life of 24,000 years. Plutonium 239 is highly fissile and can also be used as a nuclear fuel or an atomic explosive.

Uranium reserves which exist under the oceans are about a thousand times more abundant than the reserves found in high quality minerals on land. Underwater uranium, however, would be extremely costly to exploit.

TO KNOW MORE ABOUT URANIUM AND ITS USES :

– 1) : Uranium isotopes
– 2) : Uranium fuels