Why do radioactive materials like plutonium in nuclear waste continue to pose a threat for many thousands of years?

You might be uncomfortably familiar with the term “radioactive.” It might bring to mind several catastrophes that resulted in the loss of millions of lives. Such as the industrial accident in Chernobyl, Ukraine, in 1986, which left the immediate region inhabitable for 20 000 years. Or the bombings of Hiroshima and Nagasaki, in 1945, that caused the death of near a quarter-million people, and more importantly, also caused thousands to die in the following four months simply because of the effects of harmful radiations. Both these incidents had long-term implications because of one common element: radioactive materials. To get to know more about the working principles behind radioactive materials, let us first talk about radioactive elements and their history.

Almost two and half a millennia ago, in the land of ancient Greek, a philosopher named Democritus introduced us to the concept that our world is more than what we are simply able to see with our eyes. It was said that even a grain of salt could be divided into thousands of millions of smaller parts. For almost 2 000 years, this concept was forgotten, and no significant developments were made in atomic Physics. However, around the beginning of the 20th century, people started to pay more attention to the details hidden in plain sight. The concept of microscopic particles called atoms was re-established. This time, we were quickly unveiling hidden details within the atomic world. We put this knowledge into several significant practical applications, and the scientific world entered a new, unseen era. However, alongside the benefits offered by the unraveling of nuclear physics, there were also several potentially harmful effects in store for us.

In 1897, the French scientist Henri Becquerel first discovered the ability of an element to cause damage to its surroundings. When he wrapped the element uranium in a photographic plate, he observed that, after some time, the plate started to get black. He concluded from his experiments that there must be some form of invisible radiations emitted by the elements. It was soon observed that only specific elements produced such an effect. These unique elements were given the name of “radioactive elements.” Consequently, many more studies were performed on this subject, and it was agreed that the effects of radioactive elements could be harmful to the health of living creatures and could even prove fatal.

This behavior of radioactive elements was better understood at later times and was termed as the “radioactive decay” of an element. It all had to do with the structure of an atom. An atom consists of a central unit called a nucleus. Now, in most of the atoms, there are no internal conflicts, and the nucleus is stable. However, atoms that are typically larger in size tend to have an unstable nucleus. The way they resolve this issue is that they break up into at least one smaller atom. When doing so, they tend to release a lot of energy in the surroundings in the form of radiation. Three common types of these radiations are alpha, gamma and beta radiations.

The key term that determines the time period for which the radioactivity will go on is called the half-life of an element. The half-life is basically the time it takes for a particular sample of a radioactive element to decrease to half its original size. In the process, it emits a lot of radiation. Once an element decays to half its original size, it will further decay to half its new size and so on until it has decayed to an insignificant amount.

The half-life of thorium-234 is about 24 days, which is relatively safe, but for uranium-238, it’s 4.5 billion years. However, it’s plutonium-239, with a half-life of 24 100 years, which is the real cause for concern. That’s because almost all of the plutonium-239 (a lot of it) is formed as a waste product in nuclear reactors that use uranium as basic fuel. The disposal of this waste has become a serious problem because of its relatively long half-life, during which it continually gives off radiations. So, for example, 20 kilograms of plutonium-239 will decay in 24 100 years to produce 10 kilograms, which will decay in another 24 100 years to leave 5 kilograms of it, meaning that the radioactive process could virtually go on forever.