Currently, there is enough uranium available on earth to meet the energy demands of the entire world for more than 200 years. In this article I will explain how Einstein's famous equation (E=mc2) was used to calculate the tremendous energy released when uranium is split. We will also explore the advantages and disadvantages of uranium and its possible alternatives.
Generally, to break any object, we must use sharp tools, such as knives. However, when an object is really small, like the nucleus of an atom, how can we divide it? The answer is to use subatomic particles. Let's use protons as a knife and lithium nucleus as objects. If we bombard the lithium with protons, it becomes unstable and breaks it into two helium atoms. However, the difficulty in this experiment is that the proton has a positive charge, it will be repelled by protons in the lithium nucleus. Therefore, we need to provide high energy to the proton before bombardment(refer fig 1a).
In fact, this process becomes nearly impossible when the nucleus; such as uranium, has a higher number of protons. What if we use chargeless neutrons as a knife? This is exactly what the great Italian scientist Mr. Enrico Fermi did in Rome. Using neutron bombardment, he was able to successfully split uranium atoms into two parts as shown in fig 1b. This was a huge accomplishment in physics, for which Mr. Enrico Fermi won a Nobel prize.
Now consider a hypothetical case. While Fermi was doing the experiment, the genius scientist Mr. Albert Einstein entered his lab. A quick calculation of atomic mass shows that mass disappeared during this splitting process. Einstein knew that mass can be converted into energy - a lot of energy! The energy released is known as fission energy, and the process is known as nuclear fission.
In fact, all the details of the output of the fission reaction we explained so far, were discovered by a group of German scientists in 1938. They also found that the resultant elements of fission reaction were radioactive(refer fig 2).
The research paper published by the German scientists became popular and fermi had a scientific trigger in his mind when he clubbed these new revelations and Einstein’s famous equation. After this discovery, Fermi and other prominent scientists at Columbia University in NewYork rushed to the lab and began conducting experiments to measure energy release during this reaction. It was the first time anyone had measured the energy released, and the results were shocking. They found that not only heat, but also three extra neutrons are also released during this process as shown in fig 3. You may be curious about what will happen to these neutrons. If your guess is that these neutrons hit other uranium atoms, you’re correct. The same reaction will take place again and again, making it a chain reaction.
All these discoveries were kept secret for many years in the US while 160 top scientists worked on this project. The project was named the Manhattan project.
But why does the extra neutron escape and hit other atoms? Atoms are mostly made up of hollow space. So, neutrons may fly away without hitting other uranium nuclei. To ensure that the neutrons hit another uranium atom, a certain amount of uranium must be present to sustain this chain reaction. This is known as critical mass. Unfortunately, the critical mass of natural uranium is too high: 400 kilograms.
Researchers from England soon came for the rescue. They found two important isotopes of uranium: Uranium 235 and Uranium 238 (refer fig 4). Isotopes are the same chemical elements, but they have different numbers of neutrons. In Uranium-235, there are less neutrons which are unable to bind all protons, making it unstable. Therefore, Uranium-235 undergoes a chain reaction. Due to this, the critical mass of Uranium-235 is also very low at 47 kilograms.
However, naturally occurring uranium has 99.3% Uranium-238 and only 0.7% of Uranium-235. Now let's explore how to obtain natural uranium.
Uranium is mainly found in the earth's crust in the form of ore. To extract uranium, a solution is pumped into drilled holes, which mixes with uranium ore and is then pumped back to the processing unit. This solution undergoes various chemical processes, which separate uranium from other elements, and converts into Triuranium octoxide, popularly known as yellow cake. In order to extract Uranium-235 from natural uranium, a few complex enrichment processes are necessary.
If we increase the Uranium-235 to 5%, we can use this uranium as fuel in a nuclear reactor. One kilogram of uranium generates more energy than a thousand tons of coal. But if we increase Uranium-235 to 90%, it can destroy an entire city. To put it into perspective, Do you know how much uranium was dropped on Hiroshima? the bomb dropped on Hiroshima had only 64 kilograms of uranium.
As we know, we can't only rely on natural uranium for nuclear fuel (refer fig 5). High concentration uranium mines are found in very few countries in the world. Additionally, the reaction of uranium emits hazardous radiation for thousands of years. Therefore, it requires special storage facilities. This severely limits our ability to use uranium as nuclear fuel.
Is there any viable alternative available for Uranium-235? Yes! Thorium is the next best element - but it won’t undergo fission reactions immediately when bombarded with neutrons. First, it absorbs neutrons and undergoes a series of beta decays. After 27 days, it converts into Uranium-233. Uranium-233 undergoes a similar chain reaction as Uranium-235. In Fact, the average number of neutrons released during fission of Uranium-233 is more than the average number of neutrons released during fission of Uranium-235(refer fig 6).
Let's see all the advantages of Thorium over Uranium(refer fig 7). You may be wondering why, despite these technical advantages, we are not using thorium instead of uranium. The answer to this question is complex and involves a number of political and economic reasons.
I hope you have learned how Nuclear fuel and energy were invented. Also, you realize how powerful and dangerous they are.
Thanks for reading!