In recent times, especially due to the threat of a nuclear war with our neighbours, we have been very aware of nuclear bombs, and to a lesser extent, the destruction that they can wreak. Perhaps it is due to this, and also due to the stories of the accidents in major nuclear reactors and the aftermath of Hiroshima and Nagasaki, that uranium is feared by most. This is understandable, given its short, yet eventful history. But does it deserve the bad rapport it receives? We will explore it together, while I try to tell the story of uranium, and in future texts, try to explain why I feel this reputation is rather unfair.
Before we start, though, I would like to declare the conflicts of interest that I may have. Without going into details, I’ve lived near a research complex that studied uranium, and radioactivity in general. Because of this, I am a little biased towards uranium. Please keep that in mind as we continue on this journey…
Index
What is Uranium?
For any of you who are not familiar with some of the basic ideas of chemistry, this is the periodic table. It is a list of all the elements that we know of, arranged according to its properties.
Let us get to know about a few things before answering the above question “What is Uranium?”
Elements are made up of tiny particles called atoms. If you want to visualize the structure of an atom, imagine a very dense and small nucleus at the centre, with electrons revolving around it. Chemistry is the study of how the outer electrons are shared between atoms. Here, though, we will limit our discussion to the nucleus.
The nucleus of an atom is made up of two particles, protons, and neutrons. The proton is the positively charged particle and a neutron has no charge. They weigh almost exactly the same. The neutron is required to keep two protons together in the nucleus.
The number you see mentioned above each element is the atomic number. It tells us of the number of protons present in the nucleus of the atom. The number of protons is what dictates if the atom is hydrogen, or iron, or gold, or whatever else you’re looking at. It’s the identity of the element.
Another important number is the mass number of the atom. The mass number is the sum of the number of protons and neutrons in its nucleus. The takeaway is, for an atom of an element, the atomic number does not change. The number of neutrons can vary, thus two atoms with the same atomic number can have different mass numbers. These two atoms are called isotopes.
So, what’s the limit on the number of protons and neutrons that can be lumped together in the nucleus? After all, not all the elements in the periodic table are available on Earth. There’s a ratio of the number of neutrons to protons, where the nucleus is stable. If the number goes either above or below this stable ratio, the nucleus is unstable. To deal with this, and fall into the stable ratio, the nucleus breaks, either releasing smaller nuclei, or an electron, or light. Nuclei that do this are called radioactive. It just means that they are in a constant state of decay.
Uranium, represented by U, is an element with an atomic number of 92. It is the heaviest naturally occurring element on Earth. Although it has multiple isotopes, the two major ones that we will be looking at is Uranium 238 (U238) and Uranium 235 (U235). Both of these isotopes are radioactive. They break down into Thorium, which again is unstable. Around 14 or so such breakdowns later, uranium converts itself into the stable lead (Pb). All of this is spontaneous and happens because the nucleus is too unstable, it has too many protons and neutrons in it to sit at ease, if you may.
But why is Uranium such a big deal?
Remember how the Uranium nucleus is full of protons and neutrons? It takes a lot of energy to hold the nucleus together, and even then, it is groaning under the stress of all the protons repelling each other. If it were to break apart, the amount of energy released would be significant. It is this energy that makes it a useful tool and a dangerous weapon.
Breaking (the official term for this is fission) the Uranium nucleus is easier when compared to other elements. All you need is a neutron :-).
When a neutron hits the uranium nucleus, it acts like the straw that breaks the camel’s back. The nucleus becomes unstable and breaks down into smaller nuclei. But the best part? This process releases either 2 or 3 new neutrons. These new neutrons can go and break more of the uranium, which releases more neutrons, and this continues until there are no uranium nuclei left to break! If you get a large enough clump of uranium in one place, it spontaneously starts fissioning.
The above reaction is what is called a chain reaction. The energy released in this process is immense. For a kilogram of U235, a complete chain reaction would give approximately 8.2×1013 joules of energy! Practically, it doesn’t work like that. The energy obtained is far less, as not all atoms are broken. But imagine the power the person who could control such energy would have…
Such energy, much wow
So now the question arises, how do we harness this energy resource? What do we do with it?
Of course, the first thing that most people think of is an atomic bomb. To put it simply, it is an uncontrolled chain reaction. I believe the stories of Hiroshima and Nagasaki, and the destruction of Fat Man and Little Boy, tell you of the effect of such energy. I remember the first time I heard of what happened and the after-effects including the effects of exposure to radiation. People who survived the initial blast, then dying in very horrible conditions.
Using the term “uncontrolled” implies that there is a way of controlling this chain reaction. As of now, the way it is done is using control rods. These are rods made up of boron, cadmium, silver or indium, or any other material that can absorb many neutrons without themselves fissioning. They reduce the number of neutrons that goes on to break more uranium nuclei. Thus, all of the energy is not released instantly, and the reaction is controlled. This is what happens in a nuclear reactor.
The explanation above is perhaps a little oversimplified. There are different types of reactors, each with their own way of harvesting this energy. But the core principle of breaking Uranium nuclei by fission, in a controlled manner using control rods, remains the same in all. I shall explain more about how the reactor works in upcoming essays.
Thus, the nucleus is a good reservoir of energy, when harvested correctly and safely. There was a time when such a vast resource was considered to be inaccessible, that atomic bombs and nuclear reactors were just figments of fiction. In fact, the term “atom bomb’ was first coined not by scientists, but by the famous science fiction author H.G Wells in his book “The World Set Free”. He imagined hand grenades made of uranium that would explode indefinitely. He even imagined them being dropped off planes. Wells himself died in 1946, around a year after Hiroshima and Nagasaki. I wonder what he would have felt like, perhaps fascination about his writing being brought to life, and horror at the destruction it caused.
Is all uranium fissile?
Earlier, I talked about two isotopes of uranium, U235, and U238. Although both of them are radioactive, U235 can be used directly for nuclear reactions. Therefore, we call U235 fissile, since it can undergo fission reactions without much processing.
238, on the other hand, is fertile. What that means is that it cannot be used for fission reactions directly. They are made fissile by a process called breeding. Basically, fertile isotopes are bombarded with neutrons, which they absorb and turn fissile. U238 converts to Plutonium 239, which is fissile.
The uranium that we find in nature, Natural Uranium (NU), contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Separating this is a challenge since the chemical properties don’t vary vastly for isotopes. The best thing to do is exploit the difference in mass.
Uranium reacts with fluorine to form Uranium Hexafluoride UF6. The amazing thing about this compound is that it boils at close to 60˚C. So, with a little bit of coaxing, UF6 is made into a gas. This gas is then forced through special membranes, that acts as a sieve. We call such membranes semipermeable. Thus, the process of diffusion is used to separate the two isotopes. This is one method of Uranium Enrichment, basically the term for separation of uranium isotopes.
These separated isotopes are then taken away to be used in their respective areas, either fission or breeding reactors.
And so…
Created in the forge of elements in supernovae over 6 billion years ago, uranium dies the way it was born, in a massive burst of energy. We have learned of this energy, and we have done our best to harness it in a socially responsible way for our needs. Of course, the story is not all positive, and many parts of it are too complex to be simply placed in categories of good and bad. The potential for lending a helping hand out of current problems, but also destroying life as we know it, is something one has to keep in mind when analyzing the applications.
Of course, I do understand that the most widely available stories associated with nuclear energy are of accidents, of destruction and the loss of human life. That is perhaps why I was motivated to write about this. In this essay, I’ve tried to explain the basic science behind nuclear energy, with the hope that knowledge can help dispel some of the fear associated with this area of science. The other thing that I hope for is that this text motivates you to look deeper into the science and statistics of nuclear energy, and take an informed stance on the issue. Perhaps nuclear energy could help us transition from fossil fuels to completely green and renewable sources of energy, without sacrificing too much our current comforts and way of living. Perhaps it may make things worse. But the fear and paranoia associated with it have to be eliminated and replaced by knowledge.
