Old Nuclear Blog Posts

Week 1

Before we begin discussing the topic of Nuclear Chemistry, we need to ask ourselves: what is nuclear chemistry? The dictionary definition is the study of the nucleus of an atom. This definition answers the question but doesn’t tell us what it means to study nuclear chemistry. To me, nuclear chemistry is the subfield of chemistry that deals with the properties and effects of changes in an atom’s nucleus. In other words, when the nucleus of an atom changes, what do we observe?

So how does the nucleus of an atom change? There are 3 main ways that atoms decay. There are more lesser-known ways, but we can introduce them later as they come up. Before we continue, I need to introduce some nomenclature.  When we discuss radioactive atoms, it’s written in this form shown below. The top number is the number of protons and neutrons combined, otherwise known as the mass number (atomic mass). The bottom number is the number of protons. The letters are the atomic symbol as found in the periodic table.

In alpha decay, a helium atom breaks off from the original atom, creating a new element in the process.

Beta decay involves the release of an electron. However, the atom doesn’t let go of an electron currently orbiting it. Instead what it does is convert one of the neutrons in its nucleus into a proton and an electron, and then sends out the electron generated by that process. Interestingly, the end atom has a higher proton value than what we originally started with.

Finally, Gamma decay usually occurs with another form of decay, such as alpha decay. A Gamma particle is a high energy photon, and Gamma Radiation changes neither the number of protons nor the mass number of the atom. Gamma decay is really just a convenient way for an excited atom (an atom with too much internal energy) to release energy.

Today, we talked about decay, otherwise known as radiation. However, it’s impossible to talk about radiation without mentioning the harmful risk that it brings to any life it touches. Of the three types of decay which one is the most harmful? From the least dangerous to the most, it goes alpha, beta, and gamma decay. But why? I like to think of the skin as a packed highway, with each of the types of decay as different types of cars. An alpha particle is a big 18-wheeler. It can’t travel far without colliding with other cars and coming to a stop. The beta particle is a smaller car, such as a sedan. It can travel further into the crowd and make some headway, but will also eventually be stopped. The gamma particles are like motorcycles, effortlessly weaving between traffic and getting further in than the alpha or beta particles. The further into the cars, your skin,   the vehicles get, the more damage they can do to you. Gamma particles, as bursts of pure energy, can do terrible damage to your body, while alpha particles, as atoms of helium, won’t make it past your clothes and skin.

Before I finish, I want to thank you for taking the time to read this. Next week, we’ll go over the effects of radiation, and where we can bend nature to make radiation benefit us. Over the next few weeks, we’ll cover more general topics, before we go into nuclear energy, in particular, nuclear reactors.

Week 2

Last week I touched upon radiation, this week I want to dive deeper. How do we detect it? What are its uses? Let’s start with the first question. The Geiger counter is the most common and widespread method of detection. Most people are familiar with its distinguishable clicking sound and are mostly used when trying to detect radiation harmful to humans, beta and gamma decay. So how does it work? Beta and gamma decay are particularly fast-moving particles. The Geiger counter is filled with a noble gas, a gas that doesn’t chemically react with other elements, that conducts electricity when it’s exposed to beta and gamma radiation. When the radiation contacts the atoms of gas, electrons are shaken from the atom. The electrons then build up along a wire in the middle of the Geiger counter. When enough electrons have built up, then a current is generated, and an audible click is heard. As the concentration of radiation increases, the electrons build up quicker, and more clicks are heard. 

Now onto the second question: how do we use radiation in society today? One use of radiation is to date items from prehistoric times to assist us in building a more complete picture of this planet’s past. So how does this work? We track the ratio of two specific isotopes of carbon, carbon-14, and carbon-12. Isotopes are elements that contain the same number of protons, but a different number of neutrons. For example, carbon-14 contains 6 protons and 8 neutrons, while carbon-12 contains 6 protons and 6 neutrons. Carbon-14 is turned into carbon dioxide-14 in the atmosphere, where it mixes with carbon dioxide-12 and carbon dioxide-13. Plants absorb the carbon dioxide in the atmosphere, preserving the same ratio of Carbon-14: Carbon-12 found in the atmosphere within themselves. As humans and other animals eat the plants, the ratio remains intact. Thus, throughout our entire lives, the ratio of carbon-14 to carbon-12 stays the same. However, this changes once we die. Carbon-14 is an unstable isotope, meaning that it will decay into nitrogen-14 over time through beta decay. Meanwhile, carbon-12 is a stable isotope, which means it will not decay and remain intact over time. Carbon-14 also has the benefit of decaying at a steady rate. Its half-life is 5,730 years, which basically means it takes 5,730 years for half of the remaining carbon-14 to decay. This means that if there is 1/4th of the original sample of carbon-14 left, then 11,460 years have passed. 5,730 years for ½ of the original sample to decay, and then another 5,730 years for ½ of the remaining sample to decay. With this general principle, we can date objects as old as 55,000 years. 

But dating isn’t the limit to which we can use radiation. Radiation has many medical uses as well. For today, I’ll touch upon one of them: 3-D conformal radiation therapy, a form of external radiation therapy. This form of therapy uses a stream of photons (gamma radiation) to destroy the DNA in cancer cells and stop them from replicating. Unfortunately, photons can also damage the surrounding tissue, leading to harmful side effects. One way to mitigate these effects is to introduce multiple streams of photons with less power. Due to their low power, the individual streams do not do damage to the normal tissue surrounding the tumor. The streams all cross at the tumor, finally becoming powerful enough to destroy the DNA in cancer cells.

Week 3

Once, alchemists practiced in secrecy. Their methods were surrounded in mystery, but their goal remained eternal: discovering a way to convert lead into gold. That goal was impossible then, but thanks to nuclear transmutations we can achieve what they once dreamed of: the conversion of elements. 

In 1919, Ernest Rutherford, the same man that proved the existence of a dense atomic nucleus, created the world’s first man-made element. He did this by hitting a sample of nitrogen atoms with alpha particles. From this experiment, an Oxygen-17 atom and a Hydrogen atom were created. The first man-made nuclear transformation was carried out. 

For any chemical reaction to occur, one of the requirements is that the elements/molecules collide with a large enough energy, called the activation energy. For nuclear transformations to occur, this threshold of energy is incredibly high. It is so high, that to carry out most reactions, we often need to increase their internal energy, often their kinetic energy, with machines such as particle accelerators. 

Particle accelerators, as their name suggests, accelerate particles, in three easy steps. 1. Particles are generated and go through a vacuum tube so that they don’t hit any air/dust molecules. 2. The particles are accelerated via a series of electric fields, which push the particles, and electromagnets, which guide the particles in the tube. 3. The accelerated particles are then finally, directed towards a target, which they strike. Most times these collisions occur on the atomic level, so to help the scientists observe what is happening, detectors are set up near the target area. 

Coming back to our alchemists, is it possible, with our current knowledge, to convert lead into gold? Technically, yes. Glenn Seaborg successfully converted tiny amounts of lead into gold. Making money off of this scheme, however, is not plausible. Due to the cost of powering these machines, the production of gold operates at a net loss. 

So how is this useful to us? The least complicated use is in the creation of man-made elements. All elements past Uranium do not exist in any meaningfully observable way in nature. Basically, these elements may exist as isolated atoms or clusters of atoms in the universe, but we cannot observe them due to their low quantity. These elements are typically called Transuranium elements (TRUs). One of the earliest man-made elements, plutonium, was used in the Fat Man bomb that was dropped over Nagasaki on August 9, 1945. However, most of the plutonium produced today is as a result of beta decay of uranium in reactors. 

The reason we can’t observe TRUs in nature is because by the time we come around to observe them, they have radioactively decayed into other elements. If any of these elements existed when the Earth was formed, they would have long decayed by the time we created methods of detecting these elements.

So where does that leave us? We’ve talked about radiation and nuclear transmutation in the last two weeks. Next week, we’ll lay the foundation for the second half of this series, nuclear reactors.

Week 4

So far, we’ve mostly been talking big topics in nuclear chemistry. The next few weeks will be devoted to nuclear power reactors, and how they operate. We’ll dive into the actual components of a reactor and how it all comes together in a later post. For now, we are going to be talking about the science behind the reactors. What do they do? And how do they do it?

Let’s start with the first question. What do nuclear power reactors do? As their name suggests, they are devoted to generating power, which generally comes in the form of electricity and heat. So how do they achieve it? There are two primary trains of thought: nuclear fusion and nuclear fission.

We have all witnessed nuclear fusion in our lives. This process involves smashing two atoms together such that their nuclei combine, creating a new, denser nucleus. Every nucleus contains a minimum required energy to split it up into protons and neutrons called the nuclear binding energy. If the nuclear binding energy of the nucleus produced after the reaction is smaller than the energy of the nuclei before the reaction, then the nuclear fusion reaction will release energy. Typically, energy is released until iron-56 is produced. We’ve all witnessed the power of nuclear fusion, and the benefits it brings. Every active star in our universe generates power via the process of nuclear fusion. The sun combines over 600 million tons of hydrogen every second into helium. This generates over 3.8*10^26 Joules of energy per second. For comparison, a 13-watt lightbulb uses 1000 joules in 76 seconds. So where are the nuclear fusion reactors on earth? Unfortunately, they aren’t commercially viable yet. There have been successful tests of nuclear fusion in labs across the world, but scaling that up so that it can power cities has proven to be quite a challenge. During the cold war, the power of nuclear fission was harnessed in Hydrogen bombs; however, we have yet to find a way to control fission reactions so that they can be a safe source of power. For this reason, nuclear power reactors use nuclear fission. 

Nuclear fission is in some ways the polar opposite of nuclear fusion. Where fusion brought together smaller atoms to create larger ones, fission splits apart larger atoms, creating smaller ones. Similar to fusion, the source of energy in fission reactions depends on the nuclear binding energy. For energy to be produced, the sum of the nuclear binding energies of the product nuclei must be lower than the energy of the reactant nucleus.

When nuclear fission occurs, in addition to the nuclei it generates, it can also create a number of stray neutrons. For example, the reaction of uranium-235, which led to the discovery of nuclear fission, proceeds as seen above. The number of neutrons produced is critical to the science behind nuclear reactors. The extra neutrons can then be used to interact with other uranium-235 atoms, leading to a chain reaction. As seen in the equation above, if every single neutron produced interacted with another uranium atom, the rate of fission would increase exponentially. This is defined as being a supercritical situation. Conversely, if on average less than one neutron collides with a uranium atom, the reaction quietly fizzles, and it becomes known as subcritical. The goldilocks region is when on average one neutron collides with one atom, leading to a stable reaction, otherwise known as a critical state. For this to occur, a very precise amount of nuclear material is needed, known as the critical mass. This mass varies depending on the isotope, however, we know that a stable, controllable reaction occurs when we have this mass. 

The worst possible outcome for all fission reactions is a supercritical state. At that point, the reaction can no longer be safely contained, and it generates a dangerous amount of heat and radiation. You don’t have to look any further than the Little Boy and Fat Man atomic bombs to see the devastation that can be brought upon by supercritical reactions. For this reason, nuclear reactors are extremely well-regulated and operate under the strictest guidelines.

Week 5

Last week, we talked about the primary process used by nuclear power reactors to generate power, fission. Today, we’re going to explore the primary source used for that, uranium-235. However, it’s not as simple as pouring uranium in a hatch and topping off the fuel compartment. Uranium comes in many different isotopes and not all of them are useful for our purposes. 

First, we have to find the uranium in the ground. This is a relatively easy process, as we just look for spikes of radiation in the ground. Unless it’s a nuclear waste dumping site, we can be fairly confident what we find is a form of uranium oxide. The hardest part of this process is usually finding a location that makes digging in there economically viable. There are two methods used to extract the uranium from the ground. The Uranium can be dug up and dissolved in water using sulfuric acid. It can also be extracted via a process called in situ leaching. As the name suggests, water is poured into a site that has been determined to contain uranium, which then dissolves in the water, and is then pumped back to the surface. Regardless of which process is used, the uranium is dissolved into water. After this, the uranium is then filtered out of the water mixture and dried. This result is in an extremely bright yellow powder of uranium oxide, otherwise known as yellowcake. This powder isn’t very radioactive and is only made up of about 0.7% of uranium-235. To get to the necessary 3.5- 5% uranium-235, the yellowcake must be enriched. 

The enrichment process begins by converting the powder into uranium hexafluoride (UF₆), a gas. This gas is then used to increase the concentration of uranium-235 in a sample by removing some of the uranium-238. A molecule that contains uranium-235 is about 1% lighter than one containing uranium-238. Some of the uranium-238 is then physically separated from the mixture until the required 3.5- 5% composition of uranium-235 is reached. This is achieved using a centrifuge, a device that spins at a consistently high speed, pushing heavier elements to the bottom, while lighter elements remain at the top. Following this process, the enriched uranium hexafluoride is then converted into uranium dioxide (UO₂). This is the final chemical process, as the uranium dioxide is then compressed into dark gray fuel pellets. These fuel pellets are stacked into metal rods and are called, unsurprisingly, fuel rods. These rods are then grouped together to be placed into the reactor core.

Earlier, I mentioned that the uranium contains at most 5% of uranium-235, which I had described as the primary source of energy in a reactor. So why doesn’t it contain more? The statement I made earlier was only partly true. Yes, uranium-235 is important, but it isn’t the only source of fuel there. If you remember from the last post, critical situations occur when an average of one neutron is generated per fission. Uranium-235 produces 3 neutrons when it decays. Thus, if we used pure uranium-235, we would constantly go supercritical. To counteract this, we use uranium-238 as a buffer. This isotope is considered “fertile”, which means it can absorb a neutron and then become plutonium-239. Plutonium-239 and uranium-235 are both considered “fissile”, which just means they can undergo fission. It is through this delicate balance that we are able to maintain stability in the reactor core. 

Next week, we’ll dive into the other parts of the nuclear reactor. We’ll talk about what exactly comprises a reactor, and how the parts work together.

Week 6

This week’s post will be fairly brief. Today we are going to talk about the components that make up a nuclear reactor. Next week, we will get to the power generation. For now, we are going to detail the components of a nuclear reactor that help facilitate safe power generation. 

Last week, we talked about the production of uranium fuel pellets and fuel rods. These rods are then placed near each other in the reactor core, which is where the fission takes place. In previous posts, I talked about fission as if it were a single process that large atoms can undergo. However, like many things in science, it’s not so simple. There are primarily two types of fission reactors used: heat-focused and speed-focused. Most of the power reactors are heat-focused or thermal-neutron reactors (TNR). These are focused on producing a lot of heat, which can be used in more traditional methods of power generation, which will be explained next week. The other type is the speed-focused reactors, or fast-neutron reactors (FNR). When I say fast, I don’t mean the speed of the reaction per second. Instead, what I am talking about is the speed of the neutron as it collides with the uranium-235. FNRs use fast-moving neutrons while TNRs use slower neutrons. The problem is that every single fission reaction produces a fast-moving neutron. To slow down these neutrons, moderators are used. These are materials, such as graphite, that rob some of the speed of the neutron as it passes through that layer. Some other common materials used are water and heavy water, which uses Hydrogen-2 in its chemical makeup (H₂O) instead of Hydrogen-1. Moderators work by absorbing some of the kinetic (moving) energy of the neutron when it collides with the moderator. This process is comparable to hitting a billiard ball with the cue ball. At first, only one is moving quickly. After they collide, both begin moving, although they now move much more slowly than the speed the first ball originally had. 

If you remember from previous posts, the goal of every fission reaction is to achieve a critical state, or a state when the average number of neutrons created is 1. However, the fission of Uranium-235 generates 3 neutrons. So how do we prevent these neutrons from striking out and making the reactor go supercritical? Easy. We introduce control rods. These rods are placed among the fuel rods inside the reactor core. They usually contain elements such as boron and cadmium, which are used to absorb neutrons. Oftentimes, people mistake the job of the moderator with that of the control rod. They incorrectly believe that the moderator will absorb neutrons as well as slowing them down. It is easy to see where this misconception comes from. After all, if I apply the brake long enough in a car, eventually the car will stop. However, moderators are precisely designed to avoid this. Their only job is to act as a speed bump, slowing down the neutrons, but never stopping them. The control rods are there to act as the neutron sponges. Their only job is to soak in as many neutrons as they can carry. 

Finally, a few more components before we finish for the week. TNR, as their name suggests, are used to generate a lot of heat. Most of the time, this heat is exactly what we are looking for. However, like many things in life, too much of anything is bad. An excess of heat can lead to a reactor meltdown, where the core physically melts due to an excess of heat. To control the heat of the reactor, we use coolants. Typically, this is water that is pumped throughout the core that then traps the heat. Water is a fantastic coolant for many reasons. First, it’s extremely abundant, and more importantly, cheap. Second, water has the highest specific heat capacity of any liquid. Basically, it means that water requires much more energy to increase its temperature per gram than any other liquid. If I had a gram of mercury and a gram of water at room temperature, the water would require nearly 30x more energy to raise its temperature by one-degree Celsius when compared to the mercury. 

Finally, we get to the “storage” components. There are two levels: the pressure vessel and the containment. The pressure vessel is there to hold the reactor core, coolant, moderator, and any other critical components together. This is essentially a giant tank outside which engineers and scientists can interact with the core as they need to. An additional layer, called the containment, is there to shield the general public from any radiation dangers in the event that something bad happens to the reactor. This containment is extremely dense, being at least 1 meter thick in any given location and composed of both steel and concrete.

While I did say I would talk about the components of the nuclear reactor, I felt that certain components, such as the steam generator, fit better under next week’s theme of power generation. Don’t worry, I’ll get to everything by next week and we can talk about the specifics of using colliding atoms to power our homes.

Week 7

And here we are in the final stretch for the nuclear reactors. Last week, we discussed a few components regarding the fission process. This week, we’ll talk about the power generation process. 

Before we begin, I believe it’s important to detail how electricity is “made”. From there, we can explore how those general principles are used in nuclear reactors. Nowadays, many countries rely on the use of fossil fuels, such as coal and oil, as their primary sources of energy. Fossil fuels, as their name suggests, are the solid remnants of the carbon-rich remains of plants and animals that have died long ago. The coal is mined and then burned over a vat of water. This water then turns into steam and is pressurized. The pressurized steam is then fed into a turbine that spins and rotates an electric conductor. This conductor sits in a magnetic field, and when the conductor rotates, it creates a difference in charge between the two ends of the conductor, which creates an electric current.

Now, let’s apply this to nuclear power plants. If you remember from last week, I mentioned that most of the reactors today were thermo-nuclear reactors (TNRs), or heat-focused. Today, we see why this is the case. Power generation can only occur if the water gets hot enough to evaporate. Thus, maximizing the heat output of the reactor is the number one goal. So then what do we do with all of this heat? From here, there are two main types of reactors: pressurized water reactors (PWRs) and boiling water reactors (BWRs).

Let’s start with the easiest to visualize. Boiling water reactors are fairly straightforward. As I mentioned in last week’s blog post, water is used as an effective coolant for these reactors. BWRs allow the coolant to become heated enough to form steam. This steam is then pumped away from the reactor core, where it is then fed into a turbine, which creates electricity. After this, the steam is put through a condenser, which turns it back into water, and is then fed into the reactor core once again. BWRs are extremely straightforward in design, but they’re only the second most popular reactor design out there. Why? A big problem comes up when BWRs are implemented. First, the calculations required to figure out how much nuclear material is needed is much more complex than PWRs. Having steam along with water in the same chamber presents many more challenges as the chemical properties of the mixture must be taken into account. Second, BWRs do not produce as high thermal output as PWRs. Another major factor is cost. BWRs require larger pressure vessels to safely contain the steam generated by the reactor. Due to this, every BWR will cost more money than a PWR. 

Now, onto the most popular reactor, the pressurized water reactors. PWRs differ from BWRs in one major way. Instead of generating the steam inside of the reactor core itself, the heated water is pumped away from the reactor and is brought into another chamber. Inside this chamber, the heat from the reactor water is used to heat up some other water. From this, the other water turns into steam, which is then pumped into a turbine and turned into electricity. Now some of you may be asking how it’s possible to get water that hasn’t boiled to transfer enough energy to boil water. This is where the pressure comes into play. Some of you may know that heating water isn’t the only way to get it to boil, you can also lower the surrounding pressure of it. PWRs use this concept in reverse. They, as their name suggests, pressurize the water coming out of the reactor such that it doesn’t boil. When it enters the tank, it comes into contact with water at regular pressure. The temperature of the water, which was too high to boil at a higher pressure, is enough to boil at the regular pressure. PWRs are much more popular than BWRs for the exact opposite reasons: They’re cheaper, smaller, and operate much more efficiently. 

Nuclear Reactors are a fantastic source of energy, but operating with these levels of heat doesn’t come without a cost. I mentioned earlier about the use of condensers to cool down steam. This usually happens by running a pipe of colder water through a tank. The idea is the colder water will absorb the heat from the steam and cool it down, eventually condensing it into liquid water. The source of colder water is usually a nearby lake. One drawback is that the reactors frequently pump the heated lake water back into the lake, leading to thermal pollution. Thermal pollution is similar to other types of pollution, such as air pollution. However, instead of contamination via extra particles, this is contamination via extra energy in the form of heat. The increase in temperature can be detrimental to plant and animal life in, and around, the lake. 

Week 8

Last week, we finished up the state of most modern nuclear reactors. This week, we’ll look towards the future. We talked extensively about the uranium-235/238 reactors in the past few weeks, but uranium isn’t the only radioactive element on the periodic table. To illustrate what I mean, let’s look to a slightly old, yet slightly new type of reactor: the thorium reactor.

Thorium reactors, as their name suggests, use thorium as the fuel components. However, there are a lot of differences between the traditional reactors and thorium reactors. Let’s start with the chemistry itself. Thorium-232 will not undergo a fission reaction. Thorium-232 is fertile, which if you remember means that it can absorb neutrons and turn into a fissile atom. The thorium-232 converts into uranium-233, which is what brings in the power. Thorium can be used in many reactors that we see today, such as the Pressurized Water Reactor (PWR), Boiling Water Reactor (BWR), and the Fast Neutron Reactor (FNR), which have all been discussed in previous posts. But there are a few more theoretical reactors that may be able to use Thorium. The one I’d like to talk about with you today is the molten salt reactor (MSR). The thorium can be interwoven in fluorine salts, which can then be melted into the molten salts. The fluorine variation of the reactors is called Liquid Fluoride Thorium Reactor (LFTR). These salts not only function as the medium where the fission takes place, but it also doubles as the coolant for the LFTR. 

LFTRs have been theorized to be much safer than conventional reactors. They don’t produce extremely dangerous radioactive elements, such as americium and plutonium, which causes much of the radioactive waste. Thorium reactors will still produce waste; however, the waste produced is much less destructive to the environment in the long-term. This is not the only benefit to thorium. It is more common in the ground than uranium (at a rate of just over 3x as often). What does this mean? We essentially have a lot more energy from thorium available to us than with uranium. 

Surprisingly, this isn’t the first time we’ve explored the possibility of using thorium as a fuel for reactors. The US had discovered the technology back in the ’60s, but it was soon shut down in 1973. You might wonder why, and it’s for a similar reason we consider them so good today. They didn’t produce plutonium. The initial development of nuclear technology wasn’t about energy production, but bombs. Plutonium is a key component of nuclear bombs, so without any plutonium being produced in LFTRs, the US government considered them a failure. Today, this would be seen as a considerable upside, as countries would be more willing to give away nuclear reactor tech without worrying about other nations using the waste products to pad their weapon stockpiles. 

So what are the downsides of thorium reactors? The biggest hurdle is just how little we know about them right now. Incidents like Chernobyl and Fukushima show just how dangerous this technology is when bad circumstances arise. The nuclear community is justifiably concerned about rushing into a direction that may spell danger. Another important barrier is the amount of gamma radiation released from the reactor. The process of producing the uranium-233 is extremely dangerous and emits a lot of energy. I did gloss over it earlier, but the process does not go directly from thorium-232 to uranium-233. It actually contains an intermediary atom, protactinium-233, which is what decays into uranium-233. The protactinium is the problematic atom. Approximately 5 sieverts (a unit for radiation) will result in the person dying within the month. If you were exposed to 2 grams of protactinium from a distance of 1 meter, you’d receive 41 sieverts per hour. Combine this with the fact that molten salts are by their nature fairly corrosive materials, and that the nuclear community only allows its people to be exposed to 20 millisieverts of radiation per year, and it’s easy to see why the “miracle” atom remains in R&D. But just because the door has virtually closed on Thorium, for now, doesn’t mean that we won’t find something to improve on the reactor designs of today. Progress is made one step at a time, it’s just a matter of us finding our footing.