On June 27, 1954, the first nuclear power plant was connected to the power grid in Obninsk, Russia. For the first time, the fission energy of uranium nuclei was used for peaceful and constructive purposes. Today nearly 10% of our energy comes from nuclear energy, with it making up nearly 20% of our energy in advanced countries. While nuclear power plants have had many disasters, it is still 800 times less deadly than coal and 600 times less deadly than oil. Why are we so reluctant to use it? In this essay I will explain how nuclear energy works, what happens when it fails, and why it’s so useful. /p>
In this blog I’m going to explain to you how Light Water Reactors (LWRs) work. The core components of LWRs include fuel rods composed of pellets of enriched uranium oxide, a moderator to slow down neutrons (thereby increasing their likelihood of being absorbed by a uranium nucleus), and control rods (typically made of boron or cadmium) that absorb neutrons to regulate the rate of nuclear fission. Additionally, a coolant is used to remove heat generated by the nuclear fission reaction (In LWRs water is typically used as a coolant and a moderator), and a pressure vessel and containment chamber are designed to contain radioactive materials in the event of a nuclear emergency. Nuclear reactors operate on the concept of nuclear fission; when a slow-moving neutron is absorbed by a uranium-235 molecule, the molecule splits into two lighter elements, energy in the form of heat, and more neutrons. These neutrons cause a chain reaction; unlike in nuclear weapons where this chain reaction runs wild, in nuclear reactors this chain reaction is kept under control by the control rods. Now let’s talk about what precautions the reactor takes in case of an emergency. There are many reasons that nuclear reactors use uranium oxide as opposed to metallic uranium; the melting point of uranium oxide is 2 865 ℃(5 189 ℉) instead of the melting point of metallic uranium being 1 132 ℃(2 070 ℉), this provides a larger margin of safety. UO2 is chemically stable at the high temperatures present inside the reactor, as well as not corroding easily, making it easier for it to stay inside the fuel rods and not contaminate the coolant or surrounding environment. Now we’ve talked about how they work, now let’s talk about when they don’t.
The different ways that meltdowns can happen in general are a loss of coolant accident (LOCA), loss of pressure accident (LOPA), uncontrolled power excursion (A significant alteration in the neutron multiplication rate such as in case of accidentally ejecting a control rod or rapid cooling), or in reactors where there is no pressure vessel, a fire within the reactor core. A LOCA happens when there is a pipe break or leak, leading to many side effects: Water can boil and burst out of there pipes (Because of this, most LWRs have pressure operated relief valves to limit the pressure and backup supplies of cooling water). If graphite and air are present, the graphite may catch fire and spread radioactive contamination (This only happens in AGR reactors, RBMKs, such as the Chernobyl Nuclear Power Plant, Magnox reactors, and weapons-production facilities). A LOCA could cause the fuel and internal reactors to melt; if the melted blob remains critical (Able to continue nuclear fission), it could continue melting parts of the reactor and melt out the bottom of the reactor, causing a meltdown. If it remains critical for too long, it could melt through the soil into the water bed (This is known as the “China Syndrome”. This is what happened in Chernobyl). In order to prevent this virtually 100% of nuclear reactors are equipped with ECCS (Emergency Core Cooling System); In most modern plants the ECCS is triggered when the pressure or coolant flow rate drops dramatically, or when the temperature increases past a typical level. ECCS work either by injecting water straight into the core, or by simply flooding the containment chamber to quickly cool it. Another type of accident is a loss of pressure control accident (LOPA). For LWRs, a pressure vessel is necessary to prevent the water from boiling. There are many ways an LOPA can happen; The pressurizing vessel can become isolated from the reactor plant by closing an isolation valve or a pipe getting clogged (Because of these risks no commercial nuclear power plant has an isolation valve, and in order to avoid clogging the coolant is kept very clean and the pipes are designed to be short and wide in diameter.). In the pressure vessel there is a steam bubble to raise the pressure, in order to regulate the pressure the pressure vessel contains spray nozzles that spray cooler water into the steam bubble, causing it to condense and reduce the pressure. A LOPA can happen when a spray nozzle becomes stuck in the open position (Either because of mechanical wear and tear, foreign objects getting stuck in the nozzles, thermal expansion causing the spray nozzle to warp and seize, or in reactors where the spray nozzles are controlled by solenoid valves, the solenoid valve could malfunction and become stuck in the “on” position), leading to it continually spraying cool water onto the steam bubble and drastically reducing the pressure. A LOPA could also be caused by thermal stratification (the divide in temperature in the bottom and the top). Thermal stratification can happen a few ways: Insufficient mixing (The pressure ideally should be mixing constantly, either by natural convection or design features but if these do not work well enough, cold water can settle at the bottom, away from the steam bubble), low power (At lower powers, the reactor doesn’t generate enough heat, which could lead to less natural convection.), spray nozzle malfunction (If the spray nozzle sprays water unevenly, cold water can settle at the bottom), or sudden coolant temperature changes. In the case of cold water settling at the bottom of the pressure vessel, the pressure will appear to be fine, but the pressure will be slowly lowering. The pressure will appear to be fine because the pressure gauges are only measuring the temperature of the steam vessel and because the cold water isn’t initially participating in the pressure control. In case of a LOPA control, it can take multiple hours for operators to notice these trends due to the large amount of thermal inertia (Meaning it takes a long time for it to heat up or cool down). As the pressure slowly decreases, the boiling point of the water also decreases. Eventually, the pressure becomes low enough that bubbles begin to form. Once enough bubbles form, the rate of heat transfer slows down enough for the fuel rods to be generating heat faster than it is being taken away, leading to a nuclear meltdown. Because of this, nearly all nuclear reactors have systems that automatically shutdown when the pressure drops before the standard deviation. An uncontrolled power excursion happens when there is a significant spike in reactor reactivity (such as when a control rod is ejected or when there is rapid cooling). In LWRs this is very rare, as LWRs have a negative void coefficient (A void coefficient tells us how much the reactor's reactivity increases when “voids” (Steam bubbles) form. LWRs have a negative void coefficient because water acts as both the coolant and the moderator, meaning that when steam bubbles form, there is less moderator, making nuclear fission harder. A negative void coefficient is important to a reactor’s safety; In the Chernobyl Nuclear Power Plant (Which was an RBMK), they used graphite as a moderator. Graphite has an extremely high melting point, this means that when the coolant (water) began to boil, the reactor’s ability to absorb neutrons was reduced. However, since the graphite was still present and effective, this created a positive feedback loop, leading to a meltdown.). Finally, fires are extremely rare in LWRs. This is because a fire needs three things to happen, fuel, heat, and oxidizer. While LWRs do have fuel (The nuclear material) and heat (From the nuclear fission), there is no oxidizer present. Now we’ve gone at length about how nuclear reactors can fail, let’s talk about what happens when they succeed.
The biggest reason that nuclear power is so useful is its reliability. While solar panels and wind farms are very safe and have very low carbon emissions, their biggest problems are their reliability. Solar panels only work when the sun is out, meaning they don’t work at night, and are only about 80% effective during cloudy days and during winter, while wind farms only work where there’s, well, wind. Nuclear energy has nearly none of these downsides, being able to work effectively regardless of time of day or the weather. Nuclear energy also has one of the lowest carbon emissions of any power source, only being beaten by wind energy. Nuclear energy does have a few downsides. The main one is the time and money needed to get a nuclear power plant up and running. It can take ten to 15 billion dollars, as well as six to eight years to build a nuclear power plant. While the upfront costs are very high, the operating costs are very low. The cost per megawatt hour (The amount of energy used when using one megawatt of power for an hour) for nuclear power is about 30 USD, compared to about 36 USD for coal and about 24 USD for new solar. In France, where 70% of their power is nuclear, their energy costs about 0.20 euros per kilowatt hour; However in Germany, where nuclear power makes up about only 2 percent of their economy, their energy costs about 0.33 euros per kilowatt hour.
While nuclear power has many downsides, I believe it provides an important stepping stone for our transition from fossil fuels to renewables. This has been Kmitav Arishna, and I’m going to go take a nap now.