Do you know that America gets almost 20%of its electricity from nuclear energy? This is not all. Nuclear energy accounts for almost 55% of clean energy in the United States.
Despite this, it is regarded with suspicion and considered harmful and destructive. When nuclear power is the cheapest energy source for utility-scale power production, why is it regarded as evil? Despite its bad image and warnings and opposition from scientists as well as the common man, why do some states continue to rely so much on nuclear energy, generating more than half of their electricity from it?
The world has extreme views on nuclear energy – either you are an avid supporter or a strong critic. In fact, nuclear power has more critics than supporters, even among the scientific community.
We have witnessed two major nuclear power disasters – Chernobyl in 1986 and Fukushima in 2011. And, the devastating effects of these two disasters are not yet over.
After all this, it is incredible how nuclear power plants continue to generate power in the United States and across the world. What is the hold that nuclear power has over us? Why do we rely on it knowing well its dangers?
To answer these questions, you need to know how a nuclear energy plant generates electricity. This article will take you through the process, explaining it in layman’s terms. Once you understand how it works along with its pros and cons, you can make up your mind about nuclear power.
The science behind nuclear power
Atoms of certain elements are not stable enough to remain as they are. When the electromagnetic force holding the atom together is less than the electromagnetic force destabilizing it, the atom disintegrates. Under specific circumstances, they either combine with other similar atoms (nuclear fusion) or break apart into smaller atoms (nuclear fission).
During both these processes, a considerable amount of energy is released. When the energy released is channeled in the right path, it can provide a generator with the energy to produce electricity.
However, to trigger nuclear fusion, a very high temperature to the tune of 100,000,000°C is required. This is approximately 6 times the temperature of the sun’s core. In fact, the sun’s energy comes from nuclear fusion. But we haven’t yet mastered the technique to recreate the circumstances for nuclear fusion on a large scale. Also, the experimental nuclear reactors using fusion consume much more energy than they generate. There are many more practical roadblocks to overcome before we can use fusion in nuclear electricity generation.
The present-day commercial nuclear power plants use nuclear fission or the splitting of atoms to produce nuclear energy.
What is nuclear fuel?
Not all elements and their atoms are unstable enough to undergo nuclear fission. Some of the heavy elements like uranium, thorium, and plutonium are the ideal candidates for nuclear fission. They undergo spontaneous fission, a form of radioactive decay as well as induced fission, a form of nuclear reaction.
The dictionary defines nuclear fuel as “a substance that will sustain a fission chain reaction so that it can be used as a source of nuclear energy”.
For an element to qualify as nuclear fuel, it should be able to sustain the fission process indefinitely. One atom splits into two; those two atoms split further into four; these four atoms split again into eight; and so on.
Among the 118 elements known to us to date, only three display fissile properties. Among the three, thorium does not have enough fissile power to sustain a nuclear chain reaction. It can be used as a nuclear fuel only in conjunction with a fissile material like recycled plutonium. Thorium atoms need to be bombarded with neutrons to produce U-233, a highly radioactive isotope of uranium.
Plutonium, on the other hand, is at the other extreme. It is too unstable resulting in high spontaneous nuclear fission. It is still used as nuclear fuel in some countries but not in the United States. The fabrication cost of plutonium is much higher compared to uranium due to its higher propensity to radioactivity and the need for better safeguards to avoid accidents.
It is also more expensive to retrieve plutonium from nuclear fuel for reuse as is done with uranium. The process of extraction of plutonium for recycling is difficult and costly. Moreover, there are also safety and environmental concerns about this process.
This means we are left with uranium for use as nuclear fuel in reactors. Uranium is found in small quantities in rocks and also seawater. Due to its unstable atoms, uranium has numerous isotopes, three of which are found in nature. Uranium-238, the heaviest of them all, is the most abundant, comprising more than 99% of the uranium mined. Uranium-235 constitutes 0.72% of the uranium ore. Uranium-234 is found in traces.
Unfortunately, U-238 is not fissile. This means it cannot sustain a nuclear chain reaction, rendering it unsuitable as nuclear fuel. U-234 is available only in minuscule quantities. That leaves U-235 as the only fuel for the nuclear reactor.
As the percentage of U-235 in the uranium ore is limited, the quantity is increased through a process called enrichment. During this process, the percentage of U-235 increases from 0.72% to 3-5%, making it suitable for use in nuclear reactors. Some nuclear power reactors like CANDU reactors from Canada are capable of using uranium ore without subjecting it to the enrichment process.
The enriched uranium needs to undergo the fabrication process before it can be used in a reactor. It is converted to uranium dioxide powder, pressed into small pellets, and heated to form a hard ceramic material. These ceramic pellets are inserted into thin tubes or fuel rods. Fuel rods are grouped to form fuel assemblies.
Fuel assemblies may have 90-200 fuel rods, depending on the configuration of the reactor. A reactor will have many fuel assemblies. Once these fuel assemblies are loaded into a reactor, they will remain there for many years.
How does a nuclear reactor work?
The fuel assemblies are kept immersed in water inside the nuclear reactor. The water acts as a moderator as well as a coolant. It ensures that the chain reaction doesn’t go overboard and get out of control. It helps in slowing down the neutrons generated during the fission, thus sustaining the chain reaction.
Control rods are also used in nuclear power reactors to keep the fission at desired levels. These comprise elements like boron, cadmium, hafnium, silver, or indium. These elements are capable of absorbing the neutrons without decaying themselves. Control rods are inserted or withdrawn to reduce or accelerate the fission.
As nuclear fission happens in the fuel rods, a considerable amount of heat is produced. Water is pumped to circulate in the reactor core through pipes. This heat will cause the water to boil and turn into steam. As steam needs more space than water, it gains momentum while exiting the pipe.
The steam produced is directed to hit the blades of a turbine. A turbine is similar to a windmill. Just like the windmill turns when the wind blows, the steam turns the blades of a turbine. As turbine blades turn, the rotary motion is transferred to the rotor of the generator through the action of gears.
As the rotor of the generator turns, the generator produces electricity.
In a nuclear reactor, the steam after hitting the turbine cools down. It is directed through a condenser where it loses all the heat and turns back into the water. This water is pumped back to the reactor core and the process continues.
Types of nuclear reactors
In the United States, nuclear power plants use normal water as coolant and moderator. These nuclear reactors are called light-water reactors. There are two types of light-water reactors.
1. Pressurized Water Reactors (PWR)
The majority of utility-scale reactors in the United States use PWRs. In these reactors, the water is pumped into the reactor core under high pressure to prevent water from boiling. The water after absorbing heat from the fuel rods is made to pass through a heat exchanger. The heat exchanger has separate water within it. It absorbs the heat from the water circulated, turns into steam, turns the turbines, and generates electricity.
The water exiting the heat exchanger is pumped back into the reactor core for reheating and the process is repeated. While the steam after turning the turbine is cooled down and circulated back into the heat exchanger. These two water sources never mix at any point in time.
2. Boiling Water Reactors (BWR)
Though not as common as PWRs, BWRs are found in roughly one-third of nuclear reactors in the United States.
In this, there is only one water source. The water absorbs heat from the fuel rod and turns into steam inside the reactor. This steam is directly used to turn the turbine. Later on, it is condensed and the water is circulated back to the reactor core for reheating. The process is repeated.
Now, you know what happens inside a nuclear reactor and how a nuclear energy plant produces electricity. There is no denying that nuclear fission comes with its own dangers. Though safeguards are put in place to prevent the process from getting out of hand, leading to serious issues, there are times when we are helpless when a nuclear reactor malfunctions. That is why they are called accidents.
Despite the dangers lurking in nuclear power plants, it is still our best choice for a clean energy source. Unlike coal-fueled thermal power plants, there is no air pollution or carbon emissions. Though we have been searching for alternative energy sources to replace fossil fuels for the last 70 years, none of the choices can generate such vast quantities of electricity.
This is exactly the reason why nuclear power plants are still in existence.
Solar and wind power are gaining momentum in large-scale power production. The high investment they need to build the infrastructure is the main deterrent. Until these renewable energy sources gain utility-scale capabilities, the choice for us is between fossil fuels and nuclear power.
Global warming and climate change is tilting the scale in favor of nuclear power.