Nuclear energy is considered one of the cleanest sources of electricity on the planet (in terms of greenhouse gas emissions) if we don’t take into account the radioactive waste produced during the nuclear fission reaction.
Nuclear Power Plant – Definition
Nuclear power plants represent facilities built to turn the power of the atom into clean electricity by releasing a very tiny amount of greenhouse gases.
A nuclear power plant (also called nuclear power station) is a thermal power station that uses nuclear reactors to produce the heat required to generate steam.
The steam produced this way is used to spin a turbine that will produce clean electricity with the help of generators.
Nuclear power plants use nuclear reactors to generate thermal energy that will be turned into clean electricity.
Thermal energy is produced during the nuclear fission reaction that takes place in a controlled environment inside the nuclear reactors.
The nuclear fuel used in the nuclear fission reaction is usually natural uranium or enriched uranium.
A nuclear power plant usually consists of one or more nuclear reactors, a steam turbine and an alternator, two or even three circuits (primary, secondary and tertiary), and one or two cooling towers that are using water as cooling agent.
Nuclear power plants have an efficiency between 30% and 40%, and the operating lifetime is between 20 and 40 years, but the main concern is created by the radioactive waste produced during the nuclear fission reaction.
Basic Operations Taking Place In Nuclear Power Plants
I mentioned before that a nuclear power plant produces clean electricity using the nuclear fission reaction that generates heat.
The heat is used to boil water and produce steam that will spin a turbine to generate electricity.
However, the U.S. uses today in its nuclear power plants two main categories of nuclear reactors: boiling water reactors (BWRs) and pressurized water reactors (PWRs).
Boiling water reactor (BWR) boils water within the reactor, image source: ucsusa.org
Both these systems are using boiling water to generate steam, but BWRs boil water within the reactor, and PWRs boil water outside the reactor.
In both cases, the steam must be cooled after using it to spin the turbine and generate electricity.
Pressurized water reactor (PWR) boils water outside the reactor, image source: ucsusa.org
Nuclear Fuel Used By Nuclear Power Plants
Uranium is generally used as nuclear fuel during the nuclear fission reaction that takes place inside the nuclear reactor.
Being a very heavy metal, uranium can be found in soils, rocks and sea water, but is not very abundant on the planet.
There are two different isotopes that can be found in the naturally occurring uranium: Uranium-238 (U-238), which accounts for 99.3% and Uranium-235 (U-235), which accounts for only 0.7%.
These isotopes represent uranium atoms with a different number of neutrons.
Uranium-238 has 146 neutrons, and Uranium-235 only 143 neutrons.
The different number of neutrons creates different behaviors of the isotopes, so Uranium-235 can be easily split to give a large amount of energy, which makes it very suited for the nuclear fission reaction.
However, Uranium-238 (U-238) has a different behavior, even if we talk about the same element.
Uranium-238 (U-238) is less radioactive and has a longer half-life than Uranium-235 (U-235), so it requires a longer period of time to decay.
Nuclear Chain Reactions Taking Place Inside The Core Of A Nuclear Reactor
A nuclear reactor must produce more energy during the nuclear fission reaction than the energy consumed in the process, so the nuclear reactions must produce a chain reaction inside the nuclear reactor.
Uranium being a very heavy metal (chemical element with a high atomic mass) with radioactive properties, it fits very well to be used as nuclear fuel inside the nuclear reactor.
Nuclear chain reactions will take place in the core of the nuclear reactor where the tubes within the reactor that are full of fluid (refrigerant) will transport the heat (thermal energy) out of the reactor and will cooling it.
The Nuclear Reactor
The nuclear reactor is the most important device in the nuclear power plant because is the place where the nuclear fission reaction takes place.
The core of the nuclear reactor hosts the nuclear fission reaction which starts by bombarding the uranium atom with a neutron that splits the large nucleus into two smaller nuclei and releases a large amount of energy.
The nuclear chain reaction is assured by continuing the neutron bombardment.
After initiation, the nuclear fission reaction can be controlled using control rods within the reactor and a neutron moderator.
Types Of Nuclear Reactors Used Today In Nuclear Power Plants
There are three types of nuclear reactors that serve general purposes such as: civilian reactors (used to produce electricity and occasionally steam for district heating), military reactors (used to create materials that can be used to build nuclear weapons), and research reactors (used for energy production, for weapons development, for nuclear physics experimentation, for training purposes, and also to produce radioisotopes for medical technology and research.
Let’s see the civilian and military reactors in detail.
Light Water Reactors
These types of nuclear reactors rely on steam to spin a turbine and produce electricity.
1. Boiling Water Reactor (BWR)
A boiling water reactor boils the water within the reactor and is used to produce electricity.
The coolant used is water and the moderator type is also water.
The nuclear fuel used is a chemical composition: uranium-dioxide (UO2).
The level of fuel enrichment is low enriched uranium (3 to 5%).
Boiling water reactors are used by the nuclear power plants in the U.S., Japan and Sweden.
2. Pressurized Water Reactor (PWR)
A pressurized water reactor boils the water outside the reactor in a secondary heat transfer loop, and is used to produce electricity and also to power nuclear ships (by the U.S. Navy).
The coolant type and the moderator type are both water.
The nuclear fuel used is a chemical composition: uranium-dioxide (UO2).
The level of fuel enrichment is low enriched uranium (3 to 5%).
Pressurized water reactors are used by the nuclear power plants in the U.S., Japan, France, Russia and China.
3. Gas Cooled Graphite Moderated Reactor (AGR and Magnox)
A gas cooled graphite moderated reactor is used to produce electricity and plutonium.
The coolant type is gas (carbon dioxide or helium), and the moderator type is graphite.
The nuclear fuel used is uranium dicarbide (UC2) or uranium metal.
The level of fuel enrichment is slightly-enriched uranium (0.8 to 3%).
Gas cooled graphite moderated reactors (AGR and Magnox) are used by the nuclear power plants in the UK.
4. Light Water Cooled Graphite Moderated Reactor (RBMK and EGP)
A light water cooled graphite moderated reactor is used to produce electricity and for plutonium production.
The coolant type is water, and the moderator type is graphite.
The nuclear fuel used is uranium dioxide (RBMK) or uranium metal (N-reactor).
The level of fuel enrichment is slightly-enriched uranium (0.8 to 3%).
Light water cooled graphite moderated reactors (RBMK and EGP) were used by the former soviet union and in the U.S. at the Hanford nuclear site.
5. Fast Breeder Reactor (FBR)
A fast breeder reactor is used to produce electricity and for plutonium production.
The coolant type is molten, liquid sodium, and the moderator type is not required.
The nuclear fuel used is plutonium dioxide (PuO2) and uranium dioxide (UO2) in various arrangements.
In terms of fuel enrichment uses various mixtures of plutonium-239 and uranium-235.
Breeder reactors are created to produce more fissile material than they consume, and they can be found in Russia and Japan.
Heavy Water Reactor
1. Pressurized Heavy Water Reactor ‘CANDU’ (PHWR)
A ‘CANDU’ reactor is used to produce electricity and plutonium.
The coolant type and the moderator type is heavy water (deuterium oxide, D2O).
The nuclear fuel used is a chemical composition: uranium-dioxide (UO2) or uranium metal.
The level of fuel enrichment is natural uranium (not enriched).
Pressurized heavy water reactors ‘CANDU’ are used by the nuclear power plants in Canada, Romania and in Savannah River Site (SRS).
Electricity Generation In Nuclear Power Plants
The nuclear fission reaction taking place in the core of the nuclear reactor generates a large quantity of heat (thermal energy), which is used to boil water and produce steam.
After exiting the containment building, the very hot and high-pressurized steam is used to spin a turbine (thermal energy is turned into kinetic energy) that will generate clean electricity (kinetic energy is turned into electrical energy) using a generator (alternator).
Once used by the turbine, the steam that has lost its calorific energy will be sent to a condensation chamber where will be cooled down using cold water pipelines.
The steam becomes water again and with the help of a pump, the water will sent back to the nuclear reactor and the circuit will start again.
This is the reason why nuclear power plants are usually built near a water source such as: river, lake, sea etc.)
For the moment, the nuclear power plants in operation rely on the nuclear fission reaction to produce electricity, which also generates a large amount of nuclear waste every year.
Conclusion
Uranium is a finite resource, so the electricity produced by splitting the nucleus of a uranium atom is not considered renewable.
However, scientists are working hard today to develop a new type of nuclear reactor that can use the nuclear fusion reaction of hydrogen to generate huge amounts of clean electricity and no harmful waste (water is the waste product).
