Nuclear power plants (NPPs) play a critical role in the world’s energy mix. They provide a stable baseline generation that can be complimented by other carbon neutral sources, such as hydro, geothermal, wind and solar. In 2018, nuclear power accounted for 2563 TWh of the world’s total electricity production, just over 10% . Today there are over 441 operable nuclear power reactors worldwide, with 55 new reactors under construction, 109 planned and over 350 proposed! [IAEA PRIS1, world-nuclear.org]
1. International Atomic Energy Agency Power Reactor Information System
Click on the letters for more information!
A
Atom
All matter (everything around you) is made of atoms! Atoms are made of a nucleus surrounded by an electron cloud. They are the building blocks of molecules and chemical compounds.
B
Boron & boric acid
In a pressurized water reactor (PWR), boron, in the form of boric acid (HBO3), is added to the primary water. Boron absorbs neutrons. By increasing or decreasing the boron concentration in the primary coolant, the reactivity is adjusted to maintain reactor power as the fuel is spent. Boiling water reactors (BWR) do not use boron to maintain reactivity.
Boiling water reactor
Currently there are 65 Boiling light water cooled and moderated rectors in operation today [PRIS], and are the second most popular reactor type. BWRs do not have a secondary circuit. Water flows up through the core in the reactor pressure vessel (RPV), where it is heated by the nuclear fission reaction. The water is allowed to boil in the vessel. The steam turns the turbines to generate electricity.
Image credit – United States Nuclear Regulatory Commission
C
CANDU
CANadian Deuterium Uranium (CANDU) reactors are a type of pressurized heavy-water reactor (PHWR). CANDU reactors use “heavy water” (deuterium oxide, 2H2O) as their moderator and can use natural (unenriched) uranium as their fuel. CANDU reactors do not have a traditional reactor pressure vessel, but instead its horizontal fuel bundles are contained inside small pressure tubes in reactor core, or calandria. The calandria is not pressurized and remains at a relatively low temperature. To keep the calandria at a low temperature, the pressure tubes are encased in a calandria tube. The gap is filled with carbon dioxide, which mitigates the heat transfer.
Image credit – Canadian Nuclear Association
Cooling tower
A cooling tower is a large heat exchanger. A cooling tower helps to cool the water of the cooling circuit. Often the cooling towers of nuclear power plants (NPP) have a hyperboloid shape, like seen in the illustration. They can be up to 200 meters (over 650 feet) tall and nearly 100 meters wide at the top.
Not all NPPs have cooling towers, many use a “once through” cooling system, where water is drawn from a body of water nearby, used to cool the water of the cooling circuit and then pumped back to the body of water a couple of degrees warmer than before.
D
Departure from nucleate boiling
Departure from nucleate boiling (DNB) can happen on the surface of the fuel cladding of a nuclear fuel rod. Steam bubbles, are unable to leave the surface of the fuel cladding, causing the heat transfer to decrease. The temperature will continue to increase on the fuel rod surface. DNB can be avoided by increasing coolant pressure or flow rate.
Deuterium
Also known as “heavy hydrogen”, 2H. Deuterium is a naturally occurring isotope of hydrogen. It is made up of one proton, one neutron and one electron. CANDU reactors use “heavy water” (deuterium oxide, 2H2O) as their moderator.
E
Emergency Core Cooling System
An emergency core cooling system (ECCS) ensures core cooling in the event of a Loss of Coolant Accident (LOCA) and helps to limit cladding damage. It injects cool water into the core. ECCS’ often have both a high and low pressure injection systems – designed to function when the reactor is still under high pressure, and in the case of pressure loss for core spraying.
F
Fission
Fission is the splitting of an atom. When an atom is split, it releases a large amount of energy, mostly in the form of heat. This heat is used to produce electricity in a nuclear power reactor. The splitting of an atom can be initiated by bombarding the atom with neutrons. When the atom absorbs a neutron/neutrons, it may become unstable and splits into two lighter atoms – this is what happens when nuclear fuel is bombarded with neutrons.
Fuel
See “Nuclear fuel“.
Fuel assembly
Fuel rods (fuel cladding filled with the nuclear fuel pellets) are bundled together to create a fuel assembly. Each reactor type has a different design fuel assembly. One fuel assemblies can have hundreds of fuel rods, depending on design.
Fusion
Creating a heavier atom from two lighter atoms. The fusion of two atoms produces a very large amount of energy. Nuclear fusion is a topic of very high interest.
G
Gamma radiation
Gamma radiation is electromagnetic radiation from nuclear decay. It is a type of ionizing radiation that can penetrate paper, plastic and through the skin to internal organs. Shielding for gamma radiation is possible by thick concrete or lead.
The study of irradiated (active) materials is carried out at “hot laboratories” in “hot cells“. The cell walls are often made of lead or thick concrete to shield those working.
H
Hot cell
In a hot cell, “hot” (meaning irradiated) materials are handled. They are essentially a large containment box made of thick lead or concrete walls that protects users from radiation. They may be equipped with windows made of leaded glass. Hot cells are often equipped with manipulators used by personnel to handle the irradiated materials located inside the cell.
Hydrogen
Hydrogen is the lightest element of the periodic table. It is made of one proton and one electron. Hydrogen exists in nature in its stable gaseous state, H2. In a pressurized water reactor (PWR), dissolved hydrogen is added to the primary coolant to suppress radiolysis and help to protect the metallic components from various types of corrosion.
I
Isotope
Isotopes are variations of the same elemental species. They differ in their number of neutrons and, thus, in molecular weight. In the illustration, hydrogen and its two isotopes, deuterium and tritium, are shown. Hydrogen, deuterium and tritium all have one proton and one electron. An atom of hydrogen has zero neutrons, while deuterium has one neutron and tritium has two neutrons.
J
Jules Horowitz Reactor
The Jules Horowitz Reactor (JHR) is a materials testing reactor (MTR) currently under construction at the Commisariat à l’Énergie Atomique et aux Énergies Alternatives (CEA) in Cadarache, France. JHR will be a major part of European nuclear infrastructure and will support both the existing international fleet of power reactors and serve for the qualification of future nuclear technologies.(1)
(1) CEA (2016, March 9). Jules Horowitz research Reactor (JHR). Retrieved from http://www.cea.fr/english/Pages/research-areas/nuclear-energy/jules-horowitz-research-reactor-JHR.aspx
K
Kilowatt hour (kWh)
One kWh is equal to 3600 kJ. It is one of the most common units for electricity production and consumption.
Nuclear power generated 2,563,000,000,000 kWh (2563 TWh (terawatt hour)) of electricity in 2018, equivalent to approximately 10% of the world’s total production (IAEA-PRIS, world-nulcear.org).
L
Lithium / Lithium hydroxide
In a pressurized water reactor (PWR), lithium, in the form of lithium hydroxide (LiOH) is added to the primary coolant in small quantities to stabilize the pH.
In water-water energetic reactors (VVER), potassium, in the form of potassium hydroxide (KOH), is used instead of LiOH.
Loss of Coolant Accident (LOCA)
An accident that results in the loss of reactor coolant. The emergency core cooling system is designed to mitigate this type of accident. In the case of an accident or reduced coolant, the power reactor’s emergency shut down system will shut down he reactor. However, due to the nuclear decay of the fuel, heat will continue to be generated for some time. Without additional cooling, the fuel can become overheated and damaged.
M
Main control room
The area (room) from which a nuclear power reactor is operated. The control room of a nuclear power reactor is critical to safe operation and is staffed 24/7. The control room has panels and displays, which are subdivided by system, alarms and annunciators, operator support systems, etc(1). Often a control room team is comprised of 3 people – two operators and one supervisor.
(1) IAEA (1995). Control room systems design for nuclear power plants, IAEA-TECDOC-812. Retrieved from https://inis.iaea.org/collection/NCLCollectionStore/_Public/27/002/27002051.pdf.
Materials testing reactor
Materials testing reactors (MTR) are essential to ensuring the operation of today’s fleet of power reactors and vital for the qualification of fuels and materials for nuclear applications. Of the over 200 research reactors in operation in the world, only 88 are used to perform materials irradiations [IAEA RRDB1].
1.International Atomic Energy Agency Research Reactor Database
N
Neutron
There are two neutrons (red “balls” in the illustration). A neutron is a sub-atomic particle that has no net charge, unlike protons (positively charged) and electrons (negatively charged).
Neutrons are essential for nuclear power and are at the heart of the nuclear chain reaction. When a nuclear fuel is bombarded with neutrons, it emits more neutrons than it absorbs. The emitted neutrons can then collide with additional nuclear fuel and the chain reaction continues!
Nuclear fuel
The fuel that sustains a nuclear chain reaction in a nuclear power plant (NPP). The most common nuclear fuels are Uranium-235 and Plutonium-239. Uranium-235 is produced in the nuclear fuel cycle. CANDU type NPPs can operate using only natural uranium (Uranium-238).
O
Operation
During normal operation, the power reactor is operating and generating electricity.
Outage
When a nuclear power plant (NPP) is shut down it is called an outage. Outages can be either forced or scheduled. Most NPPs have a scheduled yearly outage. These outages can vary in length and can include refueling, maintenance work and/or safety inspections.
P
Pressurized water reactor
There are currently 298 pressurized water reactors (PWR) in operation today [PRIS], making them by far the most popular and common reactor type. It should be noted that VVER (water-water energetic reactors) are included in this statistic.
In a pressurized water reactor, the primary water circuit heated in by the reactor core. It is then transported in the primary loop to the steam generator (SG). The heat from the primary side is transferred to the secondary side via the SG tubes. Boiling takes place on the secondary side surface of the SG tubes. This steam then flows and turns the turbine generator to produce electricity. The secondary side steam is recondensed to water by the cooling loop (or tertiary circuit).
Image credit – United States Nuclear Regulatory Commission
Pressurizer
The pressurizer is the vessel that controls the pressure of the coolant system in a pressurized water reactor (PWR). The pressure in the pressurizer is maintained by regulating its temperature. In the picture, a pseudo-cross section of the pressurizer can be seen. Heating rods are submerged in water, and above is a blanket of steam. Emergency pressure release valves can be seen on the top.
Q
Mass flux (Q)
Mass flux is defined as mass flow per unit area.
R
Reactor pressure vessel
The reactor pressure vessel (RPV) contains the reactor core, coolant and internal components. The RPV can be considered the most important and critical part of a nuclear reactor, as it cannot be replaced. RPVs are manufactured from low alloy ferritic steel. Today these vessels are cladded with a thin stainless steel lining.
S
Steam generator
In a pressurized water reactor (PWR), the steam generator (SG) is a large heat exchanger used to transfer heat from the primary circuit to the secondary circuit via the SG tubes. Boiling takes place on the external surface of the SG tubes on the secondary side. The SG tubes act as both a heat transfer medium and a barrier to radioactive elements present in the primary side. SG tube materials vary between reactor designs, but are often nickel base alloys (PWR) or stainless steel (VVER).
T
Turbine hall
The building or room where the equipment for generating electricity from a steam cycle is located.
U
Uranium
Uranium (U) is a radioactive element. Its most abundant isotope, U-238 (146 neutrons + 92 protons), referred to as “natural uranium”, accounts for over 99% of all uranium. U-235 and U-234 account for less than 1% of all uranium in nature. Most nuclear reactors use enriched uranium, usually up to 3% with U-235, as their fuel. CANDU reactors can operate using natural uranium.
The fissile properties of uranium were first discovered by Enrico Fermi. He observed that when you bombard uranium with neutrons, it emitted beta (β-) rays.
V
Void coefficient of reactivity
The void coefficient of reactivity is the change in reactivity per precent change in void volume. A positive void coefficient can be dangerous; for example in a reactor with a water moderator, the formation of steam bubbles (voids) or the loss of coolant would lead to an increase in reactivity.
Different reactor designs have different void coefficients. Both pressurized water and boiling water type reactors have negative void coefficients, while CANDU type reactors have a very small positive void coefficient. RBMK reactors have a positive void coefficient.
VVER (Water-Water Energetic Reactor)
Water-water energetic reactors, or VVERs, are a type of pressurized water reactor that was originally developed by the Soviet Union (now Russia). Some of the major differences between VVERs and “western style PWRs” are horizontal steam generators, differences in fuel assembly shape and the absence of bottom penetrations in the reactor pressure vessel. In addition to this, there are large differences in material choices in the design; for example steam generator tubes are fabricated from a stainless steel in a VVER, while in wester style PWRs they are fabricated in Nickel-base alloys.
W
Water vapor
Gaseous water.
X
Xenon Poisoning
When a reactor is running at very low power, Xenon (Xe) can build up in the fuel rods. This effectively “poisons” the reactor by absorbing neutrons. To bring the reactor power back up, and in order to compensate for this absorption, the control rods must be nearly completely withdrawn.
Y
Yellowcake
Yellowcake (U3O8) is produced during the processing of uranium ores. After processing, drying and filtering the resulting product is yellowcake. Today, yellowcake is not always yellow – it can be black or brown color, depending on the drying temperature. Yellowcake is further processed to produce nuclear fuel.
Z
Zirconium alloy fuel cladding
Many types of fuel cladding are fabricated from zirconium (Zr) alloys. Zirconium alloys are used for fuel cladding because of the low neutron absorbing cross-section(1) of zirconium and its good high temperature mechanical and corrosion properties. Fuel pellets are housed inside the thin-walled Zr-alloy fuel cladding tubes.
(1) Meaning it absorbs few neutrons.
C. Huotilainen 