Types of Nuclear Energy: Where We Were and Where We are Today

To determine the trajectory of nuclear power we need to appreciate where we have been and where we are now. We cannot understand the issues surrounding nuclear energy without knowing something about its history, who is using it, how it works, and the basics of different reactor designs.

At the dawn of the atomic age, nuclear energy was touted as a source of almost magical wonders, however, over the last 30 years, it has fallen into disrepute. More recently we are seeing a resurgence of interest in nuclear power in response to the burgeoning demand for clean and reliable electricity. Nuclear energy is a critical part of a suite of technologies that can help to address the civilization-ending threat of climate change. Nuclear power is an emissions-free energy source that can help us to transition away from fossil fuels which are the leading cause of global warming.

Negative public perceptions regarding the dangers of nuclear energy are losing steam thanks to decades of data. The facts reveal that nuclear energy is both clean and safe. While nuclear waste is a problem, solutions are at hand. Negative perceptions of nuclear energy are attributable to 7 accidents that have occurred in more than 8 decades. While only 3 of these incidents leaked radiation, there are thousands of fossil fuel accidents every year, many of which have resulted in significant loss of life. In addition to cataclysmic explosions, accidents, leaks, and spills, millions have died due to air pollution generated by fossil fuels. Nuclear energy offers us the best chance we have of replacing fossil fuels in a timely fashion.

Nuclear power addresses the energy issues that are at the core of the climate crisis. In addition to the decarbonization potential of nuclear energy, the surge of interest is being buoyed by Russia’s war in Ukraine and the resultant rising price of hydrocarbons. A growing chorus is calling this an energy crisis. Nuclear power is attracting unprecedented investor interest in response to increasing demand, declining costs, safer designs, and the prospect of virtually unlimited power from fusion reactors. This constellation of factors is behind what many are predicting will be a nuclear renaissance.

While renewables like solar and wind are clean and safe, they are insufficient on their own. So rather than being competing solutions, nuclear power and renewable energy should be seen as complementary. Both are essential parts of the clean energy mix required to combat the climate crisis.

According to the UN, we must halve emissions by 2030 and zero them out by 2050 if we are to keep temperatures below the upper threshold limit of 1.5 – 2 degrees Celsius above preindustrial norms. Countries are not living up to their carbon reduction pledges and even if they were to honor these pledges it would not be enough to keep temperatures below the prescribed limits. Last October the International Energy Agency’s World Energy Outlook clearly stated that countries are not transitioning to clean energy fast enough to zero out emissions by the middle of the century. If we are to get to net-zero emission by 2050, we will need to ratchet up our use of carbon-free power.

Here is a brief history and breakdown by country of the leading users of nuclear energy followed by a simple explanation of how works and a summary of reactor types. This includes fourth-generation reactors led by small modular reactors (SMR), high-temperature gas-cooled reactors, fast reactors (sodium, gas-cooled, lead-cooled) molten salt-fueled reactors, thorium fission MSR reactors, and laser-generated aneutronic fusion reaction.

History of nuclear energy

Nuclear energy first entered public consciousness with the horrors of the atomic bombs that were dropped on Hiroshima and Nagasaki in 1945. Nuclear power generation started in 1938 when Hahn, Strassman, Meitner, and Frisch succeeded in bombarding uranium atoms with neutrons causing the heavy nuclei to split (fission) into two fragments emitting energy.

The first generation of reactors were fission prototypes produced in the late 1940s, ’50s, and early ’60s. The second generation was commercial light water reactors that were built from the mid-1960s to the mid-1990s. Third-generation reactors introduced more advanced features, including more reliable fuels, passive cooling systems, and reactor cores that were less prone to failure. Generation three + reactors have additional improvements and will be built into the 2030s. There are also fourth (and fifth) generation reactors that are both less expensive and safer. These reactors are differentiated largely by their use of coolants Coolants (CO2b , water, helium, liquid sodium, molten salt, lead, lithium fluoride, beryllium fluoride salts) and fuels (uranium, thorium, zirconium) and the neutron spectrum they operate (thermal neutron spectrum or the fast neutron spectrum).

The first nuclear reactor went online in 1942 but nuclear energy only became a serious source of power in the 1970s when the Middle East oil crisis caused prices to skyrocket. More than half of all the nuclear reactors in the world were built in the years between 1970 and 1985. That means that most of the nuclear reactors working today were built more than 25 years ago.

Use of nuclear power by country

Share of Nuclear Power in the Energy Mix Ranked by Country

Currently, the 30 countries operating 450 nuclear reactors generate 10 percent of the world’s electricity or about 4 percent of the global energy mix. Nuclear power generation is highly concentrated with the top 15 countries producing 91 percent of global nuclear power. In terms of percentage of the energy mix, 14 of the top 15 nuclear power-producing countries are in Europe. More than 80 percent of the world’s nuclear power comes from OECD countries.

The countries that produce the most nuclear power are the United States, China, France, and Russia. The U.S. operates 96 nuclear plants generating 789,919 gigawatts (GWh) of energy for a 30.9 percent share of the global energy mix. China operates 50 nuclear plants generating 344,748 GWh of energy for a 13.5 percent share of the global energy mix. France operates 58 nuclear plants, generating 338,671 GWh of energy for a 13.3 percent share of the global energy mix. Russia operates 39 nuclear plants generating 201,821 GWh of energy for a 7.9 percent share of the global energy mix. At 70 percent France is the global leader in terms of the percentage of energy it generates from nuclear compared to other sources.

Since the peak in the 1990s, the production of nuclear energy has slowed substantially according to the International Atomic Energy Agency (IAEA). Nuclear’s share of the world’s electricity production fell from 17.5 percent in 1996 to 10.1 percent in 2020 Rethink Energy reports. The slowing trend has ended and we are witnessing rapid and accelerating growth.

In just over two decades the UK has doubled its nuclear energy capacity. Nuclear power supplied around 21 percent of the U.K.’s energy needs in 2020, up from 9.4 percent in 2000. With more than 30 percent of the global reactors under construction, China is building more nuclear plants than any nation on Earth. China plans to build at least 150 new reactors in the next 15 years and according to Bloomberg, that is more than the rest of the world has built in the past 35 years.

The growth of nuclear energy is being driven by the interest in small modular reactors (SMR). Russia, China, the US, the UK, Denmark, Argentina, Canada, and India are all working on SMRs. The Danish firm Seaborg Technologies is building floating SMRs and Russia has already deployed two barge-mounted SMRs in the Arctic. At the start of this year, China powered up a 200-megawatt SMR that is the world’s first pebble-bed modular high-temperature gas-cooled reactor (instead of heating up water, it heats helium to produce energy) and Argentina will soon deploy its own SMRs.

The UK’s Rolls-Royce is also developing SMRs that are slated to be up and running by 2029 and Oregon-based NuScale signed an agreement with Romania’s Nuclearelectrica to build SMRs in Europe. According to the IAEA, more than 70 SMR concepts are currently under development in 18 countries with 21GW of SMRs to be added globally by 2035.

How does nuclear energy work?

Nuclear power is produced by unlocking the energy held in the nucleus of an atom. It can be obtained by two types of reactions, fission and fusion. Nuclear fission is the type of nuclear power in operation around the world today, it produces energy by bombarding uranium with neutrons that split atoms releasing heat energy. The heat from the energy created by the fission reaction is then harnessed to produce electricity (steam turns a turbine, powering a dynamo). The most commonly used fuel for the production of nuclear energy in fission reactors is uranium, however, other radioactive metallic elements can also be used as fuel in atomic reactors (eg plutonium and thorium).

How do nuclear fusion reactors work?

While fission splits the nucleus of an atom, fusion creates energy by forcing two nuclei together causing them to join and form a larger heavier atom. This is the same process that powers our Sun. During this process, some of the matter of the fusing nuclei is converted to photons which are used to produce energy.

Fusion power offers the prospect of an almost inexhaustible source of energy, that produces less radiation and waste than conventional reactors. However, creating the conditions for nuclear fusion has proven to be a scientific and engineering challenge. Keeping positively charged hydrogen isotopes very close together is at the heart of this challenge. Nonetheless, fusion reaction could be a game-changer in our pursuit of an emissions-free power source.

“It’s clear we must make significant changes to address the effects of climate change, and fusion offers so much potential,” said Ian Chapman, CEO of the UK’s Atomic Energy Authority (UKAEA). “We’re building the knowledge and developing the new technology required to deliver a low-carbon, sustainable source of baseload energy that helps protect the planet for future generations. Our world needs fusion energy.” Chapman said.

We have lived with the looming promise of nuclear fusion for decades, but last year saw some big advances that bring us closer than ever to the realization of this dream. The DOE’s Lawrence Livermore National Laboratory generated 10 quadrillion watts of power, albeit for only 100 trillionths of a second. What makes this especially noteworthy is that this demonstrated a fusion reaction in which more energy was generated from the process than was required to initiate it. Getting more energy out than we put in is the key to making fusion viable. In December, EUROfusion’s ‘tokamak’ generated 59 megajoules of sustained nuclear fusion energy for over five seconds.

Also in 2021, China’s Tokamak achieved fusion reactions for 17 minutes at 70° million° Celsius (126 million degrees Fahrenheit), which is five times hotter than the sun. France is building the world’s largest fusion reactor known as the International Thermonuclear Experimental Reactor (ITER).

What everyone is hoping for is something called ignition which is described as a tipping point/feedback process between heating and fusion. The trick is to keep the reaction under control while sustaining the reaction before temperatures approximating 150 million degrees Celsius cause the magnetic field to collapse.

While nuclear fusion has been experimentally demonstrated, there are currently no large-scale fusion reactors operating at a commercial scale. Nonetheless, we are closer than we have ever been to realizing the dream of virtually inexhaustible power.

Types of nuclear reactors

One of the ways that reactors are differentiated is by their cooling materials (water, helium gas, liquid sodium, lead, fluoride, molten salt) and the fuels that they use (uranium, uranium oxide, uranium dioxide, uranium nitride, uranium oxide, thorium, metallic alloy of uranium and zirconium and molten salt). Here are the five major types of nuclear reactors: identified by their coolants and or fuels.

Magnox advanced gas reactor: These are descendants of the original 1942 design, and they were built in Britain from 1956 to 1971. They use blocks of graphite as a moderator and CO2 as a coolant.

Pressurized water reactor: Developed in the U.S. to power submarines, this is the most common type of reactor. In this design, water is used as both a moderator and a coolant. The Three Mile Island plant was an older version of this design.

Boiling water reactor: This is a simpler, and more dangerous version of the pressurized water reactor. Water in the coolant loop is boiled and the steam directly turns a turbine, then the water recondenses and is returned to the reactor. The Fukushima plant used this design.

Reactor boiling light water: This Soviet-designed reactor is like the Magnox in that water is circulated under pressure and used to generate steam which turns a turbine. This is the design of the Chornobyl plant.

Heavy water reactor (CANDU): This Canadian-designed pressurized water reactor uses heavy water (hydrogen atoms in water are replaced by deuterium) and consequently it requires less enriched fuel. It is known for being more stable and easier to control and it is the only major reactor type that has yet to have an accident. There are 34 CANDU reactors globally, 30 of which are operable. Half of these reactors are located in Argentina, China, India, Pakistan, Romania, and South Korea.

types of nuclear reactors — Image credit: World Nuclear Association

Fourth and fifth-generation reactors

Here are ten different types of fourth-generation reactors, although this is far from the complete list of reactors that are being explored, it does include the major types of reactors that appear to hold the most promise. The most promising next-generation nuclear technologies are the smaller reactors and the fusion reactors.

Small modular reactors (SMRs) have a capacity of less than 300 MWe each, compared to up to 1,600 MWe for large reactors. SMRs are garnering the lion’s share of interest in new nuclear power. That is because they are safer, half as expensive, and faster to build than full-size reactors. Their small size also gives them a wide range of applications. SMRs can bring atomic energy (as well as heating and hot water) to people wherever they happen to be, this is particularly valuable in rural or hard-to-reach areas.

Portable nuclear microreactors: Even smaller than SMRs, this class of reactor has all the benefits of SMRs and they are even more portable. They are limited by the fact that they can generate no more than 20 megawatts of power. California-based Radiant is developing low-cost, portable nuclear microreactors (as small as 1 MWe) that can provide electricity and heat in remote communities, disaster areas, and bases.

High-temperature gas-cooled reactor is a graphite-moderated helium-cooled reactor that operates at temperatures two or three times those of conventional reactors, but unlike conventional reactors, they operate with a lower power density. The uranium fuel is encased in carbon or ceramics making it far less likely to cause a meltdown.

Fast reactors use nuclear waste as fuel. The concept has been proven by U.S. government research from the 1960s through the 1990s. According to Jess C. Gehin at the Idaho National Laboratory, the nuclear waste in the U.S. could power the entire country for a century. These reactor types typically use liquid metal coolants (helium gas, liquid sodium, lead) instead of water and they are fueled by (thorium, uranium, and zirconium).

Gas-Cooled Fast Reactors are helium-cooled but operate at a higher power density than conventional reactors. This is a type of breeder reactor that produces more fuel than it burns. It converts thorium or non-fissile uranium isotopes into plutonium or fissile uranium isotopes with fast neutrons. One version of this reactor uses ceramic uranium mono-carbide fuel to allow it to operate at high temperatures.
The Sodium Fast Reactor is cooled by liquid sodium, which has very good heat removal capability. These are small reactors are fueled by either a metallic alloy of uranium and zirconium clad in steel or uranium oxide. These fuels are recycled in the core so they can run for decades between refueling. It also has low thermal density, so if the reactor core gets too hot, it expands, causing the nuclear reaction to naturally shut down.
The lead-cooled fast reactor uses lead as its cooling element. It is fueled by uranium dioxide and more recent versions use uranium nitride. Like the sodium reactor if it begins to go out of control it will naturally shut down.

Molten salt-fueled Reactor (MSR) uses molten salt as both the coolant and the fuel. It is mixed into the fluoride salt, which flows through graphite or a similar moderator that generates slow neutrons and controls the reaction.

Fluoride-cooled high-temperature reactors are cooled by a molten mixture of lithium fluoride and beryllium fluoride salts. These reactors have a very high-power density.

Thorium nuclear reactors. China, France, India, Japan, Norway, and the U.S. are all working on thorium-powered nuclear reactors. Researchers at Oak Ridge National Laboratory pioneered a similar reactor in the 1950s. A Chinese thorium reactor prototype uses molten (fluoride) salt as a coolant.

Gas-cooled reactors. Helion is developing a fusion variant of the high-temperature gas-cooled reactor. Helion is the first private fusion company to heat a fusion plasma to 100 million degrees Celsius. The Helion system is waterless and uses deuterium and Helium-3 which are heated, accelerated through magnets, compressed, and captured as inductive current. Helion’s seventh-generation fusion generator is called “Polaris” and it is a shipping container-sized reactor that delivers industrial-scale power of around 50 megawatts of electricity.

Magnetized target fusion. The UKAEA is partnering with General Fusion to build a demonstration prototype fusion power plant which is scheduled to be completed in 2025. The process injects hydrogen plasma into a sphere of molten lead-lithium surrounded by pistons. The pistons compress the hydrogen until its atoms slam together and fuse to form helium releasing heat that boils water and turns a turbine that produces electricity. The plant will use hydrogen as a fuel and generate helium waste.

Laser generated aneutronic fusion reaction. This non-thermal laser nuclear fusion uses high power, like other forms of fusion this process involves combining two or more elements, but rather than eating hydrogen isotopes, this process involves high precision lasers to start an aneutronic fusion reaction between hydrogen and boron-11. It is safe and abundant and generates very little waste.

These technologies hold tremendous promise; however, given that we have limited time to decarbonize our energy mix, we need to start by building tried and tested reactors like modified CANDU designs. We also need to develop, refine and assess the viability of newer technologies. We have to massively increase the supply of emissions-free power, so national governments will need to make quick yet judicious decisions about which nuclear technologies they will deploy.

Related