GENERATION IV NUCLEAR POWER PLANTS

REACTOR TYPES AT A GLANCE

December 02, 2019

New nuclear power plants are set to revolutionise nuclear energy. Atomic plants using Generation IV nuclear reactors could indeed turn out to be safer and cheaper – and hopes are not being pinned on just one type of reactor. Currently, designs for eight possible reactors are in the forefront. However, most of them have only been developed on paper and have yet to be built. What’s going on here? An overview.

Please wait, the image is currently loading and will be there shortly.

Graphic: Polygraph Design

SIDE NOTES ON FUELS

Uranium: Uranium is a radioactive heavy metal that occurs naturally in the earth’s crust. In this form, it is not immediately suitable as nuclear fuel for electricity production or for the manufacture of nuclear weapons. For these purposes, it requires special processing (see “Enriched and depleted uranium”).

Enriched and depleted uranium: The most common isotope of the heavy metal uranium that occurs in nature is U-238 (approximately 99.27 percent); only 0.72 percent of naturally occurring uranium is U-235. The number signifies the particles, while the U is the abbreviation for the chemical element uranium. Unlike U-238, U-235 is fissile and can trigger a chain reaction that releases energy. Thanks to this characteristic, it can be used as fuel in nuclear power plants. Depleted uranium is the waste product of uranium enrichment. [36]

Plutonium: Plutonium is a radioactive heavy metal that occurs very rarely in nature. [37]Large quantities of plutonium are produced artificially, for example, in special nuclear reactors (fast breeder reactors) that use plutonium as fuel. This type of reactor is called a “fast breeder reactor” because it produces – “breeds” – its own fuel. There are only a few of these reactors in operation worldwide. The fission product plutonium-239 is suitable both for the production of an atomic bomb and for the generation of electricity in a nuclear power plant. [38][J1]

Thorium: This heavy metal is used in nuclear power plants both as a fuel and as fertile material, that is, material that is not itself fissile but which can be converted into fissile material by irradiation in a reactor. In nature, it occurs two to three times as often as uranium. [39]

[J1]An dieser Stelle fand ich den Originaltext unklar bzw. nicht deckungsgleich mit der Quelle https://www.bfs.de/DE/themen/ion/wirkung/radioaktive-stoffe/plutonium/plutonium_node.html. Dort heißt es: Plutonium entsteht hauptsächlich in Kernreaktoren, wenn Uran-238 in den Brennstäben einem Neutronenfluss ausgesetzt ist. Durch zivile und militärische Aktivitäten wurden bis heute weltweit etwa Tausend Tonnen Plutonium erzeugt. Plutonium wurde in größerem Ausmaß in Folge der oberirdischen Atomwaffentests, die während der 1950er und 1960er Jahre durchgeführt wurden, in die Umwelt abgegeben (sogenannter Fall-out). […] Es handelt sich dabei hauptsächlich um die Isotope Plutonium-239 und Plutonium-240. -> Das lese ich so: Plutonium-239 ist das Spaltpodukt von Uran-238.
Im Ausgangstext heißt es allerdings: Das Spaltprodukt von Plutonium 239PU ist sowohl geeignet für die Herstellung einer Atombombe als auch zur Stromproduktion im Kernkraftwerk.

[Vgl. auch Wikipedia: Für Kernwaffen geeignetes Waffenplutonium(englisch weapons grade Plutonium) muss möglichst viel ²³⁹Pu und möglichst wenig ²⁴⁰Pu enthalten. Ab einem Gehalt von etwa 92 % ²³⁹Pu gilt Plutonium als waffenfähig. Nach Ansicht der IAEO ist jedoch jedes Plutonium grundsätzlich geeignet für militärische Zwecke. Als Reaktorplutonium wird Plutonium bezeichnet, das im Normalbetrieb von Kernkraftwerken anfällt; es kann bis zu 31 % ²⁴⁰Pu enthalten. (https://de.wikipedia.org/wiki/...)]

MOLTEN SALT REACTOR (MSR)

More efficient, safer, and responsible for less or even zero nuclear waste – this is what researchers expect from the molten salt reactor. What makes it special is that the fuel – often a thorium mixture – is not cooled by water, but dissolved in liquid salt. In addition, it is envisioned that the reactor will reprocess spent fuel rods while it is operating so that these can be reused for energy generation. [1] This type of reactor is therefore a so-called “fast breeder reactor”, meaning that it not only consumes fuel but also produces new fuel simultaneously. On account of its reprocessing work, the reactor accumulates less nuclear waste, and the latter also radiates for a shorter period of time than non-reprocessed fuel rods. [2] In order to reduce the risk of overheating, the current design provides for a protective mechanism that allows the molten salt to drain off and cool down. [3] However, there’s also a downside: the weapons-grade material is not well protected. And there’s another challenge: in the case of a molten salt reactor from the 1950s – a reactor of an earlier generation – the salt caused material corrosion which resulted in a high level of radiation. [4]

Please wait, the image is currently loading and will be there shortly.

Graphic: Polygraph Design

VERY-HIGH TEMPERATURE REACTOR (VHTR)

This type of reactor operates at comparatively high temperatures (up to 950 degrees Celsius). [5] Instead of fuel rods, it runs with fuel pebbles or prismatic blocks. [6] Helium is used as coolant. All components that come into contact with the heated coolant are made of graphite, which also acts to slow down the neutrons. [7] In this reactor model, a core meltdown – that is, an accident caused by the overheating of the fuel – is deemed to be impossible. Even though the reactor is among those with the highest operating temperatures, it is designed in a way that prevents it from overheating: heat dissipation limits the temperature to an absolute maximum of 1,600 degrees Celsius, even in the event of a failure of the cooling systems. In addition, this type of reactor has a negative temperature coefficient of reactivity, meaning that the more the reactor heats up, the lower is its performance. [8] It produces hydrogen as well as electricity. [9] The high temperatures in a nuclear power plant can also generate process heat – waste heat that can be used for processes outside the power plant, for instance by non-power-generating industries. [10] According to the daily newspaper Der Tagesspiegel, the only German HTR – a reactor from an earlier generation – was shut down in Hamm in 1989 due to “serious technical defects” and inefficiency. [11]

Please wait, the image is currently loading and will be there shortly.

Graphic: Polygraph Design

GAS-COOLED FAST REACTOR (GFR)

This type of reactor owes its name to the helium cycle, which is heated to about 850 degrees Celsius, and the fast neutrons (fast electronic particles) involved. Helium serves as the coolant. What makes this reactor type special is that it reprocesses and reuses spent fuel on-site. [12] In addition to energy, it also produces hydrogen and generates process heat. [13]The design combines the fuel recycling process of the sodium-cooled fast reactor with the reactor technology and the particularly high temperatures of the very-high-temperature reactor. [14] Since 2000, the Generation IV International Forum (GIF) has been developing a design that focuses primarily on the safety concept and the reactor core. [15] It is expected to be ready for the market in 2035. [16]

Please wait, the image is currently loading and will be there shortly.

Graphic: Polygraph Design

SODIUM-COOLED FAST REACTOR (SFR)

The sodium-cooled fast reactor falls into the category of fast breeder reactors. In other words, this type of reactor breeds (produces) more fuel for electricity generation than it consumes – by converting uranium into plutonium through nuclear fission. In this way, it provides in part for its own fuel supply. Liquid sodium is used to cool the reactor core. This means that the reactor can be operated at low pressure and high performance density (thermal power per cubic metre of reactor core). [17] In the event of a leakage, the flammable sodium streams out slowly, instead of suddenly, and solidifies. [18] In this way, potential fires are meant to be extinguishable and controllable. In the past, most of this basic technology has already been put to test in other types of fast breeder reactors. [19] A nuclear power plant of this type is currently being built in India and is scheduled to go into operation this year. [20]

Please wait, the image is currently loading and will be there shortly.

Graphic: Polygraph Design

LEAD-COOLED FAST REACTOR (LFR)

This type of reactor was invented back in Soviet times [21] and was used in submarines. The Generation IV International Forum is currently conducting research to take the development of this reactor type a step further. Unlike other concepts for nuclear power plants, the LFR uses a lead-bismuth alloy for cooling the fuel rods. In contrast to sodium, this heavy metal mixture cannot start fires. However, lead can cause corrosion to the cooling system. [22]Since the alloy is rather heavy-weight, the nuclear power plant must be specially secured against earthquakes. [23] The advantage of this type of reactor is that if the reactor core is tightly sealed, it could produce energy for 15 to 20 years at a stretch. In addition to electricity, the power plant also generates process heat and hydrogen. [24

Please wait, the image is currently loading and will be there shortly.

Graphic: Polygraph Design

SUPERCRITICAL WATER-COOLED REACTOR (SCWR)

At a temperature of 500 degrees Celsius and a pressure of more than 205 bar, the water in this type of reactor serves as coolant and moderator – its high density slowing down the neutrons. Just to put it into context: one bar is approximately equal to the air pressure on the earth’s surface.

So-called light water, which is what is referred to in the reactor’s name, conducts the heat of the chain reactions, slowing down the free neutrons and even absorbing some of the latter. U-235 (uranium with 235 particles) is used as fuel. The reactor is considered to be particularly efficient and easy to build. [25] One of its advantages is that it can produce hydrogen as a by-product. It is also said to be more cost-efficient than an “ordinary” light-water reactor. [26]

TRAVELLING WAVE REACTOR (TWR)

This type of reactor is based on a concept dating back to the 1950s: [27] it uses spent fuel rods – that is, nuclear waste – and depleted uranium-238 as fuel, thus closing the fuel cycle. “Theoretically, this type of reactor could operate for an entire century,” says John Gilleland, Chief Technical Officer of TerraPower LCC, who is in charge of the company’s nuclear innovation programme. [28] Since the reactor independently breeds new fuel and keeps it under lock and key, it would be quite difficult to steal the uranium for nuclear weapons. [29]However, this type of reactor requires liquid sodium as coolant, which is considered to be highly flammable and therefore potentially problematic. [30] In the mid-2020s, the US company TerraPower plans to introduce its first operational prototype onto the market – a simulator exists already. [31] The latter enables researchers to study the operation of the reactor in theory.

SMALL MODULAR REACTORS (SMR)

Reactors with a maximum electrical power of 300 megawatts are considered small reactors. [32]For comparison: a medium-sized nuclear power plant has a capacity of about 1,400 megawatts. [33] Modular systems consist of individual reactor modules, which can also be combined to form a larger plant. They are “standardised” and produced in factories as complete units. This allows them to be transported to the desired locations, where they can be set up swiftly; and, compared to non-modular nuclear power plants, they are cheaper and can be manufactured quickly. [34] According to Germany’s Oeko-Institut, one of Europe’s leading independent research and consultancy organisations, of the more than 45 SMR concepts with different reactor types, the modular system using light water as coolant is the easiest to implement. After all, this concept ties in with already existing developments. Small modular reactors run on low-enriched uranium as fuel and produce “basically the same amount of radioactive” waste as current nuclear power plants. [35]

SIDE NOTES ON NUCLEAR POWER PLANTS

WHAT IS A REACTOR?

A reactor is the container in which nuclear fission takes place. There is a wide range of reactor models with different designs. They differ, among other factors, with regard to their cooling systems, fuel elements and sizes.

WHAT IS A NUCLEAR CHAIN REACTION?

In a nuclear reactor, the fuel – usually uranium – is bombarded with neutrons to trigger nuclear fission, releasing thermal energy and more neutrons in the process. The free neutrons cause more fission that produces further fuel. This process continues automatically as a chain reaction.

HOW DOES A NUCLEAR POWER PLANT WORK?

To put it simply, nuclear power plants are extremely large water boilers that drive turbines by means of steam and generate electricity in the process. First, a fuel, usually uranium, is used to trigger a chain reaction that releases energy. This process heats up the fuel rods containing the uranium, which, in turn, give off their heat to the liquid in which they are located. The steam produced in the process is used to drive turbines (large wheels). The rotary motion generates electricity. To see how it works in detail, click here:

Please wait, the video is currently loading and will be there shortly.

HOW IS A NUCLEAR POWER PLANT CONSTRUCTED?

A nuclear power plant consists of two parts: in the reactor’s central part (containment building), the reactor core, consisting of fuel elements, is located in a pressure vessel. The fuel elements contain thin fuel rods with fuel, mostly uranium. However, they can also contain plutonium or thorium. In this hermetically sealed space, nuclear fission takes place, generating heat in the process. This heat is fed by the condenser in the secondary side – known as the conventional side – into a closed water circuit. The water heats up. The resulting steam sets the turbine in motion, which in turn drives a generator that eventually produces electricity. The vapour is then cooled to water in a condenser – by a third water circuit – and can then be reused as cooling water. Most types of reactor use river water as coolant. Finally, the heated water from the third water circuit rises through the cooling tower and is emitted into the atmosphere as pure water vapour.

SOURCES AND FURTHER LITERATURE