Four Pathways to Fusion: Pros and Cons of Different Approaches to Nuclear Energy

Synthesis and fission: what's the difference?

Fusion and fission are different processes for producing nuclear energy.

Nuclear fusion aims at combining individualatoms into a larger one, and nuclear fission is based on the rupture of an atom (usually uranium-235) by hitting it with a neutron. Both processes release enormous amounts of energy, although fusion produces more.

This energy produced by nuclearfission, is captured inside reactors and used to heat water to steam, which turns a turbine and generates electricity. But this process creates waste that can remain radioactive for millions of years. The situation in the reactors at Fukushima and Chernobyl showed how this could turn out.

On the other hand, thermonuclear fusion will not leadto the formation of long-term nuclear waste, and the necessary materials can be recycled within 100 years. There is also no danger of an accident, because the process is based on high-temperature reactions that stop in a few seconds if the equipment malfunctions. And since these reactions use a relatively small amount of fuel, there is no danger of using them to produce nuclear weapons.

In the field of nuclear fusion research, there arescientists who strive for one goal - to recreate the processes that the Sun itself uses to produce a huge amount of energy. Enormous gravitational forces trap hydrogen from the sun's atmosphere and use intense heat and pressure to convert the gas into plasma. In it, nuclei collide at high speed, forming helium and releasing energy.

Another key factor is gravity.The sun's colossal gravitational forces are about 28 times greater than here on Earth. Scientists had to get creative with limiting the fuel for nuclear fusion reactions. The most preferred approach as it stands is to use magnetic fields that can be used to trap two heavy forms of hydrogen, deuterium and tritium, in a device called a tokamak.

Tokamak - high hopes

The word "tokamak" itself does not mean anything - it isjust an abbreviation, which later became a full-fledged word. It is used not only in Russia, but also abroad, since it was in our country that this thing was invented and it was here that they have been actively developing for a long time.

Tokamak is a toroidal chamber with magnetic coils. That's all.

The essence of the tokamak is to createthe magnetic field in which the thermonuclear fusion reaction will take place. Since the temperature of such a reaction is not just high, but literally prohibitive (several million degrees Celsius), it cannot be carried out simply inside a chamber - it will melt long before reaching the operating temperature.

This temperature is reached due to the fact thatinside the tokamak, the substance is in the fourth state of aggregation, which is achieved at such high temperatures. This condition is called plasma. On Earth, plasma in its natural environment is found only in lightning and the northern lights, in space literally everything consists of it - stars, nebulae, interstellar space.

Inside the building that will house the world's largest tokamak reactor.

The first to propose the use of thermonuclearsynthesis, including for industrial purposes, was the Soviet physicist O.A. Lavrentyev. He did this in his 1950 work. It was with his work that the study of ways to use thermonuclear fusion began.

A year later, other physicists - A.D. Sakharov and I.E.Tamm - developed the idea and said that a thermonuclear reaction should be maintained inside a closed toroidal chamber. To create a magnetic field inside the tokamak, it is composed of sections, inside which coils are wound. Since they run along the entire length of the chamber and create a kind of closed tunnel, the resulting magnetic field is called toroidal. This is the working area of ​​the installation.

Inside the China Experimental Advanced Superconducting Tokamak (EAST).

When the chamber is full, a vortex electric field is created in it, which maintains the plasma inside the chamber, and at the same time heats it up, bringing it to the same temperature of several million degrees.

Since the field and heating are created byincrease in the current in the inductor, and it cannot increase indefinitely, the time of existence of the plasma in a stable state does not yet exceed a few seconds. This is the main reason why we cannot yet use tokamaks as a source of industrial energy production. There are ways to solve this problem, including using microwave radiation, but so far work in this direction is still underway.

Many people working to achieve thisgoals, have high hopes for the next generation device that is currently under construction. The ITER, or International Thermonuclear Experimental Reactor, is one of the most ambitious energy projects ever undertaken by humankind, involving scientists and engineers from 35 countries. When completed in 2025, it will become the world's largest nuclear fusion device.

Although most of humanity's ambitions in the fieldnuclear fusion is associated with ITER, there are other exciting opportunities. Including another fusion reactor with magnetic confinement - a stellarator.

Stellator - imitating a star

Like tokamaks, stellarators are designed forconfinement of plasma flows in a closed chamber using magnetic coils, but with some key differences. Instead of a beautifully symmetrical donut shape, the stellarator sends out plasma in irregular circles that spin and spin using an incredibly complex series of magnetic coils. It seems counterintuitive, but it actually creates a lot of stability in the plasma due to the differences in the internal current, writes New Atlas.

By itself, a stellarator is a type of reactor forimplementation of controlled thermonuclear fusion. The name comes from lat. stella - star, indicates the similarity of the processes occurring in the stellarator and inside the stars. Invented by the American physicist L. Spitzer in 1950, the first prototype was built under his leadership the following year as part of the secret Matterhorn project.

Wendelstein 7-X (W7-X) - experimentalinstallation for research of high-temperature plasma, located in the city of Greifswald in Germany. Its construction was carried out by the Max Planck Institute for Plasma Physics from 2005 to 2014. The purpose of the facility is to test the industrial suitability of a stellarator-type fusion reactor, as well as to research and improve technical components and technologies in the field of controlled thermonuclear fusion.

In such fusion reactorsthe toroidal chamber has a complex tortuous shape, as well as the magnetic fields generated around it. Such an unusual design, according to the theory, helps such reactors operate in a continuous mode, spending much less energy on it than conventional "flat" tokamak reactors. In addition, the complex configuration of the magnetic field makes it possible to reliably contain the high-temperature plasma, which reduces the risk of its contact with the inner walls of the reactor chamber. However, such advantages come at a price, which is the complexity of the design and manufacture of the reactor itself.

In December 2017, W7-X completed the second stageexperiments using improved plasma heating and measurement systems. The main element that was tested at the second stage was a heat removal system, the operation of which should not affect the plasma filament. This instrument consists of a chain of magnets that generate complex magnetic fields, forcing the plasma to flow "tightly" to 10 heat dissipating plates. Any deflection of the magnetic fields entails deflection of the plasma filament, which leads to overheating of the plates and to excessive cooling and destruction of the plasma filament.

The work of electromagnets that create a magnetic field is controlled by a complex control system that uses a variety of feedbacks.

Since stellarators do not require so muchinternal current and inherently offer greater stability than tokamaks, Hole says they may well be better suited to power the grid. But this is on condition that the incredibly complex infrastructure can be built in an inexpensive way and with the same constraints, which is not the case in its current form.

Inertial hold - a radical approach

In addition to the magnetic confinement region,other approaches to nuclear fusion that fall under the so-called inertial confinement. Inertial controlled thermonuclear fusion is one of the types of thermonuclear fusion, in which thermonuclear fuel is held by its own inertial forces. The idea is to quickly and evenly heat the thermonuclear fuel, so that the resulting plasma has time to react before scattering. Thus, when using this principle, the reactor will be pulsed. This new line of research in nuclear fusion involves the use of very precisely targeted laser or ion beams to rapidly heat a fuel pellet that will be composed of deuterium and tritium.

The idea is that the impact on thesefuel pellets such a sudden and violent heating will cause enormous compressive forces that set off a chain reaction through the layers of material in which nuclear fusion can occur, releasing enormous amounts of energy. Australia-based HB11 Energy is looking to do away with the traditional deuterium and tritium recipe in favor of a non-radioactive approach that includes hydrogen and boron-b11.

Small pellet of hydrogen boron fuelplaced in a large sphere and hit by two lasers simultaneously to trigger a fusion reaction that directly generates electricity without the use of steam turbines.

The company claims to avoid many problems,which has plagued nuclear fusion for decades, largely because it doesn't try to heat its fuel to insanely high temperatures. It exposes its fuel pellets to two lasers, one to create a magnetic confinement field, and the other to trigger a hydrogen-boron chain reaction that creates particles that in turn can generate an electric current.

According to experts, this current can be almostdirect to the existing power system. There would be no need for a steam turbine generator or heat exchanger, and there would be no danger of melting. The team is very optimistic about their technology and claims that their experiments are responding a billion times better than predicted and believe their roadmap will be much faster and cheaper than other approaches.

The energy that is released as a result of suchreaction, is capable of heating the surrounding fuel and, if the temperature is high enough, it can also start a thermonuclear reaction. The purpose of such installations is the ability to achieve thermonuclear "combustion", when the process of heat release causes a link reaction that affects a significant part of the fuel. A typical ball of fuel is the size of a pinhead and contains about 10 milligrams of fuel. In practice, only a small fraction of this fuel can be used in a thermonuclear reaction, but if all of this fuel is used, it will release energy equivalent to burning a barrel of oil.

“This is an interesting science,” said Matthew Hole.nuclear fusion expert and Australian National University researcher familiar with HB11 and researchers working on the project at New Atlas - But I wouldn't say there is compelling evidence that you could turn this into a power plant on a faster scale than ITER or toroidal magnetic confinement ”.

According to Hole, the key problem with these approaches isis that these reactions happen in the blink of an eye. For a technology to be applied to a practical power plant, it must evolve from short-lived one-off reactions to something that produces a constant supply of energy, such as a burning fire.

Z-pinch - dark horse

Another interesting example of the inertial approachconfinement to nuclear fusion - Z-pinch. In fusion energy research, it is a type of plasma confinement system that uses an electric current in a plasma to create a magnetic field that compresses it.

In other words, instead of usinglarge and complex magnetic coils to hold plasma streams in place, this approach uses an electromagnetic field that is generated within the plasma itself. It can be said to be a dark horse in the nuclear fusion race. Since its inception in the 1950s, the pinch effect has promised a simpler configuration than tokamaks or stellarators. But like these devices, it is also susceptible to plasma instabilities that extend beyond the magnetic field lines.

Z-pinch retention system at the University of Washington

Scientists from the University of Washington have come up witha way to smooth out these bulges is by adjusting the plasma hydrodynamics. While a promising solution to the energy problem, like the HB11 laser approach, the Z-pinch still faces major challenges when it comes to delivering continuous power. The problem is that it is also impulsive in nature.

What's the bottom line?

ITER, toroidal magnetic confinement andtokamak designs are by far the most advanced. However, fusion researchers use different approaches, each with their own advantages and disadvantages. Unfortunately, regardless of the approach, the process still requires more energy than it returns. But solving these engineering and physical problems to generate clean energy, as incredibly difficult as it is, will be one of the greatest achievements of humanity.

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