Fusion energy

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The Sun is a natural fusion reactor.

The fusion energy is the energy released when a nuclear fusion reaction takes place. In this type of reaction, two light atomic nuclei fuse to form a heavier nucleus, releasing a large amount of energy in the process, which can be used in the hydrogen bomb and in the future in the production of electrical energy in a hypothetical reactor. Most of the existing studies for the design of a fusion nuclear power plant use fusion reactions to generate heat, which will drive a steam turbine, which in turn will activate the generators to produce electricity, similar to what currently occurs in the thermal power plants that use fossil fuels or in fission nuclear power plants, but with the great advantage that the environmental impact will be considerably less since, for example, half a kilo of hydrogen (very abundant in nature, since it is part of the water) would produce about 35 million kilowatt hours.

Research on fusion reactors began in the 1940s, but to date, no design has produced more fusion power output than electrical power input. The largest current experiment is the Joint European Torus (JET). In 1977 the JET produced a peak of 16.1 MW of fusion power (65% of the power supplied) with a power of more than 10 MW sustained for more than 0.5 s. In June 2005, the construction of the ITER experimental reactor was announced, designed to continuously produce more fusion energy than the energy supplied to it in the form of plasma.

Fusion processes require fuel and a confined environment with sufficient temperature, pressure, and confinement time to create a plasma in which fusion can occur. The combination of these figures that results in an energy-producing system is known as Lawson's criteria. In stars, the most common fuel is hydrogen, and gravity provides extremely long confinement times that reach the conditions necessary for the production of fusion energy. Proposed fusion reactors typically use heavy hydrogen isotopes such as deuterium and tritium (and especially a mixture of the two), which react more easily than protium (the most common hydrogen isotope), in order to meet the criteria requirements. Lawson with less extreme conditions. Most designs claim to heat their fuel to around 100 million degrees, which is quite challenging in producing a successful design.

As a source of energy, nuclear fusion is expected to have many advantages over fission. These include less radioactivity during operation and little high-level nuclear waste, ample fuel reserves, and increased safety. However, the necessary combination of temperature, pressure and confinement time has proven difficult to produce practically and economically. A second problem that affects common reactions is the management of the neutroness released during the reaction, which over time degrades many common materials used within the reaction chamber and causes their activation, generating radioactive waste.

Fusion researchers have studied various concepts of confinement. At first emphasis was placed on three main systems: z-pinch, stellarator and magnetic mirror. The main current designs are the tokamak and laser inertial confinement. Both designs are being investigated on a very large scale, especially at ITER in France, where a tokamak is being built, and at the National Ignition Facility (NIF) in the United States, where laser inertial confinement is being investigated. The researchers are also looking at other designs that may offer cheaper approaches. Among these alternatives, there is growing interest in magnetized target fusion and inertial electrostatic confinement, as well as new variations of the stellarator.

Description of nuclear fusion

Main article: Nuclear Fusion

The Sun, like other stars, is a natural fusion reactor, where star nucleosynthesis transforms lighter elements into heavier elements with energy release.

Link energy for different atomic nuclei. The iron-56 has the highest, so it is the most stable. Cores on the left usually release energy when they merge (fusion); those at the right end are usually unstable and free energy when they divide (fision).

Mechanism

Fusion reactions occur when two or more atomic nuclei come close enough for long enough for the nuclear force that attracts them to overcome the electrostatic force that repels them, fusing into heavier nuclei. For nuclei heavier than iron-56, the reaction is endothermic, requiring an input of energy. Nuclei heavier than iron have many more protons, resulting in a stronger repulsive force. For nuclei lighter than iron-56, the reaction is exothermic, releasing energy as they fuse. Because hydrogen has a single proton in its nucleus, it requires the least effort to achieve fusion and produces the most net energy. Also, because it has one electron, hydrogen is the easiest fuel to ionize.

The repulsive electrostatic interaction between nuclei operates at greater distances than the strong nuclear force, which has a range of about a femtometer, that is, about the diameter of a proton or neutron. For fusion to start, the fuel atoms must be given enough kinetic energy to get close enough for the strong force to overcome the electrostatic repulsion. The "Coulomb barrier" is the amount of kinetic energy required for the atoms to overcome this electrostatic repulsion. Atoms can be heated to extremely high temperatures or accelerated in a particle accelerator to reach this energy.

Cross section

The speed of the fusion reaction increases rapidly with temperature until it reaches its maximum value and then gradually decreases. The melting rate of deuterio-tritio reaches its maximum to a lower temperature (about 70 keV, or 800 million kelvin) and to a higher value than other commonly considered reactions for fusion energy.

The effective section of a reaction, usually represented as $sigma$ , measures the likelihood of a fusion reaction. It depends, among other factors, on the relative speed of the two cores. The higher relative speeds usually increase the probability, but the probability begins to decrease again to very high energies.

In a plasma, the velocity of the particles can be characterized by a probability distribution. If the plasma is thermalized, the distribution resembles a Gaussian curve, or Maxwell-Boltzmann distribution. In this case, it is useful to use the mean cross section of the particles over the velocity distribution. This is entered into the volumetric melt rate:

{displaystyle P_{text{fusión}}=n_{A}n_{B}langle sigma v_{A,B}rangle E_{text{fusión}},}

where:

${displaystyle P_{text{fusión}}}$ is the energy made by fusion, by time and volume;
$n$ is the numerical density of each species A or B, that is, the amount of particles per volume unit;
${displaystyle langle sigma v_{A,B}rangle }$ is the cross-section of that reaction, averaging over all the speeds of the two species A and B;
${displaystyle E_{text{fusión}}}$ is the energy released by that fusion reaction.

An atom loses its electrons when it is heated above its ionization energy. The resulting nucleus, now with a positive electrical charge, is called an ion. The result of this ionization is the plasma, which is a cloud made up of the ionized nuclei and the free electrons that were previously bound to them. Since both components are electrically charged, despite the fact that the total charge is neutral, plasmas are electrical conductors and have magnetic properties. This is used by various melting devices to confine the hot particles.

Lawson's Criterion

Lawson's criteria show how power output varies with temperature, density, and collision velocity for any given fuel. This equation was fundamental to John Lawson's analysis of fusion working with a hot plasma. Lawson assumed the following energy balance:

{displaystyle P_{text{out}}=eta _{text{captura}}left(P_{text{fusión}}-P_{text{conducción}}-P_{text{radiación}}right)}

where:

${displaystyle P_{text{out}}}$ is the net fusion power;
${displaystyle P_{text{captura}}}$ is the capture efficiency of the merger output;
${displaystyle P_{text{fusión}}}$ is the energy rate generated by fusion reactions;
${displaystyle P_{text{conducción}}}$ are thermal conduction losses as plasma loses energy;
${displaystyle P_{text{radiación}}}$ are radiation losses when the energy comes out in the form of electromagnetic radiation.

Clouds of plasma lose energy by conduction and radiation. Conduction occurs when ions, electrons, or other particles present in the plasma collide with other substances, usually the surface of the device where the reaction occurs, a process by which which transfer a part of their kinetic energy. Radiation is the energy that leaves the cloud in the form of an electromagnetic wave. Radiation losses increase with temperature.

Triple trouble: density, temperature, time

Reliance (left) against temperature (low) for several fusion approaches starting from 2021, assuming DT fuel

Lawson's criteria hold that a machine maintaining a quasi-neutral, thermalized plasma must generate enough power to overcome its energy losses. The amount of energy released in a given volume is a function of the temperature and therefore of the rate of reaction per particle, of the density of particles within that volume and, finally, of the confinement time, that is, the time of confinement. time the energy remains inside the volume. This is known as the "triple product": the density of the plasma, the temperature and the time of confinement..

In magnetic confinement, the density is low, on the order of a "good vacuum". For example, in the ITER device the fuel density is approximately 1.0 × 10¹⁹ m^-3, which is about a millionth of the atmospheric density. This means that the temperature and/or confinement time must be high to sustain the fusion reaction. Melting-relevant temperatures have been reached using a variety of heating methods that were developed in the early 1970s. In modern machines, the main remaining problem is confinement time. Plasmas in strong magnetic fields are subject to a number of inherent instabilities, which must be suppressed to achieve useful confinement times. One way to do this is to simply increase the volume of the reactor, which reduces the leak rate due to classical diffusion; that's why ITER is so big.

In contrast, inertial confinement systems approach useful triple product values through higher density, and have short confinement intervals. In NIF, the initial charge of frozen hydrogen fuel has a density less than that of water increasing to about 100 times the density of lead. Under these conditions, the fusion rate is so high that the fuel melts in the microseconds it takes for the heat generated by the reactions to cause the fuel to explode. Although the NIF is also large, this is due to the design of the laser used, not an inherent requirement of the fusion process that takes place there.

Energy Capture

Multiple approaches have been proposed to capture the energy produced by fusion. The simplest is to heat a fluid. The D-T reaction, which is usually the target, releases much of its energy in the form of fast-moving neutrons. The neutron, electrically neutral, is not affected by the confinement system. In most designs, it is captured in a layer of lithium that surrounds the reactor core called the breeding blanket or regenerative blanket. When hit by a high-energy neutron, the mantle heats up. It is then actively cooled with a refrigerant that transports the heat to a turbine where power is produced.

Another design proposed using neutrons to produce fission fuel in a blanket of nuclear waste, a concept known as hybrid fission-fusion. In these systems, the power output is boosted by fission events, and energy is extracted using systems like those of conventional fission reactors.

Designs using other fuels, particularly the proton-boron aneutron fusion reaction, release much more energy in the form of charged particles. In these cases, energy extraction systems based on the movement of these charges are possible. Direct power conversion was developed at Lawrence Livermore National Laboratory (LLNL) in the 1980s as a method of maintaining a voltage using the products of the fusion reaction directly. This has shown an energy capture efficiency of 48 percent.

Behavior of plasma

Plasma is an ionized gas that conducts electricity. Bulk is modeled using magnetohydrodynamics, which is a combination of the Navier-Stokes equations that govern fluids, and Maxwell's equations that govern how the magnetic and electric fields behave. Fusion takes advantage of several properties of plasma, including:

The self-organized plasma leads electric and magnetic fields. Their movements generate fields that in turn can contain it.
Diamagnetic plasma can generate its own internal magnetic field. This can reject an externally applied magnetic field, making it diamagnetic.
Magnetic mirrors can reflect plasma when moving from a low to high density field.²⁴

Methods

Enfoques of the fusion, in families codified by colors: Pinch Family (orange), Mirror Family (red), Cusp Systems (violet), Tokamaks and Stellarators (green), Plasma Structures (gris), Inertial Electrostatic Confinement (dark yellow), Inertial Confinement Fusion (ICF, blue), Pink Magneto (Black).

Magnetic confinement

Main article: Magnetic confidence

Tokamak: the most developed and most funded method. This method circulates hot plasma in a magnetically confined toroid median external electromagnetic coils and by induction of an internal current. When finished, the ITER will become the largest tokamak in the world. In September 2018 it was estimated that there were 226 experimental tokamaks planned, closed or operational (50) worldwide.
Spherical Tokamak: also known as spherical bull. A variation of tokamak with spherical shape.
Stellarator: In a stellarator attempts to create a natural path of plasma, using external magnets, avoiding the induction of a current like in the tokamaks, but at the cost of a greater complexity of the magnetic coils. Them stellerators were developed by Lyman Spitzer in 1950 and evolved into four designs: Torsatron, Heliotron, Heliac and Helias. Examples of them are the Wendelstein 7-X, a German device that is the stellerator the largest in the world, and the TJ-II located at CIEMAT in Madrid.
Internal Rings: Stellarators create a twisted plasma using external magnets, while tokamaks do so using a plasma-induced current. Several types of designs provide this torsion using conductors within the plasma. The first calculations showed that the collisions between plasma and conductor supports would eliminate the energy faster than the fusion reactions could replace it. Modern variations, including the Levited Dipole Experiment (LDX), use a solid superconductor toroid that is magnetically levitated into the reactor chamber.
Magnetic mirror: Developed by Richard F. Post and LLNL teams in the 1960s. Magnetic mirrors reflect the plasma from one side to another in one line. The variations included the Mirror in tandem, the magnetic bottle and the bionic cusp. The U.S. government built a series of mirror machines in the 1970s and 1980s, mainly in the LLNL. However, the calculations of the 1970s estimated that it was unlikely that they would ever be commercially useful.
Irregular Toroid: A series of magnetic mirrors are available from end to end in a toroidal ring. The fuel ions that escape from one of them are confined in a neighboring mirror, which allows the arbitrary lifting of plasma pressure without loss. In the 1970s an experimental installation, the ELMO Bumpy Torus or EBT, was built and tested at the Oak Ridge National Laboratory (ORNL).
Inverted field configuration: This device captures plasma in a self-organized quasi-stable structure; where the motion of particles creates an internal magnetic field that then catches itself.
Spheromak: Similar to an inverted field configuration, a semi-stable plasma structure made using the autogenerated magnetic field of plasmas. A spheromac has toroidal and poleidal fields, while an inverted field setting has no toroidal field.
Dynomak is a sferomak that is formed and maintained by continuous injection of magnetic flow.
Inverted field ray: Here the plasma moves inside a ring. This has an internal magnetic field. Leaving the center of this ring, the magnetic field reverses its direction.

Fusion Cycle

The basic concept of a nuclear fusion reaction is bringing two or more atomic nuclei close enough that the strong nuclear force (the force that holds protons and neutrons together in a nucleus) unites them to form a larger nucleus. If two light nuclei merge, they will form a single nucleus with a mass less than the sum of their original masses. The mass difference is released as energy according to the mass-energy equivalence formula E = mc². If the original nuclei are massive enough, the resulting fusion product will be heavier than the sum of their masses, in which case the reaction will require an external source of energy. The dividing line between both types of fusions, exothermic and endothermic, is established by iron-56. Above this atomic mass, energy will be released by nuclear fission; under it, by fusion.

D-T reaction

D-T reaction diagram

According to Lawson's criteria, the simplest and most promising fusion reaction is:

2
1D + 3
1T → 4
2He + 10n

Hydrogen-2 (Deuterium) is an isotope found and available in nature. The great difference in mass between the two main isotopes of Hydrogen (Protium and Deuterium itself) makes their separation easy compared to the difficulty of the uranium enrichment process. Hydrogen-3 (Tritium) is also an isotope of Hydrogen, but its natural occurrence is negligible. Due to this, it is necessary to resort to reproduction from lithium using one of the following reactions:

1
0n + 6
3Li → 3
1T + 4
2He

1
0n + 7
3Li → 3
1T + 4
2He + 1
0n

The reactant neutron is supplied by the D-T reaction above. The reaction with ⁶Li is exothermic, providing little energy gain to the reactor. The reaction with ⁷Li is endothermic but does not consume the neutron. At least a few reactions with ⁷Li are required to replace the neutrons lost to the absorption of other elements. Most reactor designs take advantage of the naturally occurring mixture of lithium isotopes.

D-D reaction

Although more difficult to produce than the Deuterium-Tritium reaction, the fusion can be done through the fusion of Deuterium with itself. This reaction produces two branches that occur with almost equal probability:

2
1D + 2
1D → 3
1T + 1
1H

2
1D + 2
1D → 3
2He + 1
0n

The optimal amount of energy to start this reaction is 15 MeV, only slightly higher than the optimum for the D-T reaction. The first branch does not produce neutrons, but it does produce tritium, so a D-D reactor will not be totally tritium-free, even though it does not require a tritium or lithium input. Most of the tritium produced will be consumed before leaving the reactor, which will reduce the amount of tritium to be handled, but will produce more neutrons, some of which will be quite energetic. The neutrons of the second branch have an energy of only 2.45 MeV (0.393 pJ), while the neutrons of the D-T reaction will have an energy of 14.1 MeV (2.26 pJ), resulting in higher isotope production and material deterioration.

Assuming all the tritium in the reactor is consumed, the reduction in the fraction of fusion energy carried by neutrons would be only 18%, so the main advantage of the D-D combustion cycle is that it does not require production of tritium. Other advantages are independence from the scarce supply of lithium and somewhat softer neutron radiation during the process. The disadvantage of the D-D compared to the D-T is that the confinement time (at a given pressure) would be 30 times longer and the power produced (at a given pressure and volume) would be 68 times less.

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