What is a Nuclear Fusion Easy Definition

Nuclear Fusion

Balasubramanian Viswanathan , in Energy Sources, 2017

Introduction

In simple terms nuclear fusion is a process in which one or more light nuclei fuse together to generate a relatively heavier nucleus in which in there is some mass deficiency that is released as energy, and the quantity of energy released follows Einstein's formula: E  = mc ², in which E is the energy in joules, m is the mass difference in kilograms, and c is the speed of light (approximately 300,000,000 or 3   ×   10⁸  m per second). If one were to look carefully at Fig. 5.1, one would notice that iron and nickel possess the highest binding energy whereas elements on either side have lower binding energy and will tend toward a more stable form. Nuclei with lesser mass will fuse together to generate stable configurations. Heavier nuclei may also fuse but these are astrophysical events that can lead to short periods of fusion, and this process gives rise to nucleosynthesis, which is the creation of heavy elements. This is not our concern in this chapter.

Let us consider a short history of nuclear fusion. The concept of quantum tunneling was first proposed by Friedrich Hund. Robert Atkinson and Fritz Houtermans in 1929 measured the masses of light elements to predict the considerable amount of energy that could be released during the fusion of light nuclei. Nuclear transmutation experiments were carried out earlier than this by Ernest Rutherford. Combing these concepts, the laboratory fusion of hydrogen isotopes was first achieved by Mark Oliphant in 1932. For nearly a decade, Hans Bethe worked out the cycle of nuclear fusion in stars. Essentially first nuclear fusion on a large scale was carried out on November, 1, 1952 in a hydrogen bomb test. This exercise is still continuing and various ways to contain the fusing nuclei in a specific volume are being attempted by various means even today.

The energy release in a fusion reaction is also the result of the interplay of the very short-range attractive force that keeps the protons and neutrons together in the nucleus and the Coulomb force that results in the repulsion of the protons. Because the nuclear forces are stronger than the Coulomb forces for atomic nuclei lighter than iron and nickel, building these nuclei from the lightest nuclei releases energy and is called the fusion energy. It is understood that a fusion reaction will be possible only when the nuclei fusing together are brought together in a confined volume (recollect the dimensions of a nucleus).

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Energy Production

Yican Wu , Sümer Şahin , in Comprehensive Energy Systems, 2018

Abstract

Nuclear fusion energy is a good choice as the base load energy in the future with many advantages, such as inexhaustibility of resources, inherent safety, no long-lived radioactive wastes, and almost no CO ₂ emissions. During the past six decades, the fusion energy research has made a great progress with the goal of producing fusion energy output in this century. In this chapter, the principle and advantages of fusion energy are briefly introduced in Section 1. The fundamentals of fusion, including plasma and confinement condition, are briefly introduced in Section 2. The systems of fusion, are illustrated in Section 3, including the approaches to controlled fusion energy by magnetic confinement and inertial confinement, the blanket system that is the key component for converting nuclear fusion energy to thermal energy, and some prospective fusion reactor concepts and fusion-fission hybrid reactor concepts. The performances of fusion energy and fusion-fission hybrid energy are described in Section 4. The results and discussion of fusion system and some key technology are shown in Section 5. The future direction and closing remarks of the promising fusion energy are described in the last two sections. Some further information about fusion can be found in Section Further Reading and Relevant Websites.

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Technologies at the experimental stages

Nasir El Bassam , in Distributed Renewable Energies for Off-Grid Communities (Second Edition), 2021

18.2 Fusion power

Nuclear fusion powers the sun and all of the stars of the universe. Harnessing fusion energy on earth would provide a practically unlimited amount of renewable energy to supply the needs of the growing world population. Harnessing fusion energy is the main goal of the International Thermonuclear Experimental Reactor (ITER) in southern France, which claims to be a key research center aiming to provide tomorrow's fusion power (Figure 18.1).

Figure 18.1. The International Thermonuclear Experimental Reactor nuclear fusion power plant prototype.

Courtesy of ITER.org.

Fusion power is produced by harnessing heat generated by fusion reactions to produce electricity. Such reactions fuse two lighter atomic nuclei to form a heavier nucleus, thus releasing energy.

Thirty-five nations are collaborating at ITER to build the world's largest tokamak, a magnetic-fusion device that has been designed to prove the feasibility of fusion on a large-scale and a carbon-free source of energy based on the same principle that powers our solar system's sun.

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From Nuclear Fusion to Sunlight

Alexander P. Kirk , in Solar Photovoltaic Cells, 2015

2.5 Quantum mechanical tunneling

The nuclear fusion process itself is also quite remarkable because one would initially assume that two protons would stridently repel each other due to Coulomb repulsion [1,3]. It is understood, however, that despite the tendency for strong repulsion of the positively charged protons, quantum mechanical tunneling (of the wave functions of the interacting protons) through the Coulomb barrier can occur as explained by Gamow [10]. The slow rate of the nuclear reaction in step 1 (Eq. 2.1) of the pp chain reaction sequence allows for the controlled and long-term (billions of years) duration of nuclear fusion in the Sun [1,3].

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Nuclear Power

Paul Breeze , in Power Generation Technologies (Third Edition), 2019

Nuclear Fusion

Nuclear fusion, the reaction that fuels the sun and the stars, has excited scientists and technologists ever since the process was identified during the 1930s. Unsuccessful attempts at fusion took place during the 1930s but halted during World War II. Experimental work restarted during the late 1940s. Since then a series of fusion reactors have been built around the world. Around 20 are in operation today.

In 1958 at an Atoms for Peace conference in Geneva, fusion research was established as an international collaborative venture and at least one strand of fusion development, that based on magnetic confinement, has remained international in flavour ever since. The necessity for this was reinforced during the 1970s when it became clear that the cost of developing fusion was likely to be beyond the resources of any one nation.

Although large fusion reactors based on magnetic confinement were being built in the United States, Europe and Japan, other developments remained hidden behind the security of nuclear armaments research. Fusion is the basis for the hydrogen bomb and so much of the research into its development and control remained secret until very recently. It is this research that has led to the idea of inertial confinement, an entirely different approach to fusion for power generation. During the last 5 years the veil of secrecy has at least partly dropped and a major programme in the United States aims to develop a demonstration power plant during the 2020s. Meanwhile the largest international magnetic confinement reactor is under construction in the south of France and should start experiments at around the same time.

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Energy Production

Sümer Şahin , Yican Wu , in Comprehensive Energy Systems, 2018

3.14.3.6.3 Space fusion propulsion

Nuclear fusion energy is also a potential candidate for space travel, especially for high-energy requirements. For the purpose of the transport of astronauts and cargo to Mars and beyond, an innovative concept with a direct utilization of fusion energy via laser ignited (D,T) capsules for propulsion has been suggested by the scientists of the Lawrence Livermore National Laboratory, ETEC/Rocketdyne/Rockwell, Jet Propulsion Laboratory, and NASA, Johnson Space Center, Gazi University with the so called VISTA (Vehicle for Interplanetary Space Transport Applications) concept Detailed description of the VISTA spacecraft is given in Refs. [65–70]. A general layout of VISTA is shown in Fig. 42 [68,70]. The primary purpose was to design a vehicle system for the baseline mission of a piloted round-trip to Mars with a 100-ton payload. For this mission, round trips totaling ≥145 days are possible with advanced DT fusion technology and a total (wet) spacecraft mass of about 6000 metric tons, where hydrogen propellant in the tanks constitutes with ~4000 t the main mass.

Fig. 42. VISTA systems layout.

Reproduced from Şahin S, Şahin HM, Şahinaslan A. Reduced shielding mass for the VISTA spacecraft. Arab J Sci Eng 2002; 24(2A):187–96 and Şahin S, Şahin HM, Acır A. Radiation shielding calculations for the VISTA spacecraft. Energy Convers Manag 2005;46(15–16):2345–58.

Such short-duration missions are advantageous to minimize the known cosmic-radiation hazards to astronauts, and are even more important to minimize the physiological deteriorations arising from zero gravity. These inertial fusion energy (ICF)-powered missions are considerably faster than those available using chemical or nuclear-electric-propulsion technologies with minimum-mass vehicle configurations.

ICF is used to provide thrust for space propulsion. VISTA spacecraft is a conically shaped vehicle similar to a whipping top in order to minimize the radiation shielding area (D,T) pellets are shot to a point on the cone axis, where a multitude of laser beam are focused simultaneously to initiate the fusion reaction. Grazing incidence liquid metal mirrors are used as the final optics for laser inertial fusion energy propulsion to reduce mirror damage [71]. Strong superconductive magnets with a peak coil field of 12 T create a magnetic thrust chamber to avoid any material contact between the hot plasma and the spacecraft structure.

Fusion emits high-speed debris plasma that is directable with a magnetic field, thereby allowing the inherently high plasma temperatures (~1 keV) of the emitted particles to be directly converted into exhaust velocities without changing the particle speeds. Fusion allows both high power/mass ratios (10–1000 W/gr) and high specific impulses (10³–10⁶ s).

The requirements for a narrow temperature distribution of the propelling plasma for the sake of high jet efficiency have suggested using a hollow spherical expellant geometry for the frozen hydrogen with a fusion target located in the center of the sphere, as shown in Fig. 43 [70]. The fuel pellets are surrounded by ~50 gr of solid hydrogen (H) expellant and are accelerated, injected and positioned in the thrust chamber at a repetition rate that is variable from 0 to 30 Hz. The (D,T) fusion fuel debris at higher plasma temperature will be mixed with hydrogen propellant at lower plasma temperature homogenously, making a plasma with uniform temperature. A multi-layer shield made of beryllium (X-ray shield), liquid lithium (neutron shield) and LiH (neutron shield) protects the superconductive magnets against γ- and neutron heating.

Fig. 43. Geometrical model of the target assembly.

Reproduced from Şahin S, Şahin HM, Acır A. Radiation shielding calculations for the VISTA spacecraft. Energy Convers Manag 2005;46(15–16):2345–58.

VISTA also offers onboard artificial gravity and propellant-based shielding from cosmic rays, thus reducing the known hazards and physiological deteriorations to insignificant levels. With aggressive progress in such terrestrial research, VISTA will be able to make round-trip missions to Pluto in ~7 years and missions to points just beyond the solar system within human lifetime. Artificial gravity is required for all flights exceeding roughly 100 days to avoid skeletal and other physiological deteriorations in crew members. Above this are cylindrical habitat modules and conical aero shell (Apollo shaped) Landers.

The vehicle is on the order of 100 m tall and 170 m in diameter (at the base of the cone) [70]. A rotation around the main axis of the cone can create artificial centrifugal gravity during the long journey through a=ω² r=4π²((ν))² r. Fig. 44 shows the variation of the artificial gravity at R=80 m as a function of rotation frequency in rpm [70]. By ((ν))~3 rpm, an artificial gravity of a ~0.9 g will be realized at the peripheral zones for the crew, nearly Earth gravity. This will give a touch of home feeling to the crew.

Fig. 44. Variation of the artificial gravity at R=80 m.

Reproduced from Şahin S, Şahin HM, Acır A. Radiation shielding calculations for the VISTA spacecraft. Energy Convers Manag 2005;46(15–16):2345–58.

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Scalar fields

J.E. Akin , in Finite Element Analysis with Error Estimators, 2005

11.15 Axisymmetric plasma equilibria

Nuclear fusion is being developed as a future source of energy. The heart of the fusion reactors will be a device for confining the reacting plasma and heating it to thermonuclear temperatures. This confinement problem can be solved through the use of magnetic fields of the proper geometry which generate a so-called 'magnetic bottle'. The tokamak containment concept employs three magnetic field components to confine the plasma. An externally applied toroidal magnetic field, B_T , is obtained from coils through which the torus passes. A second field component is the polodial magnetic field, B_P , which is produced by a large current flowing in the plasma itself. This current is induced in the plasma by transformer action and assists in heating the plasma. Finally, a vertical (axial) field, B_v , is also applied. These typical fields are illustrated in Fig. 11.101. For many purposes a very good picture of the plasma behavior can be obtained by treating it as an ideal magnetohydrodynamic (MHD) media. The equations governing the steady state flow of an ideal MHD plasma are

Figure 11.101. Schematic of tokamak fields and currents

$gradB=0,\nabla \text{p}=J\times B,\text{curl}B=\mu J$

where P is the pressure, B the magnetic flux density vector, J the current density vector, and a constant that depends on the system of units being employed. Consider an axisymmetric equilibria defined in cylindrical coordinates (r, z, θ) so that ∂/∂θ = 0. This implies the existence of a vector potential, A, such that curl A = B. Assuming that A = A(r, z) and A_θ = ψ/r where ψ is a stream function, we obtain

$B_r=-{\psi }_{,z}/r,B_z=\psi {,}_r/r,B_{\theta }=A_{r,z}-A_{z,r}=B_T$

Therefore the governing equation simplifies to

(11.40) $\frac{{\partial }^2\psi }{\partial r^2}-\frac{1}{r}\partial \frac{\partial \psi }{\partial r}+\frac{{\partial }^2\psi }{\partial z^2}=-\mu r^{2}P\text{'}-XX\text{'}=r{J}_{\theta },$

where J _θ is the plasma current, P is the pressure, X = r B_θ and where P and X are functions of ψ alone. Both J and B are vectors that lie tangent to the surfaces of constant ψ. The above is the governing equation for the steady equilibrium flow of a plasma. For certain simple choices of P and Eq. 11.40 will be linear but in general it is nonlinear. They are usually represented as a series in ψ as

$P(\psi )={\alpha }_0+{\alpha }_1\psi +\mathrm{\dots }+{\alpha }_n{\psi }^n/n$

$X^2(\psi )={\beta }_0+{\beta }_1\psi +\mathrm{\dots }+{\beta }_n{\psi }^n/n$

The essential boundary condition on the limiting surface, Γ_1, is

$\psi =K+1/2r^2{B}_{V}on{\Gamma }_1$

where K is a constant and B_v is a superimposed direct current vertical (z) field. On planes of symmetry one also has vanishing normal gradients of ψ, i.e.,

$\frac{\partial \psi }{\partial n}=0on{\Gamma }_2.$

The right-hand side of Eq. 11.40 can often be written as

(11.41) $r{J}_{\theta }=p\psi +q$

where, for the above special cases, p = p(r, z) and q = q(r, z), but where in general q is a nonlinear function of ψ, i.e., q = q(r, z, ψ). Equations 11.40 and 11.38 are those for which we wish to establish the finite element model.

A finite element formulation of this problem has been presented by Akin and Wooten [1]. They recast Eq. 11.40 in a self-adjoint form, applied the Galerkin criterion, and integrated by parts. This defines the governing variational statement

(11.42) $I=\int \underset{\Omega }{\int }\left[\frac{1}{2}\{{({\psi }_{,r})}^2+{({\psi }_{,z})}^2+\text{p}{\psi }^2\}+q\psi \right]\frac{1}{r}\text{dr}\text{dz}$

which, for the linear problem, yields Eq. 11.40 as the Euler equation when I is stationary, i.e., δI = 0. When p = q = 0, Eq. 11.42 also represents the case of axisymmetric inviscid fluid flow. Flow problems of this type were considered by Chung [6] using a similar procedure. For a typical element the element contributions for Eq. 11.41 are

(11.43) $\begin{array}{c}{S}^e=\underset{{\Omega }^2}{\int }[H_{,r}^T{H}_{,r}+H_{,z}^T{H}_{,z}+p(r,z)H^{T}H]\frac{1}{r}drdz,\\ C^e=\underset{\Omega }{\int }q(r,z)H^T\frac{1}{r}drdz.\end{array}$

These matrices are implemented in Fig. 11.102. Other applications of this model are given by Akin and Wooten [1]. The major advantage of the finite element formulation over other methods such as finite differences is that it allows the plasma physicist to study arbitrary geometries. Some feel that the fabrication of the toroidal field coils may require the use of a circular plasma, while others recommend the use of dee-shaped plasmas. The current model has been applied to both of these geometries and the following figures illustrate typical results for a dee-shape torus cross-section where the linear triangle element was employed. Biquadratic or bicubic elements would be better for some formulations which require post-solution calculations using the first and second derivatives of ψ. Figure 11.103 shows the initial relatively uniform mesh and the element level error estimates in the energy norm. The corresponding plasma B vectors appear in Fig. 11.104. Note that the initial maximum error in the energy norm is about 2.4 percent so adaptive refinements were taken to reduce the maximum error level to less than 1 percent. The two adaptive meshes and error estimates are shown in Figs. 11.105 and 106. The stream function is of less interest but the final contours of ψ are in Fig 11.107. The initial and final surface plots of ψ are in Figs. 11.108 and 109. The above results have assumed no external vertical B field so ψ was assigned values of zero on Γ₁.

Figure 11.102. Plasma element matrices evaluations

Figure 11.103. Initial Dee plasma mesh and error estimates

Figure 11.104. Initial Dee plasma planar B vectors

Figure 11.102. Plasma error estimates in ?rst adaptation (max 2.10 percent)

Figure 11.103. Plasma error estimates in second adaptation (max 0.97percent)

Figure 11.107. Stream function, Ψ, values in second adaptation

Figure 11.108. Initial Ψ surface for half symmetry model

Figure 11.109. Final Ψ surface for half symmetry model

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ENERGY AND THE ENVIRONMENT

J.F. Mathis , ... E.L. Holt , in Energy: Money, Materials and Engineering, 1982

Nuclear Fusion

The future of nuclear fusion is uncertain. Fusion research began to make substantial progress in the last decade. This has culminated in recent breakthroughs in magnetic confinement technology, and work on laser and particle beam implosion is also progressing. Although such developments are encouraging and the potential is great, much work remains to be done and significant contributions from fusion are certainly very far in the future. From an environmental standpoint, many people hope nuclear fusion will be the long-term clean energy solution. However, it may not be totally free of as yet undefined environmental concerns.

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DEFINITIONS AND SOURCES OF ENERGY

André Gardel , in Energy: Economy and Prospective, 1981

k7 Deuterium

Whilst controlled nuclear fusion has not yet been achieved, either on an industrial scale or even in the laboratory, the importance of the efforts devoted to achieving this goal justify mentioning here this source of primary energy.

Deuterium is the "fuel" of nuclear fusion reactors. It exists in enormous quantities: 0.022‰ (about 2 × 10⁻⁵) of sea water, or 3 × 10¹³ t (362 × 10⁶ km² of sea surface and 3500 m average depth, thus a volume of sea water of 1.3 × 10¹⁸ m³, of which 2/18 is hydrogen containing 1/5000 of isotope D, or 3 × 10¹³ t). If 0.1‰ of it were used, we would have available, by the D + D reaction at 90 TJ/kg (§ i4, c), 300000000 EJ (3 × 10⁸).

The corresponding energy reserve is therefore practically inexhaustible.

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NUCLEAR POWER

JERRY B. MARION , in Energy in Perspective, 1974

Publisher Summary

This chapter describes the nuclear fission and fusion processes and presents some details of nuclear reactor operations. The different nuclear forms of a particular element are called isotopes. Thus, there are three different isotopes of hydrogen—¹H, ²H, and ³H. All uranium nuclei contain 92 protons; the important isotopes of uranium contain 143 neutrons (²³⁵U) and 146 neutrons (²³⁸U). The absorption of neutrons by uranium produces a breakup or fission of the nucleus into two fragments, each with a mass roughly one-half the mass of the original uranium nucleus. It has been recognized that the fission process offers the possibility for the release of nuclear energy on a gigantic scale. When a heavy nucleus undergoes fission not only are two lighter nuclear fragments formed, but also two or three neutrons are released. If the system is designed such that, on average, exactly one neutron from each fission event triggers another event, then the fission energy can be released in a slow and controlled manner. This is the basic operating principle of the nuclear reactor.

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