What is Nuclear Fusion? Is it Possible, and Does it Matter?

Nuclear fusion is considered by some to be the "holy grail" of energy production – a virtually limitless source of clean power that could revolutionize the world. But what exactly is nuclear fusion and why is it so difficult to achieve? This in-depth guide examines what happens during fusion, its potential benefits and risks, the history of fusion science, latest progress, and why mastering fusion has remained an elusive dream for decades.

What is Nuclear Fusion?

Nuclear fusion is the process that powers stars like our Sun. It is a nuclear reaction where two light atomic nuclei collide at extremely high speeds and fuse together to form a heavier nucleus, releasing a tremendous amount of energy in the process.

Specifically, in the heart of stars like the Sun, nuclear fusion converts hydrogen into helium. Hydrogen has one proton in its nucleus. When fused, multiple hydrogen atoms add up to form more massive elements like helium, which has two protons.

In essence, fusion combines small nuclei into larger nuclei, releasing energy that comes from the decrease in mass that happens when separate nuclei fuse. A small amount of mass is converted to energy as described by Einstein‘s famous equation E=mc². Even a tiny decrease in mass results in enormous amounts of new energy produced.

How Nuclear Fusion Works

Nuclear fusion requires forcing together positively charged protons located in two nuclei so they overcome their natural repulsion and "fuse" together. For example, with hydrogen fusion – the type that occurs in the Sun – you begin with two protons (hydrogen nuclei each with only one proton). These protons must collide with enough force and speed to overcome the electrical repulsion of their identical positive charges.

Temperatures of over 15 million degrees Celsius are needed to give hydrogen nuclei enough speed and kinetic energy to fuse. Also density must be extremely high to force collisions. Under such extremes of heat and density, protons fuse together forming a heavier helium nucleus with two protons.

In stars massive enough to produce iron in their cores, even heavier elements can fuse as lighter nuclei smash together. The process repeats as atoms become heavier and heavier until fused material reaches the extreme density in the star‘s core needed to spark energy-producing fusion reactions.

Comparing Nuclear Fusion and Nuclear Fission

Nuclear fusion should not be confused with its nuclear counterpart, fission. In fission, a heavy unstable nucleus splits into smaller parts, releasing energy in the process. Fusion works in the opposite direction, with smaller nuclei fusing to become larger, heavier nuclei.

Another key contrast is that nuclear fission typically uses uranium or plutonium fuels to produce energy, while potential fusion fuels are light hydrogen isotopes such as deuterium and tritium. Fusion fuels are also far more abundant. Deuterium can be distilled from all forms of water for example.

In terms of energy released, fusion reactions release over four times more energy than fission reactions. Fusion is also cleaner with less radioactive byproducts and waste. A 1000 megawatt fusion power plant would produce about 3000 kilograms of helium and 100 kilograms of radioactive waste with low radioactivity that would decay to safe levels within 100 years.

History of Fusion Science

Humans have long studied the stars and wondered about the source of their immense energy. In 1920‘s and 30‘s, breakthroughs in physics led scientists to identify fusion reactions as the driver behind stellar power production. Research soon began into whether fusion could be harnessed for energy production on Earth.

The first milestone came in 1932 when physicists John Cockcroft and Ernest Walton designed an accelerator to artificially split lithium nuclei using protons – the first human-engineered nuclear reaction. This opened the gateway to further experiments smashing together nuclei to provoke fusion. Throughout the 1930s and 40s, additional fusion reactions were achieved using particle accelerators.

In 1951, the world‘s first fusion reactor called a tokamak was created in Moscow by Soviet scientists Andrei Sakharov and Igor Tamm. Tokamaks used magnetic fields to contain hot plasma to keep it from touching and melting the container walls. They demonstrated temperatures over 30 million degrees Celsius were possible. More advanced tokamaks were soon built and improved.

Today fusion efforts continue with multi-billion dollar collaborative projects like ITER, a magnetic confinement tokamak currently under construction in France supported cooperatively by 35 nations. Despite immense progress fusing hydrogen since early experiments, a self-sustaining fusion reaction that produces excess energy has still not been achieved after over 60 years of diligent effort.

Why is Fusion So Difficult to Achieve?

If stars can fuse atoms together to release vast energy that lights the whole cosmos, why has nuclear fusion proven so extraordinarily difficult to reproduce in labs here on Earth? There are a few key reasons:

1. Need for Extreme Heat and Pressure

As mentioned earlier, fusion depends on forcing nuclei together hard enough to make them fuse into heavier atoms like helium. But nuclei carry positive electrical charges so they naturally repel each other. Only under tremendous heat beyond 100 million degrees Celsius will nuclei get moving fast enough to smash past these electrical barriers.

Stars have immense gravity that creates the needed huge pressures and densities along with intense interior heat. But recreating even close to a star‘s 10&sup15; atmospheric pressure at millions of degrees has pushed the limits of engineering constraints. The issue becomes containing something at such extremes.

2. Plasma Containment Difficulties

One challenge is that matter at millions of degrees exists in the plasma state and touches nothing solid since it would instantly melt through. Plasma has to be suspended away from material confines using powerful magnetic fields. But the plasma resists and fights being contained, leaking out gradually like air slowly escaping from a balloon.

Years of experiments have tested different magnetic confinement chamber shapes akin to high-tech thermos bottles. Stellarators, tokamaks, magnetic mirrors, and other designs have been created and researched seeking better ways to keep the reaction hot and dense enough for long enough.

3. Instabilities in Plasma

Unfortunately in plasma various drift, fluting, rippling, and other instabilities occur, allowing hot plasma to rapidly escape containment. Powerful microwaves can counteract this by injecting more heat to restabilize reaction zones. But adding heat faster than it is constantly lost has been an persistent obstacle.

4. Need Far More Fusion Reactions

Finally, while fusion reactions have been successfully created in labs – some for milliseconds, others for minutes at a time – none have reached the point of "ignition" where reactions become self-sustaining. The output energy still falls far short of the input energy invested, something referred to as achieving a burning plasma or "energy breakeven". Until then, fusion will remain for brief bursts only, still a dream not yet realized.

Approaches to Achieving Fusion Power

Over the decades, researches have devised various methods seeking the extreme heat and densities capable of making fusion happen. While no definitive solution has emerged yet, the current front runners include:

Magnetic Confinement Fusion

This technique uses electromagnetic fields to suspend and insulate plasma away from potentially cooling solid container walls. Magnetic confinement is used by the largest fusion projects today like the ITER tokamak in construction that applies fields carefully sculpted into donut-ring spherical torus shapes. Along with heating the plasma with injected microwaves, the goal is keeping ions dense and hot for long enough durations that significant fusion happens.

Inertial Confinement Fusion

Rather than steady magnetic fields, inertial confinement relies on the inertia of the fuel‘s own mass to keep it together long enough. This is achieved using lasers or particle beams in a rapid one-two punch strategy. First energy beams rapidly heat and compress a small frozen fuel pellet from all sides causing implosion under tremendous pressure. Then in the center where most matter concentrates, fusion ignites for extremely brief instants before fuel expands again and blows apart.

Magnetized Target Fusion

This approach combines aspects of magnetic confinement and inertial confinement using magnetic fields for early plasma target compression preheating, followed by very rapid completion compression by particle beams hitting an outer solid shell enclosing the plasma. This hybrid technique can achieve compression velocity implosions while the magnetic fields help mitigate energy losses out the sides.

Other Fusion Concepts

There are also various alternative fusion schemes being investigated. One features muon-catalyzed fusion which substitutes muons for electrons orbiting the hydrogen nucleus, increasing probability of fusion without as much heat needed. Another concept called fusion-fission hybrids would wrap a subcritical fission reactor around a fusion chamber core to multiply yield. And several proprietary ideas explore fusing hydrogen with boron-11 nuclei as aneutronic fusion rather than the conventional proton-proton model.

Benefits If Fusion is Achieved

Fusion power holds tremendous promise. If the monumental scientific and engineering challenges involved can be overcome, fusion energy offers potential advantages over traditional nuclear fission or fossil fuels including:

Energy Abundance

The fusion fuels of isotopes of hydrogen used like deuterium and lithium are available in nearly inexhaustible supply. Deuterium can be extracted from seawater and only a few hundred kilograms of lithium are needed annually is needed to operate a large fusion plant. Fusion fuels are both abundant and geographically widespread.

Massive Power Output

Fusion yields about four times as much energy for the mass of fuel consumed than nuclear fission does, pound for pound. Calculations estimate that just 50 kilograms of deuterium and tritium used as fusion fuel per day could produce enough heat energy equal to a staggering 10,000 tons of coal daily.

Minimal Radioactive Waste

Though not entirely radioactivity free, fusion reactors produce waste that is thinner, cleaner, and decays far faster than waste from fission reactors. After 100 years, fusion radioactivity reduces to a hundredth while fission waste stays dangerously radioactive for 100,000+ years. This makes storage easier and reduces nuclear proliferation risks.

Greater Safety

Unlike fission reactors, fusion power systems don‘t risk dangerous runaway meltdown accidents. If containment fails the fusion reaction simply stops. And there‘s no extra fuel accumulated in the reactor during operation that could contribute to an accident. Fusion poses substantially lower threats to public safety.

Zero Greenhouse Gas Emissions

Fusion emits zero greenhouse gases. Fusion could provide copious reliable baseline grid energy without any carbon dioxide or other pollutant emissions contributing to climate change. It also doesn‘t trigger the same ecological concerns as dams needed for hydro power or wind farms that negatively impact endangered birds.

If the decades long quest to produce electricity from fusion every finally succeeds, these benefits mean it could be a game changing technology and play a major role helping curb 21st century risks posed from both climate damage and growth constraints in energy, food, and water resources.

Risks and Challenges That Must Be Overcome

Despite great potential upside, fusion power also still faces substantial obstacles and downside risks, both technical and economic:

Prohibitive Costs

So far fusion energy has already consumed tens of billions of dollars in research over 65+ years without yet reaching the breakeven point where a fusion reactor produces excess energy that could be harnessed at scale to generate electricity. Continuing research funding is still required from governments because there is insufficient private interest to finance such capital intensive but still highly speculative ventures where ultimate success remains uncertain.

Feasibility Unknowns

No one can yet guarantee fusion will ever become a practical power source. Scientists have certainly made tremendous strides in plasma research and overcome once insurmountable heat and containment hurdles. Yet actually achieving ignition where reactions are self-heating remains right over the horizon, seemingly almost within reach but still somehow elusive. There continue being unknowns.

Radioactive Waste Concerns

While far less toxic waste than from fission and with shorter half life decay periods, fusion does still generate some real radioactive waste. And the neutron radiation produced can damage structural materials inside fusion reactors. Developing suitable containment alloys resistant to such neutron bombardment is also an area still requiring work.

Weapon Proliferation Risks

Like nuclear fission, knowledge gained from fusion research could also theoretically aid development of next generation thermonuclear weapons. So policy safeguards must accompany sharing of results to prevent dual use by military interests seeking to advance weapons technologies rather than clean energy.

Cutting-Edge Progress in Fusion Goals

Today several ambitious multinational megaprojects headlined by ITER aim making the next major leaps toward establishing feasibility of fusion power generation. They hope to achieve a burning plasma and ignition, finally reaching the crucial tipping point where fusion reactions sustain themselves while also exceeding the energy breakeven point where more total energy is released than the amount required to heat the fuel.

Recent promising milestones include China‘s Experimental Advanced Superconducting Tokamak (EAST) reactor setting duration records for plasma confinement time. In 2021 it sustained a plasma temperature over 120 million degrees Celsius for an unprecedented 1015 seconds. This brings scientists tantalizingly closer to timespans needed for actual prototypical fusion energy production applications.

Meanwhile researchers continue pushing boundaries of laser-based inertial fusion as well, using the world most powerful lasers at the National Ignition Facility in California which come within striking distance of ignition thresholds. Their aim is igniting fusion from just a milligram of deuterium-tritium fuel in brief but energy-unleashing tiny thermonuclear detonations.

While past predictions have overly optimistic, many scientists are still hopeful this decade or next they will finally clear obstacles that previously seemed intractable and activate the first ever self-sustaining fusion reaction where true net energy outputs confirm feasibility. Then focus can shift to challenges of scaling up methods to build viable pilot plants.

The Outlook for Nuclear Fusion‘s Future

Mastering nuclear fusion has been a quixotic dream for eight decades now. This clean energy holy grail continues beckoning despite immense scientific and funding hurdles that constantly kick ignition goals further down the road. Yet it also holds promise so vast its potential alone motivates continued tenacious efforts.

With peak oil forecasts looming and climate change negotiations proceeding so painfully slow, fusion may arrive just in time – albeit much later than early aspirations first envisaged. But if its enormous power does get harnessed before fossil energy supplies dwindle and irreversible environmental damage occurs, then nuclear fusion still faces realistic odds of ultimately changing the world.

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