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Don't Get Excited about Nuclear Fusion Yet

Written By Briton Ryle

Posted February 13, 2014

In romance, they say that breaking up is hard to do. But in nuclear energy, getting together is even harder.
Ever since scientists successfully harnessed the immense energy released by splitting the atom, (known as nuclear fission – such as takes place inside a nuclear bomb), they have been hard at work trying to harness the greater amounts of energy released from binding atoms together (known as nuclear fusion – such as takes place inside the sun).

After more than 60 years of trying, finally we have a breakthrough. The scientific journal Nature reported yesterday that scientists at the Lawrence Livermore National Laboratory near San Francisco “have successfully triggered significant amounts of fusion by zapping a target with their laser”.

The research could one day bestow upon humankind not only the most powerful source of energy we know of, but one of the safest and cleanest too.

Since the fusion process combines two hydrogen atoms into one helium atom, its fuel source (water) is abundant and cheap, while its waste bi-product (helium) can be combusted to generate more electricity by powering steam turbines.

By contrast, the fission process used in nuclear power plants today requires mining uranium, a limited resource, and produces toxic waste that remains radioactive for hundreds of thousands of years.

The challenge for scientists, however, is that fusing has always consumed more energy than was harnessed out. If they could just develop a fusion process that is economically feasible, the world could benefit from the ultimate source of power.

This is why yesterday’s announcement is so promising. For the first time in fusion research, “We’ve gotten more energy out of the fusion fuel than we put into the fusion fuel,” LLNL lead scientist Omar Hurricane proudly announced.

The Force Behind Fusion

The processes that make nuclear fission and fusion so very different from each other stem from two forces acting against each other at the very heart of an atom, its nucleus.
Inside an atom’s nucleus you will find positively charged protons and non-charged (neutral) neutrons.

Just as two magnets will push away from each other when you line them up by similar ends, so too will the protons inside an atom push away from each other – a force known as “Coulomb force”.

This repellent force between protons would cause atoms to break apart if it weren’t for an opposing force holding the nucleus together – known as “nuclear force” – which attracts and binds protons and neutrons together.

But the strength of this attractive “nuclear force” gradually weakens over longer distances, so that it is strongest in small nuclei and grows progressively weaken in larger ones. For example, the nuclear force in a helium atom which contains two protons is weaker than the nuclear force in a hydrogen atom which contains only one proton.

This is the entire basis upon which nuclear fusion works. It all hinges on the strength of the nuclear force, which is strongest in small atoms. The smaller the atom, the stronger the nuclear force.

The goal of nuclear fusion is to force two nuclei to join together, which is resisted by the repelling Coulomb forces inside each nuclei. It’s a lot like trying to push two magnets together with both of their positive sides facing each other. The closer you bring the magnets together, the more resistance you feel.

If you could somehow push the nuclei close enough to actually touch each other, the attractive nuclear force would overpower the repellent Coulomb force, and the two would bind together. Two hydrogen atoms (containing one proton each) would then become one helium atom (containing two protons).

But remember what happens to the attractive nuclear force when the atom is larger? The force weakens. The larger helium atom now has less nuclear force than the two hydrogen atoms you started with. This results in the release of unused nuclear force, which produces a burst of energy.

That is what takes place inside our sun and other stars, where lighter elements are fused into heavier ones, releasing vast quantities of energy in the process.

Fusion Versus Fission

The following diagram illustrates the fusing of two hydrogen atoms, and the amount of energy released.

Nuclear Fusion

Source: Wikipedia

Because the attractive nuclear force is strongest in smaller atoms, fusion would produce the most energy when using the smallest atomic nucleus in existence – hydrogen, which has only one proton.

The only problem is that nearly all of the hydrogen found on Earth (99.98%) has no neutron. And neutrons are needed to combine two hydrogen atoms to produce helium, the second smallest atom in the universe. So scientists need to produce variations of hydrogen which contain neutrons, called isotopes.

The diagram above shows “deuterium” (2H: a hydrogen isotope which contains 1 proton and 1 neutron) fusing with “tritium” (3H: a hydrogen isotope which contains 1 proton and 2 neutrons) to produce a helium atom (4He: which contains 2 protons and 2 neutrons). But the count is a little bit off.

At this “neutron dance” (not to be confused with the hit song from the 1980’s) the proton girls pair up the neutron boys, leaving 1 neutron without a partner, who leaves the dance in a burst of rage. This burst of rage produces 14.1 MeV (million electronvolts), while the joining of the other four particles produces 3.5 MeV for a grand total of 17.6 MeV of power.

By contrast, the fission process used in power plants and nuclear weapons releases energy in the opposite direction, as depicted in the diagram below.

Nuclear Fission

Source: Wikipedia

The fission process depicted above starts with one lone neutron at the very top colliding into an atom of uranium-235 (which contains 92 protons + 143 neutrons = 235). The collision produces an atom of uranium-236 (92 protons + 144 neutrons = 236). [Incidentally, there is a little bit of fusion in the fission process.]

Yet the new uranium-236 atom almost instantly fissions – or splits – into krypton-92 (not from Superman’s planet) and barium-141. Yet the two bi-products contain a total of only 233 protons and neutrons. The remaining 3 neutrons are simply released into the fuel mass.

That triggers a chain-reaction. One neutron causes the release of 3 neutrons, 3 cause the release of 9, 9 then release 27, and so on until the entire mass of uranium is consumed.

Yet as efficient as this chain-reaction is, it only produces 7.6 MeV per uranium nucleon released. Since the fusing of matter releases more energy than the splitting of matter, fusion could generate much more power. If they could just get fusion to work.

Close, But Not Quite There

The colossal laser known as the National Ignition Facility developed by LLNL scientists fires 192 laser beams “creating temperatures and pressures similar to those that exist only in the cores of stars and giant planets and inside nuclear weapons”, the laboratory’s website explains.

Yesterday’s experiment fired the laser at a small gold cylinder measuring a half-inch across, which enclosed a tiny ball containing the two hydrogen isotopes described above – deuterium and tritium – in the hopes of fusing them together.

The result? It worked, in a way. Hydrogen fused into helium, releasing more energy than the amount of energy that went into the hydrogen fuel.

The problem is that only 1% of the laser’s power went into the hydrogen fuel; 99% of the laser’s power was wasted. It’s like fuelling up your car by tossing a bucket of gasoline at the gas tank. While some of it enters the tank, most of it spills on the ground.

“They didn’t get more fusion power out than they put in with the laser [overall],” explained Steve Cowley, the head of Joint European Torus, another fusion experiment in the U.K.

A Ray of Hope

But the scientists did detect something extremely promising… they achieved “bootstrapping”. There was evidence that some of the energy released during fusion went back into the fuel and caused more fusing.

“Seeing that kick-in is quite exciting,” exclaimed Hurricane, “and it does show that there is promise”.
This bootstrapping is the Holy Grail of fusion power. Simply put, it’s a chain reaction that once “ignited” will simply keep itself going until all the hydrogen fuel is turned into helium. It’s like setting a log on fire – it burns by itself until the wood is consumed.

If they could trigger this process and get the hydrogen to ignite, then it really wouldn’t matter if the laser uses more energy when getting the process started. For once you turn off the laser, the fuel would burn on its own, releasing more energy than the power that got it started.

But we’re a long way from that. Until we get there, oil and gas are our cheapest sources of energy. It’s going to be tough to shake the world off of fossil fuels. Just how tough is it? Tougher than splitting the atom. It’s as tough as fusing two atoms together.

Joseph Cafariello