SCIENCE MATTERS - Dream lives on for fusion power

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I studied medium energy nuclear physics in graduate school, but I had been tempted to try plasma physics. Why had plasma physics intrigued me 37 years ago? I was excited by the prospect of plasma physics leading to an effectively limitless source of energy, with minimal, manageable pollution issues.

It was estimated then that within 25 years a commercial fusion nuclear reactor could be built. What happened? Ask a plasma physicist now when commercial fusion power could become a reality and you will probably be told, "It is a few decades away".

All other energy sources pale in their potential relative to fusion power. The reasons it remains only a hope are many. The technical problems have surely been more difficult than anticipated. But I think a lack of resolve is at least as much to blame.

This may be due, in part, to people's fear of anything associated with nuclear power. Fusion is a nuclear reaction, but it is fundamentally different from the fission nuclear reactions exploited in power plants around the globe.

Fusion harnesses the same nuclear reaction that powers the sun. The sun does not "burn" fuel in the sense of a chemical reaction where combustible materials (coal, oil, natural gas) become oxidized in a fire. Rather, the nuclei of light atoms are slammed together at very high velocities and fused together to make a single larger nucleus.

The mass of the resulting nucleus is less than the total mass of the light nuclei. Einstein's famous equation, E = mc2, gives the relationship showing the huge amount of energy produced from the net loss of mass in such reactions.

High collision velocities are achieved in plasmas compressed and heated to extremely high temperatures. Plasmas are states of matter consisting of gases whose atoms are stripped of their electrons.

If the plasma comes in contact with the containment vessel (reactor) walls, its energy will be lost immediately. That is, its temperature will drop below that required for fusion. It self-extinguishes. Hence, it is impossible to have a dangerous run-away explosive or "meltdown" event.

Several ingenious methods have been tried with varying degrees of success. Each method takes a huge amount of energy to achieve the required temperature. To date, all fusion reactor designs have consumed more energy than could be extracted.

But, those reactors were only "proof of concept" designs. One must learn to walk before running. To build a viable reactor, scientists must be able to ignite, contain and sustain a fusion reaction while extracting surplus energy to drive a turbine.

One promising design is called a tokamak. Very strong magnetic fields are used to contain plasmas. The magnetic field suspends the plasma inside a structure shaped like a large hollowed-out donut. Then, by manipulating the magnetic fields, the plasma is squeezed into a small space.

As the plasma density rise, so does the temperature. Additional heating is provided by radiowave and neutral beam injections to achieve an operating temperature over 100 million degrees Celsius. Squeezing hot plasmas in what is effectively a magnetic bottle, never letting it touch the containment vessel walls, is truly a technological feat.

Laser inertial devices employ another method. They position light nuclei at the focus of an array of very large lasers. Laser beams simultaneously blast the target. This creates an extremely energetic shockwave that smashes the nuclei together, igniting the fusion reaction.

Such laser inertial devices don't house the "core" within huge magnets. This makes extraction of energy from the core to drive a turbine easier. It also allows use of more favorable structural materials than those used in tokamak designs.

This is important because fusion reactions produce neutrons that are absorbed by structural materials, making them radioactive.

Radioactive waste from fusion reactors is much more easily managed than the radioactive spent fuel rods from fission reactors. Radioactive isotopes of iron, which would make up much of the structure in a tokamak, have relatively short half-lives, while the extremely long half-lives of isotopes in spent fuel rods require them to be safely stored forever.

Spent fuel rods are also problematic because they can be used for weapons production.

Fuels for fusion reactors are deuterium and tritium. Deuterium is a naturally occurring isotope of hydrogen extracted from seawater in essentially a limitless supply. Tritium, another hydrogen isotope, is produced from lithium, and should not limited, either.

JET (Joint European Torus) is currently the world's largest tokamak, having produced 16 megawatts. Next is a project with a $10 billion budget undertaken by seven nations called ITER. The U.S. has committed to funding only 9.1 percent of the project, but has actually funded less.

It will be completed in the next eight to 10 years in southern France. This research reactor will sustain reactions for as long as eight minutes and generate 500 megawatts of power.

Though commercially viable fusion power is still decades away, the realization that our fossil-fuel-based economy is unsustainable has rekindled my dream for a safe, nearly limitless, fusion energy reactor.

Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at steveluckstead@charter.net.

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