In 1951, the astrophysicist Lyman Spitzer devised a way to contain a hot plasma—an ionized gas—with the hope of producing a sustained fusion reaction that could lead to electric power generation. The goal of a fusion power station has inspired generations of physicists, but the physics and engineering problems remain extraordinarily challenging.

The most promising approach is to fuse two isotopes of hydrogen—deuterium and tritium (see Plasma Power).

Deuterium is a stable isotope, but only about 0.015% of the hydrogen in water is deuterium. It is separated from water by a series of chemical reactions involving migration of deuterium from hydrogen sulfide into water and then back to hydrogen sulfide.

Tritium is an unstable isotope, with a half-life of about 13 years, and it makes up only a billionth of a percent of the hydrogen in water. Consequently, tritium must be produced separately in a breeding reaction, from neutron bombardment of lithium.

    neutron + lithium-6 → tritium + helium-4
    neutron + lithium-7 → tritium + helium-4 + neutron

The emitted neutron helps sustain the breeding reaction, and the two reactions together release additional energy. Tritium is highly radioactive, so it can pose a considerable hazard if released.

As for fusion itself, the general idea is to slam the tritium and deuterium nuclei together so they react and form helium.

deuterium + tritium → helium-5 → helium-4 + neutron

Schematic drawing of a fusion powerplant (credit: Princeton Plasma Physics Laboratory)

Most of the energy is carried off in the neutron. In a fusion power plant, the neutrons would heat a lithium blanket that, through a heat exchanger, would boil water for steam turbines in an electric power plant. In addition, reactions between the neutrons and the lithium nuclei, as described above, would breed more tritium.

The deuterium-tritium or D-T reaction is the most promising because of the forces between nuclear particles. The powerful mutual electrostatic repulsion of protons means that of all the elements, hydrogen nuclei offer the least repulsion and thereby require the lowest energy to get the reaction going. At very short distances, nuclear particles attract each other through the strong force, and the neutron in tritium adds to this attractive force, thereby promoting the fusion reaction. In this sense, a T-T reaction would seem better, but in fact it produces only 65% as much energy as D-T.

In the D-T reaction, each neutron carries off about 14 million electron volts of energy, roughly 80% of the released energy (an electron volt is the energy acquired by an electron in moving through a potential difference of one volt). These energetic neutrons constitute a considerable radiation hazard, so a fusion reactor will need a one-meter thick lithium blanket to absorb neutrons and breed more tritium. The strong neutron flux would necessitate robotic maintenance and control inside the blanket, and the radiation would make metal parts brittle. In addition, the danger of tritium leakage would require a steel container around the whole reactor.

For the deuterium and tritium nuclei to react, they must collide with very high energy. To some degree, quantum mechanics provides a way around the electrostatic repulsion of the protons, because it is possible for the two nuclei to “tunnel” through this barrier and thereby considerably reduce the necessary collision energy.

The easiest way to do D-T fusion is to accelerate deuterium nuclei into a high-velocity beam and aim it at a tritium target—tritium atoms locked in a solid. Such a machine operates as a research fusion reactor at the University of California, Berkeley. Thanks to the tunneling effect mentioned above, a 400,000 electron-volt beam of deuterium ions produces 14 million electron volt electrons. The rate of energy release is about two watts.

For a power plant, though, a sustained reaction is necessary. To keep the fusion reaction going, the deuterium and tritium must be heated sufficiently that the ions’ thermal motion will produce sufficiently energetic collisions. In this case the ions must be hot enough that they will form a plasma. As a plasma moves, its electric currents produce electromagnetic forces that act back on the plasma, so controlling and confining the plasma is a daunting challenge.

As far as safety is concerned, a fusion power plant has significant advantages over a nuclear reactor. At any time, the fusion reactor contains enough fuel for only a few minutes operation, unlike the year’s worth of fuel in a fission reactor. If the plasma container is breached, the reaction stops immediately, so there is no danger of anything like a “China syndrome” meltdown. Also, there is no problem of long-term storage of  spent fuel.