Why have scientists been unable to control nuclear fusion reactions?

Answer 1

The primary issue is one of containment.

In order to initiate a nuclear fusion reaction, you need to strip away the electron shells of the atoms, and you need to move the nuclei close enough together for the attractive strong interaction to overcome the repulsive electromagnetic interaction. Stripping away the electron shells, i.e. creating a plasma, requires ultra high temperatures. Moving the nuclei close enough together requires ultra high pressures.

Problem: We have nothing that can maintain and/or contain this temperature and pressure. No currently known material can do this. The stars do it easily, because of gravity, but a reactor large enough to take advantage of gravity would be much larger than the Earth. Not even Jupiter is large enough, though some say it is close.

So, we are left with alternative forms of containment.

One possibility is magnetic containment. Problem is, in order to do that, we need superconducting magnets, so we are faced with having ultra cold components in close proximity to ultra hot components. That, to say the least, is technologically difficult. Presently, the ITER, a tokamak design, is being constructed in Cadarache, France to attempt this. Timeline is set for first testing in 2019, with first fusion in 2026. Note, however, that we are only talking 500 MW of power, and then, only for 480 seconds. All this at a projected cost of 15 billion euros.

Another possibility is inertial containment. This is how the hydrogen bomb works, but that is an uncontrolled, destructive reaction. The NIF, a laser implosion device, has been constructed in Livermore, California (USA). It generates 4MJ pulses that can theoretically induce 45MJ fusion pulses. Problem is, that it takes 422MJ to charge the system's capacitors, so the total energy curve is backwards, and the system heats up so much that cooldown is required after each firing - they are attempting to be able to do 5 firings a day - hardly any kind of continuous output. As a result, this is only an experimental facility, though so is the ITER, described above.

Best guess - we will not achieve controlled fusion power for at least 100 years. Even the projected goals for the next 50 years do not include any kind of sustainable reaction, let alone any kind of commercial deployment.