What is limiting fusion power production

[padawan13]

Why can't we use fusion for power generation? What are the problems with fusion power, and what are the limiting factors for construction?

 

[Drakkith]

Currently we cannot produce enough fusion reactions for little enough power to produce a net gain in energy. IE if we put 500 kw into the reactor TOTAL, we only get about 100 kw of power out of the fusion. And this doesn't even factor the losses in energy generation from the reactions, but is just talking about the actual energy released from the reactions.

 

[padawan13]

Yeah, the power output being less then the input is pretty obvious, in my question i was more pointing towards what are the actual problems which cause this inefficiency.

 

[Drakkith]

Simple. We don't know how to make it work yet. It could be something as simple as a change in geometry of the coils, or something as complicated as needing to have 100,000 tiny adjustments per second in the reactor to make it work.

One of the main problems, at least with magnetic confinement, is plasma instabilities. Much work has been done over the past 5+ decades on plasma, and we have a much much better understanding of how plasma reacts to the magnetic and electric fields and currents, but it still isn't enough.

There are also issues such as particle losses from the reactor, energy losses from different types of radiation, and a myriad of other issues. We just don't know how to make it work yet!

 

[padawan13]

well they have achieved fusion with 65% of the input energy being output, but i think it was for a short period of time.

Are there any nuclear engineers who are designing fusion plants on this forum?

 

[Ryan_m_b]

There are yes, though I'm not one of them you might find this list of key research areas interesting http://en.wikipedia.org/wiki/Fusion_power#Key_design_areas.

 

[jim hardy]

There's an interesting hobbyist group i think at fusor.org.

In the late 1950's a guy named Farnsworth built a desktop fusion machine for making medical isotopes and Chrysler sold them.
He was a vacuum tube designer so it was natural for him to use electrostatic field for confinement instead of electromagnetic , meaning he used high voltage instead of high current. Presto - no huge electromagnets or supercooled conductors required. You could almost make one from an old TV set.

His design does not yet break even but plenty of hobbyists are making neutrons in their basements, some of them while still in high school.
Their hope is that the stream of energetic ions from fusion can be tapped as extremely high voltage providing electrical energy directly with no mechanical conversion.

Perhaps one of them will rewire his Tesla coil for stepdown and resonate his vacuum chamber to same note - who knows !
... that would sure embarrass the magnetic confinement research community.


search on terms Farnsworth & Fusor. His patent is a fascinating read.


have fun.


old jim

 

[JeffKoch]

It's basically confinement, in both magnetic and inertial fusion schemes. You are trying to force positively charged nucleii very close together, so that the nuclear strong force takes over - depending on the nucleii, the resulting fused mass is energetically favorable, so that energy is liberated in the process provided you put in enough energy up front to overcome the electrical repulsion of the separate nucleii. You can do this with very low efficiency very easily, just from random chance that two nucleii come together and fuse - doing this efficiently, so that you get more energy/power out compared to all the energy/power you put into all the nucleii, is the rub. Stars use gravity to overcome coulomb repulsion and generate power, but we can't use this at the scale of human-made devices.

Every time we humans have come closer to this confinement condition, we've uncovered another complicated collective phenomenon that makes it more difficult than we thought, so that fusion energy power plants have always been 20 years in the future.

Fission power plants, in contrast, are comparatively easy because they use uncharged neutrons to break up nucleii, so there's no coulomb barrier to overcome.

 

[padawan13]

I read that one of the main problems is transferring the produced energy to a medium and converting the energy to thermal. Apparently even though thermal energy is released in the D-T reaction this energy is directly used to continue the self sustaining fusion reaction. This leaves only the kinetic energies of neutrons and product nuclei. The problem arises because converting kinetic energy from neutrons to thermal energy is hard apparently. But then i read about the most likely fusion nuclear reactor design which uses these free neutrons produced to breed tritium, and thermal energy is taken from the plasma. So this clashes with the with what i said at the beginning. So what is actually going on here? Anyone familiar with fusion reactor designs?

 

[JeffKoch]

D-T fusion produces neutrons and alpha particles, mostly - I'm not so familiar with magnetic confinement, but in inertial confinement the alphas are critical to super-heating the compressed plasma and generating gain. I.e., they stay in the plasma, and the process doesn't work if they escape, so the bulk of the yield that can be used for energy production is in the form of 14 MeV neutrons. These tend not to want to stop, being energetic and uncharged, so capturing this energy, for example in the caveman cudgel approach of heating water and generating steam, is difficult. Various approaches have been proposed, and I'm sure that if it looked like we were anywhere close to achieving high-enough gain to make fusion energy seem promising and economical, many more ideas would be come up and the overall efficiency would be improved. Unfortunately we are not very close to this point.

 

[padawan13]

so what about the designs which involve capturing the neutrons with their 14 MeV of energy and using them to breed tritium. Where does the energy for power production come from?

 

[Drakkith]

so what about the designs which involve capturing the neutrons with their 14 MeV of energy and using them to breed tritium. Where does the energy for power production come from?

It doesn't take 14 MeV of energy to breed the tritium. The lithium blanket will be heated to a high temperature and this heat can be transferred to the turbines to produce power.

 

[padawan13]

but what would stop all that energy being transferred into lithium atoms?

 

[Drakkith]

but what would stop all that energy being transferred into lithium atoms?

Nothing. The neutrons would fuse with the nucleus and the extra energy is imparted as kinetic energy to the atom.

 

[JeffKoch]

One approach that's been discussed on/off for years is coupling 14 MeV neutron production from fusion to a fissionable wall. This is has some significant advantages for producing net energy, and also has been proposed as a way to burn fission reactor waste that would otherwise sit around in pools of water indefinitely (in the U.S. anyways, since for unfathomable reasons we refuse to reprocess spent reactor fuel). But the trick as always is to figure out how to make enough watts of neutrons per watt of power used to supply the whole system, and I'm personally skeptical that we'll ever get there - by the time we figure it out, I'd imagine we'll have made significant advances in other areas, like efficient battery storage and power generation, geothermal, solar, etc., not to mention efficient use of fission through reprocessing, standardized reactor designs, and legal protections to keep activists from halting plant construction for decades.