Revolutionizing Energy: The Future of Fusion Reactors Explained

[ucf-fisher21]

i was watching this show on the Science Channel and in it they talked about a new type of fusion reactor that is currently being built(construction started like 30 years ago). They said it will produce energy for entire world the same way the sun produces energy (fusing hydrogen). It's suppose to be something like 1000 times more efficient than current fusion reactors. They go on to say that water can be used to produce the energy.

Will they just put water in the reactor, or will they split water to get hydrogen and put that hydrogen in the reactor? Can anyone explain how this reactor will work?

 

[mgb_phys]

I think you mean more efficent than current FISSION reactors, all the current fusion reactors are experimental machines that can only maintain fusion for a few seconds and take more energy than they generate.

The problem with fusion is that you need incredibly high temperatures and pressures to force the positively charged hydrogen nuclei together. The centre of the sun is a good place to do this - doing it in a lab is tricky!
Although in theory you can ultimately build a H+H fusion reactor this is the most difficult, it needs the most pressure and highest temperatures. A more likely system uses one of the heavier isotopes of hydrogen ( deuterium or tritium ) these can be obtained in very small quantities from seawater - although it takes energy to do this the plants already exist since heavy water is required for some existing fission reactors.

The great thing about fusion, apart from the essentially unlimited fuel source (ie the ocean!) and the vastly greater energy output - there is much less waste.

 

[vanesch]

Was this about the ITER ?

Actually, per unit of mass, fusion is only about 4 times more efficient than fission when compared to fast breeder reactors. But indeed, the potential advantages of fusion are the rather unlimited supply of fuel (no geopolitical concerns, you "only need water"), and the low activity of waste generated, as compared to fission. Also, the much smaller potential danger, given that the amount of fuel in a fusion reactor (a few milligrams) is very very small compared to the amount of fuel in a fission reactor (200 tons or so).

The only problem with fusion is that "already 50 years, one says it is going to work 30 years from now". The tokamak was invented in 1954, and it would also take 30 years to make it work (1984...).

 

[ucf-fisher21]

the show was on the Science Channel and the title was Building the Future:the Search for Ultimate Energy. I guess it's a new series that will include maybe 4 shows with each one focusing on a different topic. They should eventually show re-runs of this episode.

They basically said that once the reactor is made, it will be like having a small sun on earth. The fusion will be self-sustaining.

 

[Orion1]

I believe that the program that you are referring to is called Building the Future: The Search for the Ultimate Energy which was aired on Jun. 10, 2007 on the Discovery Channel.

I was unable to locate a internet version of this episode, however, I noticed an interesting reference to the MIT plasma science and fusion center for a MIT electromagnetic superconducting cryostat fusion reactor featured on the Discovery Channel, video link listed in reference.

I am somewhat puzzled however, how such a device could possibly sustain a thermonuclear fusion chain reaction, given its divergence in Tokamak engineering design.

Reference:
http://dsc.discovery.com/video/play...52171&lineupId=1579871415&titleId=1632777663"

 

[ucf-fisher21]

vanesch,

Yes. they were talking about the ITER project. Anyone who wants to find out more about it can google iter and click on wikipedia link.

orion, the engineers building it must be confident that they will eventually be able to sustain a reaction because they are spending over 9 billion dollars on it. In fact, when they were talking about it on science channel show, they said it is the second most expensive engineering project, just behind the International Space Station.

 

[mheslep]

Here's one of the more noted, controversial, and long lasting attacks on ITER, Lawrence Lidsky's "The Trouble with Fusion" published in 1983. Lidsky was a noted nuclear engineering professor at MIT. Lidsky did a lot of work in the field but became increasingly skeptical of its practicality. After getting little traction for his critical questions at MIT, he wrote this.
http://www.askmar.com/Robert Bussard/The Trouble With Fusion.pdf

The most serious difficulty is the very high energy neutrons released in the D-T reaction. These uncharged nuclear particles damage the reactor structure and make it radioactive. A chain of undesirable effects ensures that any reactor employing D-T fusion will be a large, complex, expensive, and unreliable source of power... The requirement is to develop a power source significantly better than those that exist
today, and D-T fusion cannot provide that solution. Even if the fusion program produces a reactor, no one will want it...The only real hope for fusion is to take the long view...Neutron-free fusion is a quintessential example of a high-risk, high-gain area of physics that might also provide a good answer to an engineering problem.

Reply years later here by an ITER - related researcher. See section III.
http://fire.pppl.gov/fusion_critic_response_stacey.pdf

 

[Orion1]

For clarification, I was referring to the MIT cryostat fusion reactor on post #5 and video listed in that reference, called the 'Levitated Dipole Experiment' or 'LDX' reactor, listed in reference 1.

Any comments on the MIT reactor design?

Reference:
http://psfcwww2.psfc.mit.edu/ldx/"
http://dsc.discovery.com/video/play...52171&lineupId=1579871415&titleId=1632777663"

 

[ucf-fisher21]

Orion,

I watched the video, which was very interesting. This reactor is definitely on a smaller scale than ITER. I was surprised when they said that the 'donut' part of the reactor levitates during fusion. I wonder if ITER will do the same thing?

 

[mgb_phys]

I was surprised when they said that the 'donut' part of the reactor levitates during fusion. I wonder if ITER will do the same thing?

No - the designs look similar but are different.
In a tokamak (ITER) the magnets are on the outside and the plasma forms a ring inside the chamber.
The MIT design is almost exactly the opposite - the magnet is in a ring inside and the plasma forms a donut shape around the magnet. The levitating part is simply to avoid having any mechanical supports to the magnet which would interfere with the plasma.

There is some controversy about ITER - some people think it is a 'snowball' project that simply scales up previous experimental reactors without considering alternative designs.
But if you are spending $10Bn you probably want to start with a well understood conventional design.

 

[ucf-fisher21]

The funders and engineers building ITER must be somewhat confident it will work if their spending 10 billion on it. Let's hope so, because we could definitely use this to end the 'energy crisis' going on in the world right now.

 

[mgb_phys]

The funders and engineers building ITER must be somewhat confident it will work if their spending 10 billion on it.

Thats the problem "funders and engineers" not the physicists!
A project this size gets a such a momentum of administration and project management that the actual design seems almost a by-product. Once a starting point (ie scale up JET) is set, you can't consider anything different because it is a risk and the cost is so large you can't have any risk. NASA is often a good example of this problem.

 

[ucf-fisher21]

mgb_phys ,

I hope it works out. I'm sure their top priority is getting the reactor to work, but how they go about doing that, like you said, is key.

 

[mheslep]

It seems to me that there are some serious flaws with this 'internal' magnet design, namely the the floating doughnut in any realistic reactor would quickly be destroyed by the high energy products of fusion. A great deal of work has been put towards the outer walls of tokamaks: exotics for the first wall, backed up with molten lithium to carry away the heat, etc. None of this seems possible w/ a levitated internal super conductor. Am I missing something?

 

[mgb_phys]

I'm not an expert but I think that precisely one of the advantages of the floating design.
In a tokakmak it's very difficult to hold large amounts of plasma with different energies in a tight ring.
With the floating design the plasma is both held away from the ring by the field (self focussed?). The outer vacuum cryostat can be as large as you like since it isn't part of the field, so can be kept away from the plasma.

I thought extracting the energy normally requires deliberately allowing some plasma to heat the walls which you can then use to generate steam.

 

[mheslep]

Yes well I grant the idea that the floating dipole might do well at plasma confinement, that is the stated purpose. But that has little to do with the high E neutrons with which a working design must be concerned. The plasma, i.e, charged nuclei and electrons, is never intended to come into contact with anything solid. In D-D or D-T fusion, the neutrons, contained by nothing other than collisions, carry almost all of the energy produced in kinetic form at MeV levels into the walls of the reactor. That is where energy is absorbed in the form of heat by the ITER design. The neutrons also a have the side effect of making the containment vessel inevitably radioactive as Lidsky pointed out (above) while slowly destroying the first wall. (I believe ITER researchers say walls can be built that would last a year in a working reactor). I'm speculating that since the MIT dipole can not be made so tough, nor can it carry away heat, that it would be destroyed no so slowly.

 

[mgb_phys]

Good point, I didn't think of the neutrons.
A magnet quench would be interesting in the floating design as well!

 

[mheslep]

Here we go, from a paper referenced on the MIT site:
http://psfcwww2.psfc.mit.edu/ldx/pubs/presents/DPP03_kes.pdf

...The internal floating coil will operate with a high outer surface tempera-
ture (> 16000 K) so as to radiate away all of the surface and neutron heating
via black body radiation (assuming an emissivity≈1). In addition, we en-
vision that the floating coil will have internal refrigerators that will pump
to the surface the heat that is deposited directly into the superconducting
coil via volumetric neutron heating.

and

...In total we find that there is 137 MW of power deposited into the surface of the coil (DD study in Table 4). If we thermally isolate the outer and inner
shells we can use the temperature difference to drive a refrigerator.

Ok, the authors certainly recognize this as a problem to be solved. They elsewhere state the doughnut shield will be carbon boron. Still, I had thought that allowing a wall to get that hot meant substantial heavy nuclei poisoning of the plasma. It also turns out that most of the heating comes from Bremsstrahlung radiation.

 

[ucf-fisher21]

If they are able to create these new fusion reactors, how would they take the energy produced by reactor and make it usable for the world? For instance, would they use the heat generated by reactor to create electricity by means of a steam turbine?

 

[Astronuc]

If they are able to create these new fusion reactors, how would they take the energy produced by reactor and make it usable for the world? For instance, would they use the heat generated by reactor to create electricity by means of a steam turbine?

The plan would be to use thermal energy to drive a power cycle like the Rankine cycle.

Direct conversion would be ideal, but I'm not sure if it's feasible with the tokamak geometry.


Unfortunately, none of the MIT-LDX links work. However, the temperatures (> 16000 K) would be incorrect. The highest melting points would be 3890, 3880 and 3800 K for HfC, TaC and carbon, respectively, and C's boiling point is 4300 K! I believe that the temperature should be more like 1600 K.

Ref - http://www.ultramet.com/ceramic_protective_coatings.html

 

[Orion1]

Carnot cycle efficiency


According to reference 1 pg. 22, the surface input temperature is 1641 K and the internal temperature of the high temperature superconductor is 50 K, the refrigerator shell would occupy the differential radial difference.

Applying this to the Second Law of Thermodynamics for the fundamental limit on refrigeration energy and specifically to Carnot's theorem for Carnot cycle efficiency, the equation becomes:
$$T_C = 50 \; \text{K}$$
$$T_H = 1641 \; \text{K}$$

$$\eta_{th} \le 1 - \frac{T_C}{T_H} = 1 - \frac{50 \; \text{K}}{1641 \; \text{K}} = 0.969531 = 96.9531 \; \%$$

$$\boxed{\eta_{th} \le 96.9531 \; \%}$$

Current household refrigerators energy efficiency:
$$\eta_{th} \le 50 \; \%$$

No known refrigerator exists that can sustain the required MIT-LDX refrigeration energy efficiency. Therefore at its limit, such a reactor design could only operate in a pulsed mode.

Also, according to reference 1 pg. 16, the diameter of an industrial MIT-LDX type reactor design would have a diameter twice that of ITER. (see attachment)
d_iter = 30 m
d_ldx = 60 m

Reference:
http://psfcwww2.psfc.mit.edu/ldx/pubs/presents/DPP03_kes.pdf"
http://en.wikipedia.org/wiki/Thermodynamic_efficiency#Carnot_efficiency"

 

[vanesch]

In fact, when they were talking about it on science channel show, they said it is the second most expensive engineering project, just behind the International Space Station.

Well, in *any case* it will not be hard to come up with more results for the buck than the space station (unless flushing toilets in 0 gravity suddenly becomes a major issue to humanity )

ITER is for sure going to learn us something ; after all it is a scale up and improvement of JET, which already showed some fusion results. The only question is how much ITER is still away from a working, commercially viable power plant. But in any case we should know. If ITER turns out to pave the road to a good design, then it would be silly not to do it. But if ITER shows problems on that way, well, then it is good to know too.

 

[ucf-fisher21]

Is ITER just another experiment or do they plan on making it the first sustaining fusion reactor for world energy production?

 

[mgb_phys]

It's an experiment to prototype commercial reactors. It will produce about as much power as a single conventional fission reactor but only for a few minutes at a time.
But it will (hopefully) develop the technology to build and run a commercial reactor, which will probably be very similair just because it's easier to copy what you know.