Future of Fusion

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It is my belief that the next major source of energy for Earth’s population will be terrestrial fusion energy. I believe that the type of reactor that will become a mainstream source of energy initially will be the spherical tokamak.

A spherical tokamak is a tokamak with a very short aspect ratio. That is to say the height to width ratio is high relative to conventional tokamaks. The ITER reactor being built is not a spherical tokamak, it is a more conventional design and I believe this is a serious mistake. The confinement qualities improve as the aspect ratio of the plasma becomes more circular and thus a short aspect ratio spherical design can provide more fusion for the same magnetic field strength.

There is more operating experience with conventional tokamaks and I believe this is why a conventional design was chosen for ITER. I believe ITER will be completely obsolete by the time it comes online. By that time I expect that the Chinese and perhaps other countries will already have electricity producing reactors in service. It’s too little too late.

While we’ve all been dicking around over where ITER would be build, the Chinese started construction of a superconductive tokamak reactor in February of 2006 and it saw first plasma in September of 2006. Prior to this reactor no superconductive tokamak reactors had been built.

The significance of a superconductive reactor is that the coils that produce the magnetic fields are superconductive and thus create no heat from their operation. Because of this they can operate for indefinite periods. Existing research reactors use copper coils to generate the magnetic field and they overheat with more than a minute of operation so sustained operation is not possible.

ITER was to be the first superconductive tokamak test reactor to allow testing in a steady state operation. The Chinese realize the urgency of the world energy situation and built a superconductive test reactor in seven months instead of waiting the scheduled twelve years for ITER to come online.

I believe the Chinese will rapidly complete the material research that needs to be done and go on to build a full-power prototype in short order. They will work out any glitches in the prototype and then go on to produce commercial power producing reactors on a feverish scale. Meanwhile, contracts for materials for ITER will not even have been bid out yet.

The Chinese will discover reactors with a shorter aspect ratio perform better and so their design will quickly evolve into a spherical design.

It is often stated that the fuel for hydrogen fusion reactors will be so cheap that it is essentially free because deuterium constitutes one of two thousand hydrogen atoms in seawater. There is a small rub however and that is that current tokamaks can not reach the necessary temperature and pressures for deuterium-deuterium fusion to take place efficiently and so a 50-50% mix of deuterium and tritium is used.

Tritium occurs naturally as the result of cosmic ray bombardment in the upper atmosphere but at extremely small concentrations insufficient for commercial exploitation. Instead, tritium is usually bread from lithium which makes lithium a necessary fertile material from which to breed the fuel for fusion reactors.

Lithium exists in the earths crust in concentrations of only 20 parts per billion, and in seawater it is even more scarce having a concentration of only about .17 parts per billion. Presently, the price of lithium is around $27 / 100 grams, trivial considering the amount of energy that can ultimately be obtained from that 100 grams, but as the number of fusion reactors escalates, so will the demand for and price of lithium.

The tokamak design is tremendously expensive and complex. This design requires the fuel be heated to a temperature in which the average kinetic energy is favorable for a D-T reaction to occur. The problem with a thermal approach is that the nuclei all have different energies and many nuclei will either be not energetic enough or too energetic for a reaction to occur. Thus a thermal approach is inefficient.

Some potential nuclear fusion fuel cycles create neutrons, D-D and D-T are among these, and the bombardment of reactor components by these neutrons results in neutron activation rendering them radioactive. They are not nearly as hot as spent fuel, but they still represent a disposal issue.

Other fusion fuel cycles create no neutrons, boron and hydrogen nuclei create only charged products, no neutrons, and thus result in no neutron activation or embriddlement of reactor components.

However, the energy levels requires to achieve hydrogen-boron fusion are beyond those that are obtainable in tokamak reactors at present.

There are alternatives to the tokamak that have the potential for sustaining the necessary temperatures and pressures. One of these is a device called a levitated dipole reactor. The earth generates a dipole magnetic field which contains a plasma at extremely high temperatures with no problem. A recent reactor design involves magnetically levitating a superconductive magnet to provide a similar dipole field. It is believed that this reactor design may be capable of reaching the temperatures necessary for hydrogen-boron fusion.

Another design is one that was invented by Robert Bussard. Basically, this design is based upon the fusor concept but replaces the physical grids with electron clouds held in place magnetically theoretically allowing it to achieve power levels that are viable for commercial power production. Unfortunately, a full-scale reactor of this design has yet to be built, and with Robert Bussard being 78 years old, of ill-health, and without others championing this design, it is likely that it never will be.

Ultimately, one of these designs may replace the Tokamak but I believe the Tokamak reactors will be the first to go online commercially. In large part I believe this is because they are sufficiently capital intensive that existing energy companies can feel secure that they still can have a lock on the energy market.

Long term however, I believe these other designs and perhaps some we haven’t even though of will take their place owing to their ability to operate on aneutronic fuels.

Either way, lithium will become the limiting factor for tokamak reactors and boron for aneutronic reactors based upon proton-boron fusion.

Another option is He3, which is exceedingly rare on earth but less rare on the moon. However, using that fuel source would require a substantial industrial presence on the moon and a substantial space transport system presently not in place.

So overall I think fusion will be a considerable improvement over burning hydrocarbons both in terms of environmental impact and availability, however I don’t believe it’s going to approach free by any means, both because ultimately the demand for lithium will drive the price up and because the reactors will be capital intensive to build even if they are relatively inexpensive to operate.

This if coarse is not to discount exploiting the natural fusion reactor in the sky, however, the density of solar power is problematic for many applications and earthbound exploitation is limited by available land.

Category: Future

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