30 April, 2007

Fusion - Some thoughts on Tokamak

Back in January I got caught up in one of my crazy idea posts after reading about Boron fusion over at Classical Values and Power and Control.

I have been stewing on those for a while and doing some light research and what has really troubled me was the implications of this on Tokamak designs that I had never considered. I caught the beginning inclination of this in my post but have clarified it somewhat lately.

I am not saying they haven't been considered but I hadn't though of them. Some of this post is going to come off as anti nuke and anti fusion. Nothing could be further from the truth in all honesty nuclear power is our only realistic long term solution to the energy challenges we will have in the future and fission despite its advantages has some pretty significant drawbacks as well.

Fusion has always been served as the Holy Grail to solve these disadvantages and despite my background in Nuclear Physics and operation I never really questioned it. I have eagerly read about the development of toroidal field reactors and overlooked one key issue.

They have to use Neutron energy as the means to transfer energy from the fusion reaction to the power generation or transfer mechanism.

The impact of this is huge. In order to get any real power out of a fusion reaction in this manner the neutron flux would have to be insanely large. To put it in context in u235 fission reactions the neutrons produce on average less than 3 percent of the energy transfer. It results in a few degrees of heat in the primary coolant and further a few degrees in the shield tanks. While it does this it is also one of the primary problem creators for the entire reactor (of course one that by definition must be present).

It causes embrittlement and metallurgical changes in all of the reactor materials.

It is the mechanism of radioactive contamination creation.

It is the most difficult radiation to shield from with the most perplexing health impacts for people exposed.


It fundamentally alters the chemistry of the complex materials used to operate and control the reactor over time.

None of these issues go away in a Tokamak they way they identify the energy transfer mechanisms. As a matter of fact they would be about 20-30 times worse for the same thermal power output. I am not sure how anyone could ever make a viable case for a net energy producing Toroidal design and certainly not an economical one with that in mind. To go a step further it would create far more waste and more dangerous waste (admittedly only in the short term due to the lack of transuranic long lived waste) than existing fission designs.

So Dr. Bussard is quite right when he questions why we are spending the money on those approaches.

As far as the video and presentation, they make sense. I could see them adding injection fields that might use some of the toroidal design properties to help mitigate some of the electron leakage problems he mentioned in their existing designs but he has me sold.

If anyone wants to chime in and correct me feel free especially if I am missing something fundamental in the way Physicists are planning on getting power out of the Tokamaks.


Anonymous said...

Are you as sceptical about "fast Ignition Laser Fusion" (Inertial Confinement) as you seem to be about the Tokamak approach ? HiPER Laser submits to Brussels on 02 May for development funding and, although you are right that there are significant issues still to be solved, not least in the materials field, given the mess the world seems to be running into with massive and rapidly growing dependence on fossil energy, if we don't give fusion energy (both Tokamak and Laser) every chance, by the time we are forced to face the problem it will be too late to start the development work. However hard we try to make the best of renewable energy sources, there is no hope of meeting the huge energy demands without fossil fuels, even if we go for the next generation of fission reactors (which unfortunately we must)... or am I missing something ?

Jim C said...

You are completely right about the need to address fossil fuel problems from supply to politics to their impact on the atmospheric CO2 levels. Fission has some significant problems in the long run due to the long lived isotopes. This can be partially offset by using the approach that the French and Japanese of reprocessing the fuel. Unfortunately that is an option that Jimmy Carter precluded here in the US (albeit for pretty good reasons). In the long run even this approach is not going to work out. As for the Fast ignition laser fusion approach it still uses neutrons as the heat transfer mechanism. This means it is not just a matter of better materials but also of materials that stay better as they undergo neutron absorption and change to other materials. It may be possible but it will mean some incredibly sophisticated engineering to become economically viable. If every year or two half of the reactor has to be replaced due to neutron induced changes it will be a significant cost. If the half that was removed has to be isolated for 30+ years because it is highly radioactive I am not sure it would ever be feasible.

There might be a way to use the other resultant fusion products to serve as the primary energy transfer mechanisms. This would seem to be more viable in laser ignition than in a toroidal field design due to the fact that the magnetic containment essential in the Tokamak's isn't required.

In short I am not sure. I'm learning a lot though and enjoy the question.

M. Simon said...

Bussard Reactor Funded

I have inside info that is very reliable and multiply confirmed that validates the above story. I am not at liberty to say more. Expect a public announcement from the Navy in the coming weeks.

Thought you might be interested