Sunday, February 7, 2016

Recycling Fusion Reactor Stations Part 2

Recycling in a long-term stable civilization is different that recycling in a rapidly-changing civilization which has not yet reached asymptotic technology and become as stable as possible, varying only as resources are consumed and extraction costs become higher. Take transportation as an example. In a long-term stable civilization, materials from some transport equipment are obtained when some item is removed from the transportation system, and the materials can be stripped from it and recycled, and then devoted to the task of building a new item, just the same as the one that was scrapped. This creates some efficiencies in recycling. For example, if an alloy of some mixture of metals was needed in a flying car, and the car reaches the end of its life, the components made of that alloy can be taken from the car during disassembly, and sent to a recycling facility. But instead of the recycling facility having to separate the metals back to separate elements, it simply needs to re-process the old metal to bring it back to the same state that it was in when the original parts were made. After asymptotic technology is reached, no new alloys are going to be invented, and the best one was already chosen to be used wherever it was cost-efficient. This does not just hold for alloys, but for many components of different types of materials.

Furthermore, when a flying car, or anything else, is designed, the design team thinks almost first about how to both build and then un-build and recycle the car. It would be designed to be taken apart, which has implications for what materials are used, and how they are combined and put together.

Asymptotic technology applied to recycling also has other advantages. Lower level components can be recycled. Instead of the whole car being recycled, some wheels or bearing or props or tanks or whatever can be recycled, whenever it is most convenient for them. The car would be brought in to the maintenance and recycling area, and, for example, the high-pressure hydrogen tank, which might be the thing with the shortest lifetime, is taken out and sent to recycling, and a new one taken out of inventory and put in its place. This again makes recycling much more efficient when component swapping is done on a mass basis.

These basic principles of design would be used for a power plant as well, not simply mobile elements of the infrastructure. They would be even more effective there. In a fusion power plant, there might be a need for mono-isotopic materials, where the isotope chosen is one with a very small or large neutron cross-section or for some other isotope-specific quality. Then, when some component of the power plant is being recycled, and it was made of mono-isotopic elements in some alloy, the component’s materials do not have to be separated down to elements, and even more importantly, do not have to be isotopically separated again. Once isotopically separated, they stay separated.

What has to be done on the reactor side of the power plant, as opposed to the conversion side, is to recycle components to take out neutron and proton damage, as well as all the other more ordinary types of damage. Ordinary damage includes changes to materials caused by simple aging, from thermal effects which can include increased rates of aging, corrosion, abrasion, contamination, wear, oxidation, and other chemical and physical modes of degradation. Materials which flex can lose their elastic strength over time, and have to be replaced. Materials under stress gradually strain, and have to be reworked. All these things are mandated to be taken care of the combined maintenance and recycling program that keeps a power station alive for its design life, and then disassembles it down to more basic parts so another one can be built at another chosen location.

The unique types of recycling that have to be done at a fusion reactor include resolving problems of neutron damage. Any component near the fusion area will be subject to a flux of neutrons. The flux will be large in either a DD or a DT reactor. Neutrons both transmute elements in the nearby components, but also cause nodal displacements, which accumulate over time, weakening the material. Transmutation in a isotopically separated material might be thought of isotope by isotope. Some neutron caused changes alter the isotope and some alter the element. The latter is more easily taken care of in recycling, and some source of isotopically separated elements are needed to replace what was transmuted. This is a very small fraction of the total amount of materials. Isotopic changes, like what happens when Gd155 absorbs a thermal neutron and become Gd156, might be tolerated to some degree, but if the very high neutron cross-section isotopes were needed, re-separation might be necessary.

Proton damage requires some re-processing of the material to remove any absorbed hydrogen, but also to repair damage caused by the high-energy protons zooming through the material. Almost all materials are damaged, some in unique ways, from this phenomena. Metals might have to be re-annealed; polymers separated and re-polymerized; glasses filtered and recast; and so on.

Some unique efforts might have to be done to metals which were used near to the sources of high magnetic fields, if indeed, magnetic confinement in some shape does prove to be the best way to build a fusion reactor. Magnetic fields strain conductors and these might have to be recycled much more frequently than other parts, such as building materials.

There are questions for us related to nuclear fusion power plants as we on Earth proceed toward deeper understanding of how they might be designed and built, and these questions fit into two categories: is it physically possible to obtain more power out of a fusion reactor than goes into making it and running it; is it economically possible to cover the lifetime costs of a fusion reactor, from construction through disassembly and including disposal of radioactive elements, figured in some sensible currency such as energy itself. Recycling would be a dominant factor in these calculations. If there are some expensive components, figured in energy, that are needed in the reactor side of the station, which have to be recycled frequently because of neutron and proton damage, and the recycling costs are high, the plant may be infeasible.

In other words, recycling costs may make it impossible for any alien civilization to have fusion power except as a curiosity, paid for by other power sources. If this is so, we have the answer to why no aliens can get here. They wouldn’t have the energy resources to build and send the starship. Just for reference, recall that one of the rules of this blog is that magic is not invoked. There is no magic physics that transports an alien ship for no or little energy cost.

What does an alien civilization do if it finds that recycling costs for the best fusion reactor design they can come up with consume more energy that the excess that the reactor produces? Fission power is great as a bridge to fusion, but it has a lower energy density, and nothing like the almost unlimited supply of fuel that deuterium promises. Do they just give up and wait for extinction?

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