The fundamental problem with nuclear fusion as a mode of energy production is establishing a system that produces more power than it consumes. Heating and containing large volumes of tritium-deuterium plasma is an energy intensive business. As such, the sheer size of the planned International Thermonuclear Experimental Reactor is a big advantage. Just like it is easier to keep a huge cooler full of drinks cold than to keep a single can that way, a larger volume of plasma has less surface area relative to its total energy. As such, bigger reactors have a better chance of producing net power.
The other big problems that scientists and engineers anticipate are as follows:
- No previous reactor has sustained fusion for very long. The JT-60 reactors in Japan holds the record, at 24 seconds. Because ITER is meant to operate for between 7 and fifteen minutes, it will produce a higher volume of very hot hydrogen (the product of the tritium-deuterium fusion). That hydrogen could interfere with the fusing plasma. As such, it needs to be removed from the reactor somehow. ITER plans to use a carbon-coated structure called a diverter, at the bottom of the reactor, to try to do this. It is not known how problematic the helium will be, nor how effective the diverter will prove.
- Both the diverter and the blanket that surrounds the reactor will need to be able to resist temperatures of 100 million degrees centigrade. They will also need to be able to survive the presence of large amount of radiation. It is uncertain whether the planned beryllium coatings will be adequate to deal with the latter. Prior to ITER’s construction, there are plans to test the planned materials using a specially built particle accelerator at a new facility, probably to be built in Japan. THis test facility could cost about $2.6 billion – one quarter of the total planned cost of ITER itself.
- Probably the least significant problem is converting the heat energy from the fusion reaction into electrical power. This is presumably just a matter of putting pipes carrying a fluid into the blanket, then using the expansion of that fluid to drive turbines. While this should be a relatively basic change, it is worth noting that ITER will have no capacity to generate power, and will thus need to dissipate its planned output of about 500 megawatts by other means.
None of these issues undermine the case for building ITER. Indeed, they are the primary justification for building the facility. If we already knew how to deal with these problems, we could proceed directly to building DEMO: the planned electricity-generating demonstration plant that is intended to be ITER’s successor.
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Both the diverter and the blanket that surrounds the reactor will need to be able to resist temperatures of 100 million degrees centigrade. They will also need to be able to survive the presence of large amount of radiation.
Will these materials become horribly radioactive?
Helium is only found in useful concentrations in the ‘fossil’ gases emitted by relatively a small number of oil and natural gas wells with especially impermeable caps (in the US, mainly in Texas). The helium was trapped and concentrated there after it was generated by radioactive decay of uranium and thorium in the crust and mantle. Helium is on a depletion curve closely related to that of natural gas (data here and here). In 1996, the US helium reserve was privatised and is now in the process of being sold off (The Impact of the Selling of the Federal Helium Reserve, 2000, Openbook here). Helium production appears to have already peaked in the US. After helium boils out of the huge refrigerated thermos bottles around a superconducting magnet, it escapes into the atmosphere and then diffuses into space, where it is lost forever. Helium is an element and cannot be synthesized. The amount generated by a hypothetical working fusion reactor is negligible. After 20 years of high temperature superconductors, no one has come up with one that is both strong enough and capable of carrying enough current. Such a thing may not exist.
Tokamak-style fusion is predicted to become practical in a few decades. But by then, we may near to past the world peak in helium. Helium demand will soar and helium price will soar, too, but it will be too late. Perhaps it really is true that fusion is the power source of the future, and it always will be, as the joke goes.
“Problems related to tritium supply and self-sufficient tritium breeding will be discussed in detail in Section 5.2, but first, it will be useful to describe qualitatively two problems that seem to require simultaneous miracles, if they are to be solved.
“The neutrons produced in the fusion reaction will be emitted essentially isotropically in all directions around the fusion zone. These neutrons must somehow be convinced to escape without further interactions through the first wall surrounding the few 1000 m3 plasma zone. Next, the neutrons have to interact with a “neutron multiplier” material like beryllium in such a manner that the neutron flux is increased without transferring too much energy to the remaining nucleons. The neutrons then must transfer their energy without being absorbed (e.g. by elastic scattering) to some kind of gas or liquid, like high pressure helium gas, within the lithium blanket. This heated gas has to be collected somehow from the gigantic blanket volume and must flow to the outside. This heat can be used as in any existing power plant to power a generator turbine. This liquid should be as hot as possible, in order to achieve reasonable efficiency for electricity production. However, it is known that the lithium blanket temperature cannot be too high. This limits the efficiency to values well below those from today’s nuclear fission reactors, which also do not have a very high efficiency.
Once the heat is extracted and the neutrons are slowed sufficiently, they must make the inelastic interaction with the Li6 isotope, which makes up about 7.5% of the natural lithium. The minimal thickness of the lithium blanket that surrounds the entire plasma zone has been estimated to be at least 1 meter. Unfortunately, lithium like hydrogen (tritium atoms are chemically identical to hydrogen) in its pure form is chemically highly reactive. If used in a chemical bound state with oxygen, for example, the oxygen itself could interact and absorb neutrons, something that must be avoided. In addition, lithium and the produced tritium will react chemically -which is certainly not included in any present computer modeling- and some tritium atoms will be blocked within the blanket. Unfortunately, additional neutron and tritium losses cannot be allowed, as will be described in more detail in Section 5.2.
Next, an efficient way has to be found to extract the tritium quickly, and without loss, from this lithium blanket before it decays. We are talking about a huge blanket here, one that surrounds the few 1000 m3 plasma volume. Extracting and collecting the tritium from this huge lithium blanket will be very tricky indeed, since tritium penetrates thin walls relatively easily, and since accumulations of tritium are highly explosive. An interesting description of some of these difficulties that have already been encountered in a small-scale experiment can be found in reference [38].
Finally assuming we get that far, the extracted and collected tritium and deuterium, which both need to be extremely clean, need to be transported, without losses, back to the reactor zone.
Each of the unsolved problems described above is by itself serious enough to raise doubts about the success of commercial fusion reactors.”