Remaining Barriers To Fusion Energy On Planet Earth

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Producing electricity from controlled nuclear fusion would require overcoming at least four major obstacles. The removal of each obstacle would need major scientific breakthroughs before any reasonable expectation might be formed of building a commercial prototype fusion reactor.

It should be alarming that at best only the problems about plasma control, described in point (1) below, might be investigated within the scope of the ITER project. Where and how the others might be dealt with is anyone's guess.

These are the four barriers:

1. Commercial energy production requires the achievement of steady-state fusion conditions for a deuterium-tritium plasma on a scale comparable with today's standard nuclear fission reactors with outputs of 1,000 MW electric (MWe) and about 3,000 MW thermal (MWth) power.10 The current ITER proposal foresees a thermal power of only 400 MW using a plasma volume of 840 m3. Originally it was planned to build ITER with a plasma volume of 2,000 m3 corresponding to a thermal fusion power of 1,500 MW, but it was realized quickly within the fusion community that the original ITER version would never receive the required funding. Thus a smaller, much less ambitious version of the ITER project was proposed and finally accepted in 2005.

Today's 1,000 MWe fission reactors function essentially in a steady-state operation at nominal power and with an availability time over an entire year of roughly 90 percent. The deuterium-tritium fusion experiments have so far achieved short pulses of fusion power of 15 MWth for about one second and 4 MWth for about five seconds corresponding to a liberated thermal energy of about 5 kWh. The Q-value - produced energy over input energy - for both pulses was 0.65 and 0.2 respectively.

If everything works as planned, it will be 2022 (or about six years after the first plasma experiment has been performed) before ITER tries, for the first time, to achieve a power output of 500 MWth with a Q-value of up to 10 and for about 400 seconds. Compare that to the original ITER proposal which was 1,500 MWth, with a Q value between 10 and 15 and for about 10,000 seconds. ITER proponents explain that the achievement of this goal would already be an enormous success. But this goal, even if it can be achieved in 2022, pales in comparison with the requirements of steady-state operation, year after year, with only a few minor controlled interruptions.

Previous deuterium-tritium experiments used only minor quantities of tritium and yet lengthy interruptions between successive experiments were required because the radiation from the tritium decay was so excessively high. In earlier fusion experiments, such as the 1997 UK deuterium-tritium fusion project, JET,11 the energy liberated in the short pulses came from burning (fusing) about 3 micrograms (|ig) (3 x 10-6 g) of tritium, starting from a total amount of 20 g of tritium. This number should be compared with the few kilograms of tritium required to perform the experiments foreseen during the entire ITER lifetime and the still greater quantities that would be required for a commercial fusion reactor. A 400-second fusion pulse with a power of 500 MW corresponds to the burning of about 0.035 g (3.5 x 10-2 g) of tritium. A very large number when compared to 3 |ig, but a tiny number when compared with the yearly burning of 55.6 kg of tritium in a commercial 1,000 MWth fusion reactor.

The achieved efficiency of the tritium burning (that is, the amount that is burned divided by the total amount that was required to achieve the fusion pulse) was roughly one part in a million in the JET experiment and is expected to be about the same in the ITER experiments, far below the efficiency required to burn 55.6 kg of tritium per year.

Moreover, in a steady-state operation the deuterium-tritium plasma will be "contaminated" with the helium nucleus it produces and some instabilities can be expected. Thus a plasma cleaning routine that would not cause noticeable interruptions of production in a commercial fusion plant is needed. ITER proponents know that even their self-defined goal (a 400-second-long deuterium-tritium fusion operation within the relatively small volume of 840 m3) presents a great challenge. They seem surprisingly silent about the difficulties involved in reaching steady-state operation for a full-scale fusion power plant.

2. The material (called a "blanket") that surrounds and contains thousands of cubic meters of plasma (containing the active particles) in a full-scale fusion reactor has to fulfill two requirements. First, it has to survive an extremely high neutron flux (neutron radiation) with energies of 14 MeV, and second, it has to do this not for a few minutes but for many years. It has been estimated that in a full-scale fusion power plant the neutron flux (or bombardment) will be at least ten to twenty times larger than in today's state-of-the art nuclear fission power plants. Since the neutron energy is also higher, it has been estimated that - with such a neutron bombardment - each atom in the solid that surrounds the plasma will be displaced about 475 times over a period of five years.12 Second, to further complicate matters, the material in the so called first wall (FW) around the plasma will need to be very thin, in order to minimize inelastic neutron collisions resulting in the loss of neutrons (more details next section), yet at the same time thick enough so that it can resist both normal and accidental collisions from the 100-million-degree hot plasma for years.

The "erosion" for carbon-like materials from the neutron bombardment has been estimated to be about 3 mm per "burn" year, and even for materials like tungsten it has been estimated to be about 0.1 mm per burn year.13

No known material can even come close to meeting the requirements just described. Exactly how a material could be designed and tested to meet those requirements remains a mystery, because tests with such extreme neutron fluxes cannot be performed either at ITER or at any other existing or planned facility.

3. Because radioactive decay of even a few grams of tritium creates radiation dangerous to living organisms, those who work with it must take sophisticated protective measures.

Tritium is, moreover, chemically identical to ordinary hydrogen and as such very active and difficult to contain. Since tritium is also a necessary ingredient in hydrogen fusion bombs, there is additional risk that it might be stolen. So, handling even the few kilograms of tritium that are foreseen for ITER is likely to create major headaches about radiation protection and nuclear weapons proliferation.

The above challenges are essentially ignored in the ITER proposal, and the only thing the radiation and weapons proliferation concern groups have to work with are design studies based on computer simulations. This may not be of concern to the majority of ITER's promoters today, since they will be retiring before the tritium problem starts in something like ten to fifteen years from now.14 At some point, however, ITER will have to face these problems if it actually begins work on a real fusion experiment with many tens of kilograms of tritium.

4. Problems related to tritium supply and self-sufficient tritium breeding will be discussed in detail in the following section of this chapter. First, however, I will describe qualitatively two problems which it seems it would require simultaneous miracles to solve.

• 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 1,000 m3 plasma zone. Next, the neutrons have to interact with a "neutron multiplier" material like beryllium in such manner to increase the neutron flux without transferring too much energy to the remaining nucleons. The neutrons then must transfer their energy without being absorbed (for example, by elastic scattering15) to some kind of gas or liquid, like high-pressure helium gas, within the lithium carpet. This heated gas or liquid has to be collected somehow from the gigantic carpet volume and must be encouraged to flow to the outside. As in any existing power plant, this heat can be used to power a generator turbine. The gas or liquid should be as hot as possible, in order to achieve reasonable efficiency for electricity production. As we know already, however, the lithium carpet temperature can't be too hot, thus limiting possible efficiencies well below the ones from today's not very efficient nuclear fission reactors.

Once the heat is extracted and the neutrons have slowed sufficiently, they must interact inelastically with the Li6 isotope, which makes up about 7.5 percent of natural occurring lithium. The minimum thickness required of the so called lithium carpet that surrounds the entire plasma zone has been estimated to be at least one meter. Unfortunately, lithium, like hydrogen, in its pure form is chemically highly reactive. If used in a chemically bound state with oxygen, for example, the oxygen itself could interact and absorb neutrons, something that must be avoided. In addition, that the lithium and the tritium produced would react chemically and that some seven tritium atoms will be blocked within the carpet, has certainly not been included in any present computer modeling. Unfortunately, additional neutron and tritium losses cannot be allowed. Reasons will be described in more detail in the next section.

• Next, the engineers need to find an efficient way to extract the tritium quickly before it decays and without loss from this lithium carpet. We are talking about a huge carpet here, one that surrounds the few 1,000 m3 plasma volume. Extracting and collecting the tritium from this huge lithium carpet will be very tricky indeed, since tritium penetrates thin walls relatively easily, and since accumulations of tritium are highly explosive.16 And 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 envisaged success of commercial fusion reactors. But the self-sufficient tritium breeding is especially problematic, as will be described in the next section.

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