Tritium Self-Sufficiency

The blanket designs shown above can barely breed enough tritium to keep a D-T reactor going. The tritium breeding ratio (TBR) is a measure of this. Each time a T fuses with a D in the plasma, one neutron is created. This neutron has to generate more than one T to re-inject into the plasma because there will be losses in the process. In addition, extra T’s have to be stored to build up the inventory of tritium to run the reactor at a higher power or to fuel another fusion reactor. Only fusion can produce the enough tritium to build up its own industry.

The number of T’s created in the blanket for each incoming neutron is the TBR. It has not been possible to design a blanket with a TBR larger than 1.15. That means that less than a 15% margin is available. The consequence is that tritium self-suffi­ciency can be achieved only after many years. The time is long because only a small percentage of the tritium injected into the plasma actually fuses with a D; most of it goes out the divertor and is recycled. This fractional burnup is only a few percent. Figure 9.19 shows calculations of how long it will take to double the tritium inven­tory. On the vertical scale, the TBR is plotted. The bottom portion, below TBR = 1.15, is what is possible. The horizontal axis shows the fractional burnup in percent. The curve labeled 1 year shows that it is not possible to double the tritium

inventory in 1 year, since the curve never goes low enough to reach the feasible range of TBRs. The 5-year curve barely makes it if 5% burnup can be achieved. More likely, it will take almost ten years to double, and self-sufficiency can be achieved only after decades.2

In early tokamaks, before good divertors were developed, the fractional burnup was much larger, perhaps 30%, because of recycling. Ions of the plasma would hit the vacuum wall and recombine into neutral gas. This gas would go back into the plasma and be re-ionized and re-heated, thus being available again without having left the chamber. If modern divertors work well, however, ions are prevented from hitting the wall, thus preventing recycling. The ions are instead led to the divertor, where they recombine into gas and are pumped out before they can re-enter the plasma. In ITER, the fractional burnup is expected to be only 0.3%, which would be unacceptable for reactors [32]. Since burnup depends on the triple product Tm discussed in Chap. 8, this is another indication of the large step between ITER and a working reactor.

A fission reactor can produce only 2-3 kg of tritium a year, and tritium decays by 5.5% per year, so it is continually being lost. It will take 10 kg of tritium just to get DEMO started. ITER itself will use up most of the tritium available in the world [32]. There is therefore some urgency to develop breeding blankets with higher TBRs.