One of the most complex technological tasks is to manage the supply of tritium. Tritium is injected into the plasma as fuel. It leaves the plasma through the vacuum pumps, most of it going through the divertor. It is generated in the breeding blankets and has to be captured and purified. It is also a contaminant in the liquids and other materials that leave the reactor and has to be removed from them. Excess tritium has to be stored safely for future use in raising the power of the reactor or starting up other reactors. Figure 9.20 shows a simplified diagram of these paths.

To new

Inventory

Fig. 9.20

Tritium leaves the tokamak in two paths — either through the vacuum pumps, including those pumping the divertor, or through the first wall (FW) and the blanket. The vacuum exhaust goes directly to an isotope separation system which saves the T2, D2, and He and removes the impurities. Pure T2 is sent directly to Tritium Storage and Management. The tritium generated in the blanket goes first to a tritium processing plant to remove it from the breeder materials, and then to iso­tope separation. Material contaminated with irremovable tritium from both streams then goes the Tritium Waste Management. The fueling system receives recovered tritium from the two paths as well as from storage or from external sources. The fueling system then injects tritium and deuterium into the plasma. Deuterium is cheap and safe and does not have to be parsimoniously recovered.

The vacuum in the torus is maintained by cryo-pumps [13]. These are porous carbon surfaces cooled by liquid helium to 5 K; that is, 5° above absolute zero, the latter being -273°C or -459°F. At that temperature, all gases except helium are condensed and stuck to the cryogenic surfaces. To release hydrogen, including tri­tium, the cryo-pumps are periodically heated to about 90°K, and this gas is sent to the isotope separation system. To release all the captured gases, the pumps are raised to room temperature.

Fueling is done by injecting frozen pellets of tritium and deuterium at suf­ficient velocity to reach the center of the plasma. This is much more efficient than injecting DT gas at the boundary, since the gas will be ionized at the surface and will not reach the interior. There is some loss of tritium in the process, and this will appear in the pumping system. The plasma is heated mainly by neutral beam injection (NBI), the beams consisting of deuterium and tritium. This system will have its own system of tritium management.

Isotope separation is done by freezing the gases to liquid helium temperatures and selective warming in four interlinked distillation columns [13]. The tritium process­ing plant in ITER is a large seven-story building [12]. In addition, all water in the ITER installation and all air from buildings have to pass through a detritiation plant to remove the tritium. Water released back into the environment is pure H2O, and hydrogen released into the air is pure protium (H2). Tritium has to be stored until it is used. This is done in metal-hydride getter beds, each capable of holding 100 g of tritium [13]. Zirconium-cobalt (ZrCo) absorbs T2 to form ZrCoT3. The reaction is reversible upon heating to release the T2. Although techniques for tritium containment are well established in the fission industry, the amount of tritium in fusion is orders of magnitude larger. There has been no experience so far on such a large scale.