Minimizing the amount of tritium in a power plant was an important consideration in designing the indirect- and drive-direct targets.16 More ambitious ideas were proposed for the indirect-drive concept that will require additional scientific and technical development to realize: drilling a hole in the target to add the fuel (and then resealing the hole) and achieving a uniformly thick fuel layer by suspending the fuel as a liquid within a foam layer. Combined, they would reduce the tritium inventory to less than 1 kg17 by recycling tritium through the facility in less than 8 hours. The first approach adds steps to the manufacturing process and should be technically feasible; the latter approach is also technically feasible, but it is unclear whether the liquid fuel can be cooled below its freezing point and still remain a liquid, which is what has to be done to achieve the gas density required in the capsule. If this is not possible, then an alternative and lengthier process is needed to form the ice layer, which would increase the tritium inventory.
Minimizing the tritium inventory was a less important consideration for developing the direct-drive target. In any case, target tritium inventory for the direct-drive targets is much higher than for the current indirect-drive configuration. About 10 times more tritium is present in this target than in the indirect-drive target. Additionally, tritium is diffused into the capsule instead of flowing through a hole, which takes 2 to 4 days because of the fragility of the target and the quantity of fuel that has to be added.18 The process for forming the ice layer adds about 12 hours to the production cycle, which is the same process that the indirect-drive concept will use if it is not possible to subcool the liquid layer sufficiently to achieve the desired gas density.
Two main contributors to the total tritium inventory of an IFE plant will be these:
• The amount of tritium that is trapped inside the target during the target assembly phases and
• The amount that is entrained in the tritium-breeding and recovery processes (from the gaseous effluent from the reaction chamber).
At this stage, there is insufficient information to know the optimum balance between these sources and whether the effort to minimize the amount of tritium in the target assembly process is worth the added manufacturing and technical complexities.
16 M. Dunne, LLNL, “LIFE Target System Performance,” presentation to the panel on July 7, 2011.
17 M. Dunne et al., LLNL, “Overview of the LIFE Power Plant,” presentation to the panel on April 6, 2011.
18 J. Sethian, Naval Research Laboratory, “The HAPL Program to Develop the Science and Technologies for Direct-Drive Laser Fusion Energy,” presentation to the panel on September 20, 2011.