Session 2: Tritium Issues for Inertial Fusion Energy
Chairman: Ken Schultz
The Inertial Fusion Energy (IFE) session of the Tritium Town Meeting focused on discussing tritium issues relative to the similar issues for MFE. We tried to identify which tritium issues are similar to those of MFE and which tritium issues are unique to IFE. To start the discussions, we heard presentations by Scott Willms for Anita Schwendt on her models of IFE target factory tritium inventory modeling, by Neil Alexander on some innovative techniques to achieve high volume target filling for IFE, and by John Perkins on the prospects for achieving low-inventory, tritium-lean target designs. Anitašs work showed that straight-forward extrapolation of current ICF diffusion fill techniques to IFE could result in very large tritium inventories in the target factory. Neilšs work suggested that a number of innovative techniques may be possible that could significantly reduce these inventories. John summarized his target designs which show that inertial fusion targets may be able to bootstrap themselves up from DT ignition to burn of advanced fuels. This could reduce the amount of tritium in each target to ~1% of a standard DT target, with corresponding reduction in the target factory inventory. Sufficient tritium could be bred in the advanced fuel of the target itself that no breeding blanket would be needed. To be energetically attractive, this concept requires use of the fast ignition idea to reduce the required target compression energy.
Many of the tritium issues of IFE are very similar to those of MFE and it is important that we take advantage of these similarities to maximize synergism between the two programs.
Blankets. Both MFE and IFE have to breed tritium in a blanket, recover tritium from that blanket, and be concerned about the tritium inventory in the blankets and about potential accident pathways for release of that tritium. IFE appears to have an easier time achieving an adequate breeding ratio because of the simpler geometry and higher possible wall coverage without divertors. Because of this simpler geometry, there is more interest for IFE in the use of free-flowing liquid breeder blankets inside the chamber, including use of Flibe, which appears to have too high a vapor pressure for use inside MFE chambers.
Tritium Processing Systems. Both MFE and IFE must inject DT fuel into the reaction chamber, recover the unburnt fuel and fuel bred in the blanket, and process these for reinjection. IFE appears to promise higher burnup than MFE with will reduce the tritium processing systems throughput, size and inventory. IFE will introduce some unique impurities to the processing stream. These include possible chamber protection gases such as Xe, and target debris that may contain a variety of polymer and high-z materials. The use of Flibe will require very careful chemistry control to avoid evolving the bred tritium in the form of TF, which would be very corrosive.
Reaction Chamber. Both MFE and IFE will have a reaction chamber with concerns about tritium deposition and recovery. IFE is pursuing liquid blankets which will allow the bred tritium to be recovered in the vacuum system along with the recycled tritium. For dry wall chambers such as Sombrero, co-deposition of carbon and tritium is a concern, as it is for MFE. The extended vacuum boundary associated with beam tubes for IFE will make this deposition more difficult to predict and control.
Safety. Tritium is a major public safety concern for both MFE and IFE. While no "reference designs" have been selected for IFE, the two examples most used are the Sombrero dry wall chamber design and the Hylife II thick liquid wall design. Both these concepts offer very low activation levels, Sombrero due to choice of structural material and Hylife due to the thick liquid in front of the structural wall. Because of this, tritium is probably the only -- or at least dominant -- serious safety concern for IFE.
Target Factory Inventory. Direct extrapolation of proven ICF target fabrication and fill techniques lead to potentially very large tritium inventories. Conceptual techniques for tritium fill have been suggested that would have very low tritium inventories, but these lead to reliance on unproven and sometimes speculative technologies. Directions have been identified that lead to lower inventories which now must be pursued. These include non-diffusive capsule fill and cryogenic assembly of the targets.
Tritium-Lean Target Designs. The advanced IFE target concepts proposed by John Perkins rely on fast ignition and fuel layering to achieve bootstrapping to the temperatures needed to burn advanced fuels. They offer the prospects for greatly reduced tritium content and thus greatly reduced tritium inventory in the target factory.
These differences between MFE and IFE at this stage in the development of fusion energy highlight that the two programs have opportunities to learn from one another. We must maximize the benefit we gain from these opportunities.