Dr. Gouge is Head, Cryogenics and Superconductivity Group, Oak Ridge National Laboratory
I read with interest several articles in the October 1998 issue and provide some technical comments. The first article, looking Forward-8ubmarines in 2050, Part 1Wo, by Mr. J.P. Buff, was thought-provoking and interesting, especially the section on advanced propulsion. It is doubtful, however, given the volume constraints of deep-diving submarines, that fusion propulsion will be a feasible propulsion option for the foreseeable future. The physical limitations of magnetic fusion plants in terms of power density and other parameters were covered in reference 1. For inertial fusion, where a small, millimeter-scale, deuterium-tritium (OT) pellet is ignited by powerful lasers or heavy ion beams, the constraints are more severe. The proposed inertial fusion reactors run at a few to about 10 Hz (pellets/sec), so for 100 MW thermal power, this amounts to about a 10-50 megajoule explosion for each pellet injected (the interested reader can convert this into equivalent pounds of TN1). This requires a substantial containment chamber designed for pulsed loading that has to be cleared to vacuum conditions between each pellet injection. The high-power lasers or heavy ion beams will require substantial auxiliary power for the start-up as well as power conditioning systems; all these will require a large and unavailable interior volume. The radioactive tritium gas from the DT pellet will need to be recovered from the chamber exhaust, purified, and recycled, which requires another large auxiliary system. The physics requirements for ignition of deuterium only (DD) fuel are much higher than the DT cycle.
Existing pressurized light water fission reactor technology results in a propulsion plant and related machinery that occupy about 50 percent of the interior usable volume of present nuclear attack submarines. If a smaller, general-purpose attack submarine with significant warfare capability is desired, then substantial reductions in size will require new. innovative technology for the propulsion plant. A technology with promise for a revolutionary improvement in nuclear propulsion system performance in a reduced physical envelope is a high-temperature gas-cooled reactor (HTGR) as the heat source in a close gas turbine (Brayton) cycle driving an advanced electric propulsion plant. The Navy has substantial experience with open cycle, gas turbine power plants in surface combatants. Gas-cooled reactors have an extensive operating history and a closed Brayton cycle plant using a non-nuclear heat source has been operated in Germany at power levels up to 50 MW. I would argue that the next bold step in submarine propulsion, aside from nuclear-AIP hybrids, will be from the advanced fission reactor area.
In a second article, The All-Electric Ship: Enabling Revolutionary Oumges in Naval Warfare, by Mr. R.E. Leonard and Mr. T.B. Dade, I would only emphasize that this technology can be integrated into the proposed closed Brayton cycle propulsion plant discussed above. additionally, the U.S. Department of Energy has an ongoing program in partnership with the industry to develop prototypes of electrical system components based on the new class of high temperature superconducting (HTS) materials. These include power transmission cables, MW-class transformers, motors, and fault current limiters, all using HTS. In many cases, liquid nitrogen or electric cryocoolers can be used to maintain operating temperatures in the range of 20-100 K. This technology could be used in the second generation of all-electric, integrated power systems to allow even higher power densities than nearer-term components such as permanent magnet motors.
Overall, I was impressed by these articles and the technical opportunities on the horizon for the second century of service to our country.