The Department of Energy (previously known as the Atomic Energy Commission) initiated Project Sherwood in the mid-1950s to coordinate work on high temperature, magnetic confinement fusion (hot fusion) in universities, industry and the government. The objective was to show that nuclear fusion reactions could be made to occur in a high temperature gas plasma. Heat produced could be used for electricity generation. Atomic nuclei of deuterium and tritium in the plasma could be made to fuse to form helium and release energy, using the same process that powers stars like our Sun. It was determined that hot fusion requires ion temperatures greater than 100 million degrees.
Reactor Designs
Early reactor designs included the stellarator, toroidal pinch and magnetic mirror. These and other subsequent designs encountered plasma instabilities that prevented required temperatures from being achieved. Since then, many different reactor designs have been built and tested to solve this technical issue, which are discussed in technical journals, such as Fusion Science and Technology published by the American Nuclear Society. In addition, much progress has been made in reaching required ion temperatures, which was finally accomplished in the mid-1990s. None of the efforts, however, has yet completely satisfied the “Lawson criteria”, or value that needs to be obtained when multiplying temperature, confinement time and plasma density together (the triple product). When deuterium and tritium are used, this value is required to be at least 3×1021, in units of KeV-s-ions/m3, or 3.5×1028, in units of oK-s-ions/m3.
Compact Fusion Reactors
The designs have included a number of different types of “compact fusion reactors” that generally have significantly smaller plasma volumes than the traditional tokamak. These are easier to build and test, and thus less expensive, than larger tokamak machines and the International Thermonuclear Experimental Reactor (ITER) being built in Cadarache, France. Some of the compact fusion designs have been investigated by companies and universities for many years, and could reach breakeven before the ITER. By comparison, ITER has been designed to have a plasma volume of 840 m3 and to produce 500 megawatts (MW), or 590 kW/m3, of thermal output power for around twenty minutes. As indicated in previous blogs, 1 MeV = 1.6 x 10-13 joule. If each deuterium-tritium interaction were to fuse and produce 17.6 MeV of energy, the density of deuterium and tritium would need to be about 2 x 1017 ions/m3. But, plasma density is planned to be about 1-2 x 1020 ions/m3 since deuterium-deuterium interactions are not expected to fuse. Helium produced could be expected to affect reaction efficiency.
Breakeven for ITER, with fusion power output exceeding the power required to heat and sustain the plasma, is estimated to take several decades and cost $Billions. Lockheed Martin recently announced that they are on a fast pace to develop a compact fusion reactor, and that it has the advantage of combining a lot of earlier designs, such as axisymmetric mirrors, and will also be small enough to be easily transportable. If only deuterium were used, the reactor would need to operate at a higher plasma temperature and have a higher plasma density than the tokamak or ITER. Deuterium-deuterium interactions that fuse can be expected to produce a combination of energies (i.e., 3.3, 4.0, 5.5 and 23.8 MeV). Again, helium produced could be expected to affect reaction efficiency.
