to run the plant—i.e., the input to the driver and other auxiliary systems. Clearly QE = 1/f, where f is the recycling power fraction (see Figure A.2). Typically Q ≥ 10 is required for a viable electrical power plant. For a power plant with a driver wall-plug efficiency ηD, target gain G, thermal-to-electrical conversion efficiency ηth, and blanket amplification AB (the total energy released per 14.1 MeV neutron entering the blanket via nuclear reactions with the structural, coolant, and breeding material), the engineering Q is QE = ηth ηthABG (see Figure A.2). An achievable value of the blanket amplifications and thermal efficiency might be AB ~ 1.1 and ηth ~ 0.4 and should be largely independent of the driver. Therefore, the minimum required target gain is inversely proportional to the driver efficiency. For a power plant with a recirculating power f = 10 percent (QE = 10), the required target gain is G = 150 for a 15 percent efficient driver and G = 320 for a 7 percent efficient driver.
Energy gain does not, of course, guarantee commercial viability. Key challenges remain even after high gain is achieved. These are discussed in detail elsewhere in this report, but they include: