Commonwealth Fusion Makes the Physics Case For Its 400 MW Reactor

Commonwealth Fusion has published five peer-reviewed papers laying out the physics case for ARC, its planned 400 MW fusion power plant, which would follow the company's smaller SPARC tokamak now under construction. The papers suggest ARC could produce more energy than it consumes using high-temperature superconducting magnets, molten-salt heat extraction, and 15-minute fusion pulses. Ars Technica reports: ARC will be a tokamak that hosts fusion between hydrogen's two heavier isotopes, deuterium and tritium. This reaction results in a helium nucleus and releases a neutron and radiation. The helium transfers heat to the plasma, maintaining the conditions needed for fusion, but it is otherwise a waste product, referred to as "ash" in the fusion context. The neutron and radiation, however, are put to use. Part of that use is simply imparting energy into a blanket of molten salt that surrounds the fusion chamber. That energy, in the form of heat, will be used to drive a turbine that produces the electricity. The molten salt includes lithium ions; when one lithium isotope absorbs a neutron, it decays into more helium, plus tritium that can be used as fuel for the reactor. There are isotopes present that will also release additional neutrons, allowing this process to generate sufficient fuel.

Overall, the present design of ARC is expected to produce about 1.13 GW of fusion power, with 500 MW of that extracted as electricity. Some of that (100 MW) will be needed to power the plant's operations, leaving 400 MW to be sent to the grid. The rest of the energy is either kept in the tokamak to maintain the fusion reactions or lost due to inefficiencies in the heat and energy transfer of the system. There's a lot of uncertainty about these numbers; the 1.13 GW is just the center of a range of potential values running from 900 MW to 1.3 GW, so the 400 MW output may need to be adjusted up or down accordingly.

Some of that 400 MW comes during periods where fusion is not occurring. The nuclear reactions will occur within 15-minute-long periods that will be interspersed with one minute resets. The resets are meant to be kept short enough that nothing has much of a chance to cool down before it gets heated up again -- thermal inertia will let it continue generating power. That will be one of the key differentiators with SPARC, which doesn't have the heat extraction needed to maintain stable fusion for these long time periods, and so can't maintain the near constant temperatures needed for reliable power generation.

It's inevitable that parts of the device will be exposed to radiation and perhaps fusion plasma. The inner walls of the reactor will be shielded by tungsten, which will limit erosion by the conditions. Meanwhile, the vacuum vessel is designed to be replaced every one to two years. The papers note that this flexibility will allow them to make some design changes even after ARC is built. To enable this, the whole tokamak is meant to split in half for maintenance.

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