Only photovoltaic solar panels directly convert sunlight to electricity. Almost everything else has (at least one) middle step…
This is the fourth article in The Fusion Report series on critical supporting technologies for commercial fusion energy. In today’s article, we’ll look at the means to convert particle momentum and/or thermal energy from fusion reactions into electricity. But first we’re going to cover some basics about different approaches to fusion, because the type of fusion approach chosen impacts the method for performing energy conversion and fuel management.
Different Types of Fusion: Deuterium/Tritium and Aneutronic Versions
The most basic form of fusion for commercial fusion energy is deuterium-tritium (D-T) fusion. D-T fusion combines two isotopes of hydrogen: deuterium (one proton and one neutron) and tritium (one proton and two neutrons), to form helium (two protons and two neutrons with 3.5 MeV of kinetic energy), and a high-energy neutron with 14.1 MeV of kinetic energy. D-T fusion is the most popular candidate for commercial fusion energy because it requires the lowest fusion temperature (100M-150M degrees Celsius, or 13.6keV), plus the right combination of plasma pressure and time (known as the triple product).
Other relatively attractive candidates for commercial fusion are forms of aneutronic fusion, which produce no high-energy neutrons, an advantage we will touch on later. The aneutronic fusion requiring the lowest ignition temperature (58keV, or several hundred million degrees Celsius, about 2X what is needed for D-T fusion) is deuterium-helium3 (D-He3) fusion. D-He3 fusion produces a He4 (normal helium) nuclei and a proton, with a combined energy of 18.4MeV, and with no high-energy neutrons. The biggest supporter of D-He3 fusion is Helion Energy (#2 in total funds raised, with $1.5B in private funding), though others have shown interest.
Proton-boron11 (p-B11) fusion is attractive because it doesn’t require He3 (which is rare on Earth, but not on the Moon), but it requires nearly double the ignition energy (123keV) of D-He3. This is equivalent to an ignition temperature of well over a billion degrees Celsius, with some experts believing that temperatures of 2.3 to 4.6 billion degrees Celsius are required to yield a substantial net energy gain. With that said, there are at least three companies planning on utilizing proton-boron fusion, including TAE Technologies (#3 in total funds raised, with $1.3B in private funding and plans for an IPO), Marvel Fusion of Germany (#9 in total funds raised, with $415M in private funding), and HB11 Energy of Australia (#25 in total funds raised, with $32M in total funding).
The Impact of Fuel Choice on Fusion Energy Conversion to Electricity
Historically the conversion of energy (usually thermal energy to electricity, but also inertial energy to electricity) has been through rotating machinery – turbines and generators. In a D–T fusion reaction occurring in an inertial fusion energy (IFE) or magnetic fusion energy (MFE) machine, the 14.1 MeV neutron (roughly 80% of the total energy) enters the thermal blanket, where the neutron slows through collisions and deposits its kinetic energy into the blanket material as heat. In an MFE machine, the 3.5-MeV alpha particle produced by D-T fusion remains confined by the magnetic field and transfers its energy back to the plasma, helping sustain the extreme temperatures needed for continued fusion. In an IFE machine, the alpha particle is absorbed by the thermal blanket just like the neutron is.
The resulting heat is transferred to the coolant in the thermal blanket (usually water, helium, molten salt, or liquid lithium-lead), and then into a power-conversion system much like those used in other thermal power plants. In a steam-cycle design, the coolant produces high-temperature steam that spins a turbine connected to an electrical generator; gas-turbine or combined-cycle-like systems could also be used with sufficiently hot coolants. As such, a D–T fusion plant’s principal electricity pathway is indirect: fusion energy becomes neutron energy, then blanket heat, then mechanical turbine power, and finally electricity. This is largely the same mechanism that has been used for over 140 years.
For aneutronic fusion, there are a couple options: i) you can use the same indirect thermal cycle approach with a thermal blanket, steam turbine, and a generator; or ii) you can use direct energy conversion (DEC; first demonstrated a few months ago by Realta Fusion), because the energy released from the fusion appears principally as the kinetic energy of charged products (notably alpha particles), rather than in fast neutrons. Magnetic fields can guide those charged particles out of the fusion region and into an electrostatic or electromagnetic converter. In an electrostatic design, positively charged reaction products move against a deliberately imposed electric potential, slowing as they transfer kinetic energy to the electric field; electrodes then collect the resulting voltage and deliver current to an external circuit. Other candidate schemes include traveling-wave converters and magnetic-expansion systems that use changing electromagnetic fields to recover energy from directed charged-particle flows. The approach depends on specifics of the fusion mechanism used, whether MFE, IFE or a hybrid approach. However, while DEC has been demonstrated, it is not “production-ready” yet today.
Fuel Management Systems for Fusion
Fuel management in a D-T commercial fusion energy electrical plant depends on whether it is an IFE or an MFE/hybrid fusion plant. For MFE and hybrid fusion plants, fuel management is a tightly controlled, largely closed-loop process designed to deliver the correct fuel mixture to the plasma while minimizing the on-site inventory of radioactive tritium. Deuterium and tritium are stored, metered, analyzed, and injected into the fusion machine either as gas for edge fueling or as high-speed frozen pellets that penetrate farther into the plasma. Only a small fraction of injected fuel fuses during a pass, so the divertor and vacuum-pumping system continuously remove unburned deuterium and tritium, helium “ash” produced by fusion, and contaminant gases. A tritium-processing plant then purifies this exhaust, separates hydrogen isotopes to restore the required D–T ratio, and returns recovered fuel to storage and reinjection. For an IFE system, the machine must simply inject the frozen fuel pellets (for direct-drive IFE machines) or fusion-hohlraum combination pellets (for indirect-drive IFE machines) into the machine, at rates of up to 10 Hertz. The energy, in the form of high-speed neutrons and alpha particles, is absorbed by the thermal blanket, while the helium ash and other “burned” products (the fuel pellet shell, etc.) are evacuated between shots.
The fuel-management system in D-T fusion energy plants must also recover and manage tritium bred in the lithium-containing blanket surrounding the plasma. Fusion neutrons react with lithium to create replacement tritium, which must be extracted rapidly from the blanket medium, processed to remove impurities, measured for inventory control, and transferred into the fuel cycle. This requirement makes tritium self-sufficiency—not simply fuel injection—a core plant-design challenge: the breeding blanket, extraction equipment, pumps, isotope-separation units, containment barriers, and de-tritiation systems must collectively recover more tritium than the plant consumes and loses. Because tritium is radioactive and a concern for nuclear proliferation, fusion facilities use multiple physical containment barriers, continuous monitoring, air and water cleanup systems, and carefully managed storage inventories to limit worker exposure and environmental releases. Advanced concepts such as direct internal recycling aim to send purified D–T exhaust back to pellet-fueling systems without full isotope separation, potentially reducing processing equipment and the total tritium inventory held in the plant. Note that tritium recovery and management is not an issue that aneutronic fusion plants need to worry about.
Downstream, the fusion exhaust is separated by physical form and chemical species rather than discarded as a single waste stream. A representative IFE fuel-cycle scheme uses cryo-pumping to capture volatile gases, separates and vents helium ash, recovers deuterium and tritium for purification and reuse, and routes target-derived particulates to collectors. Solid or liquid debris can settle by gravity into collection regions, while captured particulate material may be oxidized or otherwise processed to release retained hydrogen isotopes before the non-fuel residue is disposed of or recycled. In liquid-wall concepts, the circulating protective liquid itself acts as a debris collector: it absorbs deposited material and transports it to external processing equipment. The goal is high recovery of valuable target materials and fuel, minimal tritium retention in waste, and a sufficiently clear chamber between pulses to sustain dependable power-plant operation.
Conclusion: Power Conversion and Fuel Management is Complex
One of the reasons that power conversion and fuel management for fusion energy is so complex is the unforgiving environment in which fusion is required to operate. Whether IFE, MFE or a hybrid fusion system, success requires a nearly perfect debris-free environment. At the same time, the tritium component in D-T fuel must be strictly controlled and accounted for. It is in many ways a chemistry problem as much as it is a physics problem or an engineering problem, and is an area that continues to develop as fusion energy nears production levels.
