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Nuclear Fusion

In contrast to nuclear fission, fusion power operates by fusing light atoms, typically deuterium and tritium, into heavier atoms. Fusion power promises cheap, stable energy production from a virtually unlimited fuel stock and with few environmental side effects, but the technology has so far proven difficult to engineer and remains far from commercialization. In this section we explore pathways to commercial fusion power.

We do not recommend continuing research and development via the ITER route, though remain open to alternative pathways to fusion energy, as discussed below.

Fusion Reactions

There are several physically plausible fusion reactions that may produce energy, and the leading three are shown below.

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Three most promising fusion reactions for power production. Reaction temperatures are given by Eliezer and Martinez-Val 1 and Ongenga 2, power density by Harms et al. 3, neutronicity (fraction of released energy in the form of neutrons) by the IAEA 4, and main advantages and disadvantages of the reactions.

The most common approach is the deuterium-tritium (D-T) reaction (deuterium and tritium are isotopes of hydrogen, with one and two neutrons respectively; most hydrogen atoms have no neutrons), due to the lower temperature and higher power density. However, high neutron production poses a major engineering challenge in mainaining a reaction vessel 5, stimulating interest in aneutronic reactions.

Overview of Fusion Technologies

The most developed fusion devices are tokamaks, donut-shaped machines that contain plasma magnetically. Spherical tokamaks, stellarators, and field-reversed configurations are variants on the geometry. Inertial confinement uses lasers rather than magnetic fields to confine a plasma, while magneto-inertial fusion is a hybrid. Z-pinch and eletrostatic fusion use electric fields to confine the plasma. Following is a brief summary.

Fusion Summary
TechnologyMajor ProjectsReactionRationaleChallenges
TokamakJoint European Torus, ITER, DEMOD-TMost developedHigh cost
Spherical TokamakTokamak Energy Ltd, MAST, Globus-M, NSTXD-TSmaller designSuperconducting magnets needed
StelleratorWendelstein 7-XD-TPlasma stabilityComplex design
Field-Reversed ConfigurationTAE TechnologiesD-T, D-He3, p-B11Compact design, aneutronic fuels, space travelMagnetic field less stable
Inertial ConfinementNational Ignition FacilityD-TSimple reactor designDisappointing experimental results, potential high cost
Magneto-InertialGeneral Fusion, Helion Energy, MagLIFD-T, D-He3, p-B11Wider parameter space than MCF or ICFPotential high cost of targets
Z-PinchZ-Machine, LPPFusionD-T, D-He3, p-B11Simple design, space travelDifficulty controlling plasma
ElectrostaticPolywellD-T, D-He3, p-B11Neutron source for medical isotopesMajor doubts of feasibility for power production

Sources are as follows: tokamaks: Clery 6; spherical tokamaks: Costley 7 and Sykes et al. 8; stellarators: Gates et al. 9 and Xu 10; field-reversed configurations: Hassan 11, Hirano et al. 12, Pancotti 13, Razin et al. 14, Steinhauer 15, TAE Technologies 16; inertial confinement: Clery 6, National Research Council 17, Keefe 18; magneto-inertial fusion: Flippo et al. 19, General Fusion 20, Helion Energy 21, Wurden et al. 22; Z-pinch: Bilbao et al. 23, Lilly 24, LPPFusion 25, Miernik et al. 26, Shumlak et al. 27, Shumlak et al. 28; electrostatic fusion: Kulcinsky et al. 29, Miley and Murali 30, Rider 31, Santarius et al. 32.

Breakeven Power

A critical milestone is the development of fusion is breakeven power: that the reactor produces more power than is required to run it. The quantity Q represents the ratio of input to output power. There are two types of breakeven: Qplasma is the ratio of input energy to the plasma to the output heat. Qelectricity is the ratio of the total electricity required to run the reactor to the electricity produced. Qplasma is more often discussed in the literature, but Qelectricity, which is generally lower, must also be greater than 1 in a functioning reactor.

The image: "q.svg" cannot be found!

JET, NIF, and ITER are, respectively, the Joint European Torus, the National Ignition Facility, and (formerly) the International Thermonuclear Experimental Reactor. Hossenfelder estimates Qelectricity by assuming that heat is converted to electricity at 50% efficiency and that the laser at NIF are 10% efficient. The performance at ITER is estimated, as ITER is still under development. Source: [18].

In December 2022, the National Ignition Facility announced that they had achieved breakeven power: 2.05 megajoules delivered to the fusion pellets and 3.15 MJ fusion power produced, for Qplasma ≈ 1.5 33. However, this falls short of the necessary Qelectricity > 1.

Tokamak Fusion via ITER

The magnetic confinement route (MCF), via tokamaks, is the most developed fusion approach. Following steady progress from the 1960s through the 1990s, state of the art tokamak performance has stalled on net energy gain and triple product, two key measures of fusion performance 34.

Today's flagship project is ITER (formerly the International Thermonuclear Experimental Reactor), expected to be the first tokamak to produce more energy than is required to sustain the reaction (in the plasma, not necessarily electrical power), to be followed by the experimental DEMO (DEMOnstration Power Plant) power plant. The ITER/DEMO schedule has suffered from delays and cost overruns, and now should not be expected to lead to commercial fusion power until at least the 2060s. We estimate that the project will cost $65 billion 35 and yield an expected $2.6 billion of benefit. See our analysis of research and development for more details.

The image: "iter_timeline.svg" cannot be found!

Past and predict development timeline of MCF fusion power via ITER and DEMO. Sources: EUROfusion 36, Ikeda 37, ITER Organization 38, Kendl and Shukla 39.

If the ITER and DEMO programs are successful, estimates based on proposed DEMO designs suggest that the cost of fusion electricity could range from 4 to 26 ¢/kWh 40, 41, 42, and for the purposes of these calculations we use a central value of 13 ¢/kWh.@Under the assumptions used here, for the U. S. Department of Energy to fund ITER looks like a poor investment. Part of the reason is that, due to the protracted timeline of ITER development, the value of electricity generated by fusion is heavily discounted. If a smaller discount rate is used, a lesser price can ultimately be attained, or if there is reason to believe that ITER has a higher probability of success, then the economics may be improved. This analysis does not apply to fusion R&D projects other than ITER.

Tritium Availability

Tritium, an radioactive isotope of hydrogen with two neutrons and a half-life of about 12 years, is a critical input for D-T reactions, and its supply is limited. All supply comes from CANDU-like fission reactors from a Tritium Removal Facility, of which two are operating in the world today: in Canada and in South Korea, and with a possible third planned in Romania 43. There should be enough tritium available for ITER, but availability for DEMO and other possible fusion projects is in question 43. Tritium breeding is a necessity for a large-scale fusion program. It may also be necessary to develop deuterium-deuterium startup 43, to stretch tritium supplies, though this would add significant cost to the reactors 44.

Private Ventures

Leading private sector fusion projects, including at Lockheed Martin 45, General Fusion 20, and TAE Technologies 16, are also far behind MFC in terms of energy ratio and triple product 6.

In a 2021 report, the National Academy of Sciences, Engineering, and Medicine estimates that a concerted effort to develop fusion with the private sector could yield commercialization by around 2050. Priorities for a pilot plant include the following 46:

  • net energy gain, both in plasma and electricity,
  • ash (byproductions of reactions, such as helium) removal,
  • expanded tritium production,
  • YCBO superconducting magnets, and
  • advances in materials.

Alternatives to Tokamaks

There are several alternatives to MCF which, though less developed, have the potential to surpass MCF by allowing smaller and leaner projects. One such approach is inertial confinement fusion (ICF). The leading ICF project, the National Ignition Facility in Livermore, California, has so far failed to achieve the self-sustaining plasma burning that was predicted by computer models 6. An estimated $10-15 billion of capital expense, and $90-150 million of annual operating expense, would be necessary for an ICF research program, with uncertain success 17.

Magneto-inertial fusion (MIF) is a hybid between MCF and ICF. Though less developed than MCF and probably not close to commercialization 47, MIF has the potential for inherently less expensive power production than MCF 22.

Cold Fusion

Cold fusion refers to any nuclear fusion process that occurs at temperatures substantially below those used in tokamak experiments. Most scientists are skeptical that the phenomenon of cold fusion is real, and a 2019 review failed to turn up conclusive evidence of cold fusion 48.

A main driver of interest in cold fusion has been the observation of anamolous excess heat with the exposure of deuterium gas to certain metal nanocomposites, such as palladium 49. The cause of this excess heat, and whether it could be useful for energy production, are unclear.

Conclusion

The most advanced fusion projects today are based on tokamaks, and yet the ITER development path is unlikely to yield commercial fusion power before 2060, and possibly not at an affordable cost. Recent technological advancements in superconducting magnets and high-performance computing offer hope that alternative fusion approaches, especially spherical tokamaks, field-reversed configurations, and MIF, could provide a faster and cheaper route to fusion, but these approaches are not developed enough to confidently assess their potential. Electrostatic fusion and any variant on cold fusion are the least likely to produce results.

For Further Reading

The World Nuclear Association outlines major fusion research projects in greater detail.

References

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