Notes on the Recent Hype about Imminence of Commercial Fusion Energy

Originally published in June 2024 and updated in January 2026.

My views on the current state of development of fusion-energy science and technology are at odds with the last few years’ breathless publicity about fusion breakthroughs of a scale that could mean achievement of commercial energy from fusion in the next 10-20 years. In essence, as I have done since starting to work seriously on fusion almost six decades ago, I very much hope for fusion’s success as a practical energy source on Earth’s surface. Even if it proves too costly to compete in that role, one may hope it finds a role in space propulsion as the only energy source now known that could carry humans beyond our solar system.  

But the recent hype about the chances for early success in the Earth-bound electricity-generation role has been far over the top, in my view, and is dangerous in the sense that large early investments in the false expectation of early commercial returns—which would be only the latest in the long cycle of overoptimism about fusion followed by disappointment―could well set back the timetable for eventual success, not least by locking in technological dead ends in premature pursuit of commercialization. The hype is dangerous, as well, in feeding the false hope that there is a technological silver bullet that will save us from the challenging problems of addressing climate change with the means now available or easily foreseeable.

The main approaches for harnessing fusion as a practical energy source fall into two categories: magnetic-confinement fusion (of which the tokamak design is the example that has achieved by far the best—but still inadequate—performance to date) and inertial-confinement fusion (of which the laser-driven National Ignition Facility at Livermore is the most successful example, albeit with performance falling short of what would be required for practical power source by a much larger factor).

The best performances in fusion experiments to date—both in a tokamak and in the NIF―have been achieved using the deuterium-tritium (D-T) reaction. Getting a true energy gain from this reaction—more fusion energy out than the energy input to the system―is inherently much easier than achieving that with any other known fusion reaction; but it is nonetheless so difficult that nearly seventy years of research on controlled fusion worldwide, at a cost of many tens of billions of dollars, have not yet led to reaching this threshold of so-called “scientific feasibility.”

As I will explain, working with tritium poses big challenges for reactor design, for maintenance, for worker safety, and for public radiation exposures, but the difficulty of achieving even bare energy breakeven with any fusion reaction other than D-T is so great that most fusion scientists believe that success with reactions that avoid tritium will come much later if it comes at all. In what follows, I first address the challenges in making a practical magnetic-confinement fusion reactor using the D-T reaction, then the even bigger challenges of doing so with a laser-driven system using D-T.

Magnetic-confinement fusion using D-T

The best performance to date using D-T in a tokamak was in a late 2023 campaign at the Joint European Torus (JET). It achieved a fusion-energy yield of 69.3 megajoules of energy over 6 seconds, for an average fusion power of 11.6 megawatts. This figure is about 33% of the 35 MW of thermal power that was injected into the plasma to maintain the reaction. That ratio is not a measure of the closeness to true energy breakeven, however, which requires that the electricity that could be generated from the fusion reactions equal the electricity that had to be supplied to the reactor system for the shot, which was about 500 MW.

Unfortunately, 80% of fusion energy from D-T reactions is carried by energetic neutrons. Unlike the charged helium-ion reaction products, whose energy can be converted to electricity at an efficiency of 90% or more, the neutrons can’t plausibly be converted to electricity at an efficiency above 50%. As a result, the electricity yield from 11.6 MW of fusion power would be about 7 MW, so the shortfall from true energy breakeven in this case would be a factor of 500 MW / 7 MW ≈ 70.

The shortfall from what would be required for a practical magnetic-confinement reactor is far larger still, because a practical reactor would need an energy gain several times larger than mere energy breakeven, and because it would need a duty factor (fraction of the time the device operates at something near rated power) far beyond the brief bursts achieved in JET. 

Bombardment by the 14-MeV neutrons produced by D-T fusion weakens most structural materials in short order, as well as turning steel, molybdenum, titanium, and most other metals intensely radioactive. The first problem could require frequent replacement of structural components exposed to fusion neutrons (quite possibly so frequent as to make the whole operation uneconomic) and the second problem greatly complicates all maintenance around the reactor innards (further threatening economic viability). How many of the today’s fusion optimists have thought about the stresses that operating continuously will impose on equipment working at the boundaries of conditions that today can be tolerated only briefly…or about how this challenge could be surmounted?

Tritium—the radioactive isotope of hydrogen—is almost nonexistent in nature. It is produced for today’s uses by neutron bombardment of lithium inserts in fission reactors. The large quantities needed for D-T fusion-reactor operation would need to be “bred” in the reactor by bombardment of lithium in the reactor blanket by the 14-MeV neutrons. Tritium’s radiation is less penetrating than that of the main activation products of concern, but as an isotope of hydrogen it is comparably volatile, and the quantities stored and flowing in and around a D-T fusion reactor would represent a significant additional radiological hazard. The degree of tritium control needed to stay within current guidelines for public radiation exposure at a nuclear plant boundary would be extremely challenging to achieve in a tritium system as complex as that in a fusion reactor.

The significant environmental/safety issues posed by tritium and activation products (which, by the way, will likely require long-term management as radioactive waste) have been ignored by practically all of the recent commentators, most of whom insist in passing that fusion is "clean."

Internal-confinement fusion

The problems that must be solved if laser fusion (or another inertial-confinement approach) is ever to become the basis of a practical power reactor are even larger than those confronting magnetic-confinement fusion. As a start, the most recent breakthroughs at the world’s most advanced inertial-confinement device—the laser-driven National Ignition Facility at the Lawrence Livermore National Laboratory—remain short of practical reactor performance by much bigger margins than those of the best magnetic-fusion devices today. The 8.6 megajoule fusion energy yield of the best NIF “shot” to date (in April 2025) would represent, if converted (optimistically) to 5 megajoules of electricity, about an eightieth of the input electricity for the shot.

But the factors by which the repetition rate and pellet cost in the NIF fall short of what would be required in a reactor are far larger. Today, with a large crew of highly trained specialists in full-time attendance, the NIF is lucky if it can get off two shots per day. A practical laser fusion reactor would require something like ten shots per second. As for the cost of the pellets that the laser beams irradiate, when last I looked they cost in the range of $10,000 each. For a practical fusion reactor, that cost would need to be no more than a few cents apiece. 

The enormous challenges posed by the use of tritium in magnetic-fusion reactors would not be less, and might be even larger, for inertial-confinement reactors. 

Conclusions

I think it’s fair to say that the scientific and technological problems that must be solved on the road to a practical fusion reactor—beyond energy breakeven in sustained operation—are not much easier than the breakeven problem itself, which is still unsolved after 70 years of costly international effort.

Furthermore, there is no guarantee that, if all the technical challenges for either the magnetic or the inertial-confinement approach could be surmounted, the result would be economically competitive with other relatively clean energy options available in the same timeframe. The frequently heard claims about fusion being “cheap” take no account of the cost of building the reactor itself. The raw fuel for fusion would be cheap, as it is for fission reactors today. But the cost of the energy from fission is dominated by the cost of building and maintaining the reactor and associated facilities, not by fuel cost, and the same will be true of fusion. The studies of the likely economics of hypothetical fusion reactors in which I’ve been involved all indicated that it will be a great challenge to make electricity from fusion less expensive that that from fission.

Avoiding the problems with 14-MeV neutrons and tritium management by relying on other fusion reactions than D-T is not likely in “early” fusion reactors (and possibly not ever). The D-T combination is so much more reactive than all the other combinations of fusion fuels that relying on any of the others will require much more severe temperatures and pressures, thus requiring even more advanced technologies than those still not yet in hand even for the D-T case.

Nobody has a clear crystal ball when it comes to the characteristics of future technologies, and I am further handicapped by not having followed fusion science and technology particularly closely since leaving the White House in January 2017. Still, if I were a betting person and if I expected to be alive to collect, I’d bet we won’t see a successful commercial fusion reactor before 2050.  

It would be good to have it then as part of society’s toolbox to address the (doubtless continuing) challenge of meeting society’s energy needs without wrecking the climate, and I fully support making the S&T investments needed to find out if we can get there. But I think over-hyping fusion energy as something we’ll have in practical form soon—and that will be dirt cheap and completely free of environmental costs—is leading to unrealistic expectations, as well as the misallocation of quite a lot of venture capital to early-commercialization schemes that have no chance of success.

John Holdren is currently Research Professor of Environmental Science and Policy at Harvard’s Kennedy School of Government and Co-Chair of the Science, Technology, and Public Policy Program at the School’s Belfer Center for Science and International Affairs. His background in fusion energy includes: masters and PhD theses at MIT (1966) and Stanford (1970) on theoretical plasma physics; first post-PhD job 1970-72 as a physicist in the Theory Group of the Magnetic Fusion Energy Division at LLNL; consultant on both magnetic and laser fusion to LLNL and the fusion office at DOE from 1974 to 1994; co-lead author of the multinational IIASA study on environmental and safety characteristics of fusion and fission breeder reactors 1974-77; chair of the DOE Committee on Environmental, Safety, and Economic Aspects of Magnetic Fusion Energy in 1987-89; member of the Fusion Energy Advisory Committee of the DOE 1990-93. He resigned all fusion consulting posts in 1994 when, as a member of President Clinton’s Council of Advisors on Science and Technology, he served as co-chair of a PCAST review of the U.S. fusion-energy program. He remained engaged on fusion-energy progress during his service as President Obama’s Science Advisor and Director of the White House Office of Science & Technology Policy 2009-2017.