HB11—Understanding Hydrogen-Boron Fusion as a New Clean Energy Source

Warren McKenzie, Dimitri Batani, Thomas A. Mehlhorn, Daniele Margarone, Fabio Belloni, E. Michael Campbell, Simon Woodruff, Jan Kirchhoff, Adrian Paterson, Sergey Pikuz and Heinrich Hora, HB11 — Understanding Hydrogen-Boron Fusion as a New Clean Energy Source, Journal of Fusion Energy 42, article 17, 2023. Reproduced under the Creative Commons Attribution 4.0 International licence stated in the article; the published version is at doi.org/10.1007/s10894-023-00349-9.

(On this site, Hora’s own road map for laser-ignited boron-hydrogen fusion is at /library/stm-b5a7103035, and the degenerate-plasma argument this paper leans on is at /library/stm-9aa4e6f741. The other live proton-boron programmes are the dense plasma focus at /library/stm-05100e66da and /library/stm-8f6027c3eb, and the first proton-boron measurements in a magnetically confined plasma at /library/stm-9e2a22de0f. The deuterium-tritium roadmap this paper is arguing against is at /library/stm-4fed2cae63, and the aneutronic propulsion studies are at /library/stm-b5e092d030 and /library/stm-08b7559cf0.)

Abstract

HB11 Energy’s mission is to realize large-scale electricity generation from the fusion of hydrogen with boron-11, the HB11 or proton-boron reaction, without the environmental problems normally associated with nuclear energy. A non-thermal approach is taken in the initiation of the reaction using high-peak-power lasers, which was the pursuit of HB11 Energy founder Professor Heinrich Hora’s career as a theoretical physicist. In the 1980s, the invention of Chirped Pulse Amplification of laser pulses by Donna Strickland and Gerard Mourou, Nobel Prize 2018, enabled the possibility of experimentally validating the earlier theoretical predictions. Several experimental demonstrations of the HB11 reaction using such lasers inspired the establishment of HB11 Energy and with it, the possibility of realizing an aneutronic nuclear energy source with easily accessible and safe fuel resources that could last thousands of years. Like all quests for fusion energy, there are significant scientific challenges remaining. HB11 Energy Holdings Pty Ltd, an Australian company, was established as the best vehicle to co-ordinate a global collaborative research effort to address these challenges and build capacity to host large-scale public private partnerships, such as those now recommended by the US National Academies of Science, Engineering and Medicine. If net-energy-gain can be achieved through HB11 Energy’s concepts, there are many engineering benefits over traditional deuterium-tritium fusion that will see a dramatically simpler and safer reactor being produced. A technoeconomic assessment of such a reactor is also discussed which presents many engineering challenges that will need to be met before commercial HB11 fusion can be deployed on a large scale.

Keywords: HB11; hydrogen boron fusion; nuclear reactions; high-intensity lasers; energy production; non-thermal fusion.

Introduction

The hydrogen-boron 11 fusion reaction, also known as proton-boron, is a most promising candidate for large-scale energy production in a bid to curb the future use of climate-impacting fossil fuels. As a nuclear process, it presents an energy density approximately seven orders of magnitude higher than chemical reactions and, with an aneutronic primary reaction, it does not induce activation in materials, leading to negligible radioactive waste. In this reaction, three alpha particles and 8.7 megaelectronvolts of energy are produced. Equation 1 in the source states it: a proton plus boron-11 gives three helium-4 nuclei plus 8.7 megaelectronvolts.

In principle, this enables the direct conversion of the kinetic energy of such charged particles into electricity, rather than through a thermal cycle. Furthermore, the primary fuel, boron, is abundant in nature with the world’s largest known mine estimated to contain about 1.2 billion metric tons of boron, of which 80 percent is the required isotope, boron-11.

As compared to classical deuterium-tritium fuel, boron targets have the advantage of being in a solid state at room temperature, removing the need for cryogenics. This would be an important point for inertial confinement fusion schemes, especially when moving to high repetition rate operations. Also, the cost of the targets is likely to be much less since they do not involve the presence of a radioactive isotope such as tritium, which must be produced by tritium breeding and recovered — burn-up fractions in deuterium-tritium fusion reactor concepts range from a few percent to about 30 percent — is hazardous, and cannot be stored for long periods of time.

Despite these advantages, relatively little attention has been given to the study of hydrogen-boron fusion. The reason for this lies in the reactivity of HB11 fuel, which indicates that the temperatures required to achieve a fusion burn are an order of magnitude higher than for deuterium-tritium fuel and far exceed 100 million kelvin, that is 10 kiloelectronvolts. Radiative losses are also larger compared with deuterium-tritium reactions, due to the higher charge, an atomic number of 5. Accordingly, the authors of earlier work held a rather pessimistic evaluation of the prospects of this fuel for energy generation.

Significantly, this difficulty was addressed through the extensive study and development of non-thermal methods for hydrogen-boron fusion. Such non-thermal approaches, coupled with the continuing improvement in ultra-high intensity lasers and the recent experimental results detailed below, inspired the establishment of HB11 Energy Holdings. The enterprise is focused on accelerating scientific and engineering development in laser boron fusion towards net-energy gain. The final aim is to realize the urgent demand for a new large-scale energy source in the face of climate change. If the quest to achieve HB11 net-energy-gain is realized, it will present a promising and attractive prospect for a new clean-energy source.

This paper outlines some of the early history of HB11 fusion, including the basic history of Professor Hora’s theoretical work and the initial target concept that inspired the establishment of HB11 Energy. The subsequent sections cover more recent experimental demonstrations of proton-boron fusion, and the approaches presently under investigation to increase reaction rates towards net-energy-gain. Finally, a high-level summary of HB11 Energy’s technoeconomic model is provided, outlining key implications in reactor design on the type and final cost of energy that it produces, and key engineering milestones required for the large-scale deployment of hydrogen-boron fusion.

History and recent results

The hydrogen-boron fusion reaction was discovered by Oliphant and Rutherford in 1933. Shortly after the discovery and early development of the laser in the 1960s, hydrogen-boron has been considered as a fuel for laser fusion.

The application of lasers to drive hydrogen-boron fusion was pursued by Professor Hora from the 1970s. An outcome of this work was that conditions required to meet the triple product threshold for proton-boron fusion were too extreme to be practical by thermal means.

During this decade some of the earliest hydrodynamic computer calculations for plasmas were performed. A simulation in 1978 suggested that the acceleration of a plasma front against the direction of a short, 100-picosecond, laser pulse could reach an extremely high value, ten to the twelfth centimetres per second squared. Accordingly, plasma-block acceleration was considered as a possible key to accelerating ions to the energies required for fusion — a non-thermal alternative to achieve fusion. Decades later, experimental results obtained by Sauerbrey in 1996 seemed to confirm such high accelerations in the plasma by measuring Doppler-shifted spectral lines.

Over the same decade, the 1990s, the developments in chirped pulse laser amplification, including the first companies making such systems commercially available, led many labs around the world to pursue experimental research programs bringing about a deeper understanding of laser-ion acceleration mechanisms. The ability to accelerate particles, including protons, to energies more than 10 megaelectronvolts — not possible with thermal mechanisms — became commonplace.

These developments led to the first experimental demonstrations of non-thermal hydrogen-boron fusion, the first of which was performed by Belyaev and colleagues in 2005, followed by many others. The progression of obtained experimental results is summarized in Figure 1.

Remarkably, a so-called pitcher-catcher concept was introduced and studied experimentally. In this concept protons were accelerated in thin foil targets, the pitchers, through the mechanism known as Target Normal Sheath Acceleration. The protons from the pitcher were then impinging on a secondary boron, or more commonly boron nitride, target, the catcher, to produce energetic alpha particles. This non-thermal fusion is also known as beam fusion because of its similarity with what takes place when an energetic proton beam produced by a particle accelerator is directed onto a solid boron target. The difference in acceleration mechanism results in protons with a larger energy spectrum in Target Normal Sheath Acceleration compared with direct proton beam irradiation. As for conversion efficiency between laser energy and proton generation, this is typically around 10 percent.

Other experiments used a different approach, directly irradiating the boron target, eventually enriched in hydrogen, with the laser. In this in-target scheme, protons are accelerated by different mechanisms including hole boring and radiation pressure acceleration.

The number of fusion reactions achieved through this approach has been impressively high, with the highest reported alpha particle flux on the order of ten to the eleventh per steradian from two key results. The first was from a nanosecond laser, PALS at ELI Beamlines, Prague. The second was a high-energy high-intensity picosecond laser pulse produced by the LFEX system at Osaka University in Japan. With an average energy of about 3 megaelectronvolts, an alpha particle flux of ten to the eleventh corresponds to a total energy of about 0.1 joule. Since LFEX delivered an energy on target of the order of 1 kilojoule, the fusion-to-laser energy efficiency is about 0.01 percent, four orders of magnitude below breakeven, the point at which the energy produced by fusion reactions equals the driver energy, the input energy from the laser pulse. While this difference is significant, the history of fusion shows how progress of many orders of magnitude is possible with a focused research program.

The application of magnetic fields. This option permits the possibility for spatial confinement of the plasma, the accelerated protons and the generated alpha particles. In this approach, the second laser of nanosecond or picosecond duration irradiates a specifically designed conducting capacitor-coil target. The laser pulse ejects hot electrons from one part of the capacitor, charging the second part. The potential difference drives an electric current in the U-turn-shaped coil, creating a sub-kilotesla magnetic field inside the loop, lasting for several nanoseconds. Then, a cylindrical target could be used, with its axis parallel to the direction of the magnetic field, which will create a flux of protons and alpha particles through the cylinder, as shown in Figure 2. Therefore, instead of being dispersed in space, the flux of protons and bulk plasma containing boron will be confined, increasing the reaction rate but also producing more localized heating of the sample. This is the basis of HB11 Energy’s initial reactor concept, where laser one accelerates ions through the cylindrical target to initiate the non-thermal fusion reaction while laser two applies a magnetic field pulse through the capacitive coil.

Quantitatively, the desired laser parameters, and corresponding anticipated magnetic field strength, proton beam energy and flux were estimated in the literature. Considering the maximum cross section is found to be above 600 kiloelectronvolts, the required proton number of ten to the eleventh is estimated. It is shown that a 1-picosecond laser pulse of 30 kilojoules, that is 30 petawatts, focused into a 200-micrometre spot and delivering ten to the twentieth watts per square centimetre optical field intensity will be required for direct drive ignition. The ignition is predicted to occur in an HB11 cylinder of 1 centimetre length and 2 millimetres diameter, thermally isolated and confined by a 10-kilotesla magnetic field generated by the capacitor-coil target irradiated with a 3-kilojoule nanosecond laser pulse.

Pathways to increase fusion gain

The current record in alpha particle generation using short-pulse lasers, about ten to the eleventh alpha particles per shot, has been obtained at the LFEX kilojoule laser in an experiment supported by HB11. The breakeven threshold corresponds to 2.15 times ten to the fifteenth alpha particles per kilojoule of laser energy, corroborating the four orders of magnitude deficit from breakeven. This is indeed a challenge considering only 10 experimental demonstrations of hydrogen-boron fusion using lasers have been made. This leaves many opportunities to increase fusion reaction rates in the quest towards net-energy-gain, as is discussed in the following subsections.

Reducing the radiation losses due to bremsstrahlung emission is another key challenge to increase reaction rates. Due to the high charge number of boron, such losses are more severe than in the case of deuterium-tritium fusion. A simple way to reduce such losses would be to use a material which contains more hydrogen than boron, as proposed by Belloni. Such a material composition is more favorable also in view of triggering a chain reaction, because it would increase the probability that the generated alpha particle collides with a light proton as compared to a heavy boron, which cannot be effectively accelerated to energies capable of triggering further fusion reactions. Also, in the context of laser-driven proton-boron fusion, target designs with layers that trap radiation to reduce losses are being considered.

Non-equilibrium plasma, where the electron temperature is different from the ion temperature and lower than it, may offer an avenue to increase reaction gains by minimizing electron collisions and radiative losses while increasing the ion reaction rates. The more recent work by Wurzel and Hsu states that bremsstrahlung power density always exceeds the power density generated by the fusion reaction when the electron temperature is at or above one third of the ion temperature, suggesting that proton-boron-11 ignition may require a non-equilibrium burn.

Degenerate plasmas. Another important issue concerns the effect of elastic collisions of suprathermal protons in the target. Such collisions are much more probable than nuclear collisions, which leads to the protons losing most of their energy to electrons before having a chance to initiate a fusion reaction. Hence, we would like to reduce the electron density in the plasma, creating a non-neutral plasma. Techniques exist to address this for low plasma densities, namely by using Penning-Malmberg traps, but the low density implies a very small number of fusion reactions. A more promising approach for fusion energy applications considers plasma degeneracy as an effective way to inhibit energy losses due to elastic collisions in high density plasmas. In degenerate matter, electrons occupy all available energy levels up to the Fermi energy. This inhibits all collisions characterized by an energy exchange below the Fermi level. To receive such energy, electrons would need to move up to an energy level which is already occupied by other electrons, which is prevented by Pauli’s exclusion principle.

Degenerate plasmas are already typical of today’s implosion experiments using deuterium-tritium cryogenic targets, which result in the production of a classical plasma hot-spot surrounded by a dense degenerate fuel. However, the extent to which the degeneracy of the material can be used to moderate elastic scattering is a complex point demanding active research. HB11 Energy is developing studies on degeneracy effects, extending Belloni’s theoretical considerations.

Target geometry. Another approach concerns the geometry of the targets, which can be optimized to improve the efficiency of the laser interaction. Strategies range from micro- and nano-structured targets increasing laser absorption, to near-surface density profiles.

Novel target materials. Most of today’s experiments have been realized using boron-nitride targets, in which hydrogen was contained only as impurities, estimated at less than 1 percent of the target composition. HB11 Energy is exploring novel target materials containing significantly more hydrogen than traditional boron-nitride targets by utilizing novel micro and nano structures. Candidates include the two-dimensional material white graphene, with surface modifications allowing its use as a hydrogen-storage material, and another two-dimensional material, borophene, which contains only hydrogen and boron. Beyond composition, these materials allow target fabrication using solution-based methods that are amenable to large-scale manufacturing. A paper including the first demonstrations of proton-boron fusion using white graphene is in preparation.

The avalanche mechanism describes the process whereby the generated energetic alpha particles undergo elastic collisions with bound protons, accelerating them and promoting further proton-boron fusion reactions. It was first proposed as an explanation for the unusually high reaction rates seen in experiments. While it has been the subject of debate, it has also been considered as one of the most promising single approaches to significantly increase gain, and was the subject of the first proposed scheme of a laser-driven HB11 reactor.

To optimize a target concept to exploit gains from the avalanche process, a deeper theoretical understanding is being explored by HB11 Energy. Points being addressed include an extension of Belloni’s work to higher ion temperatures, to degenerate plasmas, and towards more refined kinetic approaches, for example via the Boltzmann-Fokker-Planck equation. It is also important to calculate the so-called energy multiplication factor for laser-accelerated proton-streams in fast-ignition type approaches, taking into account both in-flight fusion reactions and suprathermal multiplication of the fusion products. Concerning the latter effect, the kinematic boost induced on the alpha particles by the impinging protons is particularly relevant. The recent result from LFEX also showed that alpha particles with much larger energies than produced from the fusion reaction were generated and detected, due to the direct energy transfer from accelerated protons to fusion products. This suggests yet another possibility to increase particle energies that could enhance the avalanche mechanism.

Hybrid burn. A fast-ignition-like approach is being investigated by HB11 Energy to increase fusion reaction rates by combining the non-thermal mechanisms listed above with a traditional thermonuclear burn. While this approach would represent a considerably more capital-intensive investment, the prospects for further increases in gain may provide the economic justification for its pursuit.

Thermonuclear fusion reaction rates scale with the square of the ion density, so conventional inertial confinement schemes require significant compression to minimize the energy required to ignite the fuel. Current laser-driven proton-boron experiments have all used uncompressed targets.

The hybrid burn approach combines an inertial confinement scheme including elements of thermonuclear burn and of proton-driven fast ignition. Fast ignition decouples the implosion from the generation of the initiating spark, thereby relaxing some of the requirements on implosion symmetry. Here, the idea is to implode a hydrogen-boron target and around the stagnation time inject a beam of energetic protons generated by using a short-pulse high-intensity laser, as in proton-driven fast ignition. The key difference is that in proton-driven fast ignition the laser-accelerated protons serve only to produce local heating of the fuel to the temperatures needed to trigger deuterium-tritium fusion reactions. Here, instead, not only do the protons contribute to fuel heating, but they directly induce fusion reactions. HB11 is investigating a target concept where these effects can locally heat a section of the target into the hybrid temperature range indicated in Figure 3, where the average of the cross section of the fusion reaction over the assumed Maxwellian velocity distribution of protons and boron at the given kinetic temperature is shown.

Research challenges

Relative to deuterium-tritium, the field of laser-driven proton-boron fusion is young. Consequently, there are several research challenges to be addressed by the research that will be instrumental in accelerating progress in the field.

Material properties. One basic element which is still not precisely known, despite the discovery of HB11 fusion almost 90 years ago, is the precise behavior of the proton-boron fusion cross-section. Classical data on the proton-boron cross section by Nevins and Swain has been more recently revisited by Sikora and Weller, who found higher cross sections in the range of 10 megaelectronvolts. Still, the exact shape of the cross section at energies below a few hundred kiloelectronvolts and for energies above 3 megaelectronvolts is not known. Presently, several experiments are being planned to fill these gaps, the results of which will be critical to developing the models used to simulate laser-driven proton-boron fusion experiments. Similarly, an understanding of the equation of state and opacities of boron under extreme conditions will be another critical requirement for accurate simulations, particularly under compression as proposed for the hybrid burn.

Simulations. Many experiments in the field have focused on pitcher-catcher target configurations and demonstrated quite advanced results. Experimental results have been simulated through a chain of different codes: hydrodynamic codes to simulate the effects induced by the laser pre-pulse and predict the extension of the pre-plasma; particle-in-cell and quantum-electrodynamic particle-in-cell codes to simulate the interaction of the laser beam with the pitcher and the generation of the beams of energetic protons; and Monte Carlo codes, for example GEANT4 and FLUKA, to simulate the interaction of the proton beams with the boron target and predict proton propagation, collisions, fusion reactions and propagation of reaction products.

Our understanding of direct irradiation experiments is far less advanced. Energetic protons are produced on the target front side by complex non-linear mechanisms such as hole boring. In principle these can be simulated by using particle-in-cell codes, however they usually do not include fusion reactions, especially proton-boron-11 fusion reactions. Additionally, performing three-dimensional simulations including collisions, with realistic plasma densities and with realistic space and time scales, presents severe limitations on accuracy and computation time. When a thick target is used, it becomes practically impossible to do a complete simulation using particle-in-cell codes.

One possible approach to address these challenges is to realize a close coupling of particle-in-cell to Monte Carlo codes, using the former for a description of the source and the latter to describe the propagation of hot electrons and energetic ions. However, we need to introduce the cross sections for the fusion reactions in the particle-in-cell codes and include a description of the plasma state in the Monte Carlo code. In parallel, the HB11 team is collaborating with Voss Scientific to use the Chicago simulation code, whose hybrid binary and Fokker-Planck collision operators enable a realistic model of the fusion plasma. The charged particle interactions are modeled with an accurate binary fusion algorithm. The details of the proton-boron-11 reaction including fusion product distributions are currently available in Chicago. They have also recently been implemented in versions of the open-source particle-in-cell code SMILEI.

Beyond these efforts, there is a more general need for accurate simulations of proton-boron fusion experiments, combining all aspects of laser interactions, plasmas and nuclear reactions. These simulations will be an indispensable tool to access non-measured data, complete our understanding of experiments, and optimize target designs that will maximize gain.

Diagnostics. Improvements in diagnostics are needed to obtain more effective and efficient data collection from experiments. Current experiments are mainly based on CR39 track detectors, which is extremely time-consuming and for which the interpretation of experimental results is always difficult, alpha particles being a minor component with respect to laser-accelerated protons and ions. Thomson parabolas are used to measure protons and ions, however it is difficult to detect alpha particles. Time-of-flight measurements, using several types of detectors particularly adapted to detecting alpha particles, have also been used. The drawbacks come from the fact that time-of-flight schemes give no discrimination on particles but only on their velocities, and cover only a small solid angle. Thus, it is essential to develop methods based on the indirect estimation of the proton-boron-11 reaction by detecting products of different simultaneous reactions. HB11 Energy has developed one technique based on positron decay that has been detected from carbon-11 produced in the reaction of boron-11 with a proton giving carbon-11 and a neutron. The development of additional diagnostics, including detecting several signals simultaneously, will be required to limit doubts in data interpretation and to validate models and simulations.

Commercialisation: technoeconomic model and engineering challenges

The United States National Academy of Engineering has identified providing energy from fusion as one of the 14 top grand challenges of engineering. While the realization of net-energy-gain is the primary goal from any fusion effort, another recommendation from the National Academies of Science, Engineering and Medicine was that engineering efforts on an economical reactor should be pursued in parallel to scientific programs, in order to compress the timeframe in which fusion energy can be realized and integrated into the grid.

HB11 Energy has developed a technoeconomic model to assess the engineering requirements of a reactor. Given the prospects of direct conversion of the reaction products into electricity, the markets against which the model has been tested are electricity for the grid and for electrolysis.

In theory, the direct conversion to electrical energy offers the highest efficiency, being arbitrarily close to 100 percent. In practice, the question is complex, also because there are several direct conversion approaches. Consideration of the initial conversion of the ion energy to photon energy returns the estimate of 45 percent efficiency, while direct electrodynamic conversion predicts up to 50 percent. One of the recent concepts proposes to combine plasma magnetohydrodynamic processes with a Rankine steam cycle to achieve 64 percent efficiency.

While much of the focus of other fusion efforts has been on grid electricity, electrolysis for hydrogen production has also been considered as a key market. Not included is the application of process heat, which can also be used for hydrogen production. Hydrogen has been forecast to be a larger market than grid electricity, with the potential to replace carbon dioxide emissions from coal, oil and gas across many industries including transport and steel production. These forecasts have led to significant hydrogen infrastructure investments around the world. Figure 4 shows a simple power loop for a laser-driven inertial fusion energy powerplant that has been used as the basis for HB11 Energy’s technoeconomic model.

A key feature of inertial fusion energy, which is reflected in this diagram, is that the system functions as a power amplifier and not as a power source. That is, fusion power available for conversion into electricity is proportional to the power on target multiplied by the target gain. In turn the power delivered to the target is the product of the laser power and the laser efficiency. The electrical power is determined by the generator conversion efficiency. The power available to the grid is the generated power minus the power for the laser.

The following relations are useful in evaluating the key parameters of this model. The recirculating power fraction is one divided by the product of the generator conversion efficiency, the laser efficiency and the target gain. Engineering breakeven is defined as a recirculating power fraction of one, where the powerplant produces just enough power to operate. A recirculating power fraction of 0.25 has been suggested as a starting point for nuclear fusion, and about 0.1 is typical of nuclear fission reactors. The minimum target gain for operating at a given recirculating power fraction is one divided by the product of the generator efficiency, the laser efficiency and that fraction. This relation leads to the simple rule of thumb that the laser efficiency times the gain should exceed 10. Assuming a generator efficiency in the range 36 to 40 percent corresponds to a recirculating power fraction of about 25 percent, while a laser-efficiency-times-gain of 20 drops that fraction to about 10 percent, which is desirable for achieving the lowest cost of electricity from a plant.

The market constraints used as a boundary condition in this model that reflect economic viability are a levelized cost of electricity of 35 dollars per megawatt hour, with an upper limit of 350, and hydrogen at 1.5 dollars per kilogram, with an upper limit of 2.6. While a detailed appraisal and sensitivity analysis of the technoeconomic model is beyond the scope of this paper, the range of the target gain required to achieve such economic viability varies between 100 and 300 when assuming a laser efficiency of 20 percent. Gains higher than this will both relax the engineering requirements and open the possibility for electricity generation at a cost lower than is currently paid. It may also make other energy intensive industries, such as carbon capture and storage, economically viable.

Several assumptions that have been embedded into this model represent key challenges beyond the scientific endeavors to increase gain, and should be the subject of further research and engineering.

As the fusion system operates as an amplifier of the laser power, the efficiency of the laser system is critical, which we have estimated at 20 percent. This value can only be achieved using a diode-pumped solid state laser driver. It also sets a challenge for future laser system designs that enable high average power and high repetition rates. Assuming a recirculating power fraction of 10 percent, a 500-megawatt power plant would require 50 megawatts to drive the laser system that would produce an average laser power output of 10 megawatts, ignoring energy usage by the other subsystems.

The cost of replacement of the diodes is another critical cost driver. We have assumed a lifetime for diodes of 2.2 billion shots, with a replacement cost of one dollar per watt. Increasing the lifetime and reducing the replacement cost through improvements in diode manufacturing will materially address the economics of a laser-based fusion system.

The cost of the fuel may be another major cost driver. Deuterium-tritium fusion cost analyses have assumed the material cost of the fuel is insignificant; however, in the hybrid-burn scenario, manufacturing requirements associated with the more complex targets, conducive to compression and similar to traditional direct-drive inertial confinement fusion, will add to this cost. Our modelling of the hybrid burn suggests that a target cost of several dollars per target is acceptable if a target gain of 200 can be achieved. This represents a reasonable challenge, particularly given the ease of handling the earth-abundant boron-11 isotope relative to tritium in deuterium-tritium inertial confinement fusion systems.

Significant operational costs of deuterium-tritium systems are primarily associated with the replacement of the activated reactor components exposed to high neutron fluxes. For the HB11 system, these costs are reduced for several reasons, including that there will be no need for tritium breeding, storage, handling, extraction or atmospheric recovery, or a radioactive waste treatment facility. Subject to the specific target design that is chosen, the HB11 system may not rely on a thermal conversion system. Electricity can be captured via a direct electricity conversion system. While it is anticipated that heat will be generated, this could be used as process heat, for example to complement hydrogen or electrolysis production.

The reactor lifetime is also assumed not to be limited by neutron irradiation, as the reaction is aneutronic. There is a possibility for neutronic reactions in the proton-boron chain — boron-11 with an alpha particle giving nitrogen-14 and a neutron, and boron-11 with a proton giving carbon-11 and a neutron — but at the level of about 0.1 percent, and it is not expected to be a concern. The number of neutrons produced per megawatt of electrical power would be two orders of magnitude lower than in a conventional uranium fission reactor. Accordingly, for the purpose of this model, the lifetime is anticipated to be 25 years, which we consider to be conservative. In practice, producing energy from proton-boron fusion does create energetic particles, for example from the proton-boron-10 reaction. Although these are many orders of magnitude less than for deuterium-tritium reactions, their effect on safety and costing will need to be considered against the cost of production of isotopically pure boron-11 in the fuel to ensure a truly aneutronic reaction. In the final design of the reactor, materials research will also be needed to understand the effect of alpha-particle damage to the materials and components of the reactor, to bring more certainty to reactor lifetime estimates.

Based on HB11 Energy’s technoeconomic model, some of the key goals that will enable fusion energy generation are: a target design that can reach a gain above 100; a highly efficient, high-power, high repetition rate laser system driven by cost-effective diodes; and the manufacture of fuel targets for less than dollars per shot.

To evaluate boron abundance to supply future proton-boron energetics, let us compare it with the uranium market. Uranium annual consumption is around ten to the fifth tons per annum, primarily for energy generation. Assuming every uranium nucleus fission delivers 20 times more energy than the boron reaction, and at the same time every boron nucleus is about 20 times lighter than uranium, the boron supply needs for proton-boron energetics can be roughly estimated as similar in tonnage, that is below ten to the sixth tons per year. This is 1000 times less than confirmed global boron reserves of about ten to the ninth tons, and several times less than current boron consumption for other needs.

While the end-goal of these efforts is clean, safe and virtually unlimited fusion energy, a large prize, the scientific risk and potentially long timelines cannot be ignored as they will underpin investment decisions in both the public and private sectors. A challenge for all private fusion companies will be to embrace economies of scope in their business models to mitigate some of the investment risk, which will undoubtedly open new opportunities for multi-billion-dollar industries during the pursuit of these goals.

Conclusion

Proton-boron fusion has many attractive features as a potential source of clean, safe, and abundant energy, which inspired the career of Professor Heinrich Hora as a theoretical physicist. Several experimental demonstrations of non-thermal HB11 fusion using lasers gave promise that it could become a practical reality, and HB11 Energy was founded to pursue this mission.

Relative to deuterium-tritium, the field of proton-boron fusion is young and there are considerable challenges that need to be addressed. Scientific challenges spanning the areas of theory, modelling, material properties and experimental techniques are critical to the many gain-increasing strategies that we might leverage to maximise net-energy-gain in our target concepts.

While reaching net-energy-gain is the primary initial challenge, achieving this with the non-thermal laser fusion approach being pursued by HB11 presents a significantly simpler engineering path than for deuterium-tritium. Nonetheless, there remain significant engineering challenges to generate grid electricity or electrolytic hydrogen economically using hydrogen-boron fusion. Within the context of HB11’s technoeconomic model, key challenges are identified in the areas of laser engineering, target fabrication and reactor engineering.

The magnitude of these challenges cannot be understated — it will not be possible for any one company, university, or national laboratory to achieve this mission in isolation, and large collaborative partnerships involving private fusion companies and academia will be essential, as will significant investment from both the public and private sector. Research groups around the world who can address the challenges outlined in this paper are encouraged to pursue them.

Author contributions, funding and declarations

All authors contributed to and reviewed the manuscript. Open Access funding was enabled and organized by CAUL and its member institutions. The authors declare no competing interests.

(Reference-number markers, running heads and page furniture have been dropped, the reaction equation and the power-loop relations rendered in words, and the four figures named but not reproduced; the sixty-four references and the figures are at the source.)