Nuclear energy is gained by the fusion of two hydrogen isotopes: deuterium and tritium (D-T), which yield helium. The inertial confinement reactor at the National Ignition Facility of the Lawrence Livermore National Laboratory in California achieved “ignition” in this process at the end of 2022. The international fusion reactor ITER being built in southern France is expected to become operative as a demonstrative machine using D-T fuels within the next decade. Recent studies anticipate the potential savings in the order of $1 billion in a 15-year time span by using a fusion propulsion ship instead of conventional container vessels on East Asia-to-Europe routes in the 2050s.

Vittorio Lippay spoke with Dr. Michael Ford, the associate director for engineering at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory, about the potential for nuclear fusion in ship propulsion. Ford commanded the guided-missile cruiser USS Bunker Hill (CG-52), the guided-missile destroyer USS Mustin (DDG-89), and served as lead nuclear engineer (reactor officer) aboard the USS Nimitz (CVN-68). A member of the Navy Nuclear Propulsion Examining Board, he has decades of light-water reactor operating experience.

Fusion reactors of the first generation will probably burn D-T fuel. How realistic is it to imagine a small fleet of large merchant vessels to sail on DT power by 2050 or later, using whatever type of reactor will become available by then?

It is unlikely that propulsion systems for large merchant vessels will be a viable use case for fusion energy systems by mid-century given the current pace of development for fusion technologies. Most viable fusion designs–either magnetic or inertial confinement–require significant space and support systems to operate, including [for traditional magnetic confinement systems] the requirement for cryogenics, tritium processing systems, radio frequency heating systems, et cetera.

Many magnetic confinement concepts work on a pulsed power framework that would require significant input power management systems not included in the standard electrical systems for maritime vessels. This input power demand would likely be even more challenging for inertial confinement systems absent a significant breakthrough in laser technologies.

The challenges in reaching a fusion energy gain factor [engineering gain] greater than one are already daunting. Layering the technical challenges of working in a marine environment with the attendant 3D motion, induced magnetic fields, and more challenging fuel/waste management processes make it unlikely as an early adopter use case for this technology.

While many fusion vendors are promising net gain in the next decade, it is far more likely that it will be mid-century before a commercially viable design is ready for the most likely market, which is grid electricity generation.

Smaller-scale fusion designs face greater technical challenges in many cases because, for a similar power output, they will have far greater material impacts, with neutron and thermal fluxes concentrated in smaller areas leading to accelerated material degradation.

While I always allow for breakthroughs that could accelerate development, the highest probability pathways make fusion-powered merchant vessels a late-century possibility at best.

As an experienced mariner and engineer, would you think it feasible to attenuate neutron emissions from a fusion-powered ship to protect crew, port environment, and shore personnel from the residual radiation adequately and economically?

I do not see neutron attenuation as a critical concern for fusion in a maritime use case. If using a magnetic confinement design, it is likely that there will be blanket and shielding systems that will already reduce the neutron emissions that would be of concern from a health, safety, and material damage perspective. Actually, these neutrons are needed to ensure a self-sustaining fuel cycle.

Beyond this, the inclusion of shielding systems that would further reduce risk would not be technically challenging in that the best shielding is accomplished with hydrogen-containing materials to moderate the neutrons coupled with high neutron absorption cross-section materials, such as boron. A shield tank system containing borated water would likely be a simple, low-cost solution and would not be challenging to incorporate in a marine vessel. Borated polyethylene is another well examined alternative. There is a long history of shield tank and poly usage in naval nuclear programs.

A further problem would arise because of the tritium release via the cooling cycle in port waters. In case of accidents leading to reactor vessel disruption, the resulting hydrogen combustion could disperse radioactive elements for some days.

Would it be possible to implement a credible containment system for a fusion-powered ship?

It is possible to implement a reliable, layered containment system, but tritium is a challenging isotope to contain, and it is still likely that some quantity of tritium would be released over the life cycle of a fusion system during normal operations. These releases can be minimized and most of the leakage captured through use of primary/secondary containment structures and well-managed getter systems. A well-developed array of sensors will be critical to incorporate in any ship design.

There is also risk of more significant releases from accident scenarios [e.g., loss-of-coolant or loss-of-vacuum accidents]. If designed properly, however, a marine fusion system could employ a structural defense concept that would minimize the spread of tritium or tritium-infused materials. Beyond the normal primary and secondary containments, the use of watertight boundaries typical in ship construction would serve as additional barrier to environmental release.

Expensive neutron-shielding materials, like tungsten or tungsten carbide or even high-entropy alloys, are proposed for the parts of a reactor that interface with the plasma [e.g., the central column or the diverter in tokamaks]. These expensive reactor parts would probably require frequent refitting.

Merchant shipping is a rather conservative, low-profit industry. What arguments would you suggest to convince a shipowner to experiment with a new propulsion system based on fusion?

The argument for fusion as a propulsion option would need to be based on a comprehensive cost-benefit analysis. This analysis would include not only tangible costs of the technology [capital costs, maintenance cost differentials, fuel trade-offs, etc.] but also any benefits related to operating model enhancements from a fusion propulsion system [longer transits due to limited necessity for refueling; more dedicated space on board because of more limited space requirements for fuel, etc.], coupled with reduced carbon footprint. These benefits would have to
be balanced against international regulatory and port-entry implications. A fusion design, once costs of development have reached stable and commercially viable levels, may prove a worthwhile alternative in some cases.

The bottom line, however, is that until a fusion design has been technically proven and demonstrated, it would be challenging to come up with a fully convincing narrative for a shipowner.

Would the public be ready to accept a fusion-powered ship in port or more likely to reject regular calls of such ships as happened to fission-powered merchant vessels in the past?

If fusion-powered merchant vessels can demonstrate that they have a strong safety profile and reduced environmental impact when compared with the existing fleet, then they may be able to overcome what are sure to be some perception challenges about this as a nuclear technology. This will require a concerted awareness and engagement effort by the fusion and shipping communities to ensure that public concerns [across all communities] are addressed, and the value of this transition is well understood [perhaps emphasizing the significant reduction of in-port emissions].

I think the public in most communities will be willing to accept the new technology if it is presented openly and best practices for public engagement are followed.

Photo: Dr. Michael Ford, the associate director for engineering at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), stands at the site for ITER, the multinational fusion project being constructed in France. (Credit: B. Rose Huber/PPPL)