Laila El-Guebaly

Fusion energy, the process that uses the same reaction that powers the sun and the stars, offers the promise of virtually unlimited, carbon-free energy and is approaching reality. Recently, there's been a dramatic global increase in the investment and research focused on addressing the hurdles to commercialize fusion energy. While a majority of the effort has been focused on gaps in technology, little work has been done to address the societal and environmental impacts of this technology. Three community- and environmentally-focused research priorities are identified for commercializing fusion energy: 1) understanding the environmental impacts of fusion energy across the technology lifecycle, 2) developing risk and safety assessment methodologies for fusion power plant technologies, and 3) creating a community-based socially engaged approach for fusion technology design and development. This approach will benefit private companies who wish to deploy future fusion power plants as concerns about the technology will be addressed early in the design process, thus minimizing delays in deployment that may result in increased costs for developers. Community engagement around fusion technology development must be evidence-based in order to build trust between communities and technology developers. Such an approach is grounded in informed consent is vital for the sustainable development and use of fusion technologies.

The inclusion of test modules in the U.S. Fusion Prototypic Neutron Source (FPNS) offers the opportunity to test a wide variety of materials in a representative radiation environment of the fusion Pilot Plant, DEMO, and power plant. The testing may include various generations of structural materials for conventional and advanced blanket and divertor concepts. Since all structural materials derived from the fission industry are inadequate for fusion applications (due to the more damaging effects of the 14-MeV fusion neutrons), radiation-resistant reduced-activation structural materials (reduced-activation ferritic-martensitic steel, vanadium alloy, W alloy, and SiC/SiC composites) were specifically developed for fusion and could be tested in the FPNS to qualify for the highly irradiated fusion components surrounding the plasma. The large atomic displacement and the helium and hydrogen generations by fusion neutrons are unique to fusion materials. The most important attribute for the FPNS would be the typical fusion-relevant He/displacements per atom (dpa) ratio of ~10 for steel in particular. By comparison, irradiation in the fission spectrum of the High Flux Irradiation Facility (HFIR) would underestimate the dpa and provide a very low He/dpa ratio of ~0.3 for steel, which is irrelevant to fusion. This paper reviews the neutron irradiation impacts and presents a few examples of dpa and transmutation products for steel, W, and SiC based on modeling in several fusion design studies. The operating conditions of advanced U.S. fusion power plants were considered along with the credible lifetime goal of 200 dpa and 20 MW·yr/m² fluence that could be achieved with directed research and development programs coupled with the construction of the FPNS 14-MeV neutron facility.

In recent decades, fusion designers have become increasingly aware of the large amount of mildly radioactive materials that fusion generates in comparison to their fission counterpart, which is a problem that was overlooked in early fusion studies. This radioactive waste (radwaste) problem could influence public acceptability of fusion and will certainly become a significant issue in the immediate future as fusion moves forward toward commercialization. There is a growing appreciation to revisit the 1960s decision that relegated all radwaste to the back end as only a disposal issue. In light of the challenges facing fusion in the 21st century, a thoughtful alternate approach that promotes recycling and clearance of all fusion radioactive materials is considered to stress the environmental value of fusion in utilizing natural assets efficiently, assert the fundamental premise of fusion as a nuclear energy source with minimal environmental impact, and gain public acceptability for fusion. This strategy helps to reach the common goal of several organizations that recommend recycling and clearing as much radioactive material as practically possible to reduce final radwaste burdens/risks and to maximize the use of natural resources. Recognizing the relatively early stages of commercial fusion maturity, lessons learned and worldwide industrial experiences from other nuclear fields are valuable resources for the fusion recycling/clearance approach. To make such an approach a reality, the global fusion program should be set up to accommodate the new strategy at an early stage of fusion designs and address the identified issues and needs with directed research and development programs. The absence of official fusion regulatory guidelines has been recognized for several decades, but some progress has been made in recent years, recognizing that fusion is different from fission and has a different radionuclide profile.

In recent decades, fusion energy for electricity has become an international issue with worldwide interest in several magnetic fusion concepts offering the most promising energy source for this century. From existing experiments to power plants, several next-step facilities (NSFs) must be built to bridge the large gaps in fusion science and nuclear technology. During the course of fusion studies, all power plants and NSFs require an integral nuclear assessment to identify the nuclear parameters and address key issues related to tritium breeding ratio (TBR), blanket design, selection of low-activation materials, radial/vertical build optimization and definition, magnet protection, shielding, activation, and survivability of structural materials in 14-MeV neutron environment. This paper presents our design philosophy, nuclear assessment approach, and recent research results for ARIES conceptual tokamak, spherical tokamak, and stellarator power plants as well as NSFs. Some features of the nuclear activities [such as tritium breeding requirement (overall TBR = 1.05), blanket concept, and radwaste issues] remained fixed between the various designs, while others [such as service lifetime (20 to 200 displacements per atom) and shielding requirements] were subject to change to meet the specific design needs. Emerging challenges and lessons learned from nuclear assessments performed during recent decades are highlighted throughout the paper. In particular, the cost implication of uncertainties in the TBR prediction and the large amount of low-level waste generation are important challenges facing the fusion community and should be addressed by interdisciplinary research programs.

The qualification and validation of nuclear technologies are daunting tasks for fusion demonstration (DEMO) and power plants. This is particularly true for advanced designs that involve harsh radiation environment with 14 MeV neutrons and high-temperature operating regimes. This paper outlines the unique qualification and validation processes developed in the U.S., offering the only access to the complete fusion environment, focusing on the most prominent U.S. blanket concept (the dual cooled PbLi (DCLL)) along with testing new generations of structural and functional materials in dedicated test modules. The venue for such activities is the proposed Fusion Nuclear Science Facility (FNSF), which is viewed as an essential element of the U.S. fusion roadmap. A staged blanket testing strategy has been developed to test and enhance the DCLL blanket performance during each phase of FNSF D-T operation. A materials testing module (MTM) is critically important to include in the FNSF as well to test a broad range of specimens of future, more advanced generations of materials in a relevant fusion environment. The most important attributes for MTM are the relevant He/dpa ratio (10–15) and the much larger specimen volumes compared to the 10–500 mL range available in the International Fusion Materials Irradiation Facility (IFMIF) and European DEMO-Oriented Neutron Source (DONES).

The importance of handling the sizable mildly radioactive materials that fusion generates received more attention in recent years. Disposing such sizable radwaste in geologic repositories is not a viable option. We suggest changing what is now a costly waste disposal concern for fusion energy into a valued commodity through the further development of the recycling and clearance approaches. This paper reports the outcome of two recent activities that identified the challenges of handling the radioactive materials of ARIES-ACT-2 power plant along with the required design changes and R&D programs that make the recycling/clearance approach a reality, and the development of a new detritiation code that predicts the efficiency of tritium recovery from metallic materials – an essential process before recycling.

There is a strong indication that the dual-cooled LiPb blanket is the preferred concept for many fusion power plants being designed around the world. The ability of the blanket to provide tritium self-sufficiency is among the important issues that we investigated in detail for ARIES-ACT to pinpoint the design elements that degrade the breeding the most, using state-of-the-art neutronics codes. A novel stepwise approach was developed to identify the exact cause of the degradation in the tritium breeding ratio (TBR), initially 1.8 for an ideal system, reaching 1.05 for a practical design. More broadly, this paper gives many insights into the impact that internal components of the blanket as well as essential parts of a tokamak can have on the TBR and the more damaging or enhancing conditions or changes to the breeding. To overcome the challenges of dealing with all tritium-related uncertainties in several subsystems, we suggest adjusting the Li enrichment online during operation to mitigate concerns about the danger of placing the plant at risk due to tritium shortage as well as the problem of handling and safeguarding any surplus of tritium.