ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2025 16 Fusion power will serve as a primary abundant energy source, emitting no greenhouse gases and providing global energy security. The high energy density released from nuclear fusion reactions suggests megawatts of power can theoretically be generated from just a few grams of deuterium and tritium fuel. For the last six decades, steady progress has been made in pursuit of unlocking that potential, with the aim of achieving scientific gain (Q_sci—the ratio of power output to power input in fusion reactions) greater than 1. Here, Q_sci > 1 is defined as releasing more power from the fusion reaction than the power input needed to initiate the reaction within the plasma[1]. BACKGROUND On December 5, 2022, the National Ignition Facility (NIF) achieved a scientific energy gain (Q_sci) of approximately 1.5, producing 3.15 MJ of fusion energy from 2.05 MJ of laser energy delivered to the target. Subsequently, in 2023, NIF conducted additional experiments that also achieved Q_sci > 1. Notably, on July 30, 2023, an experiment yielded 3.88 MJ of fusion energy, surpassing the December 2022 result. Recent results (April 6, 2025) yielded 8.6 MJ from a 2.05 MJ laser energy on target, translating in Q_sci of 4.2. These repeated successes demonstrate NIF’s consistent ability to achieve net energy gain in fusion reactions. As nuclear and plasma physics continue advancing toward higher scientific gains from fusion reactions, the major challenges shift to engineering solutions that can sustain these highly energetic reactions for commercial viability. One such engineering challenge is the survivability of the plasma-facing and structural materials in a commercial fusion power plant[2]. Traditional materials qualified for use in nuclear environments by ASME BPVC Section II are not expected to last more than a few years in a commercial fusion environment. A new class of materials is needed to meet the specific performance demands of fusion power plants. Commercial fusion power plants will require continuous operation at power densities of 1-2 MW/m2, with firstwall components surviving neutron fluxes of ~1014 n/cm2/s. Current pro- jections suggest demonstration plants may be operational by 2035-2040, requiring qualified materials within the next 5-10 years. This timeline demands accelerated materials development approaches, including soft materials[3], beyond traditional trial- and-error methods. FUSION FIRST-WALL PERFORMANCE CRITERIA The following sub-sections highlight major challenges facing fusion first-wall components including criteria needed to ensure successful operations. Low-Temperature Irradiation Hardening. Most materials, when exposed to neutron irradiation, experience increased hardness, accompanied by a reduction in ductility and toughness. This phenomenon is well understood and experimentally observed in a variety of steels and nickel alloys but is expected to be a problem in any new alloy systems developed for fusion energy. The hardening and embrittlement are the result of surviving vacancies and interstitials created from neutron irradiation migrating to various biased sinks that grow to become obstacles to dislocation motion. Irradiation embrittlement happens quickly and could present itself at very low doses[4]. For steels, the hardening has been observed to saturate at high doses (>20 dpa)[5]. Irradiation hardening and embrittlement of fusion first-wall materials must be well characterized to ensure the structure does not crack or fail during temperature transients. Irradiation Segregation and Phase Stability. In addition to creating point defects, irradiation damage will break existing crystalline bonds in the material and provide a driving force for the material to transform into phases not found under normal conditions. This segregation can lead to microscopic changes in precipitate composition, size, density, and grain size, which will result in macroscopic changes in the mechanical and chemical performance of the material[6]. The fusion first wall will experience large temperature gradients. Plasma-facing surfaces could experience temperatures above 750°C and drop to as low as 350°C at the interface for the breeder blanket[7]. Materials that experience significant phase change or segregation in those temperature ranges under irradiation may introduce uncertainty in material performance that is unaccounted for in the design. Therefore, any new material for the fusion first wall should characterize any phase instabilities across the relevant temperature and irradiation ranges. At a minimum, accelerated testing is needed to validate the material’s performance at the end of life. Irradiation Swelling and Creep. For high-temperature reactors exposed to a high neutron dose, irradiation swelling, and creep will be the life- limiting factors for the components. Ideally, the first-wall material should not experience steady-state swelling that puts additional stress on structural components. Swelling is the volumetric change induced by irradiation, driven by void and bubble formation in the material. It is independent of the stress state and typically shows a linear dose dependence after an incubation period[8,9]. The incubation period is highly dependent on the microstructure and temperature. New materials can be designed with high neutral sink density to enhance point defect recombination, mitigate void growth, and bubble coalescence. In contrast, creep is the high-temperature plastic deformation below the yield stress driven by dislocation loops and networks (and not void or bubble formation). Under irradiation, the accelerated formation of defects will make materials experience creep at much lower temperatures than in the unirradiated condition[10]. At tempera- tures relevant to fusion systems, combined thermal and irradiation creep can cause excessive plasticity in the first wall, becoming a lifetime-limiting failure mechanism for the component. High-Temperature Helium Embrittlement. One major difference between fusion and fission environments is the far higher helium concentration
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