The review, authored by Irfan Nazir Wani of Universidad Europea de Madrid, examines five mainstream additive manufacturing technologies for nuclear materials: laser powder bed fusion, directed energy deposition, electron beam powder bed fusion, wire arc additive manufacturing, and binder jetting. Each process leaves a distinct fingerprint on the material. Laser powder bed fusion, the most thoroughly studied technique, solidifies metal at staggering cooling rates of up to a million degrees per second, producing a hierarchical microstructure of columnar grains threaded with dislocation cells just 200 to 500 nanometers across. Wire arc systems, by contrast, deposit material up to a thousand times faster but yield coarse, weld-like grains. These process-structure-property-performance relationships, the review argues, are the key to understanding how printed parts will behave inside a reactor.

The stakes could hardly be higher. Fourth-generation fission reactors and fusion devices demand structural materials that survive temperatures up to 1,000 degrees Celsius, transient heat fluxes exceeding 10 megawatts per square meter, and neutron bombardment reaching 200 displacements per atom over decades of service. Traditional manufacturing already struggles to deliver such components, with lead times stretching to several years. Additive manufacturing offers near-net-shape fabrication, functionally graded compositions, and microstructures that could, in principle, enhance irradiation resistance. Yet the two dominant global nuclear qualification frameworks, ASME and RCC-M, apply only to conventional forged and cast materials.

One of the most striking findings concerns the thermal fragility of the very dislocation structures that give printed stainless steel its strength. In laser powder bed fusion 316L, the dense network of dislocation cells, enriched with chromium and molybdenum at their walls, begins to undergo irreversible changes at temperatures as low as 230 degrees Celsius. Above 600 degrees, the cell walls dissolve entirely, and short exposures of less than half an hour at 800 degrees can destroy the cellular structure altogether. That is an uncomfortable fact for a material intended for Generation IV reactors operating between 400 and 800 degrees. Long-term aging studies on printed 316H, however, offer a more nuanced picture: despite accelerated precipitation kinetics up to a hundred times faster than wrought material, the printed alloy retained yield strength better than its forged counterpart, and physics-informed models now extrapolate its behavior out to a century of service.

The review also exposes a genuine scientific controversy over irradiation swelling. Several studies report that printed 316L swells less than wrought material, crediting the dislocation cells with acting as sinks that annihilate the point defects radiation generates. Other work finds heterogeneous void distributions, with denuded zones near cell walls, suggesting the cell network may not be the ideal sink structure after all. The outcome hinges on a delicate interplay of grain boundary character, void bias, helium bubble nucleation from transmutation reactions, and defect recombination efficiency, all of which differ in printed microstructures. Whether cell walls act as recombination sites or as nucleation sites for damaging voids remains an open question that systematic experiments have yet to answer.

Oxide dispersion-strengthened alloys illustrate how contested the field remains. These steels, seeded with nanometer-scale oxide particles that block dislocations and trap helium, are traditionally made by costly mechanical alloying. Printing them directly from blended powders would be transformative, but studies reach opposite conclusions: some show that the extreme cooling rates kinetically trap Y2O3 and Y-Al-O phases intact, while others observe dissolution and coarsening driven by Ostwald ripening at high laser energy densities. Atom probe tomography adds a further twist, revealing that nanoclusters far more numerous than the visible oxide particles can multiply and change composition under irradiation, and that their stability depends strongly on temperature. Recent neutron irradiation data on 12Cr ODS steel at low dose showed the clusters barely changed, a reassuring but narrow data point.

Tungsten, the leading candidate for fusion divertors, presents its own drama. With a melting point of 3,422 degrees Celsius and notorious brittleness, printed tungsten cracks under the thermal stresses of fabrication. Mitigation strategies, from substrate preheating to nanoparticle doping with titanium carbide or lanthanum oxide, have produced crack-free small coupons, but scaling to divertor-relevant components larger than 100 millimeters remains unsolved. Printed tungsten also raises a fusion-specific concern: tritium retention. Studies of electron beam printed tungsten found deuterium retention comparable to conventional material but with altered desorption behavior, while laser powder bed fusion samples trapped argon from the shielding gas in nanopores, hinting that printed porosity could create additional trapping sites for scarce fusion fuel.

Perhaps the most consequential limitation is the near-total absence of neutron irradiation data for printed nuclear materials. Almost everything known comes from ion irradiation, which delivers damage up to ten thousand times faster than reactors, produces different cascade structures, penetrates only microns deep, and generates no transmutation helium or hydrogen. The few comparative studies suggest that properly heat-treated printed 316L can match wrought material’s radiation response, but as-built material may not, and extrapolation beyond 10 displacements per atom, far below the 50 to 200 that reactors will impose, is essentially unsupported. Compounding the problem, most mechanical tests run for hundreds to thousands of hours while reactor components must endure roughly half a million.

Qualification presents a regulatory labyrinth of its own. The 2025 RCC-MRx code case for laser powder bed fusion components marks real progress but covers only specific material-process combinations within the French framework, and ASME has yet to offer comparable guidance. Printed parts vary with build height and geometry in ways that conventional sampling plans may not capture, their keyhole porosity and lack-of-fusion voids behave differently from wrought defects, and traditional nondestructive inspection loses sensitivity on complex printed geometries. The European NUCOBAM project is working to draft additive-specific text for nuclear design codes, while digital twins and machine learning-enhanced inspection promise to compress qualification timelines that would otherwise stretch over decades.

The review’s roadmap is unambiguous about priorities: coordinated international neutron irradiation campaigns comparing printed and wrought materials across the full dose and temperature range, long-term thermal aging studies exceeding 10,000 hours, anisotropic property databases that respect the directional nature of printed microstructures, and validated models that embed additive-specific initial conditions. Near-term opportunities exist in less safety-critical niches, such as repairing non-structural components with directed energy deposition, or deploying printed heat exchangers in small modular and research reactors to accumulate operational experience. The central question, the review concludes, has shifted from whether additive manufacturing can serve nuclear energy to which components can be certified, under what conditions, and how quickly the evidence base can catch up with the technology’s ambition.

Subject of Research: Additive manufacturing of nuclear reactor materials under extreme irradiation and thermal conditions

Article Title: Additive manufacturing of nuclear materials under irradiation extremes: thermal-mechanical performance, qualification challenges, and future research directions

Article References: Wani, I. N. (2026). Additive manufacturing of nuclear materials under irradiation extremes: thermal-mechanical performance, qualification challenges, and future research directions. Frontiers in Nuclear Engineering, 5, Article 1894792. https://doi.org/10.3389/fnuen.2026.1894792

Image Credits: AI Generated

DOI: 10.3389/fnuen.2026.1894792

Keywords: additive manufacturing, nuclear materials, irradiation damage, laser powder bed fusion, void swelling, ODS alloys, tungsten, silicon carbide composites, reactor qualification, Generation IV reactors, fusion materials, thermal stability

The material in question is 9Cr oxide dispersion strengthened steel, or 9Cr-ODS steel, widely regarded as a leading candidate for structural components in fusion blankets and for fuel cladding in advanced reactors. Its secret weapon is a dense dispersion of oxide particles only a few nanometres across, in this case of the complex oxide YTiTaO6, embedded in a ferritic-martensitic steel matrix containing roughly nine percent chromium. These particles pin dislocations, block grain boundary migration and resist coarsening under irradiation, which is precisely why the steel retains useful strength at temperatures where conventional reactor steels soften dramatically. But the particles are only effective while they remain small, numerous and finely dispersed.

Manufacturing large ODS components requires thermomechanical processing, in which the alloy is heated and rolled to consolidate and shape it. The trouble is that the high temperatures involved, particularly rolling above Ac3, the temperature at which the steel becomes fully austenitic, can trigger recrystallization of the matrix grains and, crucially, coarsening of the nano-oxides. Earlier work by other groups had shown oxide particle growth in 9Cr-ODS steels at temperatures above about 1473 kelvin, and the field has long sought processing routes that allow industrial-scale shaping without sacrificing the nanostructure. The Hefei team set out to compare, systematically, two representative rolling routes and to track what happens to the grains, the carbides and the nano-oxides in each case.

As a baseline, the researchers first characterized a tempered sample of the steel, which displayed a heterogeneous mixed microstructure with relatively coarse carbides sitting along grain boundaries and an average grain size of 0.80 micrometres. This starting point reflects the typical state of tempered martensitic ODS steel after standard heat treatment. From there, the team applied two contrasting thermomechanical routes: one in which rolling was performed above Ac3, fully transforming the structure to austenite, and another known as intercritical rolling, carried out within the two-phase region between Ac1 and Ac3, where austenite and ferrite coexist. They then examined the resulting matrix grains, carbide distributions and oxide particles, and measured tensile properties at 650 degrees Celsius.

The results were striking. Rolling above Ac3 induced partial recrystallization of the grains, and inside the recrystallized regions the nano-oxides agglomerated and coarsened noticeably. Even more telluric for the material’s future performance, the coarsened oxides lost the specific crystallographic orientation relationship they normally share with the surrounding steel matrix. That orientation relationship matters: it reflects a degree of crystallographic coherence between particle and matrix that is associated with interfacial stability. When coarsening breaks that relationship, the particles behave more like incoherent inclusions, and prior studies of irradiated ODS alloys have linked incoherent interfaces to poorer dispersoid stability at elevated temperature.

Intercritical rolling told a very different story. By keeping the deformation within the Ac1 to Ac3 window, the process effectively restrained both grain growth and nano-oxide coarsening. The fine YTiTaO6 particles retained their characteristic orientation relationship with the matrix, indicating that their coherent or semi-coherent interfaces survived the processing intact. In other words, the two-phase region acts as a thermal sanctuary for the nanostructure: hot enough to allow the plastic deformation needed for shaping, but not so hot or so transformative that the particles dissolve, agglomerate or lose their crystallographic anchoring to the surrounding iron lattice.

The mechanical consequences were measurable. The optimally intercritically processed steel achieved a higher grain boundary density and produced finer carbides, a microstructural combination the authors associate with improved tensile behaviour. At 650 degrees Celsius, a temperature squarely relevant to reactor operating conditions, the intercritically processed material reached an ultimate tensile strength of 473.4 megapascals with a total elongation of 33.1 percent. Compared with the conventionally high-temperature rolled counterpart, that represents higher strength at essentially comparable ductility, a combination that is notoriously difficult to achieve in high-temperature structural alloys, where strength gains usually come at the expense of the ability to deform without cracking.

The study’s significance extends beyond a single alloy recipe. By demonstrating that nano-oxide stability in 9Cr-ODS steel is temperature-dependent and can be regulated through the choice of processing window, the work clarifies the mechanism by which thermomechanical processing either preserves or degrades the strengthening dispersion. It also provides practical guidance for industrial fabrication of large-size ODS nuclear components, which is one of the field’s most persistent bottlenecks. Producing ODS steels at laboratory scale is well established; producing them as sizeable, reliable components with uniform nanostructure remains a genuine manufacturing challenge for fusion and fast reactor programmes worldwide.

For the broader materials community, the findings reinforce a growing appreciation that the interface between a nanoscale precipitate and its host matrix is as important as the precipitate itself. Coherent interfaces slow diffusion, suppress coarsening and help particles survive both thermal excursions and radiation damage. The Hefei team’s demonstration that a carefully chosen intercritical rolling window can preserve those interfaces through industrial deformation suggests a path toward ODS steels that are not only superb on paper but manufacturable at scale, bringing the dream of durable fusion blanket and advanced reactor cladding materials one practical step closer to reality.

Subject of Research: Microstructural evolution and nano-oxide stability in 9Cr-ODS steel during thermomechanical processing

Article Title: Microstructural evolution of 9Cr-ODS steel during thermomechanical processing

Article References: Zhu, G., Huang, T., Hai, C., & Huang, Q. (2026). Microstructural evolution of 9Cr-ODS steel during thermomechanical processing. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13866-7

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13866-7

Keywords: ODS steel, nano-oxides, thermomechanical processing, intercritical rolling, fusion materials, Generation IV reactors, YTiTaO6, grain refinement, tensile properties, carbides, recrystallization, nuclear structural materials

Multi-principal element alloys abandon the traditional recipe of one dominant metal with small amounts of alloying additions. Instead, five or so elements are mixed in roughly equal proportions, producing a chaotic atomic landscape of lattice distortion, sluggish diffusion and chemical disorder. That disorder is precisely what excites nuclear materials scientists. Previous studies have shown that such chemical complexity can promote the recombination of radiation-generated defects, suppress void swelling, and limit the formation of dislocation loops — the very failure modes that cripple conventional reactor steels over decades of service. Ferritic body-centered-cubic steels already outperform austenitic alloys in swelling resistance, and the researchers reasoned that fully refractory body-centered-cubic alloys built from molybdenum, niobium and titanium should inherit that advantage while tolerating far higher temperatures, potentially in the 500 to 1000 degrees Celsius window targeted by Generation IV designs.

The team fabricated eight alloys by vacuum arc melting of cold-compacted elemental powders: the base MoNbTi plus quaternary MoNbTiZr, MoNbTiCr, MoNbTiV and MoNbTiAl, and quinary MoNbTiZrV, MoNbTiCrV and MoNbTiCrAl. Titanium getter chips were melted first to scrub oxygen and nitrogen from the argon atmosphere, and each ingot was flipped and remelted at least five times to maximize homogeneity. X-ray diffraction and backscatter electron imaging revealed a striking compositional dependence in the as-cast state. The base alloy MoNbTi and the aluminum-bearing MoNbTiAl emerged with clean single-phase body-centered-cubic matrices, while the remaining six alloys showed pronounced dendritic segregation. Molybdenum, the highest-melting constituent, preferentially populated the bright dendrite cores, whereas lower-melting titanium, vanadium, chromium and zirconium pooled in the inter-dendritic regions. In every chromium-containing alloy, the microscopy and hardness data also betrayed the presence of a brittle TiCr2 Laves phase seated in the dendrite cores, a classic intermetallic culprit for degraded ductility and machining difficulty.

Thermodynamics explained the pattern. Calculations of liquid-phase mixing enthalpies using Miedema’s scheme showed that zirconium-niobium, chromium-molybdenum and titanium-niobium pairs are mildly immiscible, encouraging segregation, while strongly negative titanium-aluminum, titanium-chromium and titanium-vanadium interactions lock those elements together in the interdendritic zones. Aluminum’s highly negative mixing enthalpies with titanium, niobium and molybdenum also foreshadowed its destabilizing behavior: when the team solution-treated the alloys — 24 hours at 1500 degrees Celsius for most systems, and 1300 degrees Celsius for MoNbTiZrV, guided by CALPHAD-predicted single-phase windows — MoNbTiAl actually transformed from a single-phase structure into a multiphase one, a reminder that configurational entropy does not always win. Four alloys, MoNbTiZr, MoNbTiCr, MoNbTiV and MoNbTiCrAl, achieved fully homogeneous single-phase matrices after treatment. MoNbTiZrV and MoNbTiCrV became more uniform but retained residual segregation, which the authors attribute to sluggish transformation kinetics rather than true thermodynamic instability.

The mechanical characterization painted an equally nuanced picture. Vickers hardness spanned an impressive range, from 472 VHN for the soft alloy MoNbTiV to 656 VHN for MoNbTiCrAl, the hardest of the set, with the chromium-bearing alloys consistently elevated by their Laves-phase content. Dynamic elastic properties measured by the impulse excitation of vibration technique, benchmarked against a certified 316 stainless steel reference, yielded Young’s moduli between 141 and 169 gigapascals, shear moduli of 54 to 62 gigapascals, bulk moduli of 114 to 194 gigapascals and Poisson’s ratios of roughly 0.3 to 0.35. All eight alloys sat below stainless steel in stiffness yet compared favorably with refractory alloys such as MoNbTiTaV and MoNbV reported in the literature. After heat treatment, hardness fell in the alloys that had achieved homogeneous single-phase matrices — fewer phase boundaries means fewer obstacles to dislocation motion — while hardness rose and became more scattered in the multiphase systems, consistent with the retention of hard secondary constituents.

But the headline result came from the oxidation furnace. Generation IV structural materials must resist not only radiation but also relentless attack by hot air and steam, and refractory alloys are notoriously poor oxidizers. Exposed to flowing air at 800 degrees Celsius, the eight alloys split into two dramatic camps. MoNbTiCrAl was the runaway winner, gaining a mere 0.24 milligrams per square centimeter after 24 hours and developing an oxide scale just one micrometer thick. MoNbTiCr and MoNbTiCrV followed with modest gains of 1.37 and 2.39 milligrams per square centimeter respectively, and all three chromium-bearing alloys approximately obeyed a cubic oxidation rate law, an uncommon kinetic regime associated with coupled grain-boundary diffusion, bulk diffusion and oxide grain growth. The base MoNbTi, with a parabolic trend but a 217-micrometer cracked, porous scale rich in loosely adherent TiNb2O7, Nb2O5 and TiO2, showed a worrying upward drift in mass gain that suggests trouble at longer exposures.

At the opposite extreme, vanadium and zirconium proved catastrophic. MoNbTiV absorbed a staggering 217.2 milligrams per square centimeter and was fully oxidized to a porous mass, its thick V-rich upper scale riddled with porosity because vanadium pentoxide, which melts above 675 degrees Celsius, prevents dense protective scale formation. The zirconium alloys fared worst of all: both MoNbTiZr and MoNbTiZrV disintegrated between the 12- and 24-hour marks, exhibiting the dreaded phenomenon of pesting, in which accelerated oxygen ingress through zirconium- and titanium-rich interdendritic channels produces severe exfoliation, cracking and eventual disintegration of the bulk metal. X-ray diffraction of the oxides confirmed the mechanistic story, detecting protective Cr2O3 in all chromium alloys and, uniquely in MoNbTiCrAl, a combined barrier of Cr2O3 and Al2O3 alongside the complex oxide TiNbO4 — a dual-layer defense that mirrors the strategy behind the best oxidation-resistant refractory alloys previously reported, such as TaMoCrTiAl with its CrTaO4 scale.

The authors are careful to frame the work as a screening exercise rather than a final qualification. The oxidation data represent short-term isothermal exposure in laboratory air, and the decisive questions — irradiation performance of these specific compositions in-reactor, long-term corrosion in coolant environments, and elevated-temperature mechanical behavior — remain open. The team also noted that small nitrogen-containing phases, likely picked up during powder handling, were present below roughly five percent in most samples and were judged not to compromise the conclusions. Nevertheless, by systematically connecting composition to microstructure, elastic constants, hardness and oxidation kinetics across eight alloys in a single coordinated study, the work provides exactly the kind of composition-microstructure-property map that alloy designers need before committing to expensive irradiation campaigns.

For the emerging Generation IV reactor fleet — sodium-cooled fast reactors, molten salt designs and very-high-temperature gas systems — the message is clear. Chromium is the friend, aluminum its powerful ally, and together they transform a vulnerable refractory metal into something approaching a viable core material. Zirconium and vanadium, despite their low neutron cross sections, are oxidation liabilities to be avoided or tightly managed. MoNbTiCrAl, combining low density, the highest hardness, elevated elastic moduli and an extraordinary resistance to high-temperature air attack, now stands as the leading candidate from this family, a base composition awaiting optimization and, ultimately, the neutron bombardment tests that will decide whether chaotic multi-element metals can carry the next era of nuclear power.

Subject of Research: Phase evolution, mechanical properties and high-temperature oxidation behavior of MoNbTi-based refractory multi-principal element alloys for Generation IV nuclear reactors.

Article Title: Phase evolution, mechanical properties and high temperature oxidation behavior of novel equi-atomic Mo-Nb-Ti-based refractory MPEA for Gen IV reactor applications

Article References: Karimpilakkal, A., Schulthess, J. L., Jalan, V., Wen, H., Liou, F., & Newkirk, J. W. (2026). Phase evolution, mechanical properties and high temperature oxidation behavior of novel equi-atomic Mo-Nb-Ti-based refractory MPEA for Gen IV reactor applications. Journal of Materials Science: Metallurgy, 1(1), Article 19. https://doi.org/10.1007/s44492-026-00019-9

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00019-9

Keywords: refractory alloys, multi-principal element alloys, high-entropy alloys, MoNbTi, Generation IV reactors, oxidation resistance, Laves phase, elastic moduli, pesting, arc melting, nuclear materials, cubic rate law