HFIR reactor: neutron source for material analysis in 3D printing

At the Oak Ridge National Laboratory (ORNL) of the U.S. Department of Energy, the High Flux Isotope Reactor (HFIR) plays a central role in materials research, which increasingly also includes additively manufactured components. The 85-megawatt research reactor operates in 24-day cycles and delivers a continuous neutron flux suitable for structure-resolving experiments on metals, ceramics, and polymers. ORNL describes the neutrons generated at HFIR as a tool that helps “unlock the secrets of materials and energy.”

Technically, HFIR is based on a compact uranium-235-fueled core in a pressurized vessel with a diameter of around 2.4 meters, which sits in a pool of water. About 16,000 gallons of water per minute cool the reactor and at the same time provide radiation shielding. A beryllium reflector returns neutrons escaping from the core and supports reactivity. Several independently acting control plates can bring the reactor to a safe state if necessary. Despite the rather classic appearance of the control room, the measurement and control systems are continuously modernized.

For 3D printing, the neutron scattering and neutron imaging methods available at twelve scientific instruments are of particular interest. Neutrons penetrate even massive metal components and are highly sensitive to light elements such as hydrogen. This makes it possible to characterize pore distributions, bonding defects, residual stresses, and phase transformations in additively manufactured superalloys or titanium structures without destroying samples. Such data are relevant for qualifying process parameters in laser powder bed fusion or directed energy deposition systems.

Operation is based on a tightly integrated safety concept. Reactor operators, often recruited from the U.S. Navy’s nuclear program, work together with radiological protection technicians and safety experts who analyze operating conditions and issue approvals for experiments. After each fuel change, new specimens are inserted into the core, while irradiated materials are packaged for further analysis and isotope production.

ORNL is currently planning an expansion of the Cold Guide Hall, which houses eight of the twelve instruments. Additional space for further cold-neutron instruments is intended in the medium term to enable experiments specifically targeting additively manufactured high-performance materials—from aerospace components to parts for energy technology and chemical plants.