Discover how NC State’s PULSTAR research reactor continues to drive 21st-century nuclear innovation and advanced materials testing

When the PULSTAR reactor in North Carolina State University (NC State) first went critical on 9 September 1972, the world’s first pocket-sized calculator was just released, making the slide rule obsolete overnight. Over five decades later, this 1-megawatt workhorse remains at the forefront of nuclear innovation, witnessing experiments that are 21st-century from advanced fuels, sensors, to novel materials for tomorrow’s next-generation reactors.

The PULSTAR nuclear reactor was established as the fourth in a series of reactors on the campus of NC State. It is a light-water moderated and cooled open pool reactor. The core is configured in a 5×5 array of assemblies with beryllium reflectors. Each assembly contains an array of 25 fuel pins that contain sintered uranium dioxide pellets enriched to either 4% or 6% in ²³⁵U and clad with Zirconium-based alloy. Its fuel design and configuration result in reactor dynamics and irradiation field similar to the light-water power reactors, making it a unique case among research and test reactors in the US.

The development and deployment of advanced reactor technologies face several critical challenges, including the qualification of novel structural and fuel materials, testing of unproven instrumentation and control systems, and understanding long-term material behaviour under extreme radiation environments. Tackling these hurdles through conventional testing pathways can be time consuming and cost prohibitive. The PULSTAR reactor provides a vital, versatile platform to accelerate this process. With its unique irradiation capabilities, specialised beam ports, and tailored experimental facilities, PULSTAR enables rapid testing, material characterisation, and sensor validation under controlled conditions, serving as a key bridge between fundamental nuclear research and commercial advanced reactor deployment.

The in-pool radiation facilities at PULSTAR

Exposing candidate nuclear materials, coatings, and advanced in-core sensors to the reactor irradiation field enables researchers to study radiation damage, degradation, and instrument deterioration under a similar operational condition. It helps eliminate the prohibitive costs, high regulatory barriers, and operational risks associated with testing inside active commercial power stations. The open-pool configuration in PULSTAR allows test subjects to be easily inserted into standpipes and brought to the proximity of the core under water. Some standpipes provide direct access to electronic signals during instrumentation irradiation, which gives the researchers a direct way of monitoring the devices in-situ, and can directly ‘see’ their drift and possible failure as a function of radiation dose. Several other standpipes feature highly adaptable settings tailored for specialised needs, allowing control over factors like elevated temperatures, altered neutron/gamma field ratios, or specific gas environments. In addition, post-irradiation examination is also a crucial component of the PULSTAR nuclear facility, which involves safe handling and testing of the radioactive materials. Unlike traditional testing methods, two specialised techniques are set up and used at PULSTAR.

Nuclear materials characterised at the atomic level

When talking about anti-matter, people naturally think of a sci-fi movie or scientific frontier concepts. However, what might be surprising to many people is that positron – the antimatter twin of electrons – has been used by researchers as a special tool for materials research for decades. Positrons naturally annihilate with electrons, converting both particles cleanly into energy (gamma rays) through Einstein’s famous equation E = mc². Because of their positive charge, positrons are naturally trapped in the atomic-level open spaces, e.g., vacancies, free volumes, and nano-voids, which lays the foundation of the two most important positron techniques: the positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS). Materials with distinct defects and vacancy properties would render miniscule differences in annihilation lifetime and energy. Even though the fingerprints are faint, with modern electronics such as those in the positron facilities at the PULSTAR, a difference of several pico a second and less than from the annihilating gamma rays can be detected. The detected signals can be associated with a concentration of less than one part per million (ppm) of certain vacancies that otherwise cannot be detected.

The positron source at the PULSTAR reactor has achieved a world-record production rate of one billion positrons per second, and is one-of-a-kind in the US.^1,2 Interestingly, this ‘intense’ positron source is enabled by a pair-production process in which high-energy gammas are converted to pairs of positrons and electrons – the reverse of positron-electron annihilation. By collecting the positrons using carefully designed electro-magnetic fields, an intense positron beam is formed to study the surface of a material, or a thin barrier underneath. The areas of research span from thin polymer films used as gas filtration materials, reverse osmosis membranes, radiation damage on nuclear materials, to broadly surface science, porosimetry, and fundamental materials science.

Neutrons are another particle that is abundant in the reactor and can be used as a probe. At PULSTAR, the neutron powder diffraction (NPD) uses this analytical technique to examine the structural characteristics at the atomic level.³ Similar to X-ray diffraction, the technique relies on the diffraction of neutrons that provides deeper penetration and better signals from light atoms like hydrogen. In this system, three-inch single-crystal sapphire filters are inserted into the reactor beamport to block most of the gammas and high-energy neutrons. Downstream, a double focusing silicon monochromator focuses the neutron beam onto the sample position. A position-sensitive detector array that spans 20° can be moved to cover the full scattering angle ranging from 5° to 125°. A fission chamber located in the beam upstream of the sample serves as the monitor. The detectors collect data at each section for a fixed monitor count to ensure that the number of incoming neutrons at the sample position is the same for each section. Finally, the data from the six sections are combined into a full-range scattering spectrum. This diffractometer offers low background, with a relatively high data collection rate and resolution. This facility has been used to examine various materials, including radiation-damaged graphite, magnetic materials without rare earth elements, and silicon-doped hafnia.

Though they use two distinct particle probes and physical principles, the positron and neutron facilities at PULSTAR provide a synergistic combination for nuclear materials research. The lattice structure of a material and its regularity can be examined by NPD, while the vacancies and free volumes, or the ‘irregularities’, can be accessed by PALS/DBS.

A good example is a study of nuclear graphite,^4,5 widely used in high-temperature reactors as a neutron moderator, which must deliver long-term stability under high temperature and radiation inside the harsh environment of a nuclear reactor. Materials scientists have been meticulously fine-tuning its microscopic grains to make it isotropic and robust to prevent swelling or degradation, and the evolution of its nanostructure under irradiation is crucial in understanding and projecting its performance. It is discovered that the positron lifetimes have a fairly broad distribution in both virgin graphite and the one that had undergone irradiation at a relatively low temperature (280°C), meaning the structure has a variety of vacancies with different sizes. For a sample irradiated at high temperature (700°C), a single positron lifetime dominates, indicating a stable V6 vacancy cluster (six missing carbon atoms, as illustrated) predicted by theory.

Meanwhile, the neutron diffraction pattern shows another side of the story: the unirradiated sample has a clear diffraction pattern, suggesting a largely intact graphitic structure. After irradiation at 280°C, the diffraction peaks mostly disappeared – a telltale sign of a radiation damaged lattice. Irradiation at 700°C largely restored the pattern, indicating a better order due to the annealing effect. The combined results paint a more complete and complex picture wherein lattice structure becomes amorphous under irradiation and vacancies may migrate and evolve, especially when temperature is sufficiently high to enable the carbon atoms to hop through energy barriers. While adjusting the recipe of such types of materials, exposure to both radiation and heat plays critical roles.

Fulfilling the need for advanced nuclear testing

To address the demand for testing capabilities of advanced nuclear fuels, new facilities have been designed and developed at PULSTAR over recent years. One of these is a fission gas release measurement system⁶ that measures the escape of volatile byproducts like xenon and krypton from nuclear fuels. During reactor operation, these gases migrate out of the solid fuel matrix and accumulate in the internal spacing, increasing the pressure inside the cladding. Understanding this effect is critical for evaluating reactor safety, longevity, and operational limits. The online measurement system heats the fuel particles to above 1,000°C and brings out the released gases using a stream of helium cover gas, which is fed into a highly sensitive germanium detector where different isotopes of xenon and krypton can be differentiated and quantified. By precisely measuring fission gas release, fuel behaviour under extreme conditions can be predicted and high-performance fuels can be qualified for future reactors.

Another key facility currently under development focuses on molten salt research, which is a promising technology candidate for next-generation nuclear reactors. Serving as the initial phase of PULSTAR’s molten salt research capabilities, this facility enables baseline experimentation using stationary salt melts. It is designed to evaluate material behaviour and facilitate post-irradiation examination through the use of resealable, reusable components. The system features an electric heater assembly specifically engineered for irradiation environments, housed within a standpipe outer shell. Each sample capsule can contain up to 300 grams of salt and includes three internal eyelets for suspending material samples directly in the melt. A thermocouple is integrated via a port in the capsule lid to provide direct, continuous measurement of the melt temperature during irradiation. The resealable design allows salt samples to be stored and subsequently remelted within the same container, reducing corrosion risks and preserving the ability to extract samples for later study.

Summary

For more than half of a century, NC State’s PULSTAR research reactor has served the nuclear community as a vital, open-pool testbed that bridges fundamental nuclear research and commercial advanced reactor deployment. In addition to the facilities discussed above, other advanced capabilities including neutron imaging, neutron activation analysis, ultra-cold neutron source (under development), etc. are also open to researchers around the world. As engineering leaps forward, the PULSTAR reactor continues to push boundaries, revolutionising energy research and unlocking bold new frontiers in atomic science.

References

1. Moxom, J., et al. “Performance analysis of the intense slow-positron beam at the NC State University PULSTAR reactor.” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 579.1 (2007): 534-537
2. Liu, M., et al. “The Positron User Facility at the PULSTAR Nuclear Reactor Applications in Materials Studies and Recent Upgrades.” Solid State Phenomena 374 (2025): 155-161
3. DiJulio, D. D. et al. “Performance Optimization of the Neutron Powder Diffractometer at the NC STATE University PULSTAR Reactor”, Transactions of the American Nuclear Society 97 (2007): 342-344
4. Liu M, et al. “Positron Characterization of Neutron Irradiated Reactor-Grade Graphite”. Transactions of the American Nuclear Society, 2014, 110: 15-19
5. Hawari, A. I., et al. “Facilities for Nano Materials Examination at the Pulstar Reactor.” EPJ Web of Conferences. Vol. 247. EDP Sciences, 2021
6. Liu, M., et al. “Implementation of an on-line fission gas release measurement facility at the Pulstar reactor”. (PHYSOR 2018), Sociedad Nuclear Mexicana

Please note, this article will also appear in the 28th edition of our quarterly publication.