Leveraging Glenn-Research Copper for Heat Exchanger Integration in Pressurized Water Reactors

Nuclear technology is a zero-carbon, high-output electricity source. The maturity of higher-generation reactors can drive humanity towards lower-carbon energy systems. However, inside nuclear reactors, structural materials are exposed to extreme temperatures, pressures, and neutron irradiation doses, causing rapid degradation and limiting reactor lifespans to 60 years [1,2].

Current heat-exchanger tubing in pressurized water reactors (PWRs) - the most ubiquitous Generation II and III reactors that use pressurized water to generate heat - are composed of chromium, nickel, or steel-based alloys [1]. These materials are optimized for high-strength applications but have low thermal conductivity, discerned in Fig. 1. Therefore, the combination of inefficient electricity production with short reactor lifespans inhibits the investment and development of nuclear technology, as illustrated in Fig. 2 [1,3].

This study attempts to address both issues using a Cr2Nb-strengthened copper alloy, called Glenn-Research Copper (GRCop) [4]. Used in high-temperature, high-heat-flux aerospace applications [5], GRCop could be a lightweight and efficient material for heat exchangers in PWRs. Since both reactors and rockets operate in harsh environments, GRCop could withstand intense materials degradation, similar to damage observed in PWRs.

Research Question

Can nanocrystalline GRCop, an alloy utilized in aerospace applications, be viable as heat-exchanger material in pressurized water reactors?

Objectives

To preliminarily assess the irradiation damage performance of nanocrystalline GRCop through cascade damage simulation and annealing behavior

Contribute to advancement of next-generation nuclear reactors, offering an alternative material capable of operating at higher temperatures and irradiation doses than conventional alloys, within temperature and irradiation regimes shown in Fig. 3

Methodology (Overview in Fig. 5)

First, two main samples of Glenn-Research Copper were produced: GRCop-84 (8 at. % Cr and 4 at. % Nb) and GRCop-42 (4 at. % Cr and 2 at. % Nb). Both samples were prepared using mechanical alloying (visualization in Fig. 6) in order to synthesize nanograins [8]. To prevent severe cold welding, 2.5 wt. % ethanol was chosen as the process control agent (PCA). The samples were mechanically alloyed using a planetary ball mill at 100 RPM for 1 hour followed by 350 RPM for 30 hours, with 15-minute cooldown periods every hour. X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) images were collected for the 30-hour as-milled samples and a GRCop-84 sample annealed at 500oC for two hours.

XRD was employed for microstructural analysis with a copper Kα x-ray source of wavelength 0.1504 nm. The x-ray angle sweep was between 0 and 90 degrees; corresponding intensities were measured. Microstructural properties were calculated using the Debye-Scherrer, Williamson-Hall, and Bragg equations (equations 1-3).

SEM and energy-dispersive x-ray spectroscopy (EDS), both operating at 20 keV, were utilized to analyze particulate sizes, surface morphology, and elemental compositions. ImageJ software was used to measure average particle diameters, where particles were assumed to be roughly ovular.

The Stopping Range of Ions in Matter (SRIM) simulation was used to predict the impact of thermal (low-energy) neutron irradiation on GRCop-84 and GRCop-42. Krypton ions at 1 MeV simulated the damage profile of a thermal neutron cascade [9]. A neutron cascade visualization is provided in Fig. 7. The observation window of the simulation was set to 1 μm. Displacement energies, or the energy required to knock an atom out of its lattice, for each element were calibrated according to [10-12]. Data visualization was accomplished through the Python libraries Matplotlib and Pandas [13,14].

Alloy Verification

X-ray diffraction patterns shown by Fig. 9 verified the formation of a solid solution of GRCop alloy. By calculating the Miller Indices, which describe the alignment of elements in an x-y-z plane, the characteristic face-centered-cubic copper phase peaks of (111), (200), and (220) are shown at 2θ (x-ray reflection) angles of 43.2, 50.3, and 74.0 degrees, respectively. The (222) index at 44.2 degrees indicates the presence of a Cr2Nb precipitate phase, which becomes more pronounced after annealing. This dual Cu-Cr2Nb phase is consistent with published results [19].

Illustrated by Figs. 10-12, homogenous distributions of constituent elements and surface particle morphology were observed in all samples, consistent with previous studies [19]. The average particle sizes for GRCop-84 and GRCop-42 were 58.07 μm and 167.2 μm, respectively. Trace amounts of iron and oxygen were also present in all samples.

Microstructural Analysis

A nanocrystalline state was achieved through planetary ball milling in both GRCop samples, as discerned in table 1. Nanocrystallines are associated with nanometer grain sizes, which describe groups of elements that align themselves in a similar orientation. Grain sizes for GRCop-84 and GRCop-42 were 13.0 nm and 15.0 nm, respectively. After annealing, the grain size of GRCop-84 grew by 20.8% to 15.7 nm. Such small grain sizes result in a high grain boundary density, which increases defect annihilation and enhances yield strengths through Hall-Petch and Orowan strengthening [19-21]. Grain sizes produced by milling are sufficiently large to avoid inverse Hall-Petch (<1.5 nm) [22]. Thus, as-milled GRCop-84 is predicted to have increased resistance against thermal neutron irradiation and have enhanced strength even at high temperatures due to the retention of its nanocrystalline state, something not typically reported in pure copper or PWR heat exchangers [23-27]. The calculated microstrains are typical of nanocrystallines [20], at 7.10e-3 ε and 6.15e-3 ε for GRCop-84 and GRCop-42, respectively. GRCop-84's microstrain decreased by 17.2% to 5.88e-3 ε following annealing, attributable to dynamic recovery.

Irradiation Analysis

The simulated damage profile of GRCop-84 and GRCop-42 shown in Fig. 13 is comparable to pure copper, and thermal neutron irradiation causes significantly higher lattice displacements in GRCop than current materials used in PWR components, with vacancies peaking at 65/nm-ion for GRCop-84 compared to less than 43/nm-ion for all other non-copper materials. Phonon and ion concentrations are also concentrated at the surface and are similar between GRCop and other materials. High surface phonon and ion concentrations consequently decrease the thermal conductivity of materials due to ionized impurity scattering and phonon-phonon interference in all tested materials [28,29]. Ions tend to restrict electron mobility, the primary mechanism for heat diffusion [28]. Phonons are secondary carriers of heat in metals. Therefore, the high density of surface phonons causes the destructive interference of their wave functions, causing significant thermal conductivity degradation [29]. However, SRIM does not consider the strengthening effects of grain boundaries and the Cr2Nb intermetallics of GRCop, which act as defect sinks for annihilating dislocations [19].

The low grain growth of GRCop-84 reported in this study is lower than that of past works (Fig. 14), likely from differences in milling. Low overall grain growth in GRCop may be attributed to the kinetic pinning of grain boundaries by Cr2Nb intermetallics [19]. The planetary ball milling process is one of the most scalable and inexpensive methods of creating nanocrystalline materials [10,30], which provides a promising view regarding the mass implementation of nanocrystalline GRCop as heat-exchanging material.

EDS results demonstrate that it is almost impossible to achieve theoretically perfect Cu1-3yCr2yNby samples because of environmental contaminants (for example, the stainless steel milling intermediaries). These trace contaminants have been known to occasionally construct additional alloy-strengthening particles [19]. Regardless, the deviation from GRCop's theoretical behavior should be studied further when evaluating the suitability of implementation into PWR environments.

Reductions in surface thermal conductivity from thermal neutron irradiation are also problematic for heat exchangers [1]. However, because of the high grain boundary density and Cr2Nb-strengthening intermetallics of GRCop, these factors are expected to mitigate the ion and phonon concentrations, allowing for lower thermal conductivity degradation compared to current heat-exchanging tubing in PWRs, which is especially beneficial for its performance.

Overall, the results from this study strongly support the potential implementation of Glenn-Research Copper in nuclear reactors. The significant increase in thermal conductivity compared to current materials (Fig. 1) in conjunction with its exceptional mechanical properties allows for greater reactor productivity and development, increasing thermal efficiency and leading to a more energy-sustainable future.

Four key directions for future work are proposed:

Utilize cold spraying to consolidate the powder into bulk material in order to perform strength, hardness, and modulus measurements.

Use thermal neutron irradiation test reactor facilities and intermediate voltage electron microscopy for further analysis of the effects of irradiation, such as void nucleation and coalescence.

The Debye-Scherrer and Williamson-Hall equations assume that peak broadening is only due to grain refinement and an increase in strain [16]. The Warren-Averback approach and transmission electron microscopy can increase the certainty of microstructural measurements.

Explore the feasibility of the large-scale manufacturing and implementation of GRCop

Firstly, I would like to express my sincere gratitude towards Ben Hewitt, Dr. Ahmed A. Tiamiyu, and Dr. Beatriz Garcia-Diaz for their invaluable guidance throughout the project.

Additionally, I would like to thank the MaPP lab at the University of Calgary for providing the equipment necessary to conduct the ball milling process and Moses Adaan-Nyiak for his supervision. I am also grateful to the Faculty of Science and Microscopy Facility for hosting me and supporting the XRD and SEM analyses.

Lastly, I would like to express my heartfelt appreciation to my friends and family for supporting me along my journey. They have celebrated my successes and offered comfort during hardships every step of the way.

Artificial intelligence was used for assisting in the visualization of x-ray, EDS, and SRIM data. Generative artificial intelligence technology was only used in minor revision processes of some written works in this study.

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