Heavy-liquid-metal-based heat transfer fluids for high-temperature solar and nuclear power applications
Liquid metals have been studied since the early development of fission energy as reactor core coolants for fast reactors, fusion energy blanket applications and, more recently, for both accelerator-driven systems (ADS) proposed for high-level radioactive waste transmutation and for generation IV fast reactors. Moreover, heavy liquid metals (HLM) are being proposed as target materials for high-power neutron spallation sources. In last two decades, HLM were studied also as a potential candidates for heat transfer fluids in concentrating solar power (CSP) systems.
The general selection criteria for the use of liquid metals as heat-transfer fluids include the following [1]:
-acceptable corrosion and mechanical degradation of structural materials (i.e. lifetime of equipment)
-high stability of liquid metal (limited chemical reactions with secondary coolants and air or formation of spallation products);
-moderate power requirement for liquid metal circulation ;
-high heat transfer coefficient and small size of heat exchanger;
-controllable chemical and radioactive hazards;
-simple and reliable safety measures and systems.
Besides that, in a nuclear environment, there are several more requirements related to the fast spectrum necessary for breeding, fuel conversion, and actinide transmutation [1]:
-small (fast) capture cross-section;
-high scattering cross-section (for small leakage of neutrons from the core);
-small energy loss per collision (for small spectrum softening [moderating] effect);
-high boiling temperature (for prevention of reactivity effects from boiling-related coolant voiding)
Some relevant properties of heavy liquid metal candidate materials [1, 2]
[1] Handbook on lead-bismuth eutectic alloy and lead properties, Materials Compatibility, Thermal Hydraulics and Technologies, OECD – NEA 7268, ed. 2015.
[2] D. Frazer, E. Stergar, C. Cionea, P. Hosemann, Liquid metal as a heat transport fluid for thermal solar power applications, Energy Procedia 49 (2014) 627–636.
Many engineering environments are non-ambient. In order to understand how a material reacts in a non-ambient environment, simulated and well-controlled conditions must be applied. Our research team focuses on high-temperature and liquid-metal environments, such as can be found in liquid-metal-cooled reactors and solar concentrators. The autoclaves are used to carry out long-term testing to evaluate corrosion rates. The material degradation is subsequently characterized through microscopy studies.
Current efforts are focusing on high-temperature heavy metals (e.g., lead-bismuth eutectic) in static, oxygen-controlled environments. The aim of this project is to provide sufficient oxygen in the liquid metal such that the submerged steel can form a protective oxide layer that prevents catastrophic dissolution.
In addition to liquid-metal corrosion, oxidation of reactor-grade materials in high-temperature steam environments is of interest to aid the development of accident-tolerant fuels. Fe-Cr-Al alloys as well as Mo and SiC are investigated for this purpose.
Liquid metals in solar technologies
An additional option to provide the greenhouse gas emission free power is concentrated solar power (CSP). CSP generates electricity by concentrating the sunlight (using mirrors or lenses) onto a small area and subsequently converting it to heat, which then drives a heat engine connected to the electrical generator, or generates a thermochemical reaction (Figure 2). Heat storage in heat transfer fluid allows the plants to continue to generate power after sunset, adding value to such systems compared to photovoltaic panels. Commercially used solar heat transport fluids (oils) begin to disintegrate at ≥ 600 ⁰C, leaving only gases, liquid salts and liquid metals as potential fluid candidates.
Concentrating Solar Power system
However, gases have low efficiency due to their low density, while salts have high melting points. An assessment of the liquid metals as heat transfer fluids in solar thermal power generation systems was presented recently by Pacio and Wetzel, where It was concluded that heavy liquid metals (at most LBE) are particularly promising candidates.
Oxidative passivation of metals and alloys in Pb and LBE
In molten Pb-based alloys, such as LBE, dissolved oxygen is in either molecular or atom-radical form. Initial stage of oxidation involves the chemisorption of oxygen anion on a metal surface. In oxidation process, charge transfer occurs. Metal cations, produced by oxidation of metal atoms at the substrate surface, diffuse outward through the oxide layer, along with outward migration of electrons produced by the oxidation reaction [3]. Oxygen anions diffuse inward through the oxide layer from its outer surface. The generated electrical field on surface promotes the intrusion of oxidized metal cations into the plane of absorbed oxygen ions to produce the two-dimensional monolayer which then grows into 3D structure, from the initial substrate surface both outward and inward. Thus a duplex-oxide layer forms (Figure 1), based on the model by Zhang and Li [4], but a detailed mechanism of the oxide layers formation has not been completely revealed so far. Pure Fe grows a thick and non-protective magnetite (Fe3O4) layer on its surface, while austenitic alloys are reported to form a multilayer oxide structure, with the outer layer being a non-protective Fe3O4, while the inner layer being dominantly a protective Fe-Cr-spinel (although the exact spinel crystal structure has not previously been determined) [5].
Oxide layers formed on fcc steel in oxygenated LBE at ≥500 C [4]
[3] E. McCafferty, Introduction to corrosion science, Springer (NY-Dordrecht-Heidelberg-London), 2010.
[4] J. Zhang, N. Li, “Oxidation Mechanism of Steels in Liquid–Lead Alloys”, Oxid. Met. 63, 5/6 (2005) 353-381.
[5] M. Kondo, M. Takahashi, M. Naoki Sawada, K. Hata, J. Nucl. Sci. Technol. 43 (2006) 107.
Behavior of austenitic steels in LBE
Based on many studies [xx-yy], it has been found that most of the high-Ni,Cr- stainless steels in exposure to liquid LBE containing 2 to 5 ×10¯⁶wt% oxygen at 500 to 600 ⁰C develop a multilayer structure on their surface. The outer layer usually consists of a spinel type structure (AB2O4), mostly Fe3O4 and FeCr2O4. The inner oxide layer is rather porous, consists of two phases (Ni rich and Ni depleted phase), and adapts the grain structure of the steel matrix (indicating that it forms by O migrating inward). A porous, < 1 μm Ni depleted layer forms In between the outer and inner oxide layer.
STEM image of the intermediate oxide layer (Left). Pores and Ni rich bright spots can be seen . In corresponding EDS scan (Right) the peaks in the Ni content correspond to the bright areas of the left image.
STEM HAADF images of the inner oxide layer (a). STEM image of the two phase area in the inner oxide layer (b). EDS line scan across both phases (c). (P. Hosemann et al. Corr.Sci. 66 (2013) 196-202.