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This Learn page describes engineering building blocks used in the ReCore concept — industrial materials and heat-transport practice, not a claim that every component is already nuclear-qualified for this plant. Estimated maturity sits on the technology TRL tables.
Helical shell-and-tube geometry is used in two different manufacturing classes in this concept:
| Duty | Construction intent |
|---|---|
| Primary Na→N₂ | Nuclear-grade hybrid helical, single-wall Alloy 709 bundle: internally finned + twisted tape on the N₂ bore; primary Na shell-side. Compact industrial tubes — not gyroid/TPMS or AM lattice on the Na boundary. |
| Brayton recup / precooler / intercooler | Gas–gas or gas–sink exchangers where high effectiveness and low ΔP dominate net cycle η — here the concept allows advanced 3D-printed / complex helical (e.g. computational AM) hardware. |
| Side | Fluid | Role |
|---|---|---|
| Shell | Liquid sodium, near atmospheric pressure | High HTC heat source (usually not the limiting film) |
| Tube | High-pressure nitrogen (~80 bar) | Thermally limiting side — fins + tape inside the bore only |
A double-wall (duplex) tube with a helium leak-detection gap was dropped. Internally finned helical coils at the ~7 m module scale are already a qualification article; adding a second wall, an interstitial helium path, and a second tubesheet is judged too complex to manufacture. Single-wall Alloy 709 also removes the contact / gap thermal resistance and improves the overall U-value. The 80 bar tube-rupture case is then handled by detection, isolation, and relief rather than a witness annulus — see ReCore-Reactor safety.
Nitrogen flow inside the tubes is enhanced with industrial internals:
Gyroids / TPMS were dropped for this duty: closed-Brayton η is too sensitive to gas ΔP (LEAP71-class result). Those lattices stay on the Brayton recuperator / precooler path only.
Outer (shell-side) fins are not the baseline: liquid sodium already has very high HTC, and tall outer fins often force looser packing and more sodium hold-up.
Design trade-off: maximise heat transfer per unit volume while keeping gas ΔP acceptable. First-principles sizing uses a single-wall U until a coiled Alloy 709 prototype measures it. Vibration, NDE of internally finned helices, and tubesheet machining remain open qualification work.
In the closed nitrogen cycle:
These are not sodium boundaries. The concept targets high-ε, low-ΔP advanced gas exchangers (including ambitious AM helical designs) so cycle efficiency can stay high without over-optimising the nuclear primary HX into non-industrial territory.
Alloy 709 (Fe-20Cr-25Ni-Mo-Nb-N) is the baseline tube alloy for the primary Na→N₂ heater. It is an advanced austenitic stainless steel developed for SFR high-temperature service — not a nickel superalloy.
At ~600 °C and ~80 bar internal N₂, Alloy 709 offers substantially higher creep strength than 316LN or 316H, so the same pressure duty can use a thinner tube wall. 316LN also strain-hardens severely under nitrogen, which complicates helical bending, internal-fin drawing, and tubesheet machining at the ~7 m module scale. Alloy 709 is intended to avoid that fabrication trap.
Code-case qualification (ASME Section III Division 5 class), sodium compatibility at this wall thickness, and helical-coil NDE remain open work — the alloy is an SFR candidate with an industrial pedigree, not a catalog heater already licensed for this duty.
316LN (low-carbon, nitrogen-alloyed 316) remains the baseline austenitic structural steel for this concept’s vessel and primary piping.
316LN is a standard nuclear and process industry alloy, not a research material. Nitrogen strengthens the austenite (raising allowable stress at elevated temperature relative to plain low-carbon 316L) while the low carbon limit reduces risk of weld sensitization and intergranular corrosion compared with high-carbon grades. It is widely available as plate, forgings, and seamless tube, welds with established procedures, and has a long pedigree in liquid-sodium and high-temperature water systems. For a ~600 °C sodium outlet, 316LN remains inside the usual high-temperature stainless design envelope for modular welded vessels without nickel superalloys.
Final assembly of reactor vessel modules can use electron beam welding (EBW), including local-vacuum EBW (LVEB) for thick seams with low distortion — suited to modular cylindrical sectors, bottom plate, and top plate.
Moving liquid sodium at ~600 °C is intended to use annular linear induction pumps (ALIP): no moving parts in the sodium, no shaft seals into the primary system. Electromagnetic coils drive the fluid via Lorentz forces — established liquid-metal pump technology for modern SFR concepts, still to be integrated and qualified for this plant layout.