/
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.
The sodium boundary and the Brayton exchangers are different manufacturing classes. Printed geometry stays off the sodium boundary, and off the recuperator.
| Duty | Construction intent |
|---|---|
| Primary Na→N₂ | Vertical shrouded bayonet-tube modules with removable swirl cartridges. Six identical modules × 120 MWth. Pressure boundary: seamless Alloy 709 straight tube, smooth bore, free hemispherical cap, one top tubesheet in the dry argon plenum. |
| Brayton recuperator | Gas-to-gas. Diffusion-bonded PCHE (etched channels). Not a printed lattice: roughness and trapped powder raise the ΔP that the cycle cannot afford. |
| Brayton intercooler / precooler | Gas-to-water, below the creep range. Catalog plate-and-shell or TEMA shell-and-tube. A printed TPMS lattice is a candidate only here. |
The choice is inspectability, thermal stress, and where an 80 bar break goes — the same systems questions as the rest of the plant.
A diffusion-bonded printed-circuit block is not used on primary sodium. Its channels are not practicably inspectable, and widening them loads the block in bending.
Component name: vertical shrouded bayonet-tube heat exchanger (Na→N₂). Duty: 6 × 120 MWth = 720 MWth. This is an engineering concept, not a certified exchanger.
| Item | Reference |
|---|---|
| Pressure tube | Seamless cold-drawn Alloy 709; smooth bore; hemispherical cap, free in sodium |
| Tube size | ~19.05 mm (¾ in) OD × 1.5 mm wall; active length ~4.2 m |
| Tubesheet | One, Alloy 709, in the dry argon plenum. No submerged lower sheet |
| Sodium (shell) | In ~600 °C / out ~400 °C / ~1 bar |
| Nitrogen (annulus) | In ~328 °C / out ~550 °C / ~80 bar |
| Overall U (target) | ~1,200–1,600 W/(m²·K) with swirl. Screening target, not a measured U |
| Swirl cartridge | Removable thin-walled stainless downcomer. Insulated double-wall core so cold inlet gas does not short-circuit against the hot return. Helical ribs or wire-wrap on the outside of the insert, annulus ~2.5 mm |
| Cartridge ΔP | Friction only, target under 0.5 bar. The insert sits inside the 80 bar gas; it is not the pressure boundary |
Outer (shell-side) fins are not the baseline. Liquid sodium already has a high film coefficient, and tall outer fins usually loosen the pack and hold more sodium. The Alloy 709 tube is smooth on the sodium side as well as in the bore.
Straight bayonet hardware has been run on solar-salt, chemical-synthesis, and steam-generator pilot loops. That is not this plant. Still open, and called out so they are not read as solved:
Tube-shear relief is described under ReCore-Reactor.
SFR steam-generator programmes are the precedent for caring about probe access. JSFR studied straight double-wall tubes in part so a probe could run the bore. KAERI and Toshiba 4S studied helical double-wall sodium–water generators with a helium witness gap. EBR-II operated duplex steam-generator tubes. Those machines are not a nuclear Na–N₂ bayonet, and they do not license this module.
This concept does not take that double wall as the FOAK barrier. The pressure tube is one smooth Alloy 709 bore. The second piece of metal in the bayonet is the removable cartridge: a flow insert at near-zero differential pressure, not a leak-before-mix witness gap.
In the screening cycle the recuperator carries slightly more heat than the reactor adds. Turbine exit sits only a little above the heater inlet, and the compressor discharge is much colder, so the gas-to-gas duty is large. Compressor work is a large fraction of turbine output, and a small relative pressure drop shows up as lost electrical output. That is why the recuperator and the coolers are not the same machine.
Screening still uses a high recuperator effectiveness. That number is a target. It is not a measured effectiveness of a built battery.
A station blackout stops the ALIPs and the Brayton island together. The ALIPs are electromagnetic, so they have no flywheel. The nitrogen in the bayonet tubes is a closed gas inventory: it takes a little heat and then stops being a sink. Scram is a separate function. The usual SFR arrangement is a hold magnet that releases the rods when power is lost. That mechanism is not drawn here, and dropping the rods does not remove the heat that follows.
After a long run the decay heat is still large. A simple long-irradiation approximation (Way–Wigner) on 720 MWth gives about 20 MW at one minute, 9 MW at one hour and 5 MW at one day. Those are orders of magnitude, not a safety analysis.
Storing that heat in the sodium does not work. The inventory that shares the vessel with the core is about 16 m³. Most of the 73.1 m³ sits in the external heater shells, and that sodium only circulates while the loops have a sink. Ten gigajoules — a fraction of the first hour — would lift the vessel sodium by several hundred kelvin. The boiling margin from ~600 °C to ~881 °C is 281 K. The heat has to leave.
The screening vessel is about 2.4 m outside diameter and about 6.2 m tall, roughly 50 m² of lateral surface. Radiation across a guard-vessel gap, over that whole cylinder, is on the order of 1 MW at a hot wall. That is guard-vessel air cooling (RVACS). It is below the decay heat for more than a day. Fins and a stack improve it. They do not multiply the area by ten. Heat pipes on the outside of the same wall have the same area limit, plus contact resistance.
In-vessel dip coolers put the sink in the vessel sodium, which is the right place, and they need a free annulus. Reflector and shield already fill the gap inside 2.4 m. A vessel near 4 m would make the bay and would add sodium and a heavier haul. It is not the screening baseline.
The arrangement that matches this loop plant is a small number of decay-heat loops that are already open: sodium driven by the elevation between the core and an air chimney, with a damper that fails open. No ALIP and no nitrogen compressor. The duty to cover the first hour is on the order of 10 MW, and the curve falls after that. The exchanger is ordinary sodium-to-air tubing. Two loops give a second path if one damper or one circuit fails. Nothing here is sized. The open work is the elevation head against friction, the risk of freezing sodium in an over-cooled air stack, and a test that the core flow establishes with the ALIPs stopped.
Alloy 709 (Fe-20Cr-25Ni-Mo-Nb-N) is the screening reference for the smooth pressure tube and the single top tubesheet. It is an advanced austenitic stainless steel developed for SFR service near this temperature. It is not a nickel superalloy, and it is not a stock heat-exchanger tube.
At a sodium inlet near ~600 °C and ~80 bar nitrogen inside the tube, a wall of about 1.5 mm asks for more creep strength than 316LN or 316H are usually given credit for over a plant life. The 316 path is a thicker wall, more tubes, or a lower hot-leg temperature. Any of those moves the heater and the cycle. 316LN stays the right alloy for the vessel and the pipes, where the pressure is ~1 bar and the wall is thick plate. The cartridge that carries the swirl ribs is thin stainless at near-zero differential pressure. It does not need Alloy 709.
Alloy 800H (UNS N08810, about 30–35% nickel) is the tube alloy that is actually on the market and already inside high-temperature boiler and nuclear code practice. That is why it is attractive: seamless tube, known suppliers, a code path that Alloy 709 does not yet have as a finished case.
The metallurgy does not come for free. Nickel is more soluble in sodium than iron. In a hot leg near ~600 °C and a cold leg near ~400 °C, nickel can leave the hot tube and deposit downstream. Alloy 709, at about 25% nickel, is not immune; 800H, at about a third nickel, is the worse of the two and has to be qualified for that mass transfer, and for carbon transfer, before it can replace the screening reference. Calling 800H the pragmatic choice would skip that test. It stays a candidate. The screening geometry (smooth bore, ~1.5 mm, ~4.2 m) is still reported against Alloy 709.
Code-case use of Alloy 709, sodium compatibility of whichever alloy is chosen (including the cap weld), tubesheet joints, and the in-service probe procedure remain open work. Straight seamless Alloy 709 tubing is estimated at TRL 6–7 on the code-case path. The integrated Na–N₂ cartridge is estimated at TRL 4–5. See ReCore-Reactor.
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.