Surface Fission Architecture (SFA)

Surface Fission Architecture (SFA) is a conceptual paradigm shift in sub-critical reactor design. Rather than driving fission volumetrically through a large, critical mass of fissile fuel, SFA strictly confines the breeding and fission reactions to a microscopic, two-dimensional surface layer (≤ 20 µm) of a liquid-metal matrix. By optimizing the localized spatial atomic ratio rather than bulk mass inventories, the system achieves a high energy multiplication factor (M = 50) within a highly compact, modular footprint suitable for decentralized deployment.

1. Spatial Geometry and Coulomb Vectoring

The architecture fixes the proton beam energy at exactly 12 MeV. This value represents the precise inflection point required to classically clear the Coulomb barrier of heavy nuclei, maximizing the probability of direct nuclear interactions while avoiding the high-energy spallation regimes (> 20 MeV) that cause severe structural activation and material embrittlement.

Grazing Angle Interface: The beam intersects the liquid metal target at a shallow grazing angle of 5° to 10°.

Skin Depth Confinement: This geometry reduces the orthogonal penetration depth of the protons to a skin layer of  ≤ 20 µm. Consequently, 99 %+ of the primary (p,n) reactions and subsequent radiative neutron captures are forced to occur within this ultra-thin boundary zone.

2. High-Frequency Electromagnetic Rastering

To inject high beam power (100 kW to 250 kW) into a 20-micrometer depth without causing localized boiling of the liquid lead carrier matrix, SFA utilizes a non-mechanical, electromagnetic scanning path.

The Circular Scan Path: Normal-conducting magnetic deflection coils at the accelerator exit drive the beam focus point in a high-frequency (kHz-range) circular trajectory along the surface of the liquid metal.

Thermal-Isotopic Decoupling: The rotation velocity of the beam is exponentially faster than the physical diffusion rate of the liquid metal. This ensures that while heat is conducted downward into the bulk pool and outward through the highly conductive tungsten pipe walls, the bred transuranic isotopes remain concentrated within the circular track.

3. Spatial Atomic Ratio Kinetics

SFA operates on localized probability mechanics by manipulating the local isotopic density within the scanning ring.

Target Concentration: The system requires an atomic ratio of 1% to 2% Pu 239 atoms relative to the native U 238 and Lead atoms within the 20-micrometer boundary channel.

Temporal Scaling: At a 100 kW beam power baseline, the optimized neutron economy (enhanced by a close-proximity upper Beryllium/Graphite reflector) accumulates this critical local atomic ratio in approximately 23 hours of continuous scanning.

The In-Situ Burn Phase: Once this localized ratio is established, the bred Plutonium acts as an intense fission multiplier layer. Secondary neutrons trigger a localized chain reaction confined strictly to the scanning track, transitioning the system into a net thermal exporter.

4. Mechanical Fluid Flow and Contamination Isolation

The containment and continuous refueling of the active matrix are integrated into a single structural asset: a U-shaped tungsten pipe loop.

Gravity-Fed Displacement: Raw, solid U 238 particles are introduced into the inlet leg of the U-pipe. This induces a low-velocity displacement of the bulk molten lead column.

Continuous Surface Refresh: The slow forward movement introduces fresh U 238 into the scanning ring at the exact rate the old material undergoes fission burnup.

Density-Driven Slag Purging: Fission fragments with lower densities than lead float to the surface as solid slag. The forward displacement pushes this slag to an isolated reservoir at the discharge leg of the U-pipe, continuously cleansing the active reaction zone.

System Summary and Advantages

Footprint Optimization: By utilizing a compact 5 MeV circular cyclotron injector coupled to a short HTS linear booster, the entire accelerator and U-pipe loop compress into a standard 12-meter container envelope.

Localized Radiological Footprint: Because fission is restricted to a thin, monitored surface channel rather than a high-volume volumetric core, the inventory of active fission products at any single moment is minimized by orders of magnitude, lowering the risks associated with thermal runaway or structural containment compromise.