The Spherical Tokamak (ST) is widely celebrated for its economic potential. By drastically reducing the aspect ratio (A≈1.5), the ST achieves unprecedented normalized beta, allowing for a highly compact, capital-efficient reactor island. However, when this compact geometry is combined with the conventional Deuterium-Tritium (D-T) fuel cycle, it creates an unresolvable physical paradox regarding neutron shielding and the survival of High-Temperature Superconducting (HTS) magnets.

The physics of D-T fusion violently ejects 14.1 MeV neutrons. In standard, large-aspect-ratio tokamaks like ITER, these neutrons are intercepted by meter-thick blankets of beryllium, lithium, and specialized borated steel. This massive shielding is absolutely mandatory to protect the superconducting Toroidal Field (TF) coils. If high-energy neutrons penetrate the shielding and strike the REBCO tape, they systematically destroy the crystalline lattice, rapidly degrading the critical current (Ic) and critical temperature (Tc) of the magnet.

In standard, large-aspect-ratio tokamaks like ITER, these neutrons are intercepted by meter-thick blankets of beryllium, lithium, and specialized borated steel.

Herein lies the fatal flaw of the D-T Spherical Tokamak. In an ST, the inner legs of the TF coils and the central solenoid are violently compressed into an incredibly narrow central column. There is physically no space to install a meter-thick neutron shield between the fusing plasma and the inboard magnets. If engineers attempt to widen the central column to accommodate adequate shielding, the aspect ratio increases, the normalized beta collapses, and the machine is no longer a spherical tokamak; it simply becomes a poorly optimized, conventional reactor.

Operating an unshielded or under-shielded D-T spherical tokamak means accepting that the 14.1 MeV neutron flux will irradiate the HTS central column directly. Under commercial baseload operating conditions, the radiation damage will permanently destroy the REBCO coils in a matter of months. Some startups have absurdly proposed treating the entire highly radioactive, multi-million-dollar central column as a consumable component, swapping it out via remote-handled robotics on a yearly basis. This concept destroys the Operational Expenditure (OpEx) model and renders any NOAK LCOE targets completely fraudulent.

The only mathematically and physically sound method to preserve the economic advantages of the compact spherical geometry is to eliminate the neutron flux at its source. Aneutronic fuels are not an optional upgrade for ST architectures; they are a structural prerequisite for survival.

By aggressively transitioning toward Deuterium-Helium-3 and targeting proton-Boron-11, advanced spherical tokamaks can reduce the destructive neutron output by over 95%. This ensures that >30 T inboard REBCO magnets can survive a 40-year commercial lifespan without the need for impossible meter-thick shielding in the central column.

The private fusion sector must acknowledge this geometric reality. You cannot have a compact D-T spherical tokamak that also possesses a commercial lifespan. Venture capitalists funding D-T STs are funding machines that are geometrically destined to destroy their own most expensive components. To unlock the spherical tokamak, we must definitively abandon Tritium.