DARPA wants to bypass the thermal middleman in nuclear power systems

Direct nuclear-to-electric conversion ideas

• Discussion centers on bypassing the “thermal middleman” (steam cycles) via:
  • Radiovoltaics / betavoltaics and thermo-photovoltaics that convert ionizing or IR radiation directly to electricity, but with low absorption and power density issues.
  • Direct capture of charged fusion products (e.g., Helion-style coils where plasma expansion induces current).
  • For fission, most energy is in fast, charged daughter nuclei; capturing that as electrical potential instead of heat is conceptually possible but technologically unclear.

Fusion approaches and skepticism

• Fusion is described as “infinitely harder” than fission, especially for net-positive energy and direct conversion.
• Helion’s pulsed D–D / D–He³ concept is debated:
  • Proponents highlight low neutron fraction and direct electromagnetic capture.
  • Critics argue schedules have slipped, material/energy-flux limits may be insurmountable, and many fusion startups resemble overhyped VC plays.

Nuclear batteries and betavoltaics

• Existing betavoltaic cells (e.g., Ni‑63 in diamond) offer ~100 µW in tiny volumes:
  • Energy density over decades is huge, but instantaneous power is far below phone-level needs.
  • Could suit low-duty, intermittent devices, potentially with supercapacitors.
• Safety and regulation:
  • Beta emitters are relatively easy to shield but dangerous if inhaled/ingested as dust.
  • Widespread consumer use conflicts with non-proliferation, tracking, and waste rules.
  • Some argue they’d be easier to monitor than heavy metals; others stress regulatory and security burdens.

Economics, efficiency, and waste heat

• Conventional reactors convert ~1/3 of fission heat to electricity; everything is “a big nuclear boiler.”
• Main roadblock is cost and complexity (thousands of long-lived, high-spec welds; huge skilled labor needs), not thermodynamic efficiency alone.
• Direct conversion is framed as the kind of breakthrough needed to make nuclear economically competitive.
• District heating and hydrogen production from reactor heat are discussed:
  • District heating faces infrastructure, siting, and perception barriers.
  • High-temperature electrolysis/thermochemical H₂ could use heat more directly but demands extreme temperatures and still competes with cheap intermittent renewables.

Safety, regulation, and risk perception

• Radiation fatalities are rare under strict regulation; some see this as proof safety works, others as evidence of overregulation and excessive cost.
• Comparisons are made with untracked heavy-metal pollution, suggesting societal risk trade-offs are inconsistent.

Speculative and niche concepts

• Fission-fragment rockets as high-Isp drives (and possible direct power sources) are discussed, along with RTGs, plasma scintillator schemes, and even black-hole/Hawking-radiation thought experiments.