225 kV Radiation-Hardened High-Voltage Power Supply for Neutron Radiography
Neutron radiography facilities operating near research reactors expose high-voltage equipment to integrated neutron fluences above 10¹⁵ n/cm² and gamma dose rates exceeding 800 Gy/h, yet the 225 kV X-ray converter supply must maintain regulation better than ±0.18 % and ripple below 160 V p-p to prevent image blurring from focal-spot wander. Radiation-hardened designs therefore combine extreme component selection with redundant architecture and active compensation that survives total ionizing dose beyond 8 MGy.
The generator uses a 38 kHz series-resonant inverter feeding an oil-immersed, parallel-fed six-stage Cockcroft-Walton multiplier constructed entirely from ceramic capacitors rated for greater than 12 MGy and bulk ceramic resistors with less than 0.9 % shift after 10¹⁶ n/cm² fast neutrons. All semiconductors are 200 nm silicon-on-insulator devices with annular gate layouts and ELDRS-tested to 6 MGy, while magnetic components use radiation-resistant cobalt-free amorphous cores exhibiting less than 1.1 % permeability change after 10¹⁷ n/cm².
Regulation is performed by a triple-redundant control system: three independent feedback dividers made from different resistor technologies (thin-film TaN, bulk cermet, and metal-glaze) vote in a two-out-of-three scheme implemented in radiation-hardened FPGAs. Any single channel drift beyond 0.12 % triggers automatic switch-over with zero interruption. Ripple suppression uses distributed pi-filtering with polypropylene capacitors exhibiting negligible piezoelectric response under neutron flux.
Active radiation compensation monitors total integrated dose via on-board p-i-n diodes and automatically adjusts digital gain coefficients every 400 hours to null resistor TCR shift and capacitor dielectric constant change. Long-term tests in reactor beamlines show voltage drift below 0.14 % after 14 months continuous exposure at 1.8 × 10¹⁵ n/cm².
Thermal management employs redundant oil circulation pumps with magnetic couplings (no dynamic seals) and automatic failover, maintaining oil temperature within ±1.4 °C despite gamma heating rates up to 28 kW in the tank. Oil chemistry is preserved by continuous bypass filtration through mixed-bed ion exchange and activated alumina cartridges.
Arc protection uses stored-energy limiting to 9 J combined with a fast crowbar firing in <5 µs and a mechanical series switch that isolates the multiplier during extended outages. Recovery to full voltage occurs within 1.8 seconds after beam trips.
These radiation-hardened supplies routinely deliver 225 kV with 0.09 % stability in mixed neutron-gamma fields, enabling defect detection below 40 µm in pyrotechnic components and weld inspection of irradiated nuclear fuel assemblies with spatial resolution under 22 µm.
