300 kV Self-Shielded High-Voltage Transformer for Blood Irradiation Systems

Blood and blood-product irradiation with X-ray doses of 25–50 Gy requires compact 300 kV generators that fit inside biological safety cabinets while producing <0.8 µSv/h leakage at 1 m distance. Self-shielded transformer designs therefore integrate lead and tungsten shielding directly into the high-voltage assembly to eliminate separate generator enclosures.

The transformer uses a layered cylindrical winding with 42 kV primary and 126 secondary sections immersed in dielectric oil containing 6 % dissolved SF6 for 38 % higher breakdown strength. Core and coils are encased in a 28 mm thick lead cylinder lined with 3 mm tungsten alloy at the high-voltage end, providing >96 % attenuation of 300 kV bremsstrahlung while adding only 118 kg total weight.

Voltage regulation is performed by a 52 kHz resonant converter with phase-shift control achieving ±0.68 % stability at 28 mA tube current. Ripple is <420 V p-p through distributed oil-filled capacitors and active filtering using a small linear post-regulator referenced to a shielded feedback divider.

Thermal management combines natural oil convection with an external oil-to-water heat exchanger mounted on the cabinet wall; oil circulation is driven by thermosiphon effect, eliminating pumps and seals that would increase leakage risk. Maximum oil temperature rise is <26 °C at full 8.4 kW continuous rating.

Filament supply is integrated on a separate floating deck at cathode potential, powered through a 1:1 high-frequency isolation transformer wound on the same core, reducing total connections to three low-voltage cables. Filament current is pre-programmed as a function of kV to maintain constant focal spot size across the dose range.

Leakage radiation is verified by built-in Geiger-Müller monitors at four points around the shield; any reading >0.6 µSv/h triggers automatic power reduction to 42 % until service. Dose reproducibility is maintained by closed-loop control from a transmission ionization chamber that adjusts tube current in 0.08 mA steps to hold 25.00 Gy center dose within ±0.42 %.

These self-shielded transformers routinely deliver full 300 kV continuous operation inside Class II biosafety cabinets with surface dose rates <0.38 µSv/h at 100 mm, enabling safe blood irradiation in hospital transfusion services without dedicated shielded rooms.