Dynamic Window for Accelerator-Driven Subcritical Systems: Moving Vacuum Barrier Enabling Homogeneous High-Temperature Fuel

A class of vacuum barrier designs for Accelerator-Driven Subcritical Systems (ADS) is proposed, based on a dynamic window — a moving metallic surface that continuously translates through the proton beam interaction zone. The reference implementation is an endless RAFM steel belt running on rollers between two slit-aperture blendes equipped with differential pumping, gas curtain aerosol protection, and fail-safe shutters. Conventional ADS designs use lead or lead-bismuth eutectic (LBE) as combined target, coolant, and window coolant. This creates bulk activation of hundreds of tonnes of coolant, deposition of spallation-produced noble metals (Au, Pt, Ir) and mercury on heat exchanger surfaces, and generation of volatile radiotoxins (Po-210 in LBE). These problems originate from the use of lead — which is mandated by the temperature constraints of the static beam window. The dynamic window removes this constraint: the belt temperature is governed by transit speed, not by the reactor medium, enabling reactor media whose operating temperatures greatly exceed the allowable steady-state temperature of a conventional static window. A fixed surface topology prevents direct exposure of the vacuum-side sealing face to the reactor environment during normal operation, reducing contamination-driven degradation of the sealing interface. This may remove one of the key beam-interface constraints that currently disfavor homogeneous molten-salt ADS concepts such as NaCl-UCl₃ at ~800°C, eliminating lead and its associated waste streams while exploiting uranium’s superior spallation yield. First-order thermal, radiation damage, sealing, aerosol transport, fatigue, and failure mode analyses are presented. No first-principles physical prohibition has been identified; realization depends on qualification of several coupled engineering phenomena — most critically local gap control under sliding contact, tribology and wear in the nuclear environment, aerosol exclusion from the vacuum-side sealing surface, and upset-condition response — each of which requires integrated prototype testing.