Radiation safety design of the SLAC Cryomodule Repair and Maintenance Facility using a physics-based field emission...

The Cryomodule Repair and Maintenance Facility (CRMF), currently under construction at SLAC National Accelerator Laboratory, is designed to enable on-site testing, repair, and maintenance of Superconducting Radio-Frequency (SRF) cryomodules supporting the Linac Coherent Light Source II (LCLS-II) and its High Energy (HE) upgrade. This study presents the radiation hazard analysis establishing the shielding requirements, operational envelope, and safety systems for the facility.

During cryomodule testing, field emission from niobium cavity surfaces at high accelerating gradients produces dark current electrons. The electrons interact with cavity walls and downstream components, generating bremsstrahlung photons and secondary particles, which constitute the primary radiation source term. Hazard levels depend strongly on the accelerating gradient, cavity condition, and loss location and vary across operational scenarios.

A geometry-specific high fidelity Monte Carlo model was created for radiation transport analysis in FLUKA using a physics-based, time-varying field emission source model. This model integrates the Fowler-Nordheim equation with dynamic electromagnetic field transport, enabling stochastic generation and tracking of field-emitted electrons under realistic cavity conditions. Prompt and residual dose rates, airborne and groundwater radionuclide activation, ozone production, and dose to public from CRMF were evaluated across all nominal operating configurations.

Effective dose rates at 30 cm from the shielded enclosure walls remained below 5 μSv h^-1 (0.5 mrem h^-1) under all nominal operating conditions, with localized increases at the RF penetrations and maze corners addressed through targeted shielding. Skyshine contributions to the annual public dose are negligible at the nearest publicly accessible location (≥336 m), with all azimuthal sectors well below the 1 mSv yr^-1 limit. Following beam shutdown, residual dose rates across most accessible regions were on the order of 1 μSv h^-1 (0.1 mrem h^-1) after 1 h of cool-down, with localized maxima near the Faraday cups reaching approximately 5 μSv h^-1 (0.5 mrem h^-1). Airborne radionuclide concentrations, dominated by ⁴¹Ar, ¹⁵O, ¹³N, ¹¹C, and ³H, corresponding to a total airborne dose rate of 13.5 × 10^-2 μSv h^-1 (13.5 × 10^-3 mrem h^-1) and an annual inhalation dose of 270 μSv (27 mrem), well below the SLAC dose management ALARA level. Groundwater activation levels of ²²Na, ⁷Be, and ³H, and ozone production across all scenarios, remained below their respective regulatory thresholds.

The CRMF shielding design and operational controls support safe cryomodule testing up to 26 MV/m, with all prompt, residual, and environmental dose metrics within regulatory limits. A key contribution was the use of a physics based field emission model in place of the conventional pencil beam source approximation. This approach represents the first implementation of a full physics-based field emission model, integrating stochastic electron generation with time-varying electromagnetic field transport, for the purpose of facility shielding design. The methodology and safety framework developed here apply to future SRF testing facilities operating under comparable conditions and provide a validated reference for radiation protection analysis in similar cryomodule testing environments.