Simulation Study of Seawater Tritium Measurement with Dual-Diameter Scintillating Fiber Arrays Under Variable Radionuclide Interference

Changes in radionuclide composition can bias seawater tritium measurements under fixed calibration. We evaluate dual-diameter scintillating fiber arrays combining 1 mm sensing fibers with a separately read out reference cluster. Geant4 responses for seven layouts are coupled to a scintillation–silicon photomultiplier model. Calibration transfer is assessed using 19 literature-matched Sr/Cs pairs in a fixed natural matrix, with composition withheld from the estimator. At baseline optical collection, the 5 mm central cluster reduces absolute concentration bias by 35.5–59.0% across these compositions. For the strongest-interference composition at 100 Bq L−1 tritium, the predicted mean decreases from 133.7 to 113.8 Bq L−1, and the 4 h root mean squared error decreases from 37.0 to 21.9 Bq L−1. The cluster retains 88.8% of the conventional effective response and achieves the lowest calibrated minimum detectable activity concentration (MDA) among the dual-diameter candidates: 47.3–50.6 Bq L−1 at 4 h for the calibration and strongest-interference compositions. Its high-interference error advantage persists under alternative readout assumptions. The conventional array retains lower calibrated MDA and better precision near calibration. These simulations identify a design compromise between tolerance of changing interference and retention of tritium sensitivity.