Validation of the analytic radiation load model used in the methodology for Initial STEP first wall contour designs

The Spherical Tokamak for Energy Production (STEP) pilot plant, aiming for first electricity in the 2040s, needs credible wall heat-load estimates on design timescales. The iterative concept design of the STEP first-wall contour has been guided by analytic models that estimate charged-particle and photon heat loads and automatically reshape the wall until engineering limits are met (Vaccaro et al., 2024). The radiation module, hosted within the Bluemira framework (Coleman et al., 2019), solves electron temperature and density along core and SOL flux tubes, then derives emissivities from tabulated impurity-loss data; these feed a Monte-Carlo photon-tracing solver, producing steady-state wall-radiation maps within minutes of laptop CPU time. To test its engineering fidelity, we benchmark the model against SOLPS-ITER for two STEP designs: SPR-10 (Osawa et al., 2023) and SPR-45 (Henderson et al., 2025). Upgrades, a self-consistent divertor-leg solver with momentum-loss closure and an impurity-power regression, close earlier accuracy gaps. The model matches upstream electron temperature, connection length, flux-tube Tₑ(s) and nₑ(s) profiles, detachment-front location, and wall photon flux with accuracy adequate for early engineering choices, capturing local peaks. It thus serves as a rapid front-end that guides design iterations while full SOLPS campaigns provide the definitive high-resolution view, and the approach is transferable to other divertor geometries and reactor concepts.