A Reactivity-Equivalent Physical Transformation Method for Annular Dispersed Particle Fuel Based on Equivalent Annulus Fitting

Annular dispersed particle fuel is a candidate advanced fuel form for Generation IV reactors and small modular reactors (SMRs). Conventional volumetric homogenization methods introduce significant errors when handling double-heterogeneous (DH) systems. While the existing ring reactivity-equivalent physical transformation (Ring RPT) method has been applied to cylindrical and spherical geometries, it relies on iterative searches for equivalent radii and lacks an explicit fitting formula for annular fuel elements with fixed inner and outer radii. This paper proposes a fitting-based equivalent annulus method for annular dispersed fuels. A symmetric equivalent annulus centered at the radial midline is introduced, and the inner radius, outer radius, packing fraction, and fuel kernel radius are identified through sensitivity analysis. An explicit four-parameter fitting formula for the equivalent outer radius is established, enabling rapid model construction without iterative searches. The method is validated at both single-cell and assembly levels under steady-state and burnup conditions using the DRAGON code. Numerical results show that the maximum absolute k∞ deviation is 122 pcm for single-cell steady-state cases, 172 pcm for the 13 × 13 assembly, and 164 pcm for the 17 × 17 assembly under steady-state conditions. For burnup cases, the maximum deviations are 13 pcm at the single-cell level, 80 pcm for the 13 × 13 assembly, and 112 pcm for the 17 × 17 assembly. The deviations increase when the fuel outer radius exceeds the fitted range, indicating a clear applicability boundary. Compared with conventional volumetric homogenization, which introduces errors exceeding 500 pcm, the proposed method reduces geometric-modeling complexity while maintaining acceptable accuracy within the fitted parameter space.