Runaway electron (RE) transport is affected by the presence of magnetic perturbations, either caused by internal MHD activity or externally imposed by magnetic coils (RMPs) [1]. During a typical tokamak discharge, different degrees of magnetic stochasticity are present in the plasma volume, which make the ordinary diffusive approach inadequate to the study of transport [2]. In this contribution, the fractional character of RE transport and the diffusion coefficient are evaluated in the flat-top phase of typical circular COMPASS discharges, where some internal MHD activity is observed in the form of magnetic islands, by using the guiding-center particle code ORBIT [3]. The results of particle simulations are compared with estimates from a magnetic diffusion model, and the time evolution of an initial RE beam distribution is studied by using a fractional diffusion model. Attempts to determine the parametric dependence of RE diffusion coefficient on amplitude and spectrum of the perturbation and on particle energy are ongoing. [1] O. Ficker et al., Nucl. Fusion 57 076002 (2017) [2] D. del Castillo-Negrete, Physics of Plasmas 13 082308 (2006) [3] R.B. White, M.S. Chance, Phys. of Fluids 27 2455-2467 (1984)