[en] The kinetic theory of runaway electrons is extended to account for radiation reaction and radial diffusion. It is found that the combined effects of pitch-angle scattering and radiation reaction from synchrotron radiation (caused by gyro-motion) can effectively damp a beam of relativistic electrons. This appears to explain the decay of post-disruption runaway currents in JET. A 3D Monte Carlo code simulating runaway avalanches caused by close Coulomb collisions has been constructed, which includes the effects of radiation reaction and radial diffusion caused by small-scale magnetic perturbations. The latter can prevent runaway avalanches if the transport is sufficiently strong. (author)