  We report on the microwave surface resistance of melt-textured
Y_1.6Ba_2.3Cu_3.3O_7-x samples, doped with different amount of B_2O_3 and,
subsequently, irradiated by thermal neutrons at the fluence of 1.476 \times
10^17 cm^-2. The microwave surface resistance has been measured as a function
of temperature and DC magnetic field. The experimental results are
quantitatively discussed in the framework of the Coffey and Clem theory,
properly adapted to take into account the d-wave nature of cuprate
superconductors. By fitting the experimental data at zero DC field, we have
highlighted the effects of the induced defects in the general properties of the
samples, including the intergranular region. The analysis of the results
obtained at high DC fields allowed us to investigate the fluxon dynamics and
deduce the depinning frequency; in particular, we have shown that the addition
of B_2O_3 up to 0.1 wt% increases the effectiveness of the defects to hinder
the fluxon motion induced by the microwave current.
