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This paper briefly reports on the effects of openings (heating and pumping ducts) in a breeding blanket of a fusion reactor with tokamak physics on its nuclear characteristics. Predictions are based on 3-D Monte Carlo computations using the 3DAMC-VINIA subroutine complex operating without any numerical or geometrical simplifications; the ENDF/B library data are used in extenso. The objects under study coincide with the versions considered in the U.S. "Blanket Comparison and Selection Study" (BCSS) program (Li2O and/or 17Li83Pb breeders); some results for LiAlO2 and Li-liquid metal breeders are also given. An analysis of the numerical results reveals a strong depression of tritium breeding, energy production, and an increase in neutron leakage. Analytical formulas proposed here fit all the numerical results well and permit, on the basis of numerical data for unported blankets (no openings), the prediction of the depression effects as a function of the magnitude and number of openings. The role of the position, shape, and/or number of openings is also shown.