Concrete encapsulation for spent nuclear fuel storage

Concrete systems, mixtures and methods for encapsulating and storing spent nuclear fuel. Fuel discharged from nuclear reactors in the form of rods or multi-rod assemblies is completely and contiguously enclosed in concrete having incorporated therein metallic fibers to increase thermal conductivity and polymers to decrease fluid permeability. The metallic fibers and the polymers can be distributed in a single concrete layer, or separate contiguous layers can be utilized for the conductivity and impermeability characteristics