A multi-regime model of radial particle and energy transport in tokamaks, based on the predictions of low-frequency microinstability theory, has been incorporated into a one-dimensional computer code. The code also includes background neutral gas, neoclassically (3-regime) diffusing impurity ions, adiabatic compression, and self-consistent neutral-injection heating by means of the slowing down of beam ions. A reasonable measure of agreement with experimental results from a number of present-day tokamak devices is achieved. Computations for larger next-generation tokamaks emphasize the problem of high-Z metallic impurities, and a "cold plasma blanket" technique for their control is presented.