A system, method and apparatus for providing radiation shielding to a ventilated cask for holding high level radioactive materials. In one aspect, the tubular shell is positioned to circumferentially surround the cask so that an annular gap exists between the tubular shell and a sidewall of the cask. The tubular shell includes a first air flow inlet and a second air flow inlet. An air flow barrier is placed within the annular gap, separating the annular gap into a first chamber and a second chamber. A first air flow into the first air flow inlet passes through the first chamber and into the inlet vent of the cask, a second air flow into the second air flow inlet passes through the second chamber and to an opening at the top end of the tubular shell, and the air flow barrier prohibits cross-flow of air between the first and second chambers.