It has been shown theoretically that bias-driven cavity swelling can only occur after either a critical cavity radius has been achieved or a critical number of gas atoms has been accumulated in a cavity. These possibilities merge into each other, as increasing the contained gas lowers the critical radius until at the critical number of gas atoms the minimum critical radius is achieved. With the addition of any more gas, the critical radius disappears and cavity swelling is ensured. It is found that these critical quantities are highly sensitive to irradiation conditions and material parameters. Under fixed irradiation conditions, the critical quantities are remarkably strong functions of dislocation density and bias. These results are described and their implications for the design of swelling-resistant materials are discussed.