Bob, not to worry. A few thin sheets of lead would be of little function in making or shielding a 'dirty bomb', unless the bomb itself is constructed of relatively benign (reads low energy) radioactive material like uranium ore rocks -- but what then is the point?
You aren't going to stop the radiation from a potentially harmful radiation source (reads dirty bomb) with anything less than a few inches of void free, solid, pure lead, and even then it will be likely to trip sensive survey meters in its vicinity.
A fusion bomb is quite another issue, but fortunately this remains beyond the reach most terrorists unless they can buy one.
Harry C.