For a flare stack a radiation study is normally required. Is a radiation study necessary for vent stacks as well?

Yes, for all stacks. GH2 has a minimum prescriptive height of 10 ft. There is no minimum prescriptive height for LH2. However, 25 ft has been a best practice for the industry for years. Vent stack outlets that orient the release vertically help reduce the radiation exposure at ground level. Care must be taken to consider varying weather conditions, particularly wind, as well as surrounding exposures that might be elevated in the vicinity of the vent stack outlet, such as nearby equipment, buildings, catwalks, and grade elevation changes.