Didn't I address this objection using potential advances in cellular maintanence and rejuvenation? In other words, injured cells could be periodically replaced with healthy cells generated in internal rooms which are smaller versions of your spheres with 40 foot thick walls. (See Aubrey de Grey for a rundown of coming advances in human cellular maintenance). As for the food, well, if solar panels are used to collect light, the plants could be placed in opaque rooms fitted with LED lighting. But I see your point of having to enclose the plants in large rooms. But again, if we can master cell maintanence in humans, it's foreseeable we could do it for plants, raise them in smaller rooms, then bring them out later to the large exposed farms. But, as our cells would be periodically replaced with healthy cells, it doesn't really matter if we are eating plants exposed to some radiation, does it? We'll just replace those sick cells with healthy ones generated in those much smaller protected rooms I suggested. Additionally, haven't scientists proposed hydrated plastic as an alternative shielding?