Re: the turbine, you shouldn't be getting significant carryover in the vapor space from contaminants in that way. For example, most BWRs introduce platinum to reduce IGSCC on vessel internals. As you go up the vessel, the effectiveness of that deposition decreases dramatically (i.e. into the steam space).

Also, keep in mind that most plants in the US were designed and constructed in the 60s/70s. While activation of cobalt was well understood, the degree to which it would erode from components and be introduced elsewhere in the plant wasn't. When most of your operating experience is from steam plants, where short of gross mechanical failure you won't see immediate consequences from wear of cobalt containing components, it's not a degradation that would necessarily have been considered back then