Normal reactor coolant isn't nearly that radioactive. However, there is a continuous filtration system that gets flushed from time to time, and it is extremely nasty stuff. Its possible that the yard worker was actually doused with filter media.
I actually don't think it's possible. If I remember my OPWACHEM course correctly, if the media you're thinking of did wash onto the yard worker he'd have been severely injured by radiation exposure, and not just quickly bumped up to his maximum lifetime allowable exposure. It is indeed extremely nasty stuff.
