I can't exactly answer that. We're somewhat unique, or are at least in a small family, because we operate a heavy-water-moderated reactor. It allows us to be very versatile; however, it also allows us to detect very small leaks. I really can't speak to other reactors, but generally I would say the leaks are not common, but they do occur.
On June 4th, 2009. See this statement in context.