Is it safe to fly cross-pacific right now? (within 10 days of Fukushima crisis?)?

I'm flying over Japan from America to Korea. I'm not asking if Korea is safe. I'm asking if people inside the Boeing 747 while flying over pacific ocean for 15 hours are relatively safe. I understand pengers are exposed to natural radiation from space anyway, but I don't want additional radiation exposure from Fukushima plant. The route map from the airline I'm flying shows the airplane flying over 180 km from Fukushima nuclear plant. I'm not sure how heavy or light the radioactive isotopes are. How high it will travel, etc. Obviously, it's dangerous enough for govt's to issue no fly zone. 180km where I'll be flying over doesn't seem far enough. I'm considering to postpone my itinerary because I have a young toddler and a preschooler.