Information from NRT about the radiation levels at the airport:
http://contents.narita-airport.jp/narita/en/222.pdf

I'm not a nuclear scientist, but I think this document is showing that levels of radiation at NRT are currently lower than what many people experience as background radiation in their normal places of residence.

Of course, that doesn't eliminate the possibility that some sneaky radioactive nuclides could attach themselves to a bird, which would then fly to NRT and drop a killer toxic poop on your suitcase as it comes off the plane - leaving you with underwears that snap, crackle and pop.

I'm jus' sayin'...