* U.N. agency says nuclear now safer due "robust measures" * But old reactors could affect safety, IAEA report says * Seventy percent of research reactors more than 30 years old * Fukushima disaster raised global nuclear safety concerns By Fredrik Dahl VIENNA, March 13 (Reuters) - Eighty percent of the world's nuclear power plants are more than 20 years old, raising safety concerns, a draft U.N.