(Reuters) – Ukraine on Wednesday accused Russia of exploiting its position in a nuclear power plant it had seized to target a nearby town in a rocket attack that killed at least 13 people and left many others seriously wounded. FIGHTING * Russia launched 80 Grad rockets at the town of Marhanets across the Dnipro river from the Zaporizhzhia plant on Tuesday, Valentyn Reznychenko, governor of the central Dnipropetrovsk region, said, adding that more than 20 buildings were damaged. * There was no immediate comment from Russia, which has accused Ukraine of shelling the plant, something Kyiv denies…