By John Irish NEW YORK (Reuters) -The head of the United Nations atomic agency said on Wednesday he would not abandon a plan to create a protection zone around the Zaporizhzhia nuclear power plant despite Russian plans to mobilize new troops and hold a referendum in the region. Russia and Ukraine have blamed each other for shelling at the site of Europe’s biggest nuclear power plant that has damaged buildings close to its six reactors and risked nuclear catastrophe, including by cutting power lines essential to cooling fuel in the reactors even though they are all shut down. Ukrainian staff ar…