NEW : Britain joins the US in urging residents within 50 miles to evacuate . NEW : Water is dropped from above on the Fukushima Daiichi plant 's No . 3 reactor . Nearly 13,000 are dead or missing , police say . Emperor Akihito of Japan visits a temporary shelter for evacuees at an elementary school near Tokyo.
NEW : Britain joins the US in urging residents within 50 miles to evacuate . NEW : Water is dropped from above on the Fukushima Daiichi plant 's No . 3 reactor . Nearly 13,000 are dead or missing , police say . Emperor Akihito : 'Remember everyone who has been affected ... and help '