(UPDATE-CORRECTION: the contaminated area according to the Chinese is "252,000 square-kilometer", and not "252,000 square-meter" as in the initial post.)
China sent a survey ship and taking seawater samples off the coast of Fukushima back in June and July. The State Oceanic Administration now says the contamination of the Pacific Ocean may extend as far as 800 kilometers (497 miles) off the coast of Fukushima, as reported by the Science and Technology Daily (ST Daily) in China, according to Asahi Shinbun.
(STDaily's original article in Chinese is here.)
From Asahi Shinbun (12:08PM JST 8/16/2011):
China's State Oceanic Administration cites the result of the environmental survey it did in the western Pacific Ocean off the coast of Fukushima and says a much wider area of the Pacific Ocean is contaminated with radioactive materials than the Japanese government has announced, and that the possibility cannot be eliminated that radioactive materials have entered the ocean under the Chinese control".
The Chinese paper "Science and Technology Daily (electronic version)" reported on August 15 as the State Oceanic Administration's written response to their inquiry.
According to the State Oceanic Administration, the area said to be affected by radioactive materials is a 252,000 square-kilometer [corrected] area inside the 800 kilometer off the coast of Fukushima. Cesium-137 was found maximum 300 times the level found in the Chinese coastal waters, and strontium-90 was found maximum 10 times the level.
The State Oceanic Administration conducted the survey in June and July in the Western Pacific, in response to the Fukushima I Nuclear Power Plant accident.
In a separate piece of news, 5,000 angry residents of Chengdu in Sichuan Province in China closed off the roads [to the city?] to protest the frequent power outage, according to Yomiuri Shinbun citing Hong Kong's Oriental Daily.