Four men were killed in an explosion today at a chemical plant in Japan's Osaka, Kyodo News reported. The blast occurred at a fluorine compound production plant, operated by Morita Chemical Industries Co. in Osaka's Yodogawa Ward. <br/> <br/>The explosion occurred when the four workers were cutting away the top of a chemical tank as part of efforts to clean it. The tank stores a toxic chemical material called borontrifluoride.<br/> <br/>The firefighters said part of the plant's roof was blown off in the blast.<br/>