UPDATE-2： There was a second explosion at 8AM on April 22. And there are containers of depleted uranium. See my latest post.
UPDATE: One of the two missing workers was found dead. 9 workers injured, 3 residents in the area were also injured (cuts from broken glass). It looks like part of the complex is still burning. The prefectural government does say the depleted uranium is stored at the complex, but there has been no release of toxic materials from the fire, according to the prefectural government.
Mitsui Chemical's petroleum complex in Iwakuni, Yamaguchi Prefecture had an explosion and a fire at about 2:20 AM on April 22, 2012, according to Yomiuri Shinbun (4/22/2012). Several people were injured, and window glass broken in wide areas.
Someone on Twitter has dug up information from the Ministry of Economy, Trade and Industry (METI) that lists companies that have radioactive waste on the premise. Mitsui Chemical's complex in Yamaguchi Prefecture is one of them. According to the MEXT data from 2009, the complex has 3,379 containers of nuclear waste.
There are people on Twitter saying these containers are full of depleted uranium, but I haven't found any documentation to substantiate the claim about depleted uranium yet. For now, I'd treat it as nothing but "rumor". It is possible, as in the case of Chisso's petroleum compound in Chiba that had a fire when the earthquake struck on March 11 last year. The storage unit for depleted uranium was on the premise. I'm looking for more information.
According to Mainichi, there were 7 workers working at that time in the chemical plant that manufactures "resorcin" which is used as tire adhesive. 5 workers suffered minor injuries, but 2 workers are missing. They were trying to stop the plant because of some problems but they couldn't. Explosions are still on-going, and the firefighters cannot come near the plant.