There was a "baseless rumor"(link in Japanese) that circulated on the Internet on March 12 that said "Harmful substance will come down with the rain after the Cosmo Oil fire. Do not expose your skin." Cosmo Oil and the government dismissed the rumor vehemently. Maybe the "baseless rumor" was not about Cosmo Oil's tank but about Chisso's depleted uranium...
From Chiba Nippo (7/1/2011):
劣化ウラン保管施設も延焼
コスモ石油のガスタンク火災で
Depleted Uranium Storage Facility Also Burned when Cosmo Oil's Gas Tank Exploded
６月千葉県議会は３０日、総務防災と総合企画水道の２常任委員会が開かれた。総務防災では、東日本大震災で発生したコスモ石油千葉製油所（市原市）の液化石油ガス（ＬＰＧ）タンク火災・爆発に伴い、隣接する劣化ウラン保管施設も延焼していたことが明らかになった。放射性物質の漏えいはなかったとしている。
On June 30 Chiba Prefectural Assembly held meetings of 2 standing committees, "general affairs and disaster countermeasures" and "planning and water". In the general affairs and disaster countermeasures committee, it was revealed that the LPG (liquefied petroleum gas) tank fire and explosion at Cosmo Oil Refinery in Ichihara City, Chiba also burned the adjacent depleted uranium storage facility. There was no leak of radioactive materials, according to the committee.
【総務防災】県消防課は、同製油所のガスタンクの火災・爆発事故で、隣接するチッソ石油化学の劣化ウラン保管倉庫の屋根が焼け落ちていたことを明らかにした。
The Chiba prefectural fire department disclosed that the depleted uranium storage facility's roof was burned down because of the fire and explosion of the LPG tank at Cosmo Oil. The depleted uranium storage facility belongs to Chisso Petrochemical [subsidiary of Chisso Corporation].
同課によると、同倉庫は「核燃料使用事業所」として国の許可を受け、ガス製造用の触媒として使われる劣化ウラン７６５キログラムが保管されており、放射性物質が０・３％含まれていた。
According to the fire department, the storage facility is licensed by the national government as "nuclear fuel usage location", and had 765 kilograms of depleted uranium that contained 0.3% of radioactive material [uranium]. The depleted uranium is used as catalyst for gas production.
Looking at Chisso's press releases, there is no mention of the depleted uranium storage facility at this plant location. The explosion and fire at Cosmo Oil Refinery was finally put out on March 21, at which point Chisso was able to go inside their plant to assess the fire damage.
Ichihara City is on the west side of Chiba Prefecture, facing the Tokyo Bay, and there are many industrial plants and petrochemical facilities on the reclaimed land on the Tokyo Bay. Chisso Petrochemical is marked with a Yellow balloon with a dot:
View Tokyo bay in a larger map
A minor detail, but Chisso Petrochemical is NOT adjacent to Cosmo Oil, by the way. There is Maruzen Oil in between the two. Hmmm. How would Chisso's facility catch fire when Maruzen's did not?
Here are some video of Cosmo Oil explosion on March 11, in case you missed:
http://youtu.be/TZqhc-ysHzM (Asahi Shinbun footage)
http://youtu.be/x3Hy2LV_lLo (explosion that darkens the sky after 1:40 into the video)
By the way, Chisso is the same company that caused extensive mercury poisoning in the Minamata Bay in Kyushu by dumping methylmercury in the wastewater from the company's chemical plant, and the company has been kept "alive" so that it makes profit to pay compensations to the victims. TEPCO's likely future.
(By the way, turning gossipy, Japan's crown princess is the granddaughter of the Chisso president who fought and fought the "allegation" in the 1960s that his company's plant caused mercury poisoning.)