At 0815 hours EPA received NRC # 879415. Valero Refinery reported an unknown amount of sulfur dioxide released to the air. START was dispatched to the Valero Refinery in Houston, Texas at 0830 hours. The Responsible Party (RP) reports that the roof of a sulfur tank was leaking after an explosion this morning. The fire has been extinguished and responders for the RP were foaming the tank. The shelter in place of nearby residents was instituted as a precaution. The RP reported the incident as secure at 0930 hours.