Bloomberg reports, four workers were killed when a storage tank exploded and caused a fire at Chevron Corp (CVX)’s Pembroke plant in Wales, in the U.K.’s worst refinery disaster for almost four decades. Another worker was hospitalized with serious injuries, Chevron said in a statement. Output at the 210,000-barrel-a-day refinery wasn’t disrupted and an investigation is under way. The explosion is the deadliest since 28 people were killed in a blast at a chemical plant at Flixborough in northern England in 1974, according to the U.K. Petroleum Industry Association Ltd. The Pembroke refinery, which is in the process of being sold to Valero Energy Corp. (VLO), processes crude from the North Sea, Russia and West Africa into diesel, gasoline and jet fuel. The storage tank that exploded contained no crude, minimizing the disruption to operations.