Abstract: At approximately 5:30 pm on June 7, 2010, Employees #1 though #5, of Mitsubishi Polycrystalline Silicon America Corp., were assembling the flange of a A-1 CVD. During the reactor assembly step (placing the top on the base) the exhaust lines would normally by purged with nitrogen in order for nitrogen to purge with the nozzles to limit the amount of air in the reactor space. During this assembly, the nitrogen purge had been discontinued to determine if product quality was being affected by the purge carrying hydrogen chloride gas into the reactor. The five employees were working on setting and securing the top of the reactor sitting on small roll-around stools to connect hoses and install the flange bolts. About 40 to 60 seconds after the 6 bolts had been secured a flash fire ignited inside the reactor that lifted the top of the reactor from the base and burned the five employees who were sitting on stools working on installing bolts and connecting hoses. The employees were treated onsite (first aid). One employee was transported by helicopter, and the others were taken by ambulance to USA Medical Center. All five employees were hospitalized overnight.