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At approximately 9:40 p.m. on the evening of June 29, 2010, an ignition source in a solvent sludge feed tank ignited flammable solvent vapor. The vapor was in the head space of a partially filled atmospheric tank, either tank Q and/or tank R in the E-II solvent sludge feed tank area. The explosion flame front spread to the adjacent tank, and as a result, both tank covers were removed by the force of the event. The tank cover for tank Q was peeled back to the east but still partially attached. The tank cover for tank R was jettisoned; it struck the E-II processing building to the northwest in several locations before landing on the roof of the Dock 4/5 building to the east. The subsequent tank fires resulting from the explosion were extinguished by the local fire department. The likely ignition source was determined to be ultrasonic high-level sensors within the solvent sludge feed tanks. Apparently they had separated due to solvent degradation, exposing internal wiring. There were no injuries or fatalities.