A welder was welding inside the intermediate brine tank to fix a leak on bottom of the tank. While carrying out this task, he heard a sound of air suction similar to opening of valve followed by a small amount of circulation water with oil residue entered the tank. The fluid made contact with the welding spark and caught fire. The welder immediately evacuated and alerted the crew members and crew put out the fire with portable fire extinguishers. No personal injury reported.