Last night at approx. 8:10 pm, an explosion was heard at the Stan Chem Plant on Berlin St. A short time later, the fire departments were dispatched to a fire alarm and then upgraded to an explosion with a fire. Upon arrival of the fire departments, a small fire was visible on the second floor of a building that was in the process of being de-commissioned. The alarm worked as designed, setting off the sprinkler system and shutting the fire doors, containing the fire to the second floor. The explosion occurred due to a chemical reaction of older chemicals. DEEP was requested for air monitoring and found no contaminants in the air or in the water runoff.  The fire was extinguished and confirmed there is no environmental hazard or air quality hazard.