The 209-E Critical Mass Laboratory is located in the 200-East Area of the Hanford Site.  Construction of the facility began in 1958 and was completed in 1960.  The building was in use from 1961 until 1983 and features an administrative wing and a wing dedicated to processing and experimentation. 

Scientists needed the 209-E facility to work with plutonium and uranium solutions.  The facility features a mixing room with glove boxes and a mixing hood, and a control room from which experiments could be remotely monitored and controlled.  Criticality experiments, where a nuclear chain reaction becomes self-sustaining, were also conducted.  In addition, 209-E was a research facility where methods of protecting workers in the event of a nuclear accident were tested.

No research or experiments have been conducted in the building since 1983.  However, the radioactive nature of the work that was done in 209-E has resulted in some parts of the building becoming contaminated. 

In 1988, the Department of Energy directed that the laboratory be prepared for unoccupied status.  This resulted in hazardous materials being removed from the building and tanks that were used in experiments were flushed, emptied, and ventilated.  Since 1989, some of the administrative areas of 209-E have been used intermittently, but the only entries into the laboratory of the complex have been limited to decommissioning, characterization of materials or equipment, surveillance, and maintenance.