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The Argonne National Laboratory (ANL) broke ground for the Transient Reactor Test (TREAT) facility in February of 1958. By April, the basic foundation had been laid and progress was running smoothly. The TREAT reactor was created to conduct transient tests. During these tests, nuclear fuels and materials were subjected to brief, yet highly intense, neutron pulses to simulate different reactor conditions.
The construction was completed in November of 1958 and the reactor reached its first criticality on February 23, 1959. This means that the reactor reached a configuration under which it could maintain and operate at a steady power level.
In its 37 years of operation, the TREAT reactor performed over 2,500 tests that provided vital information on how a nuclear fuel will respond during accidents ranging from a mild upset, such as loss of power, to severe accidents, such as natural disasters or loss of coolant. These tests covered fast reactor fuels, as well as light-water fuel reaction and even some exotic special purpose fuels, like those found in space reactors.
The reactor was shut down in April 1994, a time period during which there was no apparent need for transient testing, and was placed in a safe standby mode. The reactor itself remained fueled during this time and the facility's material condition was maintained.
Resumption of Transient Testing was officially underway in December 2010, but it was not until February of 2014 that TREAT was selected by the Department of Energy as the preferred restart choice. The original proposal set the end date as 2018, but TREAT was ahead of schedule and officially restarted in November 2017.