Transient Reactor Test Facility

The TREAT fuel assemblies are approximately 9 feet long, and 4 inches square in cross section. The fuel is a graphite uranium mixture, with 1 part uranium to 10,000 parts graphite. The active portion of the fuel assembly is about 48 inches, with a graphite reflector of about 24 inches above and below the active portion. The active portion of the fuel assemblies are encased with Zircaloy. There is also a graphite axial reflector, composed of two parts. The first part of the axial reflector consists of movable assemblies similar to the fuel assemblies, but containing only graphite and no fuel. The second part of the axial reflector consists of permanent blocks of graphite, approximately 24 inches thick, stacked outside the core cavity. This permanent reflector was reclaimed from Chicago Pile-1, the world's first nuclear reactor. The core may be loaded to a size of 5 feet by 5 feet (nominal) up to 6 feet by 6 feet (maximum), depending on the needs of the experiment.[12][13]

As described above, the fuel is composed of a mixture of graphite and uranium. The uranium is in the form of uranium oxide particles that are approximately 20 microns in size, and are in direct contact with the graphite moderator. The graphite, in addition to being the neutron moderator, also acts as a large thermal heat sink. The time lag of the heat transfer is on the order of 1 millisecond, much faster than the heat transfer to a liquid coolant flowing past fuel assemblies. Also, when the graphite is heated it creates a sizable negative moderator temperature coefficient. These characteristics allow TREAT to produce large ‘self-limited’ transients, which are limited by the fuel negative moderator coefficient without control rod movement.[13]

TREAT is capable of a wide range of operations and test conditions. TREAT can operate at a steady state power of 100 kW, produce short transients of up to 19 GW, or produce shaped transients controlled by the TREAT automatic reactor control system and the Control Rods. A test assembly can be inserted in the center of the core. The test assembly is a self-contained vehicle, that can contain fuel or materials for a variety of reactor types.[14] These test assemblies, also referred to as test vehicles or test loops, can simulate the conditions of a light water reactor, heavy water reactor, liquid metal fast breeder reactor, or a gas cooled reactor.[15]

In some experiments, provisions were made to make high speed film recordings of the experiment, such as these videos.

On November 14, 2017, the TREAT reactor achieved criticality for the first time since 1994. This was accomplished 12 months ahead of schedule, and about $20 million under budget. This is an important milestone toward the testing of new nuclear fuel, which is expected to begin in 2018.[24][25][26][27][28][29]

On September 18, 2018 TREAT completed the first experiment with a small sample of light water reactor fuel.[30][31][32][33][34] This was a very significant milestone for the TREAT reactor, and is a major step toward one mission of TREAT - validating new accident tolerant nuclear fuel for commercial power plants. The senior Senator from Idaho, Senator Mike Crapo, read the statement below into the Congressional Record.

RECOGNIZING THE TRANSIENT REACTOR TEST FACILITY RESTART

Mr. CRAPO. Mr. President, along with my colleagues Senator James Risch and Representative Mike Simpson, today I wish to call attention to an important event taking place today at the U.S. Department of Energy's, DOE, 890-square-mile site in Idaho. Today, Idaho National Laboratory, INL, personnel ran the first experiments in the Transient Reactor Test, TREAT, facility in nearly a quarter century. 

Idaho National Laboratory is our Nation's lead nuclear energy research, development, and demonstration laboratory, the place where 52 original nuclear reactors were constructed and demonstrated. One of those reactors was the TREAT facility, which operated from 1959-1994, and remained fully fueled while on standby status. Transient testing focuses upon testing nuclear fuel under accident conditions. TREAT is one of the most capable and flexible transient test reactors in the world.
  Following the accident at the Fukushima-Daiichi Power Plant in Japan 7 years ago, Congress directed the DOE to develop reactor fuels that could better withstand accident conditions. During TREAT's 35 operating years, the reactor performed 6,604 reactor startups and 2,884 transient irradiations. Given this history, it made more sense to restart the facility than build a new reactor. That decision paid off when, on August 31, 2017, the Resumption of Transient Testing Program was completed more than 1 year ahead of schedule and approximately $17 million under budget.
This highly successful restart at the TREAT facility was recognized in August, when a joint DOE-INL team won the Secretary of Energy Award. This award recognizes DOE employees or contractors who accomplish significant achievements. It is the highest nonmonetary internal recognition that can be achieved at the DOE. U.S. Secretary of Energy Rick Perry highlighted the TREAT restart team's effort and efficiency, and recognized the importance of the facility to nuclear energy scientists and engineers as they work to develop advanced nuclear fuels and reactor technologies.
  Congratulations, INL and DOE, on the TREAT restart and for bringing back online an important national asset in the effort to develop the advanced nuclear reactors so vital to our economy, environment, and national security.[35]

Idaho National Laboratory TREAT site - https://transient.inl.gov/SitePages/Home.aspx

Idaho National Laboratory "360 degree tour" of the TREAT facility - https://inl.gov/360-tour-map/#treat

Stacy, Susan M. (2000) Proving the Principle: A History of the Idaho National Engineering and Environmental Laboratory, 1949-1999. United States Government Printing. pp. 136, 268. ISBN 0160591856.