• Significance: After World War II, one of the highest priorities of the Atomic Energy Commission (AEC) was nuclear reactor research. The AEC needed a "high flux" research reactor, an instrument subjecting materials to intense radiation: the Materials Testing Reactor (MTR). Scientists used the MTR to learn how radiation affected materials potentially useful for cooling systems, fuels and structural support for later reactors.

The AEC applied MTR findings to propulsion reactors for warning systems and to commercial power plants, which required reliable, continuous, and safe operation in locations near populated urban areas. One of the first projects built at the new Nation Reactor Testing Station in Idaho, the MTR operated between 1952 and 1970. The MTR subjected every conceivable substance to neutron flux in its test holes and loops, logging 125,000 hours and 19,000 irradiations. It "mothered" most of the military and commercial reactors subsequently built in the United States (and many other countries). Demand for higher neutron flux and larger test holes, particularly by the U.S. Navy, resulted in the Engineering Test Reactor (ETR), built next to the MTR, which continued and refined the materials testing mission. It operated between 1957 and 1981.

• Survey number: HAER ID-33-G
• Building/structure dates: 1950-1951 Initial Construction
• Building/structure dates: 1956-1957 Initial Construction