Across the parking lot from the EBR-1 outside Arco, Idaho, you’ll find the decommissioned HTRE-2 and HTRE-3 in large test assemblies. These Heat Transfer Reactor Experiments were tests of nuclear propulsion in aircraft. General Electric J47 turbojet engines were modified (renamed X-39) to use heated compressed air from a heat exchanger as part of the nuclear reactor rather than from combusting jet fuel. The X-39 engines would have been used in the proposed Convair X-6.

The reactor and heat transfer system was tested on a Convair NB-36 (converted B-36 Peacemaker) with 47 recorded flights between 1955 and 1957. The reactor was turned on through many of these flights not to power the aircraft but to test and collect data on the feasibility of a sustained nuclear reaction on a moving platform.

The HTRE-2 used vertical control rods with a removable core. The HTRE-3 was built to test horizontal control rods to accommodate the orientation in an airframe. The test assemblies were going to be decontaminated and decommissioned for burial in the Radioactive Waste Management Complex. However, preservation was chosen instead, and the assemblies went on display on May 22, 1989. This includes the lead-shielded locomotive used for the test assemblies and that would have towed the proposed planes inside the hangar.

The program was canceled on March 28, 1961, by President Kennedy, due to public safety concerns, advances in ballistic missiles, and aircraft design innovations, after spending more than a billion dollars developing the concept.