Neutron Sciences Directorate

The High Flux Isotope Reactor (HFIR) was constructed in the mid-1960s to fulfill a need for the production of transuranic isotopes—heavy elements such as plutonium and curium. Since then, its mission has grown to include materials irradiation, neutron activation, and, most recently, neutron scattering.

The status of the transuranium production program was critically reviewed by the US Atomic Energy Commission (AEC) Division of Research at a meeting on January 17, 1958. From that review, the AEC embarked on a program designed to meet the anticipated needs for transuranium isotopes by undertaking certain irradiations in existing reactors. By late 1958 it became apparent that the program’s pace needed to be accelerated. Following a November 24, 1958, meeting in Washington, DC, the AEC recommended that a high-flux reactor be designed, built, and operated at Oak Ridge National Lab (ORNL), with construction to start in 1961.

Authorization to proceed with the design of a high-flux reactor was received in July 1959. The preliminary conceptual design of the reactor was based on the “flux trap” principle, in which the reactor core consists of an annular region of fuel surrounding an unfueled moderating region or “island.” Such a configuration would permit fast neutrons leaking from the fuel to be moderated in the island to produce a region of high thermal-neutron flux at the center of the island. This reservoir of thermalized neutrons would be “trapped” within the reactor, making it available for isotope production. The large flux of neutrons in the reflector outside the fuel of the reactor would be tapped by extending empty beam tubes into the reflector, to allow neutrons to be used in experiments outside the reactor shielding. Finally, a variety of holes in the reflector would allow irradiation of materials for later retrieval.

In early 1965, with construction complete, final hydraulic and mechanical testing began, and criticality was achieved on August 25, 1965. The low-power testing program was completed in January 1966, and operation cycles at 20, 50, 75, 90, and 100 MW began.

From the time it attained its design power of 100 MW in September 1966—a little over 5 years from the beginning of its construction—until it was temporarily shut down in late 1986, HFIR achieved a record of operation time unsurpassed by any other reactor in the United States. By December 1973, it had completed its 100th fuel cycle, each running approximately 23 days.

In November 1986 tests on irradiation surveillance specimens indicated that the reactor vessel was being embrittled by neutron irradiation at a rate faster than predicted. HFIR was shut down to allow for extensive evaluation of facility operations. ORNL spent nearly 2.5 years conducting a thorough evaluation, making modifications to extend the life of the plant while protecting the integrity of the pressure vessel, and upgrading management practices and then restarted the reactor for fuel cycle 288 on April 18, 1989. HFIR operated initially at very low power levels (8.5 MW) until all operating crews were fully trained and continuous higher power operation could take place. Documents were updated, and new ones were generated where necessary. Technical specifications were amended and reformatted to keep abreast of design changes as they were accepted by DOE. Not only were the primary coolant pressure and core power reduced to preserve vessel integrity while maintaining thermal margins, but long-term commitments were made for technological and procedural upgrades.

Following the April 1989 restart, a further shutdown of 9 months occurred due to a question of procedural adherence. Following permission by then-Secretary of Energy James Watkins to resume startup operations in January 1990, full power was reached on May 18, 1990. HFIR has remained in operation since then. Ongoing programs have been established for procedural and technological upgrades of HFIR during its operating life.

In 2007, HFIR was refurbished, and a number of new instruments were installed, including a cold neutron source. Improvements included an overhaul of the reactor structure for more reliable, sustained operation, significant upgrading of the eight thermal-neutron spectrometers in the beam room, new computer system controls, installation of the liquid hydrogen cold source, and a new cold neutron guide hall.

Although HFIR's main mission shifted to neutron scattering research, one of its original primary purposes was to produce californium-252 and other transuranic isotopes for research, industrial, and medical applications. HFIR is the western world’s sole supplier of californium-252, an isotope used for cancer therapy and detection of pollutants in the environment and explosives in luggage. Beyond its contributions to isotope production and neutron scattering, HFIR also provides a variety of irradiation tests and experiments that benefit from the facility’s exceptionally high neutron flux.

In 2014, the American Nuclear Society designated HFIR as a Nuclear Historic Landmark in recognition of its vital role in the history of the nuclear age and its continued importance to the United States for isotope production, neutron scattering research, and national security.

In 2017, the International Atomic Energy Agency (IAEA) designated ORNL as an International R&D Hub because of HFIR’s continued importance for post irradiation testing of materials, neutron scattering, and processing of radioisotopes. The IAEA designation made the United States just one of four countries in the world that have been identified for unique capabilities and excellence in nuclear research.

Notable Accomplishments

Neutron activation analyses conducted at HFIR have informed work of the semiconductor and environmental remediation industries and the US Food and Drug Administration.

The Fusion Energy Program has been supported by HFIR in three major areas: neutron-interactive materials (structural materials and ceramics), high heat flux materials, and plasma-interactive materials.

HFIR neutron scattering facilities have provided support to basic research programs involving neutron scattering from polymers, colloids, magnetic materials, alloys, superconductors, and biological materials.