History of the High Flux Isotope Reactor

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

In 2007, HFIR completed the most dramatic transformation in its 40-year history. During a shutdown of more than a year, the facility was refurbished and a number of new instruments were installed, as well as a cold neutron source. The reactor was restarted in mid-May; it attained its full power of 85 MW within a couple of days, and experiments resumed within a week. Improvements and upgrades to HFIR include an overhaul of the reactor structure for 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. The upgraded HFIR will eventually house 15 instruments, including 7 for research using cold neutrons.

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

Why Was HFIR Built?

The status of the transuranium production program was critically reviewed by the U.S. Atomic Energy Commission (AEC) Division of Research at a meeting on January 17, 1958. At that time the AEC decided to embark 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 acceleration of this program was desirable. Following a meeting in Washington, D.C., on November 24, 1958, the AEC recommended that a high-flux reactor be designed, built, and operated at ORNL, with construction to start in FY 1961.

As a result of this decision ORNL submitted a proposal to the AEC in March 1959. 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 permits fast neutrons leaking from the fuel to be moderated in the island and thus produces a region of very high thermal-neutron flux at the center of the island. This reservoir of thermalized neutrons is "trapped" within the reactor, making it available for isotope production. The large flux of neutrons in the reflector outside the fuel of such a reactor may be tapped by extending empty "beam" tubes into the reflector, thus allowing neutrons to be beamed into experiments outside the reactor shielding. Finally, a variety of holes in the reflector may be provided in which to irradiate materials for later retrieval.

In June 1961, preliminary construction activity was started at the site. In early 1965, with construction complete, final hydraulic and mechanical testing began. 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, the 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, approximately 23 days each.

Notable accomplishments resulting from HFIR operation include the production of californium-252, which is used for reactor startup sources, scanners for measuring the fissile content of fuel rods, neutron activation analysis, and fissile isotope safeguards measuring systems. In addition, californium-252 is used as a medical isotope to treat several types of cancer. Also, neutron activation analysis at HFIR has been used by the semiconductor industry, environmental remediation operations, and the Food and Drug Administration.

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

The neutron-scattering facility at HFIR has provided support to basic research programs involving neutron scattering from polymers, colloids, magnetic materials, alloys, superconductors, and biological materials.

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. The HFIR was shut down to allow for extensive reviews and evaluation of the operation of this facility. Two years and five months later, after thorough reevaluation, modifications to extend the life of the plant while protecting the integrity of the pressure vessel, and upgrades to management practices, the reactor was restarted. Coincident with physical and procedural improvements were renewed training, safety analysis, and quality assurance activities. Documents were updated, and new ones were generated where necessary. Technical specifications were amended and reformatted to keep abreast of the 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.

After a thorough review of many aspects of HFIR operation, the reactor was restarted for fuel cycle 288 on April 18, 1989, to operate initially at very low power levels (8.5 MW) until all operating crews were fully trained and it was possible to operate continuously at higher power. Following the April 1989 restart, a further shutdown of nine months occurred as a consequence of a question as to procedural adequacy. During this period, oversight of the HFIR was transferred to the DOE Office of Nuclear Energy (NE); previously, oversight was through the Office of Energy Research (ER). Following permission by Secretary of Energy James Watkins to resume startup operation in January 1990, full power was reached on May 18, 1990. Ongoing programs have been established for procedural and technological upgrade of the HFIR during its operating life.