The High Flux Isotope Reactor (HFIR) is a nuclear research reactor at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, United States. Operating at 85 MW, HFIR is one of the highest flux reactor-based sources of neutrons for condensed matter physics research in the United States, and it has one of the highest steady-state neutron fluxes of any research reactor in the world. The thermal and cold neutrons produced by HFIR are used to study physics, chemistry, materials science, engineering, and biology. The intense neutron flux, constant power density, and constant-length fuel cycles are used by more than 500 researchers each year for neutron scattering research into the fundamental properties of condensed matter. HFIR has about 600 users each year for both scattering and in-core research.
The neutron scattering research facilities at HFIR contain a world-class collection of instruments used for fundamental and applied research on the structure and dynamics of matter. The reactor is used for medical, industrial, and research isotope production; research on severe neutron damage to materials; and neutron activation to examine trace elements in the environment. Additionally, the building houses a gamma irradiation facility that uses spent fuel assemblies and is capable of accommodating high gamma dose experiments.
The next major shutdown for a beryllium reflector replacement is planned for 2028-2029. In parallel, this program foresees in an extension to the cold guide hall to allow for reconfiguring and optimizing of the cold neutron instruments, and provide space for new instruments. HFIR is projected to continue operating through 2040 and beyond.
In November 2007 ORNL officials announced that time-of-flight tests on a newly installed cold source (which uses liquid helium and hydrogen to slow the movement of neutrons) showed better performance than design predictions, equaling or surpassing the previous world record set by the research reactor at the Institut Laue–Langevin in Grenoble, France.Data suggest world record at Oak Ridge reactor, By Frank Munger, Knoxville News Sentinel, November 26, 2007
History
In January 1958, the U.S. Atomic Energy Commission (AEC) reviewed the status of transuranium isotope production in the United States. By November of that year, the commission decided to build the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory, with a fundamental focus on isotope research and production. Since it first went critical in 1965, the in-core uses for HFIR have broadened to include materials research, fuels research, and fusion energy research, in addition to isotope production and research for medical, nuclear, detector and security purposes.
A 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 five years from the start 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 lasting ~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 review and evaluation of the facility. After thorough reevaluation last over two years, 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 at 85 MW. 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 the U.S. Department of Energy (DOE), formerly the AEC. The primary coolant pressure and core power were reduced to preserve vessel integrity while maintaining thermal margins, and 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. After the April 1989 restart, a further shutdown of nine months occurred due to a question as to procedural adequacy. During this time, oversight of HFIR was transferred to the DOE Office of Nuclear Energy; previously, oversight was through the Office of Energy Research. Following permission by Secretary of Energy James D. 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.
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 new instruments were installed, including a cold neutron source. When the reactor was restarted, it attained its full power of 85 MW within a couple of days, and experiments resumed within a week. Improvements and upgrades 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, 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 detection of pollutants in the environment and explosives in luggage.
Technical description
HFIR is a beryllium-reflected, light-water-cooled and -moderated, flux-trap type reactor that uses highly enriched uranium fuel. The preliminary conceptual design of the reactor was based on the "flux trap" principle, where the reactor core consists of an annular region of fuel surrounding an unfueled moderating region or "island". Such configuration permits fast neutrons leaking from the fuel to be moderated in the island and so gives 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, allowing neutrons to be beamed into experiments outside the reactor shielding. A variety of holes in the reflector may be provided in which to irradiate materials for experiments or isotope production.
The original mission of HFIR was the production of transplutonium isotopes. However, the original designers included many other experiment facilities, and several others have been added since then. Experiment facilities available include:
Four horizontal beam tubes, which originate in the beryllium reflector. The hydraulic tube irradiation facility, in the very high flux region of the flux trap, which allows insertion and removal of samples while the reactor is running. 30 target positions in the flux trap, which normally contain transplutonic production rods but which can be used for irradiation of other experiments (two of these positions can accommodate instrumented targets). Six peripheral target positions at the outer edge of the flux trap Many vertical irradiation facilities of various sizes, throughout the beryllium reflector Two pneumatic tube facilities in the beryllium reflector, which allow for insertion and removal of samples while the reactor is running for neutron activation analysis Two slant access facilities, called "engineering facilities", on the outer edge of the beryllium reflector Spent fuel assemblies are used to provide a gamma irradiation facility in the reactor pool.
Reactor core assembly
The reactor core assembly is in an 8-ft (2.44-m)-diameter pressure vessel in a pool of water. The top of the pressure vessel is 17 ft (5.2 m) below the pool surface. The control plate drive mechanisms are in a sub-pile room beneath the pressure vessel. These features give the necessary shielding for working above the reactor core and greatly facilitate access to the pressure vessel, core, and reflector regions.
The reactor core is cylindrical, about 2 ft (0.61 m) high and in diameter. A 5-in. (12.70-cm)-diameter hole, the "flux trap," forms the center of the core. The target is typically loaded with curium-244 and other transplutonic isotopes and is positioned on the reactor vertical axis within the flux trap. The fuel region is made of two concentric fuel elements. The inner element contains 171 fuel plates; the outer element has 369 plates. The fuel plates are curved in the shape of an involute, providing constant coolant channel width. The fuel (93% 235U enriched U3O8-Al cermet) is non-uniformly distributed along the arc of the involute to minimize the radial peak-to-average power density ratio. A burnable nuclear poison (boron-10) is included in the inner fuel element primarily to flatten the radial flux peak providing a longer cycle for each fuel element. Average core lifetime with typical experiment loading is ~23 days at 85 MW.
The fuel region is surrounded by a concentric ring of beryllium reflector ~1 ft (0.3 m) thick. This in turn is subdivided into three regions: the removable reflector, the semi-permanent reflector, and the permanent reflector. The beryllium is surrounded by a water reflector of effectively infinite thickness. In the axial direction, the reactor is reflected by water. The control plates, in the form of two thin, nuclear poison-bearing concentric cylinders, are in an annular region between the outer fuel element and the beryllium reflector. These plates are driven in opposite directions to open and close a window at the core mid-plane. Reactivity is increased by downward motion of the inner cylinder and upward motion of the four outer quadrant plates. The inner cylinder is used for shimming and power regulation and has no fast safety function. The outer control cylinder consists of four separate quadrant plates, each having an independent drive and safety release mechanism. All control plates have three axial regions of different neutron poison content designed to minimize the axial peak-to-average power-density ratio throughout the core lifetime. Any single quadrant plate or cylinder is capable of shutting down the reactor.
The reactor instrumentation and control system design reflects emphasis on continuity of and safety of operations. Three independent safety channels are arranged in a coincidence system that requires agreement of two of the three for safety shutdowns. This feature is complemented by an extensive "on-line" testing system that permits the safety function of any one channel to be tested any time during operation. Additionally, three independent automatic control channels are arrayed so that failure of one channel will not significantly disturb operation. All these factors contribute to continuity of operation in HFIR.