|Fig. 1: HFIR cross-section. Source: Wikimedia Commons
For decades, the United States has depended on the High-Flux Isotope Reactor (HFIR) for fundamental insights into condensed matter and the production of certain radioisotopes. Located at Oak Ridge National Laboratory (ORNL), the HFIR was planned and constructed during the 1960s. Its primary objective was to meet the nation's need for transuranium isotopes. Its reactor design, based on the "flux trap" principle, employs an annular, outer region of fuel that surrounds an inner moderating region. The water-cooled core is then encapsulated by a concentric beryllium reflector. This setup captures thermalized neutrons in a reservoir inside the reactor and allows them to be used for isotope production. In fact, the design of the HFIR reactor was originally optimized for producing transplutonium isotopes. Most notable among these is Californium-252. At a steady-state neutron flux of 85 MW, it has become one of the most powerful (85 MW) research reactors in the world .
In addition to the thermalized neutrons "trapped" for isotope production, the HFIR can conduct scattering experiments with the neutrons that escape the reactor shielding into the surrounding fuel. These particles are captured via one of four beam tubes that are located in the reflector region. Holes have also been incorporated into the reflector to allow some of these scattered neutrons to be used for materials irradiation studies and neutron activation analysis
Despite the over 400 fuel cycles (21-27 days each) completed over the past 40 years, the HFIR has only experienced two extended shutdowns. The first, during the late 1980s, occurred when neutron irradiation damage to the reactor vessel led to a multi-year shutdown of the HFIR. This lull in research, however, allowed for a period of re-evaluation of the procedures used at the facility. The primary result was a decrease in HFIR's operational power level, from 100 MW to 85 MW, as a way to considerably extend the service lifetime of the reactor .
A more dramatic, though less lengthy, period of modification occurred in 2007. In addition to general improvements in hardware and instrumentation, a liquid hydrogen cold neutron source was installed into beam tube number four (HB-4). This re-circulating coolant system, which moderates neutron energies by colliding thermalized neutrons with cold hydrogen atoms, creates a population of neutrons at much lower energies. These are then directed to one of four separate guides, each containing their own specialized instrumentation. These lower energy particles are ideally suited for long length-scale structures such as polymers, nanophase materials, and biological samples. As a result of this profoundly significant jump in research capabilities, the focus of HFIR shifted from isotope production and materials irradiation studies to neutron scattering using both thermalized and cold neutrons .
Currently, the HFIR has 14 ongoing experiments contained in its four beam tubes. These investigations cut across a variety of disciplines ranging from physics, chemistry and materials science to biology and medicine.
Isotope production has continued to be an area of research at the HFIR. The materials generated here have been to an ever-growing number of scientific fields. The HFIR continues to be the world's largest supplier of Californium-252, Einsteinium-253, and Berkelium-259. The high-flux beams also allow for lighter, more specialized isotopes, such as Tungsten-188, Holmium-166, and Lutetium-177 to be produced. This program also aided Russian researchers in the discovery, and now production, of the new element number 117 .
Irradiation studies have also remained an ongoing endeavor. These include the use of spent fuels from the HFIR core for gamma irradiation experiments. Projects have looked at the effects of long term exposure on materials, treatments for the AIDS virus, and the evaluation of hardware for nuclear applications. The breadth of the neutron spectra offered by the reactor core and reflector setup also allows for a variety of non-gamma irradiation studies to be performed.
However, modern improvements to the HFIR have shifted the primary focus of this facility from isotope production and irradiation studies to neutron scattering. Such investigations look at a variety of phenomena and research needs as identified by the scientific community. As of September 2010, the suite of instruments installed in the beam tubes and their research areas are briefly summarized below . These experiments can best be grouped by the tube they occupy in the HFIR facility.
Beam tube one (HB-1) is specifically designed to allow for polarized beam experiments. Studies of the magnetic excitations in materials with colossal magneto-resistance and high-temperature superconductivity are conducted using spectrometers. HB-1 can also be used for non-polarized studies of materials ranging from alloys and films to quantum materials. High-temperature condensed matter investigations are also performed to investigate crystallographic or magnetic structures and transitions of materials .
The second beam tube (HB-2) contains three experiments that use diffractometers. The applications of these studies, however, are quite varied. One setup explores the structural properties of powdered samples such as catalysts, conductors, ceramics, and radioactive waste forms. A second set of instruments uses neutron diffraction to look at time-resolved phenomena such as phase transition, structural fluctuations, and crystallization. The third setup endeavors to use neutron scanning to quantify the residual stresses in engineering materials. This non-destructive technique has been used in both materials selection and as an effective diagnostic tool .
Beam tube three (HB-3) features both spectrometry and a diffractometry. A high-flux thermal neutron spectrometer is used to study energy and momentum transfers across a range of scales in single crystal structures. The diffractometer is used for problems encountered in neutron crystallography, such as structure refinement and charge or nuclear density mapping .
The cold beam source, located in tube 4 (HB-4 or CG-1), can be split into four separate beams. The test stations, used in conjunction with this source, are primarily used to develop new instrument concepts. These stations have also looked been applied to neutron imaging. Additional experimental setups in this tube explore the structural evolution of industrial and manufacturing equipment. Additional studies have focused on magnetic effects in low excitation materials. Cold neutrons have also been applied to biology as a way to investigate the properties of biofuels . Medical research has found its way into research portfolio of the HFIR. Low energy neutrons have been used as a way to evaluate pharmaceutical casings at the molecular level as well as in the search for cures to diseases such as Huntington's .
Neutron activation analysis, a non-destructive way to measure trace elements in materials, has also become a popular source of experiments. This field has lent itself to investigations in forensics, nuclear nonproliferation, and fundamental condensed matter research .
Despite the reactor's age, it is estimated that over a billion dollars would be required to construct a new facility capable of fully replacing the HIFR . Hence, millions of dollars in upgrades are scheduled to keep it operational for the foreseeable future. Many of these improvements and additions are already planned. Most significantly, the incorporation of a second cold source in the most intense neutron beam tube available at HFIR would greatly increase the number and scope of experiments provided to scientists . Near term research also promises to be closely coupled with investigations planned at the Spallation Neutron Source (SNS). This world-class, accelerator-based pulsed neutron source, also located at ORNL, has become a premier neutron science facility. Working together, these two facilities offer a unique neutron research capability that could produce significant discoveries for the next several decades.
© Zachary Vane. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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