Although the United States pioneered in the development and use of early neutron sources, the Europeans and Japanese have capitalized on this early experience and developed newer sources that have been the best in the world for the past 15 to 20 years. In the meantime, the research uses and practical applications of neutron sources have exploded in science and industry. The need for a brighter source of neutrons in the United States is urgent, and because it will take at least six to ten years to design and construct such a source, it is essential to start now to meet the nation's future needs in neutron science.
The Department of Energy has worked closely with the science community since the early 1970s in planning for the future needs for neutron sources. In the 1993 report of DOE's Basic Energy Sciences Advisory Committee (BESAC), Neutron Sources for America's Future, the scientific community strongly recommended to DOE the construction of the Advanced Neutron Source (a new steady-state reactor) and a complementary spallation neutron source. The report eloquently summarized the broad uses of and future opportunities for neutrons in science and technology. In the 1996 budget, DOE recommended cancellation of the Advanced Neutron Source project because of the cost ($2.9 billion) and provided $500,000 to ORNL in FY1995 for initial scoping studies on the spallation neutron source. In a reassessment of priorities, BESAC recommended in 1996 the design of an accelerator-based spallation neutron source that could begin operation at a beam power of approximately 1megawatt. The design should be sufficiently flexible that it could be upgraded to significantly higher powers in the future to meet the continually growing needs for an intense source. The report also recommended that the source be highly reliable, that it have high availability, and that it possess the inherent design capability to provide future capabilities for the neutron user community.
In the FY1996 budget, DOE recommended and Congress appropriated $8 million to begin work on this new source. DOE directed the funds to ORNL to initiate research and development (R&D) and a conceptual design. The department has since provided an additional $8 million in the FY 1997 budget to complete the conceptual design for the new spallation source. On September 6, 1996, Vice President Gore announced that President Clinton would recommend $23 million to Congress for FY 1998 to accelerate design of this new spallation neutron source. These funds have been included in the 1998 DOE budget requests, and DOE's Office of Energy Research has designated neutron science as a top priority for FY 1998.
Spallation Source Collaboration
To complete the design of the proposed $1.33 billion neutron source, ORNL has organized a collaborative design effort called the National Spallation Neutron Source (SNS) project, which involves ORNL, Argonne National Laboratory (ANL), Brookhaven National Laboratory (BNL), Lawrence Berkeley National Laboratory (LBNL), and Los Alamos National Laboratory (LANL). Other laboratories, U.S.industrial firms, and universities are already involved in the conceptual design, and more will join as the project matures.
This truly collaborative design effort is a relatively new approach to designing and constructing a major DOE project. Although ORNL has responsibility for coordinating the design and construction and will ultimately operate the SNS, the other participating laboratories are totally responsible for designing, constructing, and delivering their parts of the system for the final facility. The project focuses on the strengths of the individual DOE national laboratories and avoids costly duplication of expertise and subsequent downsizing by the lead laboratory when the project is finished. This collaborative approach was taken to gain access to the best technical expertise available, to initiate a conceptual design process that made the most efficient use of DOE manpower and resources, to incorporate results of a number of previous feasibility studies conducted by the collaborating laboratories, and to consolidate community consensus. In addition, memoranda of understanding have been signed with the European Spallation Source and several European laboratories that will allow access to research results and technology developments that could further leverage the SNS design effort. Similar cooperative technology-sharing arrangements have been initiated with laboratories in Japan, as well.
The SNS Design
The reference design proposed for the SNS consists of a high-energy linear accelerator (linac) injecting negative hydrogen ions from an ion source into an accumulator ring that produces short bursts of protons at extremely high energies and power levels. These proton pulses will be directed onto a heavy metal target, which will be a container of liquid mercury. In the spallation process, protons interact with and heat the mercury nuclei, which then release neutrons. The free neutrons will be moderated and guided through beam lines to areas containing special instruments. There the neutrons can be used in a wide variety of experiments.
BNL, LANL, and LBNL are responsible for the accelerator system design, which will be coordinated by an accelerator design group at ORNL. LBNL is responsible for the ion source and front-end system that produces the hydrogen ion beams. LANL is responsible for the linac, which accelerates this beam to energies of 1 billion electron volts (GeV). BNL is responsible for the accelerator ring, which bunches and intensifies the ion beam into proton pulses of about 1 microsecond in duration and then delivers these pulses onto the heavy metal target to produce the neutron beams. ORNL is responsible for the coordination, installation, and operation of the new source; for project management; conventional construction; design and construction of the liquid mercury target; andwith ANLthe experimental facilities and instruments that will be used by the neutron science community.
The goal of the SNS is to complete the conceptual design in time for a construction line item request to DOE for FY1999. The conceptual design of the SNS will take three years to complete, and assuming a 1999 construction start, it should be in operation by the year 2005. The completed facility will occupy a site of approximately 110 acres. The SNS will produce the highest-flux, pulsed, neutron beams in the world for neutron scattering. The SNS is designed to make future upgrades to higher powers extremely cost effective, so that the United States should be able to stay at the forefront of neutron science for many years to come.
Joint Institute for Neutron Sciences
The Joint Institute for Neutron Sciences will provide meeting facilities, offices, laboratories, and housing for researchers throughout the world.
It is estimated that the SNS will attract 1000 to 2000 researchers each year from U.S. universities, industries, government laboratories, and other nations. The governor of Tennessee has committed $8 million for ORNL and the University of Tennessee (UT) to construct a Joint Institute for Neutron Sciences (JINS). Funds were included in the state of Tennessee's FY1996 budget to begin design of the JINS. This facility will enhance the utility of the SNS and ORNL's High Flux Isotope Reactor by providing meeting facilities, offices, laboratories, a communication center, and housing for scientists and engineers from universities, industries, and the international community. It will also be a focus for expanding neutron science R&D with UT, other regional universities, and industrial collaborators. It will serve as an interface and economic-development gateway for outside access to ORNL facilities.
Now that the intensity of activities to restore U.S. leadership in neutron science has picked up, we hope to stay on the path to many new scientific breakthroughs.Bill Appleton, director of the National Spallation Neutron Source.
ORNL's High Flux Isotope Reactor (HFIR), which provides the highest thermal neutron flux in the world, is a national resource that has missions in four important areas: radioisotope production, neutron scattering research, neutron activation analysis, and irradiation testing of materials. To perform these important missions even better, modernization of the HFIR is a must.
The reactor was originally designed primarily to produce the transplutonium isotopeselements that do not occur naturally on earth, beyond plutonium in the periodic table of elements. However, over time, the neutron scattering mission has grown in scientific and economic importanceproviding, for example, much of our knowledge about the molecular and magnetic structure and behavior of the new high-temperature superconductors. The package of upgrades (some already under way) would enhance our capabilities in all four areas, but the biggest impact would be on neutron scattering.
One enhancement proposed is a return to the original 100-megawatt (MW) design power level instead of the 85-MW level that has been used since 1989. This change will benefit all missions, because the neutron production rate, which is proportional to reactor power, will be increased by nearly 20%. Likewise, modernizing instrumentation and electrical equipment and replacing certain other components of the reactor system will reduce maintenance downtime, making more operating time, and hence more neutrons, available for all users.
Neutron Scattering Improvements
For neutron scattering, a cold source will be installed in one of the reactor's four beam tubes. The source will be an aluminum chamber of very cold hydrogen, at a temperature of only 20 K (about 420°F and 253°C). At this temperature, the hydrogen molecules move fairly slowly, so neutrons that enter the chamber are slowed down by collisions with those molecules. These very slow neutrons have a larger wavelength than the faster ones, so cold neutrons are more useful for studying large molecules, including big polymer chains (plastics) and biological structures (e.g., proteins). The best cold neutron beams in the world are those at the Institut Laue-Langevin (ILL) in France, but with higher power at the HFIR (85100 MW vs 59 MW at ILL) and new cold source design concepts, our cold beams will be as bright or brighter. Although space limitations mean that we will have fewer beams and instruments, the ones we do have will be world beaters.
The second change will be to install larger beam tubes for neutron scattering and to put neutron guides at an existing beam port called HB-2. Neutron guides work exactly like fiber-optic guidesthey are rectangular conduits, typically about 5 × 18 centimeters (2 × 7 inches), whose inside surfaces are coated with a layer or, for greater effectiveness, multiple layers of a material (e.g., nickel) that will reflect neutrons that strike the surface at a glancing angle. Thus, the guides can bring neutrons from close to the reactor, in a series of ricochets, to a distant instrument with little loss of neutrons. This is important for two reasons: first, there isn't enough room close to the reactor for many big instruments. Second, close to the reactor is considerable background radiation, including gammas and fast neutrons, that tend to swamp the small signals scientists are looking for. However, the guides reflect only fairly slow neutrons, so this undesirable background radiation escapes through the wall of the guide, where it can be absorbed by shielding, before reaching the experimental sample and the detector at the far end. These guides will provide additional spaces for instruments in the HFIR beam room. Eventually, we hope to extend the guides outward from the reactor into a new "guide hall" where up to nine additional neutron scattering instruments could be placed.
The HFIR already has the most intense beams of thermal neutrons (those that have slowed down to the equivalent of room temperature or so) in the world. The enlarged beam tubes and guides will make them brighter still and, by increasing the number of beams and instruments, will also raise the number of users that can be accommodated.
Neutron Activation Analysis Upgrades
Neutron activation analysis (NAA) is a sensitive, quick technique for determining the elemental composition, including trace elements, in a sample (such as contaminated soil in a small box). In general, the sample is inserted in the reactor for a predetermined, usually short, period: The intense neutron bombardment will make many of the elements radioactive. The elements can be identified by detecting the characteristic signatures in the emitted artificial radioactivity. An important example is the analysis of samples from the floodplain of East Fork Poplar Creek in Oak Ridge for environmental pollutants such as mercury. The upgrade proposal includes more, and larger, facilities that will allow improved access to the reactor so that measurements of more irradiated NAA samples can be taken.
The HFIR, because of its very high neutron flux, is able to produce radioisotopes of very high specific activity. Some are needed for diagnostic and therapeutic medical applications (including treatments for patients having ovarian or prostate cancer). Other isotopes find industrial applications, for example, iridium-192, which is used as a source of gamma rays for the radiographic inspection of welds. The upgrade proposals include additional irradiation positions and an improved neutron spectrum for isotope production.
Materials Testing Capabilities
The development of new fusion and fission reactor systems depends on the development and testing of materials that can retain their strength, ductility, and shape, among other characteristics, even under intense radiation. Because of its high flux and versatility, the HFIR is the preferred reactor to conduct these materials tests. We propose to provide improved capabilities for handling and dismantling the test capsules.
Because neutrons are now so important to so many fields of science and engineering, the upgrade program has been carefully planned to minimize the time the HFIR will be out of service. It is planned that much of the new equipment will be installed during a planned outage of HFIR, expected to begin in 1999 to replace a large beryllium neutron reflector, an essential part of the reactor.
The HFIR is a research reactor whose operations are essential to the nation. It provides some of the world's best capabilities for isotope production, neutron scattering, NAA, and materials irradiation testing. The planned upgrades will make the world's highest-flux research reactor even better.Colin D. West, director of the Neutron Sciences Program Office at ORNL.