Originally, ORNL was considered a nuclear lab. Now, it's an energy and environmental research lab, but our missions include protecting nuclear materials, producing electricity from nuclear energy, isolating the hazardous nuclear materials from the environment, and making the beneficial ones available for medical purposes.
Our nuclear programs have changed over the past half century. At first we were dedicated to helping the nation build an atomic bomb to end a terrible war. Then our major goal was to develop nuclear energy as a safe and reliable source of part of the nation's electricity. Now, our most pressing goals are to support the conversion of bomb-grade uranium and plutonium from U.S. and Russian weapons into fuel suitable for power-generating reactors—and to confirm that the conversion occurred.
Converting Nuclear Weapons to Energy
At the end of the Cold War, the United States and the former Soviet Union agreed to dismantle a large number of nuclear weapons, creating a surplus of plutonium and highly enriched uranium. Such large inventories of nuclear material in both countries are potentially dangerous. Outlaw groups could divert this material for use in making nuclear weapons.
Weapons-grade plutonium. Last year, based on the results of ORNL studies, the Clinton administration decided to pursue a dual-track strategy for dealing with weapons-grade plutonium. One approach is to immobilize some of it in glass or ceramic logs. The second is to mix the rest with depleted uranium to form a mixed oxide (MOX) fuel for use in power reactors.
"ORNL is participating in experiments with several national laboratories to demonstrate technologies needed for such a plutonium disposition program," says Gordon Michaels, director for Nuclear Technology Programs at ORNL. "This past year we designed and managed a program to test nuclear fuel fabricated with plutonium extracted from nuclear weapons."
MOX fuel may harbor small amounts of gallium contained in the plutonium. Gallium may interact with the reactor fuel's zirconium cladding enough to damage it. Thus, tests must be conducted to determine if a gallium problem exists. In addition, the effects on fuel performance of other unique isotopes found in weapons-grade plutonium must be evaluated. Also, it must be determined whether it is safe to use MOX fuel containing plutonium oxide powder produced from a metal alloy using a "dry" conversion process. This process is being considered, if sufficient purity can be obtained, because it may produce less waste than the "wet" plutonium-production process used in commercial MOX fuel fabrication facilities in Europe.
This past year, under ORNL management, reactor fuel made from weapons-grade plutonium was tested for the first time. Eleven fuel capsules containing MOX pellets were fabricated at Los Alamos National Laboratory. The capsules, each slightly larger than a pencil, were then taken to Idaho National Engineering and Environmental Laboratory, where they are being irradiated with neutrons for different lengths of time at the Advanced Test Reactor. The first set of partially irradiated capsules was shipped here in November 1998. An ORNL group led by Steve Hodge, who designed the capsule irradiation hardware, is now examining the first set of irradiated fuel. "So far," Hodge says, "we have seen no sign of harmful effects caused by gallium. But the full story won't be known until the planned irradiation is completed in 2000."
Weapons-grade uranium. To honor a bilateral agreement between the United States and Russia, Russia's uranium processing facilities are blending (diluting) the highly enriched weapons-grade uranium (HEU) from dismantled nuclear weapons with low-enriched uranium (LEU) to produce reactor-grade fuel. The goal is to make a blend that is below 5% uranium-235 that is suitable for use as nuclear fuel for power reactors. Under the agreement, the United States will purchase this material from Russia for use in U.S. nuclear power plants. But how can we be sure that the purchased blend actually came from dismantled Russian weapons rather than from enrichment facilities? ORNL has the answer.
Drawing upon our many years of experience in noise analysis, Jose March-Leuba, Jim A. Mullens, John T. Mihalczo, and others in the Instrumentations and Controls Division developed a technique using californium-252 neutron sources for activating the gaseous HEU streams as they flow through blending points in Russian facilities, thereby confirming that the blend-down is occurring. The technique measures the velocity and concentration of fissionable uranium-235 flowing into and out of the blending points, thus determining if the HEU gas mixture has been blended down to the desired LEU product level. ORNL engineers, under the leadership of Jim McEvers, designed and built these systems to not only accomplish the required measurements but also to comply with Russian facility safety and radiation regulations. The system was successfully demonstrated with flowing uranium hexafluoride gas in the uranium enrichment facility at Paducah, Kentucky. Under the continued leadership of Bill Sides, current plans are focused on installing and placing into operation three HEU flow monitor systems at Russian facilities at Novouralsk and Zelenogorsk. Two complete systems were installed at Novouralsk in January 1999. This work was performed in support of the U.S.-Russian Highly Enriched Uranium Purchase Agreement in which the Department of Energy's Office of International Nuclear Safety and Cooperation, HEU Transparency Implementation (NN-30), is responsible for implementation of the negotiated transparency measures at the Russian facilities.
|Schematic of ORNL's blend-down fissile mass flowmeter showing how highly enriched uranium is converted from weapons-grade to reactor-grade fuel.|
Identifying Nuclear Materials
When a nuclear weapon is dismantled, how do we know that all the fissile material has been removed? How can we be sure that the removed uranium-235 or plutonium-239 is present in the designated storage area? How can it be verified that a nation has no more nuclear weapons or nuclear material than it declares and that it is complying with bilateral treaties? How can a facility's managers be sure that a shipment contains the nuclear material ordered or that nuclear material is absent from a container? How can we be certain that highly enriched uranium is in a storage vault or that a train carload of spent nuclear fuel won't go critical, causing an inadvertent release of radiation?
|Eric Breeding holds the electronic board that is the heart of the more compact version of the nuclear materials identification system, while he and Jim Mullens examine its detector.
To answer these questions reliably, a group of ORNL researchers including Mihalczo, T. E. Valentine, J. K. Mattingly, Jim McEvers, and others have developed a nonintrusive nuclear materials identification system. It uses neutrons from a fissioning californium-252 source to induce fission in fissile material present in a target container; two detectors on the opposite side of the container detect the emitted gamma rays and neutrons, which indicate the type and amount of nuclear material present, if any.
"Over the years," says Mihalczo, an ORNL Corporate Fellow, "we have made this system more sensitive, more portable, and easier to use. This technology has been transferred to the Russians so they can use it to verify that fissile material has been removed from their dismantled nuclear weapons."
Therapeutic Isotope from
ORNL Stockpile of Uranium-233
ORNL has its own stockpile of uranium, which is deemed a potential boon to health. We are storing more than 400 kilograms of uranium-233, which had been intended for use as fuel in our old molten salt breeder reactor program and for tests of other reactor concepts. Uranium-233 is valuable because it decays and forms thorium-229, which, in turn, decays and forms actinium-225. We have learned to recover thorium-229 chemically to meet the demand for actinium-225. When placed at the top of an ORNL-developed radioisotope generator, actinium-225 decays to form a continuing supply of bismuth-213, a rare emitter of high-energy alpha particles, which is being investigated as a radioimmunotherapy agent for treating cancer patients. "So far," says Jerry Klein, who leads ORNL's isotope program, "early results with leukemia patients in clinical trials at Sloan-Kettering Cancer Center have been promising." ORNL's Brad Patton notes that, as the demand for the radioisotope rises both for medical research on animals and for the treatment of patients, the only significant source of bismuth-213 in the western hemisphere is ORNL's uranium-233 stockpile.
|Rose Boll, Greg Groover, and Dairin Malkemus discuss plans for extracting actinium-225 from the thorium column in a hot cell to meet a request of a research project sponsored by the National Institutes of Health. Inset: closeup of the thorium column (yellow tube) in the hot cell from which actinium-225 is produced for nuclear medicine research. |
Isotope Technique Helps Heart Patients
A heart attack may strike when a coronary artery has been narrowed by the accumulation of fatty deposits. Some heart patients are treated with balloon angioplasty to clear the blockage, widen the artery, and restore full blood flow to the heart muscle. But, six months later 30 to 40% of the 450,000 Americans who have the procedure each year face a different type of blockage. Their coronary arteries become reclogged by the buildup of smooth muscle cells in response to balloon-induced vessel damage, a condition known as restenosis. Thus, these patients must undergo additional angioplasty or heart bypass surgery to unclog their reclogged arteries. These second operations add $1 billion to the nation's health care bill.
One promising approach to preventing restenosis is to couple angioplasty with radiation from rhenium-188, which is expected to inhibit smooth muscle cell proliferation, based on animal studies. This isotope can be easily produced for hospital patients by a radiopharmacy-based tungsten-188/rhenium-188 generator system developed at ORNL under Russ Knapp, head of ORNL's Nuclear Medicine Group in the Life Sciences Division. The source of the tungsten-188 is ORNL's High Flux Isotope Reactor.
"The idea is to use a highly concentrated rhenium-188 solution to inflate the balloon at low pressure following high-pressure balloon inflation with saline to unclog the artery," Knapp says. "At ORNL we developed techniques for concentrating the rhenium-188. We provide generators that are being used for restenosis studies at various sites under physician-sponsored protocols approved by the Food and Drug Administration or similar regulatory agencies in other countries."
|Robert Spencer (left), a cardiologist in the Knoxville Cardiovascular Group, and Russ Knapp, head of the ORNL Nuclear Medicine Program, discuss their approach for a new collaborative project between the University of Tennessee Hospital and ORNL for inhibiting restenosis.|
An initial six-month follow-up study of the results of this experimental treatment for 25 patients in Perth, Australia, found that only 4% of the patients developed restenosis, or about one-tenth of the expected rate. The Nuclear Medicine Group considers these results encouraging.
Conversion Process Designed
In March 1994, analysis of two samples drawn from the off-gas piping at ORNL's long dormant Molten Salt Reactor Experiment (MSRE) yielded surprising results. The gas was saturated with radioactive uranium hexafluoride (~10% 233UF6), and 50% of it was fluorine, a highly reactive gas. This gas was found to have migrated to an adjoining underground charcoal bed where it formed potentially explosive compounds (partially fluorinated carbon) and a uranium deposit that posed a significant nuclear criticality risk. Employees working in the reactor building were immediately evacuated and relocated because of fears of a criticality incident or chemical explosion that could release U-233 to the environment.
Cleanup of Molten Salt Reactor
Since the discovery of this condition, a number of important steps have been taken by ORNL's Chemical Technology Division (CTD) to minimize the potential hazards posed by mobile uranium and fluorine. By the end of 1998 CTD had eliminated the possibility of a criticality accident or chemical explosion in the charcoal bed and had removed and trapped the volatile UF6 and fluorine in forms suitable for interim storage. Almost all of the volatile UF6 was removed and chemically sorbed on sodium fluoride (NaF) traps in secure containers. Configuration changes were made to eliminate the potential for nuclear criticality. A novel chemical treatment of the charcoal with ammonia (devised by CTD's Bill Del Cul, Mac Toth, Darrell Simmons, and Lee Trowbridge) removed the possibility of a chemical explosion.
A longer-term remediation activity is under way to address the causes of this unsafe condition so it won't appear again. Now that the uranium deposit has been stabilized, preparations are being made to remove it from the charcoal bed. Because of its exposure to radiation, the reactor fuel salt is still generating fluorine gas that could convert the uranium tetrafluoride (UF4) in the fuel to additional UF6. The reactive-gas-trapping technology used in the first remediation phase will support the eventual stripping of the remaining uranium from the fuel salt. In the final remediation phase, the fuel salt will be stripped of its uranium, removed from the reactor system, and packaged for final disposal.
The final technical challenge confronting ORNL is to make the U-233 packages generated by the remediation suitable for long-term storage. The UF6 sorbed on NaF traps and the uranium deposit on the charcoal bed must be converted to a stable oxide prior to storage. Conventional aqueous processes for converting fluorides to oxides are not appropriate for highly radioactive MSRE uranium. To efficiently meet the need for a highly contained system that prevents uranium losses and produces little secondary waste, a CTD team (Del Cul, Alan Icenhour, Simmons, and Jeff Rudolph) developed an integrated conversion process that will begin operating in Building 4501's hot cells in August 2000.
In a closed system, the UF6 will be recovered from either an NaF trap or a charcoal deposit batch and then converted to a stable uranium oxide (U3O8). The converted MSRE uranium will be stored at ORNL's U-233 repository.
Novel Ion Exchange Resin Removes
ORNL and UTK researchers have prepared a novel material to cleanse groundwater of two persistent pollutants, one of which comes from nuclear operations. They have developed the BiQuat bifunctional anion exchange resin, which effectively and selectively removes trace levels of two hazardous groundwater contaminants, pertechnetate (TcO4-) and perchlorate (ClO4-). The new resin can cleanse five times more groundwater than the best water-treatment resin on the market.
These groundwater contaminants are present at parts-per-billion concentrations and as negatively charged ions, or anions. Pertechnetate and perchlorate at trace levels are not efficiently removed by routine cleanup methods.
The leader of the resin development effort funded by DOE was Gilbert Brown, of ORNL's Chemical and Analytical Sciences Division (CASD). His collaborators are Baohua Gu at ESD, Spiro D. Alexandratos of the University of Tennessee at Knoxville, and Peter V. Bonnesen and Bruce Moyer, both of CASD.
Because the BiQuat resin is so selective, it removes both compounds, making the treatment process more efficient and cost effective. It does not alter the water quality by adding undesirable secondary by-products or removing desirable minerals. "The resin is bifunctional," Brown says, "because it has two different kinds of exchange sites. One site is highly selective for pertechnetate anions and the other swaps chlorine ions quickly with the anions the first site attracts."
Pertechnetate is the chemical form of radioactive technetium-99, a fission product of enriched uranium used to fuel research, production, and power reactors. This beta emitter, which has a half-life of 213,000 years, is present in groundwater at many DOE sites, including Paducah, Kentucky; Portsmouth, Ohio; and Hanford, Washington.
Perchlorate, which comes from solid rocket propellants, is present in many groundwater contaminant plumes and surface water in California, Nevada, and other parts of the United States. Perchlorate, which has a chemical structure and properties similar to those of pertechnetate, is just now being recognized as a groundwater contaminant of concern because potentially it poses a threat to living organisms. Lockheed Martin Corporation came to ORNL for help in solving the problem.
A recent field trial demonstrated that the BiQuat resin can remove 60 parts per trillion pertechnetate and 50 parts per billion perchlorate to below detection limits. Larger-scale field studies using resin are being conducted by Purolite International to gather data to aid the design of a longer-lasting resin.
ORNL has made progress in finding ways to identify and protect nuclear materials, remove them from the environment, and convert them to energy and medical treatments.