Sunday, May 11, 2014

Transmutation: Making Wastes Nonradioactive

Partitioning and Transmutation: Making Wastes Nonradioactive 

By Gordon E. Michaels, Chemical Technology Division, Oak Ridge National Laboratory

The word transmutation originates from the never-realized goal of ancient alchemists to transform, or transmute, the base metals into gold. Today scientists seek ways to transmute radioactive waste into nonradioactive elements, thereby eliminating the radiological hazards and waste disposal problems.

In accelerator transmutation of waste, an intense source of neutrons transmutes long-lived radioisotopes in less harmful waste.

An example of two radioactive isotopes that can be transmuted into less hazardous forms are technetium-99 and iodine-129. Both of these isotopes are very long-lived and require disposal strategies that will isolate them from the environment for long periods of time. Both iodine and technetium are considered difficult to isolate because they dissolve readily in groundwater and move easily throughout the ecosystem. Irradiation of the long-lived technetium-99 isotope by neutrons will cause it to absorb a neutron and become technetium-100, which undergoes complete radioactive decay into stable ruthenium within minutes. Similarly, the iodine-129 isotope can be transformed by neutron absorption into stable xenon isotopes.

Another class of radioactive wastes that can be transmuted into less hazardous forms are the actinide elements, particularly the isotopes of plutonium, neptunium, americium, and curium. When irradiated with neutrons in a nuclear reactor, these isotopes can be made to undergo nuclear fission, destroying the original actinide isotope and producing a spectrum of radioactive and nonradioactive fission products. Isotopes of plutonium and other actinides tend to be long-lived with half-lives of many thousands of years, whereas radioactive fission products tend to be shorter-lived (most with half-lives of 30 years or less). From a waste management viewpoint, transmutation of actinides eliminates a long-term radioactive hazard while producing a shorter-term radioactive hazard instead.

A challenging aspect of this waste management strategy is the required waste partitioning. Just as household wastes must be partitioned into categories, such as paper, glass, and aluminum, before they are recycled, radioactive waste must also be sorted before being recycled back into nuclear reactors.

One particularly challenging partitioning task involves the actinide and lanthanide (rare earth) elements. Actinide and lanthanide elements are chemically similar and, thus, very difficult to separate efficiently. Most lanthanide isotopes are nonradioactive, and the few radioactive lanthanide isotopes are long-lived, so there is little incentive to invest neutrons in transforming them into stable elements. However, lanthanide elements tend to absorb neutrons efficiently (they are so-called neutron poisons) and will prevent the efficient transmutation of americium and curium if they are intermixed. Improved methods of separating lanthanides from actinides are needed to reach the goal of actinide transmutation.

The largest program at ORNL focusing on partitioning and transmutation is investigating methods for accelerator transmutation of wastes (ATW). Conceived by scientists at Los Alamos National Laboratory, ATW uses a linear accelerator system to produce neutrons for transmutation of excess weapons plutonium and other radioactive DOE wastes, such as technetium-99 and iodine-129. Activity at ORNL is centered on developing chemical separations technology, including processes for performing actinide-lanthanide separations, and on studying, designing, and ultimately testing the reactorlike flow loops in which the transmutation occurs.

A second program is developing technology for using a nuclear reactor to transmute the actinides in spent nuclear fuel from light-water reactors (LWRs). This program offers an alternative to direct disposal of LWR fuel in geological repositories. ORNL researchers are developing techniques for the front-end processing of LWR fuel to prepare it for introduction into the chemical partitioning system and are examining wasteform technologies for immobilizing some of the unique waste streams produced during the partitioning process. Additionally, we have performed several studies to characterize the benefits and disadvantages of recycling spent nuclear fuel.

Recently, interest at ORNL has turned inward to see whether partition-ing and transmutation offer any near-term advantages for management of some of our own radioactive wastes. Transmutation of highly radioactive europium isotope wastes (currently stored at ORNL solid waste area groups) into nonradioactive gadolinium isotopes appears to be practical and is being studied further. These europium wastes pose the dominant health risk to the public in certain environmental scenarios, and their elimination might be beneficial to the Laboratory. As proposed, the transmutation device would be the High Flux Isotope Reactor or the proposed Advanced Neutron Source. The physical and chemical partitioning of the radioactive europium from nonradioactive europium and gadolinium represents a key technology that must be developed for this task.

Ultimately, the potential of partitioning and transmutation to waste management is this: If a radioactive waste stream no longer exists, then it poses no radiological hazard. More than anything else, this simple fact has spurred the recent resurgence of interest in partitioning-transmutation technology.


Related Story: Accumulation Of Nuclear Waste Turning Into Atomic Time Bomb

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"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars." -- Carl Sagan

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