Fuel Cycles and Radioactive Waste
Fuel Cycles and Radioactive Waste
Radionuclide Transport in Disturbed Zone between Engineered and Natural Barriers of Deep Geologic Disposal of High-Level Radioactive Wastes
Geological disposal systems for high-level radioactive wastes confine radioactivity by engineered barriers and the natural barrier. Engineered barriers usually consist of multiple components, such as waste solid, metal canister, and a certain backfill material such as bentonite. The natural barrier is the thick geologic formation, such as granitic
rock, rock salt, and sedimentary rock.
In the host rock adjacent to the engineered barriers, i.e., the near field, disturbances will be introduced during the construction and waste-emplacement period. Geochemical, hydrological, and rock-mechanical properties should be different from those of undisturbed host rock. The near field is important because this region directly affects the hydrological and geochemical environment of the engineered barriers and behaves as a barrier through which residual radioactivities might be weakened to sufficiently low levels.
In this project, rock fracture networks are generated in a computer based on the statistics of fractures in a host rock, and then a water-flow analysis through the fracture network and a mass transport analysis are performed. The output of the model is the radionuclide
release rate across the boundary of the near field to the far field around the repository. Bentonite is a material that may be used for backfill of the excavated disposal pit. Bentonite can affect the chemistry of water in the engineered barrier strongly, and swell into fractures. Thus, the bentonite expansion into fractures is also modeled to determine the environment for radionuclide transport in the near field. These models are integrated into a repository-wide mass transport model by applying an object-oriented approach
A mass flow analysis for the system is performed to determine the fraction of each radionuclide lost as waste from the system that must be disposed of in a geologic repository. After the masses of radionuclides to be disposed of in the repository, a detailed mass transport analysis is performed to quantify the environmental impact of the buried radionuclides to the public. The results can be compared with the environmental impact for the case where the spent fuel is to be disposed of in the repository without any transmutation of radionuclides.
The results of the analysis can be applied to develop technological solutions for repository capacity expansion with acceptable environmental impact, and for optimizing nuclear fuel cycles in view of environmental impact minimization.
This area of research involves studies of different alternatives for the transmutation of the potentially hazardous long-lived isotopes produced in the nuclear fuel in the process of generation of nuclear energy. By “transmutation” we mean conversion of the hazardous long-lived isotopes to non-radioactive or to short-lived isotopes. Two general types of reactors are being considered: critical reactors and accelerator-driven sub-critical reactors. The latter use a combination of a high-energy proton accelerator and a sub-critical core that is “driven” by accelerator-generated neutrons. The accelerator generates a beam of protons that have hundreds of MeV of energy. These very high-energy protons impinge on a heavy target such as lead and generate, via spallation reactions, several dozens of fission-like neutrons per proton. The thrust of our research is to search for reactor core (either critical or accelerator-driven sub-critical) design and fuel cycle that will maximize the benefit from the transmutation. The impact of the transmutation on the expected performance of the Yucca Mountain Repository (YMR) is being assessed as well. The ultimate goal is to minimize the nuclear waste to such an extent that will eliminate the need for repositories other than the YMR. Participating in this work are Professors Ahn, Vujic
While nuclear electricity generation in the United States has not increased for more than a decade, nuclear energy capacity is steadily increasing in far-eastern Asian countries such as Japan, South Korea, and Taiwan. Large-scale nuclear development is started in China.
Nuclear activities in this area will certainly influence the global energy balance and our environment, as well as the US nuclear industry and national policies on non-proliferation. For the last three years, symposia were held once a year with close cooperation with Tokai
University of Japan on this subject.
The radioactive waste and materials management program includes development of chemical and nuclear processes for better waste treatment, development of waste disposal technologies, long-term performance assessment for disposed wastes, and institutional and international-political analyses. Institutional and international-political aspects can be studies in collaboration with the Center for Nuclear and Toxic Waste Management, whose activities are led by faculty from public policy, law and political sciences as well as Nuclear Engineering Faculty. Participating in this work are Professors Kastenberg, Peterson
Overview: Nuclear stewardship will require long-term management of increasing quantities of nuclear materials. In addition to protecting the environment and public health by preventing release of nuclear materials during use and following disposal, two additional issues have received less study: the potential for accumulation of critical configurations of fissile material and the subsequent response, and safeguards and security issues for geologic repositories.
Flammable gases can be generated in DOE high-level waste tanks, including radiolytic hydrogen, and during cesium precipitation from salt solutions, benzene. Fig. 1 shows a typical underground high-level waste tank found at the Savannah River Site. Under normal operating conditions the potential for deflagration or detonation from these gases is precluded by purging and ventilation systems, which remove the flammable gases and maintain a well-mixed condition in the tanks. Upon failure of the ventilation system, due to seismic or other events, however, it has proven more difficult to make strong arguments for well-mixed conditions, due to the potential for density-induced stratification which can potentially sequester fuel or oxidizer at concentrations higher than average. This has complicated the task of defining the safety basis for tank operation, and in the
case of cesium precipitation, has led to delays in cesium processing which are quite expensive. Improved tools for predicting tank mixing processes following loss of ventilation, coupled with mixing experiments designed specifically for DOE waste tank conditions, have the potential to both strengthen the safety basis for tank operation and to prevent schedule delays in tank operations.