Research Activities
At
Nuclear Waste Research Group
Department of Nuclear Engineering, University of California, Berkeley
June 1999
Nuclear waste issue has become very controversial and difficult in any country of the world, where nuclear materials are being utilized either for civilian or military purposes. In the nuclear waste research group at UC Berkeley, studies are being performed by focusing mainly on nuclear wastes from commercial utilization of nuclear power. The research interest can be divided into three categories.
In one category, efforts are being done to understand mechanisms of waste isolation by geologic disposal. The studies in this area are directly contributing for realization of permanent geologic disposal of radioactive wastes that already exist. We are developing performance assessment models for geologic disposal at different levels of scale; from one waste canister to the entire repository, and to the surrounding region around the repository. Radionuclide transport is the major subject for study, because risk may arise due to redistribution of radionuclides disposed of in the repository over a long time period.
In the second category, efforts are being done to search for technologies with which waste generation itself could be significantly reduced, resulting in significantly smaller risk from nuclear energy utilization, especially from geologic disposal. To achieve this goal, we reconsider the current fuel cycle scheme. The spent nuclear fuel from current light-water reactors (LWRs) still contains significant amount of fissile materials. The current reprocessing process for spent nuclear fuel extracts only uranium and plutonium, leaving most actinides in the waste stream. There is a possibility of reducing risk arising from geologic disposal by recovering and utilizing actinides as energy sources and transmuting long-lived fission products into short-lived species. To optimize the material recycling system in terms of environmental impacts, we need to understand better the geologic repository system, which is required with any recycling systems.
Impacts of a next-generation nuclear power system will not be so important if the system is not suitable for as many countries as possible to adopt. Economy in Asia/Pacific region is expanding rapidly, emitting huge amount of carbon dioxide. Major oil exporters in this region will turn to be importers in the next decade. In order for those newly-emerging countries to adopt nuclear energy, nuclear power systems must satisfy conditions such as ease of utilization, competitive cost, and acceptable environmental risk. Such conditions are being clarified by communicating researchers in this region.
Detailed descriptions follow.
Any geologic disposal systems that have been proposed so far consist of multiple barriers around the waste form. The inner-most barrier is the waste form, which contains initially all radioactivities in some solidified form. The waste form is encapsulated in a metal canister. Sometimes, there is an overpack around the canister. The canister is placed in a repository tunnel, a few hundred meters below the surface. The canister may be surrounded by some backfill material to prevent water penetration and to maintain favorable chemical environment for a long time period. The system including the waste form, the canister, the overpack if any, and the backfill is called the Engineered Barrier System (EBS).
A repository would contain 10,000 to 50,000 canisters. Because canisters generate intensive heat in early times, canisters are placed in the repository at a certain spacing in order to meet temperature criteria imposed on the materials in the EBS. The repository has an areal extent of a few kilometer by a few kilometer.
We categorize the repository safety into three kinds. The first one is the radiological safety. For a long term, radionuclides in the repository may be released and transported by groundwater, which will eventually enter the food chains reaching human beings. Radiological safety can be measured by the exposure dose rate caused by radionuclides ingested or inhaled by human beings. The second kind is the criticality safety. In the waste, fissile radionuclides exist. If material configuration alters into a critical configuration, there may be energy release by fission chain reactions. This scenario can be studied by transport and accumulation of fissile materials. The third kind is attractiveness to proliferating countries and groups. In the wastes, there exist weapons-usable materials. Inspection by IAEA may be necessary to make sure that no weapons-usable materials are recovered for weapons production, if the repository contains significant amount of weapons-usable materials.
Thus, for safety assessment of geologic disposal, it is the key to analyze spatial distribution of radionuclides that are initially confined in waste canisters as a function of time.
Engineered barrier performance
In this area, we study radionuclide migration in the area containing one waste canister, the surrounding EBS and near-field rock. The near field rock is the host rock in the vicinity of the EBS. This region of the host rock may be disturbed by excavation of tunnels, and have different hydrological and geochemical conditions as the undisturbed host rock has.
Radionuclide transport in EBS and near-field rock
More fractures would be added into the near-field rock. Fracture network is generated based on statistics of fractures in the host rock. Percolating fracture clusters are identified, which connect the EBS surface and the regional water flow around the repository. Water is supplied into the EBS through percolating fracture clusters. Water flow through the percolating fracture cluster is analyzed by applying the finite element method.
Based on the water flow field obtained, radionuclide migration is simulated. Sorption retardation and radioactive decay are included. This model gives the release rate of radionuclides into regional groundwater flow. Analyses are made by analytical solutions in case the transport medium is assumed continuous (Ref. A1(2), A1(9), A1(11), B1(7), B1(16), B2(9), B2(11), B2(19)) or by numerical schemes for discrete fracture networks (Ref. A1(6), A2(4), A2(6), B2(15), B2(16), B2(18), B2(24), B2(25)). Geochemical reactions are taken into account in some analyses (Ref. A1(7), A1(15), A2(2), A2(9), B2(30)).
Long-term behavior of bentonite backfill (Ref. A2(18), B1(18), B1(20))
If the EBS is backfilled by clay mineral such as bentonite, it is expected that the backfill material will swell by water uptake. Fractures intersecting the EBS will be filled with extruding bentonite, which may function as an additional barrier to the EBS. A simulation study is being performed for bentonite extrusion in fractures.
Repository-wide performance model (Ref.
A2(14), A2(17))There are thousands of waste canisters in a repository. For the overall repository performance assessment, we need to integrate the models developed for the one-canister configuration into a repository-wide model.
An object-oriented code has been developed for such integration. The repository region is divided into multiple compartments, each of which contains one waste canister, associated EBS, and the near-field rock. Radionuclide transport in the repository is analyzed starting with radionuclide release from the canister. Multi-member decay chains are incorporated into the code. The code is written in c++. The code can be executed on multiple Windows-NT workstations in a distributed fashion. The Parallel Virtual Machine (PVM) technologies have been adopted. The code can be upgraded by replacing objects with more advanced ones. With more workstations, calculations can be executed faster.
Far-field transport analysis
The repository-wide model gives information such as the rate of release of radionuclides from the repository into the surrounding far-field region. The far field region is natural geologic formation, for which less information is available than for the repository region. A radionuclide transport code has been developed based on exact analytical solution for multi-member decay chains (Ref. A1(1), A1(5), A1(17), A2(3), A2(7), A2(8), B1(3), B1(4), B1(5), B1(6), B2(1), B2(3), B2(4), B2(5), B2(6), B2(7), B2(8), B2(20)). Path for radionuclide transport is assumed to be a parallel fracture surrounded by porous rock matrix or a homogeneous porous medium.
Sensitivity/Uncertainty Analses and Integrated Performance Assessment
These safety assessment models are developed to obtain the performance measures for the repository. In Ref. A1(8), A1(10), A1(12), A2(5), B2(14), B2(21), B2(22), B2(23), B2(29), examples of such integrated performance assessments are shown.
Due to required longevity for safety assessment and heterogeneity in geologic formations, safety assessment results contain uncertainties. References A1(4), A1(19), B1(19), B2(2), B2(13), B2(27), B2(31), B2(34) show methods developed for sensitivity analyses and their applications to radiological safety and criticality safety.
Autocatalytic criticality event
Among three kinds of repository safety, analyses have been done for criticality safety in detail. Two cases have been studied. One is the case with weapons-grade plutonium disposition at the proposed Yucca Mountain repository (Ref. A1(14), A1(16), A2(10), A2(11), A2(16), B1(15)), and the other is the case with vitrified high-level wastes from reprocessing of spent commercial nuclear fuel disposed of in a conceptual repository in water-saturated geologic formation (Ref. A1(18), A1(19), A2(12), A2(13), A2(15), B1(17), B1(19)).
In either case, bounding analyses have been made to obtain the theoretical maximum of fissile material accumulation in geologic formation. The resulting fissile material accumulation is compared with the minimum mass required for over-moderated criticality in rock-water system, which has been obtained by static neutronics analysis by the Monte Carlo code, MCNP.
Impacts of nuclear materials recycling on geologic disposal have been a contentious issue for a long time. Observations have been made from the repository performance (Ref. A1(13), B1(10), B1(13), B2(28)). More quantitative studies have started recently.
Impact of Accelerator-driven transmutation of waste on geologic disposal (Ref.
A2(21), A2(22), B1(21))Spent fuel (SF) from light-water reactors (LWR) contains uranium, plutonium, other actinides, and long-lived fission products that require the long-term isolation. The Accelerator-driven Transmutation of Waste (ATW) system has been proposed for separation and transmutation of long-lived actinides and fission products in the SF from commercial LWR. Because the ATW system generates wastes that require a geologic repository for the disposal, the extent to which the ATW system can reduce the difficulty of geologic disposal of SF is determined by comparing the performance of YMR for the case of disposal of SF with that for the case of ATW waste disposal.
A mathematical model is established to quantify the mass of an individual radionuclide that comes out of the ATW system as waste destined for YMR. The aforementioned mathematical models have been used for estimating the radiological hazard at the boundary of the accessible environment 5 km distant from the repository. The mass of fissile materials existing in the repository is also estimated as a measure for criticality safety and proliferation resistance.
The differences in the repository performance between by LWR SF disposal and by the disposal of waste from ATW are presented in terms of the radiological hazard and the radionuclide inventory in the repository give insights for optimization and improvement of the ATW concept.
Impact of Pu recycling with LWR reactors (Ref.
A2(19), A2(20), A2(23))In this work, first, radionuclide compositions, masses and volumes of wastes are estimated, based on an assumed fuel cycle scheme, where plutonium is recovered by PUREX reprocessing from spent uranium-oxide fuel, and is fabricated into mixed oxide (MOX) fuel for PWRs. In addition to vitrified HLW and spent MOX fuel, a significant volume of wastes contaminated by trans-uranic (TRU) elements is generated from spent fuel reprocessing and MOX fuel fabrication. TRU wastes are assumed to be disposed of in a deep geologic repository.
Then, radionuclide transport analyses are performed for geologic disposal of TRU, spent MOX fuel and HLW coming from this fuel cycle. The amount of spent uranium-oxide fuel produced by an equivalent once-through scheme is also estimated, and its environmental impact is compared to that of the wastes from the aforementioned fuel cycle.
Impacts of low-level waste disposals will also be incorporated into the analysis, based on the previous analysis results (Ref. A1(3), B2(12)).
While nuclear electricity capacity in the United States has not increased for more than a decade, nuclear energy capacity is steadily increasing in Japan, South Korea, and Taiwan. Large-scale nuclear development has started in China and is about to start in Indonesia. Japan, South Korea, and Taiwan adopted nuclear technologies mostly from the United States, and have maintained good and close relationships with the United States. Nuclear policies, the development of new technologies, and public perception of nuclear energy in the United States have been directly influencing the Asian countries (Ref. B1(11), B1(12)).
In Novermber 1997, the President’s Committee of Advisors on Science and Technology, Panel on Energy Research and Development, issued the Report to the President on Federal Energy Research and Development for the Challenges of the Twenty-First Century. The report points out that world leadership in nuclear energy technologies and underlying science is vital to the US from the perspective of national security, international influence, and global stability. It is also recognized that there will be a firm basis for maintaining nuclear power’s significant, no-carbon contributions to the energy supply of the US and the world.
Regarding the future of nuclear energy in the Asia/Pacific region, it can be foreseen that:
In this study, the future of nuclear energy in the Asia/Pacific region is investigated by:
Collaboration of scholars in energy related fields (nuclear, fossil, renewable, etc.) in major Asian countries has been made through three symposia: the first on the Berkeley Campus as the Nuclear Engineering Session of the Industrial Liaison Program (ILP), 19th Annual Conference, March 1997, sponsored by College of Engineering, and the second in Honolulu in March 1998 co-sponsored by Tokai University and the Nuclear Engineering Department of UC Berkeley, and the third in March 1999 in Taipei co-sponsored by the Institute of Nuclear Energy Research of Taiwan, Tokai University, and the Nuclear Engineering Department at UC Berkeley.