Influence of Defect Interactions on Diffusion Processes in UO2+x: a Key Issue for Understanding the Behaviour of Spent Nuclear Fuel.

2004 ◽  
Vol 824 ◽  
Author(s):  
Georgette Petot-Ervas ◽  
Gianguido Baldinozzi ◽  
Pascal Ruello ◽  
Lionel Desgranges ◽  
Georgeta Chirlesan ◽  
...  
Keyword(s):  
Electrical Conductivity ◽  
Diffusion Coefficient ◽  
Nuclear Fuel ◽  
Spent Nuclear Fuel ◽  
Diffusion Processes ◽  
Transport Processes ◽  
Accurate Information ◽  
Chemical Diffusion Coefficient ◽  
Large Defect ◽  
Matter Transport

AbstractThe transformation of UO2 into U3O8 is of technological and academical interest because of the severe consequences on the spent nuclear fuel management. The structural mechanism responsible for the isothermal transformation of UO2 into U3O8 seems still unclear. Several phases (UO2+x, U4O9, β-U3O7, α-U3O7, U3O8 were reported but their true structures, phase boundaries between their existence domains and matter transport processes are still a matter of debate. Gathering accurate information on the behaviour of uranium oxide is a key issue for understanding the behaviour of spent nuclear fuel. The chemical diffusion coefficient ( ~ D) of UO2+x was determined by electrical conductivity experiments. Measurements were performed in transient state for departure from stoichiometry in the range 0<x<0.17 (10-11<P(O2)<10-8 atm.)and for 973<T<1673 K. We have found that ~ D is a decreasing function of the departure from stoichiometry x. This behaviour was attributed to the presence of singly charged (2:2:2) Willis defects as suggested by equilibrium conductivity measurements. The decrease of Dchim can be explained by transport processes occurring via a dynamic exchange between isolated mobile defects and complex defects frozen in clusters or domains. At higher P(O2), near U4O9, the time to reach an equilibrium electrical conductivity value becomes increasingly long. This suggests the presence either of large defect aggregates or of complex defects arranged into domains. Furthermore, the analysis of the transport processes in non equilibrium conditions has allowed us to show that the results of ~ D are consistent with those of the oxygen diffusion coefficient within the P(O2) and temperature range of stability of the [2:2:2] clusters.

Assessment of the Evolution with Time of the Instant Release Fraction of Spent Nuclear Fuel in Geological Disposal Conditions

2002 ◽  
Vol 713 ◽  
Author(s):  
Christophe Poinssot ◽  
Patrick Lovera ◽  
Marie-Hélène Faure
Keyword(s):  
Nuclear Fuel ◽  
Quantitative Assessment ◽  
Spent Nuclear Fuel ◽  
Diffusion Processes ◽  
Driving Forces ◽  
Geological Disposal ◽  
Physical And Chemical ◽  
Evolution With Time ◽  
New Framework

ABSTRACTUnder the geological disposal conditions, spent nuclear fuel (SNF) is expected to evolve during the first thousands years while being maintained isolated from the biosphere before water comes in. Under those circumstances, several driving forces would lead to the progressive intrinsic SNF transformations within the rod which would basically modify the physical and chemical state of the fuel and the subsequent release of radionuclides in solution. In this paper, we briefly summarize the mechanisms we estimate to be significant and propose a new framework for the quantitative assessment of the radionuclide (RN) inventory we estimate to be associated to the classically referred to “Instant Release Fraction” (IRF). We hence demonstrate that in this framework, significantly high IRF values have to be expected for the long term due mainly to the presence of athermal diffusion processes.

Sample Manipulation in Microfluidic Devices With Electrical Conductivity Gradients

2004 ◽  
Author(s):  
Carolyn L. Ren ◽  
Dongqing Li
Keyword(s):  
Electrical Conductivity ◽  
Diffusion Coefficient ◽  
Electrical Potential ◽  
Analytical Techniques ◽  
Microfluidic Devices ◽  
Transport Processes ◽  
Individual Species ◽  
Sample Shape ◽  
Conductivity Gradient ◽  
Bulk Transport

A common application in microfluidic devices is on-chip capillary electrophoresis (CE). In this process, sample species are transported by electroosmotic flow and separated based on their electrophoretic mobilities. Separated analytes are typically detected using laser-induced fluorescence. It has been found that the sample shape and size, which is critical to the later detection processes or the quality of other analytical techniques, depends on many parameters, such as the sample diffusion coefficient, the applied voltages, and the electrical conductivity difference between sample and buffer. The conductivity difference can alter the electric field strength, which is the driving force behind both the electroosmotic bulk flow and the electrophoretic velocity of individual species. Therefore, the manipulation technique is required to consider the transport processes with conductivity differences. A numerical model presented in this paper is used to simulate the sample transport process with the consideration of conductivity gradient in order to develop the sample manipulation techniques. There are two situations studied here, which are sample pumping (where bulk transport is increased and analyte separation is delayed using a relatively high conductivity sample), and sample stacking (where bulk transport is decreased and analyte separation is expedited using a relatively low conductivity sample). The effects of applied electrical potential, sample diffusion coefficient and the extent of conductivity difference on the sample control are investigated through the developed model. The simulation results show that the sample transport with the consideration of conductivity gradient differs significantly from that of uniform conductivity case.

scholarly journals Post-Closure Safety Calculations for the Disposal of Spent Nuclear Fuel in a Generic Horizontal Drillhole Repository

Energies ◽  
2020 ◽  
Vol 13 (10) ◽  
pp. 2599 ◽  
Author(s):  
Stefan Finsterle ◽  
Richard A. Muller ◽  
John Grimsich ◽  
John Apps ◽  
Rod Baltzer
Keyword(s):  
Nuclear Fuel ◽  
Spent Nuclear Fuel ◽  
Uncertainty Propagation ◽  
Radioactive Decay ◽  
Transport Processes ◽  
Sensitivity Analyses ◽  
Annual Dose ◽  
Performance Metric ◽  
Safety Function ◽  
Wide Range

The post-closure performance of a generic horizontal drillhole repository for the disposal of spent nuclear fuel (SNF) is quantitatively evaluated using a physics-based numerical model that accounts for coupled thermal-hydrological flow and radionuclide transport processes. The model incorporates most subcomponents of the repository system, from individual waste canisters to the geological far field. The main performance metric is the maximum annual dose to an individual drinking potentially contaminated water taken from a well located above the center of the repository. Safety is evaluated for a wide range of conditions and alternative system evolutions, using deterministic simulations, sensitivity analyses, and a sampling-based uncertainty propagation analysis. These analyses show that the estimated maximum annual dose is low (on the order of 10−4 mSv yr−1, which is 1000 times smaller than a typical dose standard), and that the conclusions drawn from this dose estimate remain valid even if considerable changes are made to key assumptions and property values. The depth of the repository and the attributes of its configuration provide the main safety function of isolation from the accessible environment. Long-term confinement of radionuclides in the waste matrix and slow, diffusion-dominated transport leading to long migration times allow for radioactive decay to occur within the repository system. These preliminary calculations suggest that SNF can be safely disposed in an appropriately sited and carefully constructed and sealed horizontal drillhole repository.

The electrical conductivity of model melts based on LiCl-KCl, used for the processing of spent nuclear fuel

2017 ◽  
Vol 257 ◽  
pp. 510-515 ◽  
Author(s):  
Alexander Salyulev ◽  
Alexei Potapov ◽  
Vladimir Khokhlov ◽  
Vladimir Shishkin
Keyword(s):  
Electrical Conductivity ◽  
Nuclear Fuel ◽  
Spent Nuclear Fuel
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