During the high-level radioactive waste (HLRW) disposal, because the very low hydraulic permeability of compacted clay, molecular diffusion is the predominant mass transfer process in these barriers. Is there any fluid flow (rapid) rather than diffusion (slow) phenomenon in clay barriers during the HLRW disposal? I mean whether the fluid flow (rapid) is possible during the HLRW disposal?
It is highly unlikely that the migration rate can be much higher than the diffusion rate since the barrier is designed and tested for low porosity, which prevents the leakage of the materials.
Dear Kheng
Thanks for your suggestions, however, due to the heat-emitting characteristics of HLRW waste, the clay may probably bear the thermal load or chemical reactions due to the alkaline reaction, hence, fracture may be created, however, due to the self-sealing properties, these fracture may be closed. Therefore, my question is, generally, how long will the fracture last?
Regards
The duration of the fracture can only be simulated and tested in the lab, as the consequence of the thermal change and reactions are difficult to predict. However, it may be expected that the hydro-static pressure in the bulk fluid may just force the fluid to fill the gap quite instantly when crack appears.
My experience with mid level radioactive trenches seem to suggest that your design with good safety factors will prevent any leakage. Ultimately a simulation and lab test will verify your assumptions in the design.
Dear Kheng
Thanks for your kind suggestions, do you mean that "there still exists an opening about the seal-sealing time after the creation of fractures due to the creation of EDZ or thermal-induced fractures"?
I read one article by (Tsang et al. 2005) , which may be a help for this question, here it refers " The Excavation Damaged Zone (EDZ) is a zone with significant irreversible processes and significant changes in flow and transport properties. These changes, for example, can include one or more orders of magnitude increase in flow permeability. (Tsang et al. 2005) "
Regards
Dear Kheng
FYI:
"clays show a high capacity for self-sealing. Any fractures and fissures that occur, in particular those created by excavation activities, close quite rapidly; " (Cited from 2017 Radioactive waste confinement-clays in natural and engineered barriers–introduction),
Also, based on the modelling results, it can be concluded that the extent of the EDZ around the HLW emplacement rooms will not exceed a thickness of one room diameter and that the average hydraulic conductance of the EDZ around emplacement rooms, access tunnels and shafts will not exceed a value of 10-7 m3/s. Self-sealing of the EDZ and low hydraulic gradients along the tunnels will result in negligible radionuclide transport by the EDZ pathway. (cited from 2016 Technical report-High-level waste repository-induced effects)
Therefore, we can draw the conclusion that the seal-sealing time is short for the EDZ, so my question has changed to "What about the seal-sealing time of thermal-induced fractures or generated-gas propagate-induced fractures"?
Regards
Ma,
"What about the seal-sealing time of thermal-induced fractures or generated-gas propagate-induced fractures"?
This is a tough issue, but you can simulate the phenomena using CFD for various simple cases. BUbble rising and impingement of walls etc...... causing cracks, very exciting research. Good luck. kk
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