What's the difference between advective movement of gas and diffusion of gas in high-level radioactive waste (HLRW) disposal?
In a deep geological disposal facility for radioactive waste, precompacted bentonite is proposed as a sealing material for the isolation of boreholes, disposal galleries and deposition holes. The advective movement of repository gas in bentonite has been linked to the development of new porosity and propagation of dilatant pathways.
During the molecular dynamics simulations, how to distinct the advective movement from diffusion? Is the velocity of gas different from each other or this difference can be characterise at other aspects?
FYI:
Article Gas network development in a precompacted bentonite experime...
Corrosion of ferrous materials under anoxic conditions, combined with the radioactive decay of waste and radiolysis of water, lead to the formation of hydrogen, carbon dioxide, hydrogen sulphide and methane within a repository depending on the waste composition, availability of water and disposal concept. Determination of the primary mode of gas migration is a complex issue, dependent on both repository concept and evolution. As such, there remains a degree of uncertainty as to the relative importance of diffusion versus advection. However, in scenarios where the rate of gas production exceeds the rate of gas diffusion through the EBS or host rock, a discrete gas phase will form (Weetjens and Sillen, 2006; Ortiz et al., 2002; Wikramaratna et al., 1993, Sellin and Leupin, 2013; SKB, 2006; Norris, 2015). Under these conditions, a free gas phase begins to accumulate until its pressure becomes sufficiently large for it to move, through advection, in the surrounding material (Sellin and Leupin, 2013; Graham et al., 2012; Harrington and Horseman, 1999, 2003; Horseman et al., 1999). Previous studies (Angeli et al., 2009; Skurtveit et al., 2012; Harrington et al., 2009, 2012a,b; Cuss et al., 2014a,b; Gerard et al., 2014; Rodwell, 2000) indicate that in the case of plastic clays (Whitlow, 2001) and in particular precompacted bentonite, advective gas flow is associated with the development of new pressure induced pathways leading to a complex coupling between gas pressure, stress state and volumetric strain.
Very briefly and basically, the difference is that advection is transport by bulk motion, while diffusion is the movement of particles driven by concentration gradients. For the modelling of advection, you need to describe the motion of a scalar field being advected by a velocity vector field. That is described by a differential equation different from the diffusion equation.
https://books.google.com.tr/books?id=ga0qBQAAQBAJ&pg=PA64&lpg=PA64&dq=the+difference+between+advective+movement+of+gas+and+diffusion+of+gas+in+high-level+radioactive+waste&source=bl&ots=gddMbqswKg&sig=CPKzW0DqvxX6iorCoqPleXZsgX8&hl=tr&sa=X&ved=0ahUKEwjO7InX74jYAhWEmbQKHVV4CfwQ6AEIWTAI#v=onepage&q=the%20difference%20between%20advective%20movement%20of%20gas%20and%20diffusion%20of%20gas%20in%20high-level%20radioactive%20waste&f=false
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