Boundary Loss Term in the Diffusion of Radioactive Volatile Fission Products

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Abstract
The diffusion equation for volatile radioactive fission products in sintered nuclear fuels is investigated. All known effects that may affect the rate of diffusion to the grain boundaries are taken into account: simultaneous diffusion of the radioactive precursors, radioactive decay, sink trapping, and radiation resolution.Starting from the analysis of the spatial transport equation, an expression for the boundary loss term to be used in the simpler reaction rate equation is deduced.For practical applications the boundary loss term in the absence of resolution effects can reasonably be assumed to be independent of time. This is not generally true if resolution effects are present; in this case the release calculations become more complex than it was assumed so far.Finally, a discussion on the properties of the boundary loss term as functions of the physical parameters involved follows, and details of the calculations are presented.

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