Transport of helium and fission product gases in porous/fractured geometry

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Measuring and quantifying the transport of helium and fission products in reactor materials is a critical element in the future success of Very High Temperature Reactors (VHTR's). While the extreme temperatures in VHTR's, which may be in excess of 950 [degree sign]C, allow for potentially very high efficiency, they also can be problematic due to the increase in fission product transport. The aim of this work was to address safety issues concerning VHTR's by developing better understandings of the flows and transport of fission gases through microcracks and nanopores in the fuel coatings and in the moderators. An additional purpose was also to improve the understanding of the transport of helium through these structures since, in a VHTR, helium will be very abundant. We used an existing system to obtain measurements of slip coefficients, which are useful in determining transport of helium gas mixtures, for varying mixtures of gases that could be present in a VHTR. The mixtures we have studied are helium and air, helium and nitrogen, helium and argon, and helium and nitrogen, with varying trace amounts of xenon. We also constructed a new system to aid in understanding the transport of helium and fission products through reactor grade graphites. This later work has permitted us to obtain results for the permeability and diffusion coefficients of various graphites.