Please review the [https://www.ifixit.com/Wiki/Breville_BCG800XL_Smart_Grinder_Troubleshooting?revisionid=HEAD| Breville Troubleshooting page] first to be sure your grinder does indeed need an impeller repalcement.
Many users of the BCG800XL SmartGrinder experience jamming / clogging and a horrible gear-stripping, clacking sort of sound during use at some point. This is usually due to either a worn out impeller or stripped drive gear. This guide will show you how to replace the impeller on the grinder.
Breville unfortunately doesn't sell replacement parts, so this tutorial has been made possible only recently by 3D printer technology. I've designed an improved impeller for the BCG800XL and made it available here: [https://www.shapeways.com/product/NASLAGCCP/breville-coffee-grinder-impeller-upgrade?optionId=59828416|Shapeways Impeller Source]
[https://youtu.be/zB5F9ha1-zs|Here's] a quick two minute overview of this replacement part.
If you have a BCG'''600'''SIL or BCG'''400'''SIL then [https://www.ifixit.com/Guide/BCG600SIL+Grinder+Jamming+due+to+Worn+Impeller/96493|go here instead].