From my admittedly limited knowledge gleaned from occasional PopSci articles posted here on HN over the years, don't they use plasma trapped in magnetic fields to contain the reaction and not a particular metal?
The heat itself is entirely absorbed by the walls. As well as the previously mentioned neutron flux.
Also, heres one of many sources- this was found as part of looking up HFIR's name.
https://arpa-e.energy.gov/sites/default/files/B03-Wirth-Fusi...