[danielbln]

It's about the radiation, which isn't contained by the magnetic field. The latter "just" keeps the plasma from melting the walls.

[telgareith]

Well, the magnetic or inertial confinement keeps the reactants dense/close enough to react.

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...