The core fusion plasma must not actually touch the first wall.
ITER and many other current and projected fusion experiments, particularly those of the tokamak and stellarator designs, use intense magnetic fields in an attempt to achieve this, although plasma instability problems remain.
Even with stable plasma confinement, however, the first wall material would be exposed to a neutron flux higher than in any current nuclear power reactor, which leads to two key problems in selecting the material:
It must withstand this neutron flux for a sufficient period of time to be economically viable.
It must not become sufficiently radioactive so as to produce unacceptable amounts of nuclear waste when lining replacement or plant decommissioning eventually occurs.