Some of the more whackadoodle fusion plant designs (which nonetheless have millions in funding and early prototypes built) get around this by doing the fusion i

Some of the more whackadoodle fusion plant designs (which nonetheless have millions in funding and early prototypes built) get around this by doing the fusion in a blanket of liquid metal, or by using aneutronic fusion (which requires ridiculously high temperatures), or by using the neutron activation to breed more fuel.