They already do this with ITER[1] for the "bioshield" to protect humans from radiation, the facility is semi underground as well. However by the sounds of it,

They already do this with ITER[1] for the "bioshield" to protect humans from radiation, the facility is semi underground as well. However by the sounds of it, a reasonable fraction of the output power is in the form of neutrons. If you had no lithium blanket and just concrete, the energy of the neutrons would go into heating the concrete, where it would either be wasted, or require the concrete to be actively cooled which would be difficult as concrete is not a great conductor. [1] https://f4e.europa.eu/mediacorner/newsview.aspx?content=941 https://f4e.europa.eu/mediacorner/newsview.aspx?content=941