The terrorism threat has always been more media then reality with reactors though. The idea of these small reactors is that they're not big complicated structures, so there protection needs are different - i.e. the reactor body can be built into the ground and concreted over until its time to decommission it.

Such a unit would be pretty close to impenetrable to any terrorist threat - there's no building to crash into, no structure or employees to attack. The literal attack surface is greatly reduced - and any operation to penetrate the unit would be large, highly visible, and take time.