This paper describes an assembly designed to monitor atmospheric corrosion inside the concrete reactor vessel of a retired nuclear power plant in Spain during the waiting period (latency) of its decommissioning. Atmospheric corrosiveness was determined according to International Organization for Standardization (ISO) 11844-1 and 11844-2 standards, using gravimetric specimens and electrical resistance probes arranged on special racks inserted in the reactor vessel through three upper penetrations. Results are reported for the first three years of exposure of the racks inside the reactor vessel. Very low corrosion rates (<0.02 μm/y) were found for all three carbon steel types tested.