We investigated the physical/chemical phenomena that a slow loss of CO2 inventory into sodium after the sodium-CO2 boundary failure in printed circuit heat exchangers (PCHEs), which is considered for the supercritical CO2 Brayton cycle power conversion system of a sodium-cooled fast reactor (SFR). The first phenomenon is plugging inside narrow sodium channels by micro cracks and the other one is damage propagation referred to as wastage combined with the corrosion/erosion effect. Experimental results of plugging shows that sodium flow immediately stopped as CO2 was injected through the nozzle at 300~400 °C in 3 mmID sodium channels, whereas sodium flow stopped about 60 min after CO2 injection in 5 mmID sodium channels. These results imply that if pressure boundary of sodium-CO2 fails a narrow sodium channel would be plugged by reaction products in a short time whereas a relatively wider sodium channel would be plugged with higher concentration of reaction products. Wastage by the erosion effect of CO2 (200~250 bar) hardly occurred regardless of the kinds of materials (stainless steel 316, Inconel 600, and 9Cr-1Mo steel), temperature (400~500 °C), or the diameter of the CO2 nozzle (0.2~0.8 mm). Velocities at the CO2 nozzle were specified as Mach 0.4~0.7. Our experimental results are expected to be used for determining the design parameters of PCHEs for their safeties.