Comparative study on the thermal behavior of structural concretes of sodium-cooled fast reactor: Siliceous concretes

Thermal behaviors of two different siliceous concretes used in a sodium-cooled fast reactor were comparatively investigated in a temperature range from room temperature to 1900 K for obtaining fundamental information required for establishing a plant simulation system for safety assessment under a postulated accidental condition. Silica crystals and Portland cement were identified as the major component of the aggregate and cement portions of the concrete samples, respectively. The thermal decomposition of the cement portion exhibited partially overlapping multistep reaction comprising the thermal dehydration, thermal decomposition processes of Ca(OH)₂ and carbonate compounds including CaCO₃. TG–DTG curves recorded for the multistep thermal decomposition process of the cement portion were analyzed using the kinetic deconvolution analysis, and the contributions and kinetic parameters of each reaction step were determined. The kinetics of comparable reaction steps between two samples were practically identical, while the difference between the samples was found in the content ratio of Ca(OH)₂/CaCO₃. The melting behavior of the siliceous concretes was revealed by the complementary interpretation of TG–DTA curves and the morphological observation of the sample heated to different temperatures. The softening and melting behaviors of the siliceous concretes initially occurred in the thermal decomposition product of the cement portion at a temperature range of 1400–1600 K. The subsequent melting behavior of the aggregate portion that occurs at a higher temperature was different between the samples, owing to the different compositions of the aggregates and the possible interaction of the aggregate with the molten cement portion.