Thermal behavior of sodium hydroxide–structural concrete composition of sodium-cooled fast reactor

Abstract

Under postulated accidental condition of sodium-cooled fast reactor (SFR), liquid sodium spill into the floor may lead to fail the steel liner. Consequently, sodium–concrete reaction (SCR) comes to occur by direct contact of liquid sodium with structural concrete. Significant phenomena of SCR for safety assessment of SFR are hydrogen release, energy release by exothermic heat generation, and concrete ablation by chemical corrosion. SCR consists of two reaction stages: (1) dehydration of concrete and hydrogen release by the reaction of evolved water vapor with Na as initial stage and (2) ablation of concrete by reactions between concrete compositions and Na compounds as secondary stage. In this study, the sodium hydroxide (NaOH)–silica (SiO2) reaction as one of the possible secondary stage reactions was investigated for safety assessment of SFR. Thermal behavior of NaOH–SiO2 reaction such as reaction onset was determined using a differential scanning calorimetry (DSC). As a result of DSC measurement, it was revealed that NaOH–SiO2 reaction occurs as rapid reaction right after NaOH melting at 583 K. Therefore, it was expected that NaOH–SiO2 reaction is dominant in the time frame of secondary stage of SCR if significant amount of NaOH has been generated during the initial stage reaction. Meta sodium silicate (Na2SiO3) was detected as the major solid product of NaOH–SiO2 reaction from XRD analysis. Considering the phase diagram of NaOH–SiO2 reaction system, likely overall reaction pathway of NaOH–SiO2 reaction was discussed.