Hiroshi Seino

Kinetic study on liquid sodium–silica reaction for safety assessment of sodium-cooled fast reactor

In this study, the kinetic behavior of the sodium (Na)–silica (SiO2) reaction was investigated for an assessment method of reactivity/stability of siliceous concrete against the sodium–concrete reaction (SCR) by postulating a severe accidental condition in the sodium-cooled fast reactor (SFR). The reaction behavior was tracked using a differential scanning calorimetry (DSC) equipped with a videoscope for viewing the changes in the sample during the reaction. The reaction was characterized as a partially overlapping multistep process controlled by physico-geometrical reaction scheme. The kinetic behavior in view of the change in the temperature at the maximum reaction rate with heating rate was analyzed using the simplified Kissinger method and the approximated Ozawa method. Further by separating the kinetic data for the overall reaction into five component reaction stages, the kinetic information for the second and third reaction stages was extracted. By comparing the kinetic results, it was revealed that the kinetic results determined from the kinetic data at the maximum reaction rate can be interpreted as is for the major reaction stage (the third reaction stage) controlled by an autocatalytic reaction. In addition, the second reaction stage was characterized as a possible premonitory process for the major reaction stage. Through the comprehensive interpretation of the kinetic results, significance of the kinetic analysis for the Na–SiO2 reaction using DSC for the reactivity/stability evaluation against SCR and for the safety assessment of SFR is discussed.

Experimental study and kinetic analysis on sodium oxide–silica reaction

ABSTRACT In a sodium-cooled fast reactor (SFR), if considering hypothetical severe accidental condition such as the steel liner failure of structural concrete caused by intensive leakage of liquid sodium (Na) coolant, the liquid sodium–concrete reaction (SCR) may take place. The major consequences of SCR are hydrogen release, energy release and concrete ablation. Thus, it is important to understand the phenomenology and kinetic behavior of SCR. As a part of a series of studies on SCR, this study focused on the reaction between sodium oxide (Na2O) and silica (SiO2), which is dealt with possible formation of Na2O as the result of sodium–water reaction in the initial stage of SCR. Through thermoanalytical and X-ray diffraction measurements, it was revealed that Na2O–SiO2 reaction to form sodium orthosilicate (Na4SiO4) occurs at significantly lower temperature in comparison with Na–SiO2 reaction. The reaction is kinetically characterized as a largely overlapping multistep reaction, which is composed of at least three reaction steps. On the basis of the observations, the impact of Na2O–SiO2 reaction in the overall SCR and the significance of the conventional kinetic analysis using the Kissinger method are discussed.

A study on self-terminating behavior of sodium–concrete reaction

ABSTRACT A sodium–concrete reaction (SCR) is one of the important phenomena to cause the structural concrete ablation and the release of hydrogen (H2) gas in the sodium (Na) leak accident. In this study, the long-time SCR test had been carried out to investigate the self-termination mechanism under the condition to keep the temperature of Na on the concrete more than the reaction threshold temperature during 24 hours. The test results showed the SCR terminated by itself even if enough amount of Na remained on the concrete. In addition, quantitative data were collected on the SCR terminating behavior such as the temperature, the concrete ablation depth, the H2 generation behavior and the concentration profiles of Na, silicon (Si), aluminum (Al) and calcium (Ca) in the reaction products after the test. In the concentration profiles, the calculation by the sedimentation diffusion model of the steady state was comparable with the experimental results. Though the reaction products were suspended by H2 bubbling and Na ablated the concrete surface with the high H2 generation rate, the reaction products gradually settled down with decreasing of the H2 generation rate. Therefore, the Na concentration decreases at the reaction front with time and the SCR terminates of itself.

Kinetic Study of Sodium-Water Reaction Phenomena by Differential Thermal Analysis

In a sodium-cooled fast reactor (SFR), if a heat transfer tube in the steam generator (SG) is failed, high pressurized water vapor blows into the liquid sodium and sodium-water reaction (SWR) takes place. SWR may cause damage to the surface of the neighboring heat transfer tubes by thermal and chemical effects. Therefore, it is important to clearly understand the SWR for safety assessment of the SG.From recent study, sodium (Na)–sodium hydroxide (NaOH) reaction as secondary surface reaction of the SWR phenomena in a SFR was identified by ab initio method [1]. However, kinetics of this reaction is a still open question. It is important to obtain quantitative rate constant of sodium monoxide (Na2O) generation by Na-NaOH reaction because Na2O may accelerate the corrosive and erosive effects.Differential thermal analysis (DTA) provides us with the valuable information on the kinetic parameters, including activation energy, pre-exponential factor (frequency factor) and reaction rate constant. Thus, kinetic study of Na–NaOH reaction has been carried out by using DTA technique. The parameters, including melting points of Na and NaOH, phase transition temperature of NaOH, Na-NaOH reaction temperature and the decomposition temperature of sodium hydride (NaH) were identified from DTA curves. Na, NaOH, and Na2O as major chemical species were observed from the X-ray diffraction (XRD) analysis of the residues after the DTA experiment. It was inferred that Na2O could be generated as a reaction product. Based on the measured reaction temperature, the first-order rate constant of Na2O generation was obtained by the application of the laws of chemical kinetics. From the estimated rate constant, it was found that Na2O generation should be considered during SWR.The results can be the basis for developing a chemical reaction model used in a multi-dimensional sodium-water reaction code, SERAPHIM, being developed by the Japan Atomic Energy Agency (JAEA) toward the safety assessment of the SG in a SFR.Copyright