In the nuclear fusion demonstration reactor of helical type, FFHR designed by NIFS, supercritical carbon dioxide (S-CO2) is assumed as a coolant in the secondary loop system. A molten salt, FLiNaBe, assumed as a coolant in the primary loop system has low solubility for hydrogen isotopes. Therefore, tritium permeation from the primary system to the secondary system thorough a heat exchanger is concerned.
For tritium safety management, it is important to understand tritium behavior in S-CO2. The objective of this research is understanding mass transfers and chemical reactions between S-CO2 and metal interface. Gaseous components produced in S-CO2 including H2 enlcolsed in a stainless steel SUS316 container at 400 Celsius degrees was investigated. Metal samples such as SUS316, SUS430 and Monel was exposed to CO2 at 400 Celsius degrees, and elemental composition and chemical state of metal surfaces were analyzed. From these results, it was discussed chemical reactions and mass transfer phenomena occurring between S-CO2 and metal interface.
Regarding the measurement of reaction products in S-CO2, CO and CH4 were detected as gaseous products in addition to H2 which was initially enclosed with CO2. In SUS316 and SUS430, elemental analysis showed oxide formation and carbon deposition. XPS analysis showed the most abundant oxide was Fe2O3 from the elemental bonding states. In Monel, metallic luster remains, and oxide formation was not detected.
Based on the measurement of gaseous products and the metal samples analysis, chemical reactions between S-CO2 and metal interface were assumed, and the reaction rate equation for each component was proposed based on this assumption. Then, the changes of concentration of CO2, CO, H2 and CH4 in S-CO2 including H2 enclosed in SUS316 were simulated by solving the reaction rate equations.
It was found that CH4 decomposition reaction was slow, and the formation reaction was fast. Considering the operation in an actual fusion reactor, it was suggested that tritium permeates continuously into CO2 in the secondary cooling system and reacts with carbon deposited by the oxidation reaction of Fe on the metal surface to produce tritiated methane on a steady basis.
When the S-CO2 secondary cooling system using SUS316 as the structural material is applied to the helical demonstration reactor FFHR, tritiated methane is generated continuously by the reaction between the carbon deposited by the oxidation reaction of Fe on the metal surface and the tritium permeating from the primary cooling system. For tritium safety management, the extraction system for tritiated methane is required.

In the nuclear fusion demonstration reactor of helical type, FFHR designed by NIFS, supercritical carbon dioxide (S-CO2) is assumed as a coolant in the secondary loop system. A molten salt, FLiNaBe, assumed as a coolant in the primary loop system has low solubility for hydrogen isotopes. Therefore, tritium permeation from the primary system to the secondary system thorough a heat exchanger is concerned.
For tritium safety management, it is important to understand tritium behavior in S-CO2. The objective of this research is understanding mass transfers and chemical reactions between S-CO2 and metal interface. Gaseous components produced in S-CO2 including H2 enlcolsed in a stainless steel SUS316 container at 400 Celsius degrees was investigated. Metal samples such as SUS316, SUS430 and Monel was exposed to CO2 at 400 Celsius degrees, and elemental composition and chemical state of metal surfaces were analyzed. From these results, it was discussed chemical reactions and mass transfer phenomena occurring between S-CO2 and metal interface.
Regarding the measurement of reaction products in S-CO2, CO and CH4 were detected as gaseous products in addition to H2 which was initially enclosed with CO2. In SUS316 and SUS430, elemental analysis showed oxide formation and carbon deposition. XPS analysis showed the most abundant oxide was Fe2O3 from the elemental bonding states. In Monel, metallic luster remains, and oxide formation was not detected.
Based on the measurement of gaseous products and the metal samples analysis, chemical reactions between S-CO2 and metal interface were assumed, and the reaction rate equation for each component was proposed based on this assumption. Then, the changes of concentration of CO2, CO, H2 and CH4 in S-CO2 including H2 enclosed in SUS316 were simulated by solving the reaction rate equations.
It was found that CH4 decomposition reaction was slow, and the formation reaction was fast. Considering the operation in an actual fusion reactor, it was suggested that tritium permeates continuously into CO2 in the secondary cooling system and reacts with carbon deposited by the oxidation reaction of Fe on the metal surface to produce tritiated methane on a steady basis.
When the S-CO2 secondary cooling system using SUS316 as the structural material is applied to the helical demonstration reactor FFHR, tritiated methane is generated continuously by the reaction between the carbon deposited by the oxidation reaction of Fe on the metal surface and the tritium permeating from the primary cooling system. For tritium safety management, the extraction system for tritiated methane is required.

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