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Tritium release behavior from solid breeder pebbles in a blanket module is strongly influenced by not only diffusion in grains but also surface reactions. A part of tritium released to a purge gas permeates into a coolant. Nishikawa et al. proposed the tritium release model from solid breeder materials that includes tritium diffusion and surface reactions such as isotope exchange reaction, water form reaction and absorption/desorption of water vapor on grain surface [1]. Recently, tritium release behavior from Li2TiO3 pebbles by DT neutron irradiation was experimentally investigated by Edao et al. and a ratio of HT and HTO in released tritium was reported [2]. On the other hand, the release behavior of water vapor from Li2TiO3 pebbles manufactured by the same method was reported [3]. However, the relationship between HT/HTO ratio and the release behavior of water vapor has not been discussed yet.
In this work, numerical calculation of tritium release behavior from Li2TiO3 pebbles was performed applying Nishikawa’s model and calculation results were compared with experimental data such as [2]. Then, it was shown that HT/HTO ratio is relatively influenced by isotope exchange reaction rate and the amount of released water vapor generated by Li2TiO3 pebbles and H2 in the purge gas. Additionally, tritium behavior in a water cooled solid breeder blanket module was numerically estimated considering tritium permeation loss into cooling water. Tritium mass transfer parameters such as diffusivity and isotope exchange reaction rate were referred from literature data for several kinds of solid breeder material such as Li2TiO3 and Li4TiO4. The influences of temperature in the solid breeder pebble bed and grain size of solid breeder on chemical form of released tritium were quantitatively shown. Tritium inventory in grain bulk, grain surface, purge gas, cooling pipe or cooling water was estimated respectively. Tritium inventory in grain bulk was largest among these components.
[1] M. Nishikawa et al., J.Nucl.Mater., 325 (2004) 87-93.
[2] Y. Edao et al., Fusion Eng. Des., 112 (2016) 480-485.
[3] R. Yamamoto et al, “Li mass loss from Li2TiO3 with excess Li pebbles fabricated by optimized sintering condition”, SOFT2016, Sep.5-9, 2016, Prague, Czech Republic.
| Eligible for student paper award? | No |
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