Compatibility of structural materials with Li2BeF4 molten salt breeder

doi:10.1016/S0022-3115(98)00371-7

Journal of Nuclear Materials

Volumes 258–263, Part 1, October 1998, Pages 513-518

https://doi.org/10.1016/S0022-3115(98)00371-7 Get rights and content

Abstract

Compatibility of structural materials (ferritic steel, vanadium-based alloy and molybdenum) with molten LiF–BeF₂ mixture under the atmosphere containing O₂, H₂O and HF was studied using thermodynamical calculation of chemical equilibria at 823 K. It was clarified that the oxidation of structural materials would have precedence over fluorination with coexistence of oxidizing species though fluorination was found under the atmosphere containing HF. Then, these materials would have satisfactory corrosion resistance if generated oxides function as protective scales.

Introduction

A mixture of LiF–BeF₂, Flibe, can be considered as a candidate for tritium breeding material in a liquid blanket system of a fusion reactor. Flibe has favorable characteristics such as low electric conductivity, chemical stability, etc., and it can be utilized as a coolant.

The liquid blanket system using Flibe is utilized in the conceptual design for Force-Free Helical Reactor, FFHR 1, 2, 3. In this design, ferritic steel (e.g. Fe–9Cr–2W) or V-based alloy (e.g. V–4Cr–4Ti) is considered to be utilized for the structural material of the blanket, as is shown in Fig. 1. Then, the corrosion of such materials is one of the critical issues; the compatibility of structural materials with molten fluoride strongly influences the durability of the system.

In the present work, compatibility of materials with Flibe was estimated using thermochemical calculation under the conditions which are difficult for us to control practically in experimental systems.

Section snippets

Thermodynamical data

Recent integrated thermochemical database systems enable us to calculate complex chemical equilibria. In this work, thermodynamic database system, MALT2 4, 5was utilized for the calculation of chemical equilibria for Flibe/structural material systems.

Flibe itself has higher chemical stability as a fluoride than those of ordinary metals which would be put to practical use for fusion reactors, and has almost no reactivity with structural materials. In case of this work, however, Flibe generates

Ferritic steel

Fig. 3(a) shows the effect of the initial amount of H₂O to the product system. Fluorination of ferritic steel Fe–9Cr–2W was not found under the O₂–H₂O-added atmosphere except in case of very large H₂O fraction; the steel seemed to be merely oxidized by H₂O in spite of coexistence of Flibe.

Ferritic steel can be considered to have good compatibility with molten Flibe under the atmosphere containing oxygen if the amount of moisture is properly controlled. In case of Fe–9Cr–2W, BeWO₄ was found to

Conclusion

Compatibility of structural materials with Flibe under the atmosphere containing O₂, H₂O, HF was simulated using the thermochemical calculation. It was found that structural materials would be ready not to be fluorinated but to be oxidized if oxidizing species existed. Though fluorination was found under the atmosphere containing HF, structural materials such as ferritic steel, V-based alloy and molybdenum would have sufficient corrosion resistance when those oxides functioned as protective

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FLiNaBe is a promising liquid blanket material for a fusion reactor. Since hydrogen solubility for FLiNaBe is low, one concern is the permeation of bred tritium. In order to increase effective solubility of hydrogen, the addition of Ti powder was proposed. In this study, the influence of Ti addition on the behavior of tritium generated inside FLiNaBe by neutron irradiation was investigated. The samples of FLiNaBe and FLiNaBe mixed with Ti of 5 wt% and 0.5 wt% were prepared and the solid-state samples were irradiated by thermal neutrons at Kyoto University Research Reactor. The neutron-irradiated samples were heated to 700 °C in an Ar gas flow. The released tritium was collected by a water bubbler and the temporal variation of TF and HT release was observed. Initially, the most of tritium was released as HT from both FLiNaBe with and without Ti but eventually, the most of released tritium was HT after long time heating from FLiNaBe with Ti. This result suggests that the addition of Ti can decrease the release of TF very much. The amount of tritium released from FLiNaBe with Ti was lower than that from FLiNaBe without Ti. This result suggests that the addition of Ti can suppress the release rate of tritium from FLiNaBe.
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