Molten-salt reactor corrosion

Another problem is corrosion due to the hot fluoride melt. Previous experience with the Molten Salt Reactor Experiment (MSRE) at the Oak Ridge National Laboratory (Weinberg 1970) seem to indicate, however, that these problems can be overcome. 

A molten salt reactor (MSR) is a reactor in which fluorides of fissile and fertile elements such as UF4, PuFg, and/or Thp4 are combined with carrier salts to form a fluid fuel. MSRs can operate as simple burner reactors with high fuel economy or with the addition of online fission product removal and can achieve breeder status. Typical operation sees molten salt flowing between a critical core and an intermediate heat exchanger. A secondary coolant salt then transfers heat to a steam or closed gas cycle. The majority of work has involved fluoride salts, as corrosion-resistant alloys have been shown to be compatible with these salts. 

Specifically, Hastelloy-N has been specially developed as a basic structural material for reactors with molten salt coolants. Corrosion resistance of this alloy is defined by the presence of the impurities in salt compositions, such as soluble oxides, traces of moisture, fission products, etc. 

The coolant is a liquid (molten) salt however, the AHTR is not a molten salt reactor (MSR). The term molten salt reactor refers to reactors that have the fuel dissolved in the coolant (i.e. MSRs are liquid fuel reactors). The liquid salt in the AHTR is a clean coolant with a coolant cleanup system to remove impurities. The coolant cleanup system ensures low rates of equipment corrosion and minimizes radioactive contamination outside the reactor core. The AHTR uses a traditional solid fuel. The Appendix to Annex XXVI provides more detailed information on the characteristics of liquid salts as coolants and the relevant industrial experience. 

P. Calderoni, C. Cabet, Corrosion issues in molten salt reactor (MSR) systems, in D. Feron (Ed.), Nuclear Corrosion Science and Engineering, Woodhead Publishing Ltd., 2012. 

A molten-salt reactor system requires structural materials which will effectively resist corrosion by the fluoride salt mixtures utilized in the core and blanket regions. Evaluation tests of various materials in fluoride salt systems have indicated that nickel-base alloys are, in general, superior to other commercial alloys for the containment of these salts under dynamic flow conditions. In order to select the alloy best suited to this application, an extensi e program of corrosion tests was carried out on the available commercial nickel-base alloys, particularly Inconel, which typifies the chromium-containing alloys, and Hastclloy B, which is representative of the molybdenum-containing alloys. 

IXOR-8 has excellent corrosion resistance to molten fluoride salts at temperatures considerably above those expected in molten-salt reactor service further, no measurable attack has been ob.served thus far in tests at reactor operating temperatures of 1200 to 1300°F. The mechanical properties of IXOR-8 at operating temperatures are superior to those of many stainless steels and are virtually unaffected by long-time exposure... 

At the temperature of interest in molten-salt reactors, that is, 1250°F, the same trend of relative corrosiveness of the different salts may exist for Inconel, but the low rates of attack observed in tests preclude a conclusive decision on this point. Similarly, if there is any preferential effect of the base salts on IXOR-8, the small amounts of attack tend to hide it. 

MSO is unsuited for treating materials with high inert content, such as asbestos, concrete, soils, and rubble. There is concern over emissions from MSO relating to particulate mercury content and radioactivity. MSO is inappropriate for wastes with high tritium levels. MSO pilot programs have encountered problems with carbon monoxide (CO) emissions. The corrosion of reactor materials by molten salt has remained a concern for the long-term operability of the system. The viscosity and volatility of the melt have to be controlled. There have been problems with material from the melt plugging air exhaust and feeder systems. 

Structural material selection. The AHTR requires high-temperature corrosion resistant materials. Materials are the greatest challenge for all high-temperature reactors, including the AHTR. Materials have been identified that allow operation with molten salts to 750°C (Hastelloy-N). 

One important source of impurities in electrochemical reactors is the anode, which invariably suffers from corrosion over the operating life of the plant. Corrosion is affected by the nature of the electrolyte, temperature, and current density. If any of these change on scale-up, the rate of contamination could change. Systems which use aggressive media such as molten salts are generally susceptible to contamination risk. 

Corrosion phenomena induced by molten salts in Generation IV nuclear reactors... 

As can be seen from the consideration above there are different potential applications of molten salts for Generation IV nuclear reactors. Although MSR concepts are focused on different baseline options (MSFR, MOSART, TMSR, and FHR), large commonalities in basic R D areas exist the Generation IV framework is usefid to optimize the R D effort. AH of the concepts are similar, at least in terms of general chemistry, materials, and corrosion phenomena. 

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