Refractory metals interstitial impurities

Refractory metals and alloys are generally dissolution-resistant in liquid metals and corrosion is often controlled by reactions with impurity/interstitial elements [13,14,27]. In the case of refractory metal alloys based on niobium or tantalum, the concentration of oxygen in the alloy is an important parameter with respect to corrosion in alkali metals, particularly lithium [34-38]. As little as 300 wppm of oxygen in niobium will induce catastrophic penetration of the niobium by lithium. Interstitial oxygen will also cause penetration of niobium tmd tantalum by sodium or potassium, but the threshold of oxygen concentration is higher. 

Results from a thermochemical analysis between core and plant structural materials showed a strong driving force for the mass transport of impurities, such as carbon, from nickel-base superalloys to refractory metal alloys that would promote interstitial embrittlement of the latter. Mitigation of this effect may require lower operating temperatures, the development of protective coatings, or the use of alternate materials of construction. Further details of this work and the results can be found in Reference 14-6. 

Figure 14-1 shows a representation of the material options for Material Case 1. This case shows the refractory metal alloy core completely contained within a Ni-base superalloy vessel. Refractory metal alloys suffer from interstitial embrittlement after adsorption of impurities such as oxygen, carbon and nitrogen, so precluding their exposure to external environmental conditions was a major goal of the NRPCT. Protection of the refractory metal core from both micrometeoroids for the flight unit and Earth s atmosphere for the Ground Test Reactor (GTR) is provided by the He-Xe coolant within the Ni-base superalloy vessel. However, due to their sensitivity to interstitial impurities likely to be present in the coolant, core refractory metal alloy components would likely need to be coated for protection. 

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