Guoping Cao

High-Temperature Corrosion of UNS N10003 in Molten Li2BeF4 (FLiBe) Salt

Corrosion testing of UNS N10003 in molten fluoride salt was performed in purified molten 27LiF-BeF2 (66–34 mol%) (FLiBe) salt at 700°C for 1,000 h, in pure nickel and graphite capsules. In the nickel capsule tests, the near-surface region of the alloy exhibited an approximately 200 nm porous structure, an approximately 3.5 μm chromium-depleted region, and MoSi2 precipitates. In the tests performed in graphite capsules, the alloy samples gained weight because of the formation of a variety of Cr3C2, Cr7C3, Mo2C, and Cr23C6 carbide phases on the surface and in the subsurface regions of the alloy. A Cr-depleted region was observed in the near-surface region where Mo thermally diffused toward either the surface or the grain boundary, which induced an approximately 1.4 μm Ni3Fe alloy layer in this region. The carbide-containing layer extended to approximately 7 μm underneath the Ni3Fe layer. The presence of graphite dramatically changes the mechanisms of corrosion attack in UNS N10003 in molten FLiBe salt. In t...

Stress Corrosion Cracking of Austenitic Alloys in Supercritical Water

Tapered uniaxial tensile samples under constant load and bent ring samples under constant strain were used to investigate the stress corrosion cracking (SCC) of Alloy 600 and 690 in supercritical water. To verify the effectiveness of the tapered tensile sample design, the SCC of two 304 stainless steels with high and low carbon content were tested in supercritical water at 400°C, 25MPa and 10–15ppb dissolved oxygen. Both testing methods are effective to initiate SCC in 304SS, Alloy 600 and 690. Based on the bent ring SCC testing, the SCC resistance of Alloy 690 in supercritical water is lower than Alloy 600. This unexpected lower SCC resistance is probably due to the much lower tensile ductility (less than 10%) of the tested Alloy 690 as compared with the standard 40–50% ductility and the severe plastic deformation and strain in the bent ring samples. The large TiN particles and their inhomogeneous distribution in the alloy may promote the SCC initiation in this specific Alloy 690 in the bent ring samples. SCC is more likely to initiate from the Electrical Discharge Machined (EDM) or end mill machined surfaces than from ground and polished surfaces.

Electrochemistry in Molten LiF-BeF 2 Salt for Fluoride Salt-Cooled High Temperature Reactor Applications

A dynamic beryllium reference electrode has been developed for in situ electrochemistry studies of molten LiF-BeF2 (66-34 mol%) (FLiBe) salt which is being considered as the primary coolant for the fluoride salt-cooled high temperature reactor (FHR). Measurement and control of the salt redox potential can mitigate corrosion which is an important challenge in high temperature molten fluoride salts. This three electrode probe has demonstrated good reproducibility in the measurement of the salt’s redox potential with an error of ±4 mV at 500 °C. The response of the probe to changes in salt chemistry was investigated by the intentional additions of fluorides of nickel, iron, and chromium, which are corrosion products of common structural materials. The equilibrium potentials of these metal impurity fluorides were measured in solutions containing approximately 250 ppm of each impurity using cyclic voltammetry. This approach is being extended to study electrochemical purification of molten FLiBe.

Emissivity of Candidate Materials for VHTR Applicationbs: Role of Oxidation and Surface Modification Treatments

The Generation IV (GEN IV) Nuclear Energy Systems Initiative was instituted by the Department of Energy (DOE) with the goal of researching and developing technologies and materials necessary for various types of future reactors. These GEN IV reactors will employ advanced fuel cycles, passive safety systems, and other innovative systems, leading to significant differences between these future reactors and current water-cooled reactors. The leading candidate for the Next Generation Nuclear Plant (NGNP) to be built at Idaho National Lab (INL) in the United States is the Very High Temperature Reactor (VHTR). Due to the high operating temperatures of the VHTR, the Reactor Pressure Vessel (RPV) will partially rely on heat transfer by radiation for cooling. Heat expulsion by radiation will become all the more important during high temperature excursions during off-normal accident scenarios. Radiant power is dictated by emissivity, a material property. The NGNP Materials Research and Development Program Plan [1] has identified emissivity and the effects of high temperature oxide formation on emissivity as an area of research towards the development of the VHTR.