A Functionally Graded Composite for Service in High-Temperature Lead- and Lead-Bismuth–Cooled Nuclear Reactors—I_ Design

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Short and Ballinger A COMPOSITE FOR HIGH-TEMPERATURE LEAD- AND LEAD-BISMUTH–COOLED REACTORS—I Downloaded by [Australian Catholic University] at 03:26 17 August 2017 Fig. 4. Micrographs of s-Cr precipitation and its effects in alloy HK-40, an Fe-Cr-Ni alloy, after exposure to 8508C for no more than 4000 h ~Ref. 45!. ~a! Optical micrograph of s-Cr forming on prior austenitic grain boundaries and as highly oriented platelets. ~b! SEM image of a crack proceeding directly along s-Cr phase boundaries, showing severe weakening of the material at the phase boundary. have at least 10 to 12 wt% Al and 4 to 5 wt% Cr in order to adequately resist corrosion in gaseous NO x environments. This composition falls below the weldability line in their study. 56 In addition, adding TiC to prevent cracking ~by acting as a hydrogen sink! results in the precipitation of TiC on grain boundaries in the interdendritic liquid as the melt cools, 55 leading to microstructural inhomogeneity in the weld. The cladding layer for the FGC was therefore based on the Fe-Cr-Si system. In choosing the composition of this alloy, the benefits of added corrosion resistance were considered along with microstructural, macrostructural, and radiation considerations to find the best balance of properties. The ultimate goal of this process was to ensure that the cladding remains intact over decades of fast reactor operation. As mentioned before, more chromium and silicon increase the corrosion resistance of F0M steels. Therefore, the Fe-Cr-Si chemistry was chosen to maximize the concentrations of chromium and silicon without compromising radiation performance, corrosion resistance, microstructural stability, or ease of processing. II.B.1. Chromium Concentration Fig. 5. Ellingham diagram for metal0metal oxide systems found in this study. 51 Oxides are stable above each solid line, and metals are stable below each solid line. Oxygen concentrations and corresponding hydrogen–to–water vapor ratios are shown with dotted lines. welding of these alloys. 55 The weldability of similar Fe-Cr-Al alloys with lower carbon is worse, resulting in a shift of the curve to lower Cr and Al concentrations. 56 Regina et al. have shown that an Fe-Cr-Al alloy must Studies have shown that corrosion resistance increases with added chromium content. 57 The addition of too much chromium can lead to s-Cr precipitation, as mentioned earlier. However, a chromium concentration of 1 to 2 wt% over the solubility limit is not expected to produce severe enough s-Cr precipitation to adversely affect a thin layer of cladding material. Therefore, based on the desire to increase corrosion resistance and on reasonable assumptions about the prevention of secondary phase precipitation, a chromium concentration of 12 wt% was chosen for the cladding layer of this composite. 372 NUCLEAR TECHNOLOGY VOL. 177 MAR. 2012
Short and Ballinger A COMPOSITE FOR HIGH-TEMPERATURE LEAD- AND LEAD-BISMUTH–COOLED REACTORS—I Downloaded by [Australian Catholic University] at 03:26 17 August 2017 Fig. 6. Weldability map for Fe-Cr-Al alloys, showing which alloys crack during gas tungsten arc welding due to hydrogeninduced cracking. TiC is added as a hydrogen sink to mitigate this problem. 55 II.B.2. Silicon Concentration Increasing the silicon level is also desirable, as it has been shown to encourage the formation of SiO 2 beneath the Cr 2 O 3 oxide layer. Lim has shown that an increasing Si concentration leads to the formation of a higher volume fraction of SiO 2 and FeSi 2 O 4 precipitates. 58 The total oxide layer thickness has also been shown to vary inversely with silicon content. These data can be interpolated beyond those collected by Lim to suggest that further increasing the silicon concentration should theoretically produce a fully dense, thin layer of SiO 2 . This effect was recently observed by Short. 29 The speed of formation of this layer is also expected to be proportional to the silicon content as well as the chromium content. This is because a higher chromium content would lead to faster formation of a thinner oxide layer, allowing little enough oxygen to enter so that only SiO 2 can form underneath the layer of Cr 2 O 3 . Once this second layer is complete, a very effective, stable barrier to oxygen and dissolution will remain regardless of the fate of the iron oxides facing the liquid metal. This is because oxides act as powerful diffusion barriers to ion transport as compared to metals. The diffusion coefficients for iron in oxides are between 3 and 12 orders of magnitude lower than their counterparts in relevant metals, as can be seen in Fig. 7 ~Ref. 29!. In addition, forming a thinner oxide layer results in a smaller depletion zone of Cr and Si directly beneath it, as forming the oxide layer consumes less Cr and Si from the bulk. A thinner oxide layer therefore results in less time required for Cr and Si concentrations directly beneath the oxide to return to their original levels by diffusion from the bulk. This results in a faster ability to repassivate should the oxide layer be mechanically removed more than once. Adding too much silicon does have its drawbacks. Experimental studies performed by Miller et al. have suggested that silicon can induce radiation embrittlement via preferential segregation at dislocations and grain boundaries, probably as a result of physical displacement by radiation. 59 The presence of too much silicon can also lead to the precipitation of Si-rich precipitates, mainly Fe-Si intermetallic compounds, that would lead to localized depletion of silicon in the matrix. 60 Enhanced liquidmetal embrittlement of silicon-enriched Fe-Cr bainitic steels has been confirmed by Van den Bosch et al. 61 This is not expected to be a concern in the context of this composite, as the corrosion-resistant layer is specified to be completely ferritic, thereby avoiding problems of lack of ductility that could be enhanced by lead-bismuth. Therefore, based on concerns relating to radiation embrittlement due to silicon segregation and the desire to avoid Si-rich phase precipitation, a maximum silicon concentration of 2.0 wt% was chosen for the cladding layer. At the same time, a lower limit of 1.25 wt% was chosen based on the work of Lim as being the minimum Si concentration, in combination with 12 wt% Cr for adequate corrosion resistance in lead-bismuth. The final composition of this composite’s cladding layer was therefore chosen to be Fe-12Cr-2Si. II.C. Comparison of the Cladding Alloy with Previous Work Fe-Cr-Si–based alloys have been developed in the past with the intent of meeting all the requirements for NUCLEAR TECHNOLOGY VOL. 177 MAR. 2012 373
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A Functionally Graded Composite for Service in High-Temperature Lead- and Lead-Bismuth–Cooled Nuclear Reactors—I_ Design

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