Austenitic stainless steels are also a key component for MFR systems because of many of the properties and vast experience in fission nuclear systems described above. An important example of rapid alloy development is presented, which is part of the US contribution to the international fusion demon­stration project in France, called ITER, and in­cludes ^20% of the first wall (FW) and shield components. The ITER project could include nearly

100 modules from austenitic stainless steel (316LN — ITER Grade or — IG) each weighing ^3.5 T and 366 FW panels (SS/CuCrZr/Be). An example of the shield wall module is shown in Figure 18. Traditional machining of the cooling channels shown in Figure 18 results in a loss of ^30% of the raw mate­rial during fabrication. A US industry manufacturing assessment indicates that casting the shield modules (including the cooling channels) results in major cost savings when compared to fabrication via welding together quarter modules machined from large for­gings. However, because casting produces a large grain size, low dislocation density, and extensive seg­regation of alloying elements, the strength properties of such cast components are frequently inferior to those of conventionally forged and annealed compo­nents. Additional R&D has been performed27 in recent years to ensure that the properties of cast 316L(N)-IG equivalent grades meet ITER Structural Design Criteria,28-30 which require cast steel performance that is similar to or no worse than wrought equivalent material.

On the basis of past development experience, archive material analysis, and simulations, several improvement strategies were identified as part of this effort to modify and upgrade the properties of the standard CF3MN cast stainless steel grade (which is described in more detail in Busby et a/.27) (Table 3). The primary strategy identified for boost­ing the YS was increased strengthening by additions of N and Mn; N is the most powerful solid solution strengthener (0.1 wt% should increase strength by 50 MPa). However, Mn increases are also required to raise the solubility limit of N. In addition, Mn is also an austenite stabilizer, and increases both strength and strain-hardening rate. Industrial part­ners were involved in the fabrication of test alloys

to help speed scale-up to larger test articles. Alloys with the most minor alloying additions were studied most extensively, with one alloy showing the greatest performance, which is designated CF3MN-US (Table 3). Mechanical testing (tensile, impact, and fracture toughness) was performed along with examinations of physical properties, porosity, weldability, and resistance to stress-corrosion cracking.

To accelerate the transition to heavy-section cast­ings, tensile tests were conducted on both cast keel blocks and specimens cut from the larger cross­section as-cast ingots (from both the surface and center regions). These different specimen locations help illustrate the potential differences in mechanical performance. The results from room temperature test­ing demonstrated no systematic difference between types of specimens, locations of testing, or locations or types of specimen used.

Elevated temperature tests were also performed. The yield stress results for samples in the as-cast condition are illustrated in Figure 19 and are com­pared to the minimum requirements for use in ITER applications. At all temperatures, the CF3MN-US exceeds the minimum required strength and meets the ITER acceptance criteria.

An evaluation of impact properties on the CF3MN-US was also conducted. Initial testing was performed using a drop-weight machine setup with a maximum capacity of 325J potential energy for ini­tial screening tests. Two tests of CM3F-US with the drop weight machine set at 325J were performed.

Only one specimen at —196 °C (liquid nitrogen tem­perature) broke. To demonstrate the excellent tough­ness of the materials in the temperature range of interest for the ITER shield module applications, additional testing was performed at higher tempera­tures. Tests were performed at room temperature (27 °C), 100, 200, and 300 °C, again using a drop — weight machine. All tests at all temperatures were fully ductile and very tough, and none of the speci­mens tested fractured. Figure 20 shows a photograph of a Charpy specimen tested in the drop-weight machine at 300 °C. It is clear that the specimen did not fracture when tested with a maximum potential energy of 270J, so its actual impact toughness is higher than that. As indicated earlier in this section, wrought 316 and 347 steel typically have Charpy impact toughness of 100—150J, so the CF3M-US cast stainless steels exhibit excellent impact tough­ness, even at the liquid nitrogen temperature (—196 °C). The stated minimum impact toughness for the ITER shield module materials is 60 J, whereas the tested specimens exhibited impact toughness values ranging from 140 to 262 J at —196 °C.

Finally, testing of fracture toughness properties on the CF3MN-US was performed. The 12.5-mm thick compact tension (0.5T C(T)) specimens were tested at room temperature and at 90 and 190 °C. At least two specimens of each alloy were tested at each temperature in general accordance with the ASTM E 1820-06.31 As expected from the simpler Charpy
impact data reported previously, all alloys exhibited very high fracture toughness at all test temperatures. Moreover, none of the specimens exhibited crack extension regardless of test temperature. The value of critical J-integral, J1C, was above 800 kJ m—2 at all tested temperatures, comparable or better than that of the equivalent wrought austenitic steel, meeting the ITER acceptance requirements.

While not shown in detail here, testing and evalu­ation of the most promising alloy under development, CF3MN-US, has been completed for several proper­ties. Composition, ferrite content, microstructure, porosity, mechanical properties (tensile, impact, and fracture toughness), irradiation performance, stress — corrosion cracking performance, and weldability have all been found to meet ITER acceptance cri­teria. This combination of past experiences, expertise, and new tools demonstrates new opportunities for rapidly developing improved austenitic steels for advanced reactor applications such as ITER. It is also reasonable to expect that the new CF3MN-US steel may have attractive properties in either the cast or wrought condition for advanced LWR core or structural support designs and applications.