2.09.4.1 FBR Application

Type 316 stainless steel was the most commonly used steel for FBR applications, and was used in the early prototype and demonstration reactors in the United States and around the world in the mid-to-late 1960s, until void swelling was discovered in 1967. As shown in Figure 15, type 316 is very prone to void swelling. Alloy D9 is an advanced austenitic alloy that was developed during the US National Cladding and Duct Development Program in the 1970s and 1980s.23 This program was designed to provide advanced materials for the liquid metal fast breeder program with a primary goal of reducing swelling at high relative to types 304 and 316 stainless steels.

D9 is a Fe-15Cr-15Ni alloy with Ti added to pro­duce TiC particles during reactor irradiation or higher temperature creep. Slight variants on this composition have been used in nuclear reactor appli­cations in the United Kingdom, France, Germany, Japan, Russia, and most recently, India. A variant of D9 has currently been used successfully as cladding and for other components in both the Phenix and SuperPhenix reactors. The D9-type austenitic stain­less steel has a clear advantage in void swelling resis­tance compared to 316 steel, but at high doses, voids form and swelling occurs (Figure 15). Several advanced austenitic stainless steels, including the creep-resistant HT-UPS steel, based on much more stable nano-dispersions of MC-precipitate micro­structures relative to the D9-type steel may have better void swelling resistance than D9

steel.10,11,14,24 However, FBR irradiation data are

needed on the HT-UPS steel to establish such benefits.

Currently, FBR technology in the United States is one of the advanced reactor options being considered by the Gen IV Nuclear Energy Systems Initiative.26 Advanced austenitic steels like D9 have higher maximum allowable design stresses for structural
components that are in the sodium-cooled reactor compared to standard 316 steel but are not exposed to the highest radiation doses found for fuel cladding and duct components. Figure 17 shows a comparison of the allowable stress benefits of the Ti-modified D9-type alloy, on the basis of higher values of UTS at lower temperatures and design rules that define maximum allowable stress as 33% of the UTS. At higher temperatures, creep-rupture strength is more limiting that tensile strength, so the maximum allow­able stress is defined as 66% of the creep-rupture stress for rupture after 100 000 h. While creep — rupture of the D9 alloy is only modestly better than that of type 316 (Figure 6), the design window defined by UTS and creep-rupture properties is larger. The HT-UPS steels are austenitic stainless steels developed from the same austenitic steel alloy composition as D9, but with a combination of addi­tions of Ti, V, and Nb rather than just Ti, and minor additions of B and P (Table 3). These composi­tional modifications to the HT-UPS steels produced unusually stable nano-dispersions of MC-carbide precipitates for much better creep-resistance than the D9 steel at 700-800 °C (Figure 6). 10,11 The creep-rupture resistance and strength of the HT-UPS steels are far superior to that of 316 and 347 steels, better than that of other advanced creep-resistant steels, such as the Nb-stabilized 347HFG, Super 304H, and NF709 austenitic stain­less steels and alloys, and comparable to that of the solid-solution strengthened Ni-based superalloy, 617, as shown in Figure 5. The creep-resistance of the HT-UPS steel at 700 °C and 170MPa is several orders of magnitude better than that of the D9 steel, as shown in Figure 6, so it should provide even larger design benefits for advanced FBR applications. Since FBR technology has recently evolved to include small, modular reactor systems as well as the more traditional larger reactor systems, advanced steels such as the HT-UPS could provide reactor designers with attractive options to improve or optimize FBR systems without dramatically increasing cost.