Nuclear Science User Facilities 58 The clustering of Si, Mn, and Ni is a notable irradiation induced chemistry change.The formation of similar clusters containing Si, Mn, and Ni is often reported in F/M steels such asT91, HCM12, and HT9 and in austenitic stainless steels, but rarely in ODS steels. Several cluster formation mechanisms have been proposed: • Si, Mn, and Ni form oxide phases such as SiO2 , Mn3 O4 , or NiO and nucleate in conjunction with the existing oxide phases. • Si, Mn, and Ni form silicides such as Mn4 Si7 or Ni3 Si2 and nucleate as their own precipitates, independent of the existing oxides. • Si, Mn, and Ni diffuse towards sinks (such as oxides) due to radiation induced segregation (RIS). The first possible mechanism must consider the enthalpy of formation of the Fe, Cr,Y,Ti, Si, Mn, and Ni oxides relevant for 9-Cr ODS.All of these elements have a high affinity for oxygen, with the Si and Mn having enthalpy of oxide formation on the same order as that of Fe, Cr, andTi. Yet because Si, Mn, and Ni oxides or clusters are not present in the as-received condition, their clustering is irradiation induced. In addition, the oxide formation mechanism does not explain why Si, Mn, and Ni also cluster upon irradiation in other F/M and austenitic stainless steels that do not contain pre-existing oxide compounds.Therefore, this is not likely to be the primary mechanism of Si, Mn, and Ni clustering. The second possibility considered is that the Mn and Ni form silicide compounds and precipitate out of solution.Although all of these silicides have a negative enthalpy of formation, these enthalpies are less favorable than those of the oxides considered in the first possible mechanism. Further- more, if silicide formation were the primary mechanism for Si, Mn, and Ni clustering in ODS and F/M steels, it would be expected that Mo would also cluster in other F/M steels due to the negative Mo-silicide forma- tion enthalpy, yet this has not been observed in irradiated F/M alloys. Thus, silicide formation is not likely the primary mechanism of Si, Mn, and Ni clustering in ODS and F/M steels Finally, the possibility of RIS is consid- ered as the driving mechanism for Si, Mn, and Ni clustering. Si, Mn, and Ni are found in higher concentrations within the oxides following irradia- tion.These same species are known to segregate toward grain boundaries in commercial F/M and austenitic stainless steels.Thus, the RIS mecha- nisms reported in literature appear to be consistent with the solute enrich- ments at oxides observed in this study, suggesting that the clustering of Si, Mn, and Ni is the likely result of RIS toward existing point-defect sinks within the matrix, such as oxide nanoclusters. This work was performed primarily by Matthew Swenson and Corey Dolph, under the research advisorship of Janelle Wharry.At CAES, the students were assisted byYaqiaoWu, Joanna Taylor,Alyssa Bateman, and Jatuporn