JAEA manufactured 12Cr-ODS steel cladding (1DK and 1DS) and Argonne National Laboratory in the United States qualified a welding process that employs PRW. Fuel pins composed of 12Cr-ODS steel cladding and MOX fuel pellets were successfully fabricated and qualified, and irradiated up to 35 dpa at EBR-II.73 The ODS cladding with high smear density solid pellet MOX fuel did induce some diametral strain, demon­strating some in-core ductility. This program demon­strated the viability of ODS steel as a potential cladding material for long-life advanced FRs.

4.08.8.3.2 DT2203Y05 in Phenix

Fuel pins with DT2203Y05 cladding were irradiated in an experimental capsule placed in a special subas­sembly in Phenix. The process by which they were manufactured was described in Section 4.08.6. The dose reached at midplane was 81 dpa and the temper­ature along the fuel pin ranged from 400 to 580 °C.

It was observed by TEM that the uniform distribu­tion of fine oxides totally disappeared, and a few large oxides were also fragmented into smaller ones. The recoil resolution of particles is a process where the atoms that compose particles are ballistically ejected by an impinging neutron. Dubuisson63 pointed out that the atoms ejected from oxides by ballistic dissolu­tion depend on radiation-enhanced solute diffusivity and enhanced solubility under irradiation.

A uniform distribution of tiny particles <10 nm in size and with a density higher than the original oxide
density, was observed in the lower part of the fuel pin at temperatures <500 °C. These precipitates were found to be a’-phase, as shown in Figure 43(a).63 At irradiation temperatures above 500 °C, precipita­tion ofw-phase was uniformly distributed throughout all grains, as shown in Figure 43(b). Their chemical composition was slightly different from that of the intergranular w-phase that was present before irradi­ation. At a high temperature and low dose, w-phase is replaced by the thermal precipitation of Laves phase, as shown in Figure 43(c).

From the results of tensile tests at levels corres­ponding to the fissile column, rupture occurred with­out striction, and uniform and total elongations were equal. The elongation values reached 0.2% close to the maximum dose. These results indicate that DT2203Y05 cladding was highly embrittled by irra­diation. At the bottom of the fuel pin, where the temperature is below 500 °C, a’ precipitation, oxide redistribution and dislocation loops are the main features of the microstructure. Dubuisson63 pointed out that dislocation glides on the dislocation denuded bands in the hardened materials, that the deformation is all localized in these bands, and that this heteroge­neous shear could nucleate cracks that then propa­gate along these channels. At higher temperatures, w precipitation induces a loss of ductility.

4.08.7 Summary

The formation process of nanosized oxide particles through decomposition by MA and subsequent pre­cipitation by annealing was reviewed. Based on
information concerning the irradiation embrittle­ment of DT2203Y05 cladding produced by CEN-SCK Mol due to the formation of a0-phase below 500 °C and w-phase above 500 °C, 9Cr-ODS and 12Cr-ODS steels containing low Cr and low Ti were developed. The manufacture of cladding and improvement of the creep rupture strength in the hoop direction were successfully achieved by introdu­cing a—g reverse transformation or recrystallization. 9Cr-ODS steel has a unique structure consisting of tempered martensite and residual ferrite that induces superior strength through finely dispersed oxide par­ticles, which are promising candidates for advanced SFR fuel cladding. 16Cr-4Al-ODS steels present an advantage due to their superior resistance to corro­sion and oxidation in LBE and SCPW environments. There is still uncertainty concerning the irradiation performance of ODS steels, such as the oxide particle dissolution due to their diffusion-based mechanism. In order to substantiate the use of cladding materials as advanced fast reactor fuels, abundant ODS cladding fuel pins should be irradiated, and their results should provide feedback contributing to the further improve­ment of ODS steels.