Influence of Composition and Condition on In-PWR Behavior of Zr-Sn-Nb-FeCrV Alloys

Abstract

Corrosion and dimensional behavior of Zr-Sn-Nb-FeCrV alloys with varying conditions and compositions having been tested out-of-pile and after irradiation in “hot” PWRs for long exposure times (up to 9 annual cycles) and very high burnups (98 MWd/kgU). The exploratory program for alternative zirconium-based alloys performed on fuel rod cladding and corrosion coupons allowed an understanding of the separate effects of composition and fabrication and yielded Zr alloys appropriate for high-burnup, high-fuel-duty application. In the alloy system Zr-Sn-FeCrV, the corrosion rate decreases with decreasing Sn and increasing Fe+Cr+V content, much more pronounced in-PWR than out-of-pile. The addition of Nb exhibits a similar effect as Fe+Cr+V, but increases corrosion in general if Sn > 0.4%. Zr-Sn-FeCr alloys are sensitive to the thermal treatment during fabrication and therefore have to be fabricated according to a well-defined A-parameter to achieve high corrosion resistance. Nb and V containing Zr-Sn-Fe alloys allow fabrication in a larger A-parameter range. The beneficial effect of high cold work and a low degree of recrystallization on corrosion is significantly reduced if the Sn content is low. At high fluences, some alloys exhibit an increase in the corrosion rate. With decreasing Sn content, the onset of the acceleration is shifted towards higher fluences and its extent is reduced. Alloys containing Sn < 0.2% do not exhibit increased corrosion induced by high fluences, at least up to 1.6E22 cm-2, E > 1 MeV. Irradiation induced and thermal creep depend on the content of Sn, Nb, and O in solid solution, on the grain size, and on the degree of recrystallization. Growth of stress-relieved and partially recrystallized fuel rod claddings strongly depends on the Sn content: it is low if Sn < 0.5%. In addition, the C content and the grain size affect growth of fuel rods.

The characterisation of second phases in the Zr-Nb and Zr-Nb-Sn-Fe alloys: A critical review

“…In an early characterisation of Zr-2.5Nb (wt.%) pressure tubing it was suggested that Fe exists in the form of ZrFe binary intermetallics, and an EDS spectrum was published to that effect in the energy range 5-19 keV [47]. Whilst binary ZrFe intermetallics are known in Zr-Sn-Fe-Cr type alloys [48], and have been demonstrated in Zr-Nb alloys of low Nb and Fe content [13], Zr2Fe and Zr2Si both have body centred tetragonal allotropes and so may be confused with one another when characterised by electron diffraction alone; indeed, it was later reported that Fe-coated zirconium silicides might be mistaken for binary ZrFe phases if Si is not explicitly looked for in the EDS spectrum [49] and it is therefore unfortunate that the authors of the initial study [47] did not publish the full spectrum to include the Si Kα and Kβ emission lines at 1.7 and 1.8 keV. Further, the precipitate assumedly Zr2Fe in Figure 2 of that publication has a periphery that diffracts differently to the main body of the precipitate, suggesting that it is indeed coated with another phase or region.…”

Radiation Effects in Zirconium Alloys

“…and Garzarolli [346] who proposed to use a 'SNO' parameter (wt%Sn+2×wt%Nb+6×wt%O) to illustrate the effect of these alloying element on creep. Concerning in-reactor creep, the dependence in this SNO parameter appears to be lower than that of thermal creep.…”

Autoclave study of zirconium alloys with and without hydride rim

“…for the oxidation of zirconium hydride), it has been suggested that this could result in lower compressive stresses in the oxide. 44 A previous work has shown that a reduction in tin content delays the transition to breakaway corrosion in Zr–Sn–Nb–FeCrV alloys, 45 and it is difficult to see how the two arguments above can be affected by alloy chemistry. Note that alloy chemistry may indirectly impact breakaway by controlling hydrogen pick-up and the propensity for hydride rim formation.…”