Influence of Transition Elements Fe, Cr, and V on Long-Time Corrosion in PWRs

Abstract

Two major factors that affect corrosion behavior of Zircaloy-type alloys have been evaluated in a PWR cladding development program ongoing since the early eighties. One of the important parameters was found to be the alloying concentration and distribution of transition elements. The influence of transition elements (Fe, Cr, and V) was studied on corrosion coupons and experimental fuel rods. Isothermal long-time corrosion tests were performed out-of-pile (1 to 3 years in 300 to 370°C) pressurized water and 400°C steam with and without LiOH additions) and in PWRs (up to 6 years at 310 to 335°C). Experimental fuel rods were irradiated in PWRs up to nine annual cycles. The experiments show a large reduction of corrosion rate with increasing content of transition elements in all environments. Above a certain transition element content no further improvement in corrosion is observed. The test results clearly suggest the use of transition element concentrations higher than specified by ASTM for Zircaloy-2 and -4. The beneficial effect of transition elements increases with increasing exposure time. The most significant effect of corrosion behavior was seen after six years of PWR exposure, especially at high temperatures. Different transition elements influence corrosion differently, depending on the environment. Out-of-pile in 350 to 370°C pressurized water and 400°C steam, a beneficial effect is exhibited only for Fe, but not for Cr and V. In PWR and in diluted LiOH autoclave tests, all three transition elements reduce corrosion. for Zr-based alloys with transition element (TE) concentrations meeting the ASTM Zircaloy-4 specification range, the fraction of corrosion hydrogen picked up by the metal increases with increasing corrosion resistance. Zr-based alloys with enhanced transition element concentrations (above the ASTM limits) exhibit reduced hydrogen pickup fractions.

Corrosion of Zirconium Alloys Used for Nuclear Fuel Cladding

“…Moreover, there is a correlation between f t H and oxidation kinetics for different alloys (56), suggesting a common oxidation and hydrogen pickup mechanism. These results, supported by previous studies (108)(109)(110), are plotted in Figure 17. An inverse relationship between the oxidation kinetics and f t H is clearly observed: the lower the n, the higher the f t H and vice versa.…”

Effect of Alloying Elements on Hydrogen Pickup in Zirconium Alloys

“…If we assume that the electron transport is rate limiting, the hydrogen pickup mechanism will be linked to the corrosion kinetics through the electron flux: the higher the oxide electronic conductivity, the higher the electron flux (and thus the higher the corrosion rate) and the lower the hydrogen pickup fraction. This is also in accordance with results reported on binary Zr-Fe alloys [59] and on Zr-Sn-Fe-Cr alloys [60].…”

Charged species redistribution at electrochemical interfaces: a model system of the zirconium oxide/water interface

“…Experimental evidence has shown that these elements influence the degradation rate through a coupled chemical and mechanical mechanism. 62,63 The space-charge profiles presented here contribute to the decoupling of these effects. Given the above analysis, we conclude that both the water chemistry and the solid-state defect chemistry contribute non-negligibly to the surface space charge effect.…”