Experimental and Theoretical Studies of Parameters that Influence Corrosion of Zircaloy-4

Abstract

Waterside corrosion of Zircaloy cladding in pressurized water reactors (PWRs) is largely dependent upon the operating parameters and microstructure of the zirconium alloys. The impact of these parameters on the corrosion kinetics of Zircaloys is investigated on the basis of empirical data and experiences that can be interpreted using existing corrosion models. The influence of thermo-hydraulic data, heat flux, local boiling conditions, and of the growing oxide films has been studied from corrosion tests performed in static autoclaves or in out-of-pile loops. These parametric investigations are described as well as the models that were developed. The impact of microstructure is studied from the comparison of the corrosion behavior of different Zircaloy-4 specimens corroded in out-of-pile tests. In particular, a poor corrosion resistance of an experimental Zircaloy-4 material is analyzed as a function of the microstructure close to the metal/oxide interface. The impact of the alloy composition and primary coolant chemistry on the corrosion kinetics of Zircaloy-4 is modeled empirically or uses a mechanistic approach that proposes a series of chemical equations with a mathematical representation of the kinetics. These proposed models are then used to investigate the corrosion behavior of Zircaloy-4 cladding in 17 by 17 plants for rods irradiated at high burnups. Higher PWR operating cycles, core average coolant temperature, power, and elevated primary coolant lithium concentrations (3.5 to 4 ppm) are then simulated and discussed in terms of Zircaloy corrosion resistance considerations.

Lithium and boron analysis by LA-ICP-MS results from a bowed PWR rod with contact

“…The enhanced Li content thus seems to be related to the outer surface of the oxide, and not to the deeper parts of the oxide or the clad-oxide interface. These results contrast with the flatter Li oxide depth profiles from experiments on Li enhanced rapid corrosion of Zry-4 at high local voids [2,4]. Keeping in mind that the cladding in this study is the Nb containing M5 TM alloy, the results do however have similarities with Li and B profiles from other Li and B corrosion tests [8].…”

“…Local overheating in itself was also deemed insufficient to account for the accelerated corrosion. The increased clad oxidation rate was, however, explainable by proposed Li induced corrosion enhancement under local boiling [2,3]. Enhanced concentrations of Li and B due to conditions of local boiling in the crevice-like rod-to-rod contact area was thus hypothesised to explain the accelerated corrosion.…”

Can corrosion and CRUD actually improve safety margins in LWRs?

“…However, in the transition and post-transition stages the oxide layer develops nanoporosity at the grain boundaries, and microcracks parallel to the cladding surface; this evolution could be due to cracking from accumulation of compressive stresses in the oxide, or from crystalline phase transition transformation of ZrO 2 (from tetragonal to monoclinic), or from oxidation of the Zr(Cr,Fe) 2 precipitates [42]. These pores/cracks range from 1 to 500 nm in size [45] and can be interconnected, thus letting the coolant percolate to the metal substrate, which compromises the corrosion protection function of the oxide layer [42]. Typical oxide thicknesses at fuel discharge (at 60…”

“…MWd/kg HM ) are between 10 and 60 m [42]. Roughness <0.5 m and porosity of up to 15% are typical for the thick oxide layer at discharge [45]. The volume fraction of the larger cracks appears to be order of 20-30% ( Fig.…”

Corrosion of Zr Alloys in Water and Steam

“…[106][107][108]. Semi-empirical models of uniform corrosion have been developed to model the corrosion processes in Zircaloys in PWR reactors under working conditions, e.g.…”