The H-Zr (hydrogen-zirconium) system

An L/sub 1/ norm minimization scheme is applied to the determination of the impulse response vector h of flaws detected in practical examples of ultrasonic nondestructive evaluation in CANDU nuclear reactors. For each problem, parametric programming is applied to find the optimum value of the damping parameter that will yield the best estimate of h according to a quantified performance factor. This performance factor is based on a quantified analysis of the transitions in estimates of h as the damping parameter is varied over a wide range of possible values. It is shown that for the examined cases in which the true impulse response is a sparsely filled spike strain, the L/sub 1/ norm provides significantly better results than the more commonly used L/sub 2/ norm minimization schemes. These results are shown to be consistent with theoretical predictions. >

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Recovery of a sparse spike time series by L/sub 1/ norm deconvolution

Zirconium alloys are currently used in nuclear power plants where they are submitted to hydrogen pick-up. Hydrogen in solid solution or hydride precipitation can affect the behavior of zirconium alloys during service but also in long term storage and in accidental conditions. Numerical modeling at mesoscopic scale using a “phase field” approach has been launched to describe hydride precipitation and its consequences on the mechanical properties of zirconium alloys. To obtain realistic results, it should take into account an accurate kinetic, thermodynamic, and structural database in order to properly describe hydride nucleation, growth, and coalescence as well as hydride interaction with external stresses. Therefore, an accurate structural characterization was performed on Zircaloy-4 plates and it allowed us to identify a new zirconium hydride phase called ζ. The ζ phase has a trigonal symmetry and is fully coherent with hcp αZr. The consequences of this new zirconium hydride phase on hydride transformation process and stress-reorientation phenomenon are discussed. A first attempt to numerically model the precipitation of this new zirconium hydride phase has been undertaken using the phase field approach.

Characterization of Zirconium Hydrides and Phase Field Approach to a Mesoscopic-Scale Modeling of Their Precipitation

The aim of this work is to model the ductile fracture of Zircaloy-4 sheets containing various amount of embrittling hydride precipitates. The proposed model is based on the Gurson–Tvergaard–Needleman model which is extended to take into account plastic anisotropy and viscoplasticity. The mechanical behavior is identified by conducting tensile tests and the damage nucleation rate (hydride cracking) is measured using quantitative metallography. The model is then used in a Finite Element software to represent crack propagation in Center Crack Panel specimens. Results are strongly mesh size dependent. The mesh size has to be identified by comparison with experimental results. Finally the model is validated by simulating crack initiation and growth in moderately complex structures (sheets containing holes).

An anisotropic Gurson type model to represent the ductile rupture of hydrided Zircaloy-4 sheets

Behavior and failure of uniformly hydrided Zircaloy-4 fuel claddings between 25 °C and 480 °C under various stress states, including RIA loading conditions

A critical review is presented of published data on hydrogen, hydrides, and delayed hydrogen cracking (or hydrogen-induced delayed cracking, HIDC) in zirconium and its alloys. It is shown that factors such as solution temperature, cooling rate, and applied or residual stresses greatly affect the hydride morphology and orientation. The most detrimental hydride morphology is shown to be that when the hydride is oriented normal to the applied stress and has the morphology of a continuous sheet. Although knowledge of the mechanism(s) of delayed hydrogen cracking has improved greatly in the past five years, further research is needed to provide a better understanding of the process. The review concludes with a discussion of delayed hydrogen cracking in the practical case of nuclear-reactor pressure tubes, and presents possible remedies to ensure no further reoccurrence of the phenomenon in commercial reactors.

U. Kosasih

Papers

Hydrides and delayed hydrogen cracking in zirconium and its alloys