Predicting High-Temperature Transient Deformation from Microstructural Models

Abstract

We have developed a multicomponent model to describe the transient plastic deformation of Zircaloy fuel sheathing during postulated loss-of-coolant accidents (LOCA). From deformation maps, we identify three creep mechanisms, which in principle, occur in all three phase fields of Zircaloy-4 (α, α + β, β): diffusional creep, dislocation creep and athermal strain. Diffusional creep (phase boundary or grain boundary sliding) is modeled in the α and α + β phase fields. Dislocation creep is modeled in all three phase fields and controlled by a work-hardening/recovery expression. Athermal strain has practical significance only at lower temperatures (< 900 K). The sum of the strain rates of the three deformation mechanisms is the total plastic strain rate. At a particular point in the stress-temperature field, one mechanism usually dominates. Microstructural changes that affect deformation are also taken into account; these are changes in grain structure, recrystallization, and the α ⇌ (α + β ) ⇌ β phase transformation. When two components are present (α + β or recrystallized and unrecrystallized), they are assumed to deform with equal rates according to the appropriate single-phase model. The individual components of the model give excellent agreement with the isothermal data on which they are based, and the entire model gives very good agreement with transient data over a range of temperatures from 700 to 1600 K, a range of heating rates from 0 to 100 K/s, and a range of strain rates from 10-5 to 10-1 s-1. Several applications of the model in evaluating the history dependence of deformation are given.

FEMU based identification of the creep behavior of Zircaloy-4 claddings under simulated RIA thermo-mechanical conditions

“…The α grain size 575 does not evolve significantly with time while the β grains grow from very small grain size (nucleation) to several tens of microns. These results are consistent with the results of the literature [30,33]. Figure 12 compares the radial displacement normalized by the initial inner radius obtained experimentally and 580 numerically at the center of the ROI.…”

“…While the stress exponents identified are in the expected order of magnitude under the tested conditions, the activation energies identified following a very fast heating are much higher 490 in the two phases than the ones identified in the literature in isothermal conditions. The activation energy in the α phase for a diffusion mechanism is usually estimated around 150kJ/mol [27,21,20] for isothermal tests, and the one in the β phase for a dislocation mechanism is about 140kJ/mol [28, 29,30,21].…”

“…To obtain physically acceptable results the initial grain size was constraint to be inferior than 10 µm and the maximal grain size at 100s was bounded to be inferior than 50µm. These boundaries were set following the microstructure data of 560 [30] for Zircaloy-4 and of [33] for M5 alloy). The idea here is to show that a grain growth contribution in the creep law enables well reproducing the hardening of the material.…”

Characterizing High-Temperature Deformation of Internally Heated Nuclear Fuel Element Simulators

“…Basic geometry of the fuel pin model. creep term from the AECL's ELOCA code Zircaloy creep model [6]. The creep rate is given by:…”

“…This model is just part of the microstructural creep model first developed for determining high temperature circumferential creep and implemented in the ELOCA code [4,6]. The original form of the model included tracking of the microstructure of the Zircaloy sheathing to account for grain growth, strain For personal use only.…”