Experiment and Modeling of Advanced Fuel Rod Cladding Behavior Under LOCA Conditions: Alpha-Beta Phase Transformation Kinetics and EDGAR Methodology

Abstract

The behavior of PWR fuel rod cladding under LOCA conditions strongly depends on the interactions between the metallurgical evolution (phase transformation) and the thermomechanical behavior at high temperature. FRAMATOME needs for the qualification of advanced zirconium alloys have motivated an extensive study of the phase transformation kinetics and the development of a new test facility, EDGAR-2, to perform thermomechanical tests. The first part of the paper deals with an experimental study and modeling of the α↔β phase transformation kinetics. High-temperature/high-sensitivity calorimeter and fast dilatometer facilities have been used to measure the on-heating α⃗β and on cooling β⃗α phase transformations from near equilibrium conditions up to LOCA conditions (heating-cooling rates up to 100°C/s). The equilibrium fraction of α/β phase as a function of temperature is derived from calorimetric measurements using the Zhu and Devletian model and described using a modified Johnson-Mehl-Avrami equation. Modeling of the α↔β kinetics is given by the differential Holt equation. The present results show that the Holt model gives a good description of the α⃗β kinetics upon heating but is not able to describe accurately the β⃗α phase transformation upon cooling, probably because it does not take into account the occurrence of the partial martensitic transformation during cooling, especially for the higher cooling rates. An important feature of the current study is that, despite the α/β equilibrium temperatures for M4 (ZrSnFeV) and M5 (ZrNbO) alloys being lower than that of Zy-4, for kinetic reasons the α to β phase transformation occurs in the same temperature range for the three alloys in the case of fast thermal transients (i.e., 10°C/s). This observation could be related to the slower thermal diffusion of Nb and V atoms compared to that of Fe and Cr. This slower diffusion explains why the thermalmechanical behavior of new M4 and M5 alloys is quite equivalent to Zy-4 in LOCA conditions. The second part of this paper deals with the thermomechanical tests. The EDGAR-2 test facility performs single rod tests under any internal pressure and clad temperature transients in steam environment. The advanced zirconium alloys M4 and M5 have been tested under steady-state conditions of pressure and temperature, continuous-heating and constant-pressure conditions for various heating rates and pressure, and some LOCA representative pressure temperature transients. The experimental program covers the LOCA conditions, and the rupture occurs from a few seconds up to 2000 seconds. The results are compared with those of the SRA optimized Zy-4 clad using the Monkman-Grant correlation, rupture ductility versus burst temperature, and burst criteria. EDGAR models for the prediction of the evolution of the transformed β-phase volume fraction and of the diametral deformation, time to rupture, and uniform diametral rupture elongation in typical LOCA conditions are derived for advanced alloys from the two parts of this study. These models may easily be implemented in most accident simulation computerized codes for safety analysis.

Identification of the steady-state creep behavior of Zircaloy-4 claddings under simulated Loss-Of-Coolant Accident conditions based on a coupled experimental/numerical approach

“…Eccentricities Ξ were evaluated between 1 and 3% using equation (2). This rather small eccentricity prevents the specimen from early non-axisymmetric localization during the test.…”

“…The full analysis of a LOCA requires complex coupled physical modeling including thermal-hydraulics and mechanical behaviors of fuel rods. During its operating life, the water in the primary loop of a Pressurized Water Reactor (PWR) is pressurized at [2]. Cladding micro structure is modified by irradiation under normal operations, however, irradiation damages are annealed early in the ballooning phase of a LOCA [3].…”

Modelling and Simulation of Reactor Fuel Cladding under Loss-of-Coolant Accident Conditions

“…Eqs. (11)- (12), are all based on a uniform concentration of oxygen in sample cladding tubes. More elaborate oxidation models and/or correlations could be introduced, but that would require accurate experimental data, which specify the creep and burst as a function of local oxygen concentration.…”

“…IV). Other workers have recommended phase mixing rules for calculating the creep rate 11) and burst stress 12) in the two-phase coexistence ð þ Þ domain of Zr alloys. Hence, it is worthwhile to utilise the output of our phase transition model, that is, the fraction of the -phase as a function of time (temperature), to calculate the creep rate and the burst stress in the ð þ Þ domain, through Eqs.…”

“…4) and 5), which however, did not include a phase transformation kinetics model. More recent modelling efforts are the works of Forgeron et al 12) and Brachet et al 13) and the implementation of such models in a fuel performance code. 11) Although our general modelling approach is similar to that presented in Refs.…”