Experimental study of the fracture kinetics of a tubular 16MnNiMo5 steel specimen under biaxial loading at 900 and 1000 °C. Application to the rupture of a vessel bottom head during a core meltdown accident in a pressurized water reactor

N Tardif,M Coret,A Combescure

doi:10.1016/j.nucengdes.2011.01.026

Abstract

This study proposes an experimental analysis of the creep, crack initiation and crack propagation phases in a 16MnNiMo5 steel subjected to thermomechanical loading representing a core meltdown accident in a pressurized water reactor involving the transfer of a molten corium bath to the bottom head. The experimental setup enabled a biaxial mechanical loading (internal pressure+tension) to be applied to a tubular specimen at 900 and 1000°C. In addition to the usual temperature, load, displacement and pressure measurements, the specimen was observed by two high-speed numerical cameras and an infrared camera. The crack's initiation and propagation conditions and the depressurization law were inferred from these measurements. At such temperatures, creep induces very large strains prior to the occurrence of the cracks which, in the worst-case scenario, can propagate at velocities as high as several meters per second. The design of the experiment enabled us to study the influence of the temperature (magnitude and hoop distribution), of the toughness of the steel (two grades were studied) and of the volume of pressurized gas. The results show that creep and crack propagation are highly dependent on temperature, and also that crack initiation and propagation are highly dependent on the degree of heterogeneity which is responsible for the localized initiation of the crack.

Highlights

This study was undertaken in the context of problems raised by severe accidents in French pressurized water reactors
We are considering the case in which the degradation of the core leads to the transfer of a molten corium bath, which is a highly caloric mixture of an oxidized phase U-Zr-0 and a metallic phase derived from the structural materials of the core, toward the bottom head
A simplified experimental model was implemented in order to study the creep, crack initiation and crack propagation phases in the bottom head of a 1350 MW pressurized water reactor subjected to thermomechanical loading representing a core meltdown accident involving the transfer of a highly calorific corium bath to the bottom head

Summary

Introduction

This study was undertaken in the context of problems raised by severe accidents in French pressurized water reactors. We are considering the case in which the degradation of the core leads to the transfer of a molten corium bath, which is a highly caloric mixture of an oxidized phase U-Zr-0 (the product of the melting of the Zircaloy of the cladding and the dissolution of the fuel) and a metallic phase derived from the structural materials of the core (steel, Inconel, etc.), toward the bottom head. At this point, the integrity of the vessel can be jeopardized due to various mechanisms. Even if the vessel successfully survives that transient flow phase, it can subsequently experience creep rupture when the corium bath settles to the bottom head

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Conclusion