Résumé

Any incident can arise in a pressurised water nuclear reactor. It can provoke a temporary increase of the temperature. In these conditions, fission gases diffuse rapidly and form bubbles specially in grain boundaries. These bubbles participate in the swelling of the irradiated material.  These increases of temperature have been simulated in laboratory by thermal treatment on irradiated materials. A morphological study is necessary to understand the phenomenon and to allow the construction of a model of swelling.

The goal of this paper is to propose a method based on automatic image analysis that allows to quantitatively characterise the micro structure of the polycrystalline uranium oxide after intergranular swelling.

Irradiated and annealed samples are polished and observed by scanning electron microscope.

An automatic threshold method is used to obtain the binary bubble image. Grain boundaries are reconstructed from bubbles by using mathematical morphology operators (closing, distance function, watershed...). One can thus count bubbles per boundary length unit and know the specific boundaries perimeter of grains.

An observation of the intergranular fracture surface shows that the distribution of bubble size is narrow.  Their shape is approximately constant.  That allows to use a stereometric relationship and to estimate the number of bubbles by unit of grain boundary surface from polished sections.

These stereological parameters are introduced in the model of swelling which has been validated experimentally.

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