An improved methodology to determine pore size distribution and pore density in the high burnup structure

In the periphery of UO2 fuel pellets, a restructuring of the fuel occurs, leading to the formation of the high burnup structure (HBS). Main characteristics of the HBS are the refined grain structure following polygonisation/recrystallization and the steep increase of porosity. The newly formed micron-size intergranular pores trap most of the created fission gas. The study of the pore size-distribution and its evolution with burnup is fundamental to estimate the fission gas pressure in the pores and can influence the burst release in accidental conditions. Quantitative image analysis is usually employed to quantify the pore size-distribution. It is determined mainly in two-dimension (2D) using a histogram. Few attempts have been made in the past to derive also three-dimensional (3D) pore size-distribution employing the Schwartz-Saltykov method. In this work, we determine experimentally the pore size-distribution in the HBS and apply a new procedure to determine the distribution, both in 2D and 3D. A detailed characterisation of the pore size distribution and pore mean diameter is carried out in the burnup range 80-200 GWd/tHM. The evolution of the total pore density and mean pore diameter as a function of the local effective burnup give evidence of pore coarsening starting from 100 GWd/tHM, confirming previous investigations. The present data set the basis for modelling of bubble growth and coarsening in the HBS with a level of complexity suitable for applications in integral fuel performance codes like TRANSURANUS.

CAPPIA Fabiola;  RONDINELLA Vincenzo; 

2016-12-14

American Nuclear Society (ANS)

JRC100310

http://www.ans.org/store/browse-transactions/,    https://publications.jrc.ec.europa.eu/repository/handle/JRC100310,