NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Talk 2.2: Investigation of mechanical and local properties of high burnup (>60 GWd/tHM) UO2 fuel with focus on the pellet rim

Fabiola Cappia1,2, D. Pizzocri3, P. Van Uffelen1, A. Schubert1, R. Macián-Juan2, V.V. Rondinella1
  • 1: European Commission, Joint Research Centre, Institute for Transuranium Elements (ITU), Hermann-von-Helmholtz Platz 1, PO Box 2340, DE–76125 Karlsruhe, Germany
  • 2: Technische Universität München, Boltzmannstraße 15, 85747 Garching bei München Germany
  • 3: Politecnico di Milano, Nuclear Engineering Division, Via La Masa 34, 20156 Milan Italy

Abstract

During in-pile operation, the pellet-cladding mechanical interaction (PCMI) is strongly affected by the fuel mechanical properties and its microstructural changes. Upon reaching a certain burnup and temperature level, fission gas accumulation into bubbles causes fuel swelling and gap closure. At the pellet periphery, the steep increase of gas-filled porosity typical of the high burnup structure (HBS) formation affects the PCMI at high burnup. It is important to understand the consequences of such porosity levels under accident conditions.

Mechanical testing on irradiated fuel is very difficult due to extensive cracking and radial heterogeneities. Micro-indentation allows local micro-hardness measurements with radial resolution sufficient to tackle some fuel heterogeneities. In this work, we present Vickers micro-hardness indentation results obtained from light water reactor (LWR) UO2 fuel in the local burnup range 60–200 GWd/tHM. The micro-hardness profiles are related to porosity distribution profiles measured by scanning electron microscopy image analysis, with particular focus on the pellet rim, where the HBS is present. At the pellet periphery, a noticeable decrease of the micro-hardness is seen, primarily related to porosity build-up, which decreases the load-bearing area.

The current data provides the first step for an assessment of micro-hardness and other mechanical properties in relation with local fuel microstructure and supports the development of predictive models in fuel performance codes like TRANSURANUS, with which the consequences of such material changes can be evaluated.

Keywords: Vickers micro-hardness, porosity, high burnup UO2 fuel, PCMI, HBS.