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 6.3: A semi-empirical model for the formation of the high burnup structure

Davide Pizzocri1,2, F. Cappia1,3, L. Luzzi2, G. Pastore4, V.V. Rondinella1, and P. Van Uffelen1
  • 1: European Commission, Joint Research Centre, Institute for Transuranium Elements (ITU), Hermann-von-Helmholtz Platz 1, PO Box 2340, DE–76125 Karlsruhe, Germany
  • 2: Politecnico di Milano, Nuclear Engineering Division, Milan, Italy
  • 3: Technische Universität München, Munchen, Germany
  • 4: Idaho National Laboratory, Fuel Modeling and Simulation Department, Idaho Falls, United States

Abstract

In the rim zone of UO2 nuclear fuel pellets, the combination of high burnup (>60 GWd/tU – i.e., high radiation damage and fission product concentration) and low temperature (<1000°C – i.e., limited thermal recovery of the radiation damage) drives a microstructural change, leading to the formation of the high burnup structure (HBS). In this work, we propose a preliminary model to describe the formation of the HBS. This process always includes four characteristics phenomena: (i) first, dislocations pile-up forming an entangled network, followed by (ii) the polygonization/recrystallization of smaller grains, together with (iii) the decrease of the intra-granular fission gas concentration (depletion), and (iv) the formation of a novel population of spherical bubbles. These processes are not strictly sequential, but may be thought as (partially) concomitant. The present model embraces phenomena (i-iii), describing them as inherently related. Based on experimental observations, we assume an exponential reduction of the average grain size (i-ii), paired with a simultaneous depletion of intra-granular fission gas driven by diffusion (iii). The assumption that a reduction in the average grain size can represent both the dislocation pile-up and the actual polygonization/recrystallization is based on the experimental observation of dislocations acting as sinks for fission gas. As regards the diffusion process, the determination of the intra-granular diffusion coefficient in the HBS is based on the combination of EPMA measurements of fission gas concentration and SEM measurements of the grain size. The underlying physical interpretation is that HBS formation can be seen as a phase transition from a structure with low surface to volume ratio (i.e., big grain size ≈10 µm) to a structure with much higher surface to volume ratio (i.e., small grain size ≈300 nm), the second being able to endure more radiation damage. In this picture, the depletion of intra-granular fission gas (iii) and the formation of new bubbles (iv) can be related to the increased surface to volume ratio, which enhances diffusion and promotes HBS pore growth.