Evolution of the Nuclear Fuel Mechanical Properties at High Burn-Up - an Extensive European Experimental Program

Abstract

Computer codes have been developed in order to simulate the nuclear fuel rod mechanical behaviour, and therefore compare the evolution of the main parameters against a certain number of safety criteria, for reactor class1 and class 2 operating conditions. Concerning the fuel material (UO₂, MOX or UO₂ with additives) the mechanical properties have been determined on non irradiated samples. One can expect that these properties evolve with burn-up, due to the transmutation, the evolution of the oxygen potential, the accumulation of fission defects and in some case the material restructuring (High Burn-up Structure).In order to provide the fuel thermo-mechanical calculation with more accurate mechanical properties, a large experimental project has been launched since several years by CEA, EDF and ITU ; furthermore a recent collaboration has been started with Studsvik, Sweden, concerning the possibility of performing in pile creep measurements. The program is indeed organised in three folds, which can be described as follows:

1 - Acquisition of mechanical properties on non irradiated materials (UO₂, UO₂+Gd, UO₂+Cr, MOX) with axial creep tests, three points bending tests up to 1700 C, acoustic measurements at room temperature, instrumented micro-indentation tests and Vickers test from room temperature to 1200 °C. This allowed the development of a mechanical behaviour law available for these materials in the non-irradiated state.

2 - Acquisition of mechanical properties on irradiated materials in hot cells, using a micro-indentation machine developed especially in TUI, Vickers tests with the same machine, and a focused acoustic technique developed by the LAIN laboratory in the Montpellier University (France). The target is to define the evolution of the elastic properties, of the yield stress and of the thermal creep properties.

3 - More recently with the Studsvik Laboratories, the design of a specific rig for in-pile indentation has been completed. This aims at assessing the evolution of the irradiation creep properties, mainly in the HBS material.

Mechanical behaviour models are already achieved for non-irradiated materials and are presented. Concerning the acoustic methods, several presentations have already been published [2 to 7]. The main results are summarised. The micro indentation technique from room temperature to 1200 °C is actually under qualification on non-irradiated samples. The state of the art is made. Finally an in-pile indentation concept is presented.