Presentation of the xM3 Test Case of the P2M Simulation Exercise and Modeling with the Fuel Performance Code ALCYONE

This paper presents simulations of the xM3 power ramp with the fuel performance code ALCYONE performed during an international simulation exercise organized within the Organisation for Economic Co-operation and Development/Nuclear Energy Agency Power to Melt and Maneuverability project. The xM3 test involved a large-grain UO₂ fuel from Mitsubishi Heavy Industries cladded with Zirlo and pre-irradiated in a Spanish pressurized water reactor up to an average burnup of 27 GWd/tU⁻¹. It was then submitted to a staircase ramp protocol in the R2 reactor at Studsvik (Sweden) with 10 successive steps of 5 kW·m⁻¹ up to a ramp terminal level of 70 kW·m⁻¹. The fuel rodlet did not fail, and detailed post irradiation examinations performed during the Studsvik Cladding Integrity Project II evidenced recrystallization of the pellet center around a central hole, interpreted as signs of fuel melting.

In this paper, simulations with ALCYONE of the xM3 power ramp, including an advanced model for fuel melting based on thermodynamic equilibrium calculations, are detailed. The model relies on the determination of the liquid fuel fraction evolution with temperature that is used to obtain a continuous description of the material properties during phase change. In consequence of the incorporation of rare earths and actinides in the bulk of the fuel, distinct solidus and liquidus temperatures are estimated. It is shown that the observed central hole and recrystallized central part of the pellet could be the consequence of totally melted fuel (liquidus is reached), partially melted fuel (solidus is reached), or pore migration only.