Modeling of the P2M Past Fuel Melting Experiments with the Fuel Performance Code CYRANO3

Abstract

CYRANO3 is the thermal-mechanical industrial code developed and used by Electricité de France (EDF) to simulate nuclear fuel rod performance under normal and transient conditions (power ramp tests) in pressurized water reactors and during transport and storage periods, as well. This code has already been successfully used by EDF for the last 30 years to justify normal operations and category 2 transients, covering various types of fuels: UO₂, UO₂ + gadolinium, mixed oxide, and various claddings, as well those proposed by nuclear fuel suppliers: Zircaloy-4, Zirlo,™ and Optimized Zirlo.™

The CYRANO3 code was recently improved to allow for modeling fuel melting. In this paper, a global overview of CYRANO3’s ability to simulate past power-to-melt (P2M) ramps is presented with a focus on recent developments carried out to assess fuel rod behavior under these conditions. CYRANO3 is demonstrated to be a powerful tool to provide reliable values of melted radii.

As part of validation of these development works, CYRANO3 calculations have been used to assess two P2M ramp experiments carried out in the BR2 experimental core in Belgium (HBC-4 P2M ramp), and in the R2 experimental core in Sweden (xM3 P2M ramp). The main objectives of the work are to expand knowledge of the thermal-mechanical behavior of high-burnup fuel under P2M ramps by making interpretations of test simulations and to validate newly developed computational models for fuel melting that have been implemented in an extended version of the CYRANO3 fuel code.

For both rods, the steady-state irradiation power history was captured and modeling was performed. The key results of the steady-state irradiation modeling are reproduced with fair accuracy by means of CYRANO3 simulations. The results demonstrate the good ability of CYRANO3 to simulate P2M ramps. The melted radii and conditions of failure are well predicted. Calculated melted radius at ramp terminal Linear Heat Generation Rate (LHGR) is in good agreement with the experimental measurements performed after the experiments.

Keywords:

• CYRANO3
• fuel behavior
• PWR thermomechanical transients
• power ramps
• fuel melting

Acknowledgments

The authors would like to thank the OECD/NEA FIDES framework and the Core Group CEA, EDF, and SCK·CEN for the organization of the simulation exercise that contributed to enhancing the understanding of past fuel melting experiments and allowed for a better calibration of the fuel codes regarding fuel melting. The authors would like to acknowledge MNF, Studsvik, and SCK·CEN for the extensive data used during this simulation exercise on the xM3 and HBC-4 power ramps. This work was funded by EDF through grant P11LY.

Disclosure Statement

No potential conflict of interest was reported by the author(s).