Analysis of Phébus FP experiments in Korea

• Phébus FP tests were simulated with MELCOR 1.8.5.
• Reasonable estimations of important variables were obtained.
• Improved models to treat absorber materials, high burn-up fuel and iodine are needed.
• FP data helped to develop MELCOR-RAIM; the RAIM part simulates iodine behavior.
• RAIM containment iodine models will be developed to include organic iodide formation.

In the Phébus FP program, the phenomenology of severe accidents in water cooled nuclear reactors have been studied, especially fuel rod degradation and the behavior of fission products released via the primary coolant circuit into the containment. This paper is focused on the analysis of Phébus FPT-1 and FPT-3 test results using the severe accident analysis code, MELCOR. Since FPT-1 was adopted as the OECD ISP-46 and FPT-3 as the SARNET2 WP8.3 benchmark problem, a variety of code-to-code and code-to-experiment comparisons are possible. The two tests, FPT-1 and FPT-3, were carried out under low system pressure and using fuel irradiated in a reactor to about 23 GWd/tU, and in a steam-rich atmosphere for FPT-1 and in a steam-poor atmosphere for FPT-3. In FPT-3, the control rod was made of boron carbide (B4C) instead of silver–indium–cadmium (SIC) as in the previous test FPT-1. Intensive analysis of the FPT-1 and FPT-3 data highlighted major differences between them in the aspects of the fuel bundle degradation, material relocations, hydrogen production, and moreover fission product behavior, especially iodine behavior in the reactor coolant system and containment. The analysis results for two tests using MELCOR1, which is the main tool for assessment of Korean nuclear plants, were synthesized in this paper. Comparison between code estimates and experimental data demonstrated the code’s ability to simulate the whole sequence with reasonable accuracy, and assessed detailed models in the code. Also uncertainties, particularly regarding modeling of core degradation and fission product behavior, were identified showing that further model improvements are required in the code. In addition, MELCOR-RAIM, a coupled code, was used for simulation of the iodine behavior for both FPT-1 and FPT-3. The coupling was achieved by replacing the pool chemistry model of MELCOR with RAIM, which had been developed as a standalone model based on the IMOD code to analyze the behavior of iodine including organic iodide.