This paper presents the results of posttest calculations of the phebus FPT2 experiment. While the exercise concentrates mainly on code-to-code benchmarking, a comparison is also made with selected experimental results. The test scenario with the appropriate initial and boundary conditions was provided by the Institut de Radioprotection et de Sûreté Nucléaire. For the analyses, seven severe accident analysis codes were used: ASTEC, ATHLET-CD, MELCOR, ICARE2, ICARE/CATHARE, SCDAP/RELAP5, and RELAP/SCDAPSIM.
The calculations focused on the following phenomena occurring in the FPT2 bundle: thermal behavior; hydrogen production, mainly due to cladding oxidation; severe degradation of irradiated fuel; and the release of fission products, control rod, and structure materials.
Using the same postdefined boundary and initial conditions, the code-data differences are typically within 10% for most parameters, and not more than 25%. More importantly, the codes were able to capture the major features of the transient evolution. Given that Phebus FPT2 exhibited almost all of the major low-pressure severe accident phenomena except for core cooling by water injection and late-phase core melt behavior in the lower head, the results engender a degree of confidence in the code predictive capability for sequences similar to FPT2.