Conclusions from V&V studies on the German codes PANAMA and FRESCO for HTGR fuel performance and fission product release

In modern HTGR reactors, the fuel element represents the principal barrier against release of the fission products generated during reactor operation. Both the acquired experience from HTGR operation and experimental data from accident simulation testing of the fuel have always been accompanied by intensive efforts of mathematical fuel performance modeling taking into consideration as far as possible the physical phenomena that may occur. The computer codes FRESCO and PANAMA both developed at the Research Center Jülich in Germany in the early 1980s have become essential tools to predict the fission product release from spherical fuel elements and the TRISO fuel performance, respectively, under given normal or accidental conditions. These two codes are also presently being used for a conservative estimation of the source term, i.e., quantity and duration of radionuclide release, for the Chinese demonstration project HTR-PM. A description of the comprehensive efforts in the past on code verification and validation (V&V) including the most recent studies of code-to-code benchmarking studies and code-to-experiment comparisons as part of the IAEA directed CRP-6 project will be given. The conclusions drawn from those activities will be given with regard to the codes’ ability to allow for a conservative quantification of the source term as part of safety and risk analyses for pebble-bed HTGRs. Recommendations for further model improvement have been taken into account in the new code development, STACY, combining the PANAMA and FRESCO model approach and other features, as a module of the HTR Code Package (HCP).