ALEPH - A MONTE CARLO BURN UP CODE

Monte Carlo burn up codes are powerful and accurate tools but they are also quite demanding on calculation time because the reaction rates required for burn up calculations are always calculated within a Monte Carlo code. By studying the Monte Carlo simulation process and the calculation of reaction rates by Monte Carlo in particular, we have identified a more efficient approach to reaction rate calculation with Monte Carlo. This allows us to reduce a single burn up step to a single spectral Monte Carlo calculation without reaction rates which decreases the calculation time significantly (a factor 33 decrease for a fuel pin model has been observed). We have implemented this approach into ALEPH, a Monte Carlo burn up code using MCNP or MCNPX, ORIGEN and NJOY. We have benchmarked ALEPH on the NEA-OECD BUC IV-B benchmark and on the ARIANE experimental data in the framework of the VALMOX project. It has also been shown that a statistical error of around 0.5 % on the total flux in a cell is sufficient to obtain reliable results on the isotope composition.