Improvement in cristal criticality standard calculation schemes
Congress title :ICNC 2007
Congress location :Saint-Petersbourg
Congress date :28/05/2007
In nuclear criticality safety, standards are often used to provide critical or sub-critical parameters as, for example, minimal masses or volumes corresponding to a given neutron multiplication factor. Thus, these parameters must be provided quickly with a sufficient accuracy and especially have to be conservative.
The French criticality package CRISTAL, developed by the CEA and the IRSN in collaboration with the AREVA company, makes it possible to carry out standards calculations. The criticality standards are worked out from deterministic computations with the APOLLO2 code using the Sn method. In the framework of CRISTAL, the calculation schemes were optimized on a criterion of computing time minimization while providing sufficient quality results. Thus, they are relatively basic schemes: cell calculation, rather coarse spatial and energetic mesh, whatever the fissile medium. The computing and experimental validation of these calculations schemes showed that, for most of the cases, the critical parameters computed by the CRISTAL package are provided in a very satisfactory way. Nevertheless, in some particular cases as for example, low dimensions geometries or fissile media with a heterogeneous distribution of the moderator, the recommended computing options are not well suited.
Therefore, the CEA and the IRSN set up an intensive work to update the CRISTAL recommended computing options. It mainly concerns:
-generation of the macroscopic cross sections provided by collision probability Pij method (Pij geometry choice, self-shielding options)
-SN method computing parameters (quadrature order, anisotropy order, energy and spatial mesh).
These studies made it possible to improve the critical parameters prediction quality by specifying the Pij calculation options according to the fissile mediums, and by evaluating biases due to the various options of the SN method.
New recommendations resulting from this work will be proposed. Moreover, the new criticality standards calculation procedure will be integrated into the future version of CRISTAL package in order to use automatically the most relevant computing options according to the studied configuration.