[en] The direct use of Computer Aided Design (CAD) models in criticality and shielding codes has been a long standing goal for Sellafield Ltd. Such functionality could offer several advantages over the traditional method of text based modelling systems. Analysts would be able to take advantage of the advanced Graphical User Interface based modelling features provided by solid modelers, potentially reducing the costs associated with creating models in a format suitable for the analyst's criticality and shielding code. A prototype system has been developed that allows CAD models created in Autodesk Inventor or Solidworks to be used in criticality and shielding calculations. The system is based on the ANSI Initial Graphics Exchange Specification 5.3 standard and models are exported from the CAD software in Trimmed NURBS format. The format retains much more of the model's geometrical information than a format based on solid meshing techniques and avoids many of the associated problems such as large memory costs, surface approximations and void spaces. The time consuming and complex meshing process is also avoided. Runtime intersection calculations are performed using either a Bezier clipping process for NURBS based surface definitions, or by transforming the coordinate system through which the ray tracks for Surface of Revolution calculations. NURBS surfaces are therefore converted to Bezier form as the model is imported. In addition, the SR generatrix is, in general, converted to a 'strip tree' representation, allowing the SR intersection calculations to be performed with arbitrary generatrix shapes. Details of recent improvements to the Bezier clipping process are provided. Reduction in runtime of SR based Solidworks models over equivalent NURBS based Autodesk Inventor models is also demonstrated. (authors)