Accuracy of the Quasistatic Method for Two-Dimensional Thermal Reactor Transients with Feedback

The quasistatic method was compared with a direct finite difference method of solving two-dimensional thermal reactor transient problems with thermal-hydraulic feedback. Calculations using both methods were performed for a cylindrical (r-z), D₂O-moderated and -cooled uranium-fueled reactor. This study shows that the quasistatic method is capable of producing highly accurate results, relative to the direct finite difference method, for two-dimensional thermal reactor transients with feedback. The quasistatic method also offers the flexibility of using larger time steps between flux shape calculations, without encountering numerical problems, than the direct method. The quasistatic and direct method codes used in this work are comparable with respect to accuracy and computing costs for the subprompt critical transients considered in this work except for transients with weak spatial effects. For such transients, much larger time steps can be used in the quasistatic code than in the direct method code to achieve a specified accuracy, which, in turn, provides a considerable savings in computing costs.