A main-steam-line-break accident analysis for Three Mile Island Unit 1 is performed with point kinetics and three-dimensional kinetics with RETRAN-3D MOD002. These analyses were performed to demonstrate differences in results that can be expected due to different reactor kinetics models. To illustrate the difference in kinetics models, the RETRAN-3D models used for both analyses were the same with the exception of the reactor core modeling. The key assumptions and methods used to model loop mixing are described.

The point-kinetics analysis demonstrates a significant return-to-power following the reactor trip while the three-dimensional kinetics case does not. This study shows that three-dimensional core transient modeling provides margin to recriticality over a point-kinetics approach. Such margin is desirable to allow for power uprate and extended refueling cycles.