Modeling of Reactivity Effects and Transient Behavior of Large Sodium Fast Reactor

In the paper, the reactivity characteristics of the core of the large sodium fast reactor Superphenix (SPX) were evaluated and compared with available experimental data. The analysis was performed using the TRACE system code modified for the fast reactor applications. The simplified core model was developed aiming to overcome the lack of detailed information on design and realistic core conditions. Point kinetics neutronic model with all relevant reactivity feedbacks was used to calculate transient power. The paper focuses on challenging issue of modeling of the transient thermal responses of primary system structural elements resulting in reactivity feedback specific to such large fast reactor, which cannot be neglected. For these effects, the model was equipped with dedicated heat structures to reproduce important feedback due to vessel wall, diagrid, strongback, control rod drive lines thermal expansion. Peculiarly, application of the model was considered for a whole range of core conditions from zero power to 100% nominal. The developed core model allowed reproducing satisfactorily the core reactivity balance between zero power at 180C and full power conditions. Additionally, the reactivity coefficients k, g, and h at three power levels (about 20, 50, and 80% of the nominal power) were calculated and satisfactory agreement with experimental measurements was also observed. The study demonstrated feasibility of application of relatively simple model with adjusted parameters for analysis of different conditions of very complex system. Reducing some differences with experimentally observed behavior of feedback coefficients,
would require more sophisticated approaches on fuel pin model, more detailed information on management of control rods during power rise, more complicated models of primary system, its structural elements, and flow paths.