DYNSUB: A high fidelity coupled code system for the evaluation of local safety parameters – Part II: Comparison of different temporal schemes

DYNSUB is a novel two-way pin-based coupling of the simplified transport (SP3) version of DYN3D with the subchannel code SUBCHANFLOW. The new coupled code system allows for a more realistic description of the core behaviour under steady state and transients conditions, and has been widely described in Part I of this paper. Additionally to the explicit coupling developed and described in Part I, a nested loop iteration or fixed point iteration (FPI) is implemented in DYNSUB. A FPI is not an implicit scheme but approximates it by adding an iteration loop to the current explicit scheme. The advantage of the method is that it allows the use of larger time steps; however the nested loop iteration could take much more time in getting a converged solution that could be less efficient than the explicit scheme with small time steps. A comparison of the two temporal schemes is performed. The results using FPI are very promising and represent a very good option in order to optimize computational times without losing accuracy. However it is also shown that a FPI scheme can produce inaccurate results if the time step is not chosen in agreement with the analyzed transient.

► A fixed point iteration (FPI) is implemented in DYNSUB.
► Comparisons between the explicit scheme and the FPI are done.
► The FPI scheme allows moving from one time step to the other with converged solution.
► FPI allows the use of larger time steps without compromising the accuracy of results.
► FPI results are promising and represent an option in order to optimize calculations.