Numerical diffusion and the tracking of solute fields in system codes: Part I. One-dimensional flows Original Research Article

Advances in neutronics and thermohydraulic modeling have resulted in system codes capable of describing local interactions between the core neutronic behavior and the thermohydraulic conditions inside the vessel with full 3-dimensional real time coupling. Making use of these advances in the analysis of boron dilution transients requires a good description of the boron field inside the core, and of its transport along the primary system. However, the relatively low accuracy displayed by advanced system codes in the simulation of solute transport as a result of numerical diffusion is a major obstacle to performing accurate boron dilution studies. Implementation of high order numerical methods in system codes can considerably improve their accuracy when modeling solute transport by reducing the numerical diffusion to a level that is less than the physical diffusion expected from the turbulence of the flow; even when using relatively coarse noding schemes. In order to show this is feasible, the explicit QUICKEST-ULTIMATE scheme for 1-dimensional flow was adapted to the integration procedures used in system codes and implemented in TRAC-PF1/MOD2. Numerical tests were used to assess the performance of the methodʹs implementation. A statistical methodology adapted from its original experimental formulation to the quantitative characterization of numerical diffusion in system codes was used for the analysis of the results. They showed that, for flow conditions commonly found in nuclear system simulations, high order tracking of a solute field can provide results whose diffusion is considerably less than that expected from the turbulence and characteristics of the flow field.