Consistent and accurate schemes for coupled neutronics thermal-hydraulics reactor analysis

Conventional coupling paradigms currently used to couple different physics components in reactor analysis problems can be inconsistent in their treatment of the nonlinear terms due to the operator-split (OS) strategies employed. This leads to the usage of small time steps to maintain accuracy requirements, thereby increasing the overall computational time. This paper proposes some remedies to OS techniques that can restore consistency in the coupling of the nonlinear terms and explores high-order mono-block nonlinearly consistent techniques with time step control. The performance of the methods was studied for several transient scenarios using a 0D point-kinetics/thermal-hydraulics lumped model and a 1D neutronics/heat conduction/enthalpy balance model. The results prove that consistent approximations can be made to enhance the overall accuracy in conventional codes with simple nonintrusive techniques. Additionally, an analysis of a mono-block coupling strategy (without having recourse to an OS strategy) is carried out to assess automated time stepping control using higher order Implicit Runge–Kutta (IRK) schemes. The conclusions from these results indicate that nonlinearly consistent adaptive time stepping methods can provide better accuracy and reliability in the solution fields than constant time stepping methods, even for transients with rapid and discontinuous variations.