MC21/COBRA-IE and VERA-CS multiphysics solutions to VERA core physics benchmark problem #6

The Virtual Environment for Reactor Applications (VERA) core physics benchmark problem #6, 3D Hot Full Power (HFP) assembly, from the Consortium for Advanced Simulation of Light Water Reactors (CASL) was simulated using the MC21 continuous energy Monte Carlo code coupled with the COBRA-IE subchannel thermal-hydraulics code using the R5EXEC coupling framework. The converged MC21/COBRA-IE solution was compared to results from CASL's VERA-CS code system, MPACT coupled to COBRA-TF (CTF). MPACT is a three-dimensional (3D) whole core transport code, executed in a 2D/1D approach employing planar method of characteristics (MOC) solutions with SP3 in the axial direction, and CTF is a subchannel thermal-hydraulics code designed for Light Water Reactor analysis. Eigenvalues agreed within 63 pcm, axially-integrated normalized radial fission distributions agreed within ±0.2% (root mean square (RMS) difference of 0.1%), local volume-averaged fuel pin temperatures agreed within +8.8/-4.3 C (RMS difference of 3.9 C), and local subchannel coolant temperatures agreed within +0.8/-1.5 C (RMS difference of 0.5 C). A sensitivity study to guide tube heat transfer indicated that a statistically-significant increase in reactivity and shift in radial pin power distribution occurred within the assembly when guide tube heating was enabled.