Nek5000 is an open source CFD software with highly-scalable solvers for multi-dimensional heat transfer and fluid dynamics. It is a flexible code with minimal dependencies (in perms of message passing interface and compilers) to allow easy integration of user coding. The DNS, LES, and URANS modeling options for turbulence can produce high quality reference solutions on high-performance computers; but, the range of fidelity it covers also include porous media and momentum-source approaches to reduce the level of computational complexity to allow reliance on much smaller systems. It has a broad user and validation base for both nuclear and non-nuclear applications with advanced capabilities including moving meshes, adaptive mesh refinement, overlapping multi-domain simulations, and ensemble averaging (10x acceleration of DNS/LES in terms of time to solution). One of its variants, Nek2P, also feature modeling capabilities for Eulerian two-phase flow (supported and available through NEAMS RPL).