Hi all I am very much a layman when it comes to Thermal Hydraulics and this may be a naive questions but here it goes... On the job, i use various codes to examine two phase flow in the core loop. Despite being vetted and validated numerous times over the course of many years (TRAC, RELAP etc...), these codes cannot resolve some of the more complex higher order effects of flow, two phase behavior in particular. Many of the codes I use have origins in the 60s-70s and things have come a long way since then, and with CPU power being more accessible I had the following questions: 1) Why is implementing a multi-phase code that uses the NS instead of a momentum balance so much more difficult? 2) What could be gained by switching schemes? I know CFD has its own issues, but I feel it could help with some problems if enough man power is behind it. 3) Both equations represent a momentum balance (NS and conservation of momentum) but what are the key differences? Thanks in advance.