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System Thermal-Hydraulics (SYS-TH) Codes began to be developed towards the end of the 1960's as a response to the requirements, or knowledge targets, put by the Regulatory Body in the United States, US Atomic Energy Commission (US AEC), now US Nuclear Regulatory Commission (US NRC. Huge amount of investments in nuclear reactor safety research and developments, including V&V (Verification and Validation), brought to the nuclear community the availability of the mature computational tools towards the end of 1990's, when those codes were classified as BE (Best Estimate) due to the use of number of mechanistic models in the codes.

The application of the SYS-TH Codes within the licensing process required the deployment of suitable BEPU (Best Estimate Plus Uncertainty) approaches, thus implying the development of uncertainty methods. Efforts aiming at planning BEPU applications are presently in progress as of 2010.

Research and development to improve SYS-TH Codes have never stopped, thus moving the precision targets within the V&V framework and also the extending the application ranges and areas of the SYS-TH codes.

In addition, during the last thirty years, a variety of numerical tools under the acronym CFD (Computational Fluid Dynamics) has been developed for single phase flow and came under the scrutiny of the nuclear industry with the additional need of extension to two-phase and multi-phase flows.