[en] In the past, EVALUATION type codes have been used exclusively by the NSSS suppliers for plant licensing. This approach was dictated by the absence of rigorous physical models to deal with complex plant phenomena, and by the concern to provide a conservative EVALUATION of the plant behaviour for a particular phenomenon (e.g. DNB, LOCA PCT). The drawback of these codes is that they concentrate on a few critical phenomena and can hide the real process. Following 2 decades of intense research in the area of THERMAL-HYDRAULICS, the so called 'BEST ESTIMATE' codes have been developed and are being assessed on a wide variety of experimental data from both small scaled facilities. These codes are now available for performing a realistic and physically based thermal-hydraulic analysis of the reactor systems behaviour, and also for licensing. Indeed, in 1988, the USNRC revised the emergency core cooling system (ECCS) licensing rule to allow best estimate computer codes for calculation of the ECCS behaviour, provided an uncertainty quantification is performed (Regulatory guide 1.157). This change in the regulatory practice opens the potential to use these advanced best estimate codes also for a large number of licensing calculations, provided an acceptable methodology is developed and accepted by the safety authorities for each application. Chapter 2 presents the Belgium context and the fundamentals of the BELGATOM methodology on the basis of which a practical application is applied and discussed in chapter 3