Best Estimate Plus Uncertainty Approach in nuclear reactor safety and licensing: brief history and elements

The best es ti mate plus un cer tainty is, at the same time, an ap proach, a pro ce dure and a frame - work in nuclear thermal-hydrau lics and nuclear reactor safety and licensing. The motiva tion at the ba sis of the best es ti mate plus un cer tainty is the lack of knowl edge in the ar eas of sin gle and, mainly, two-phase tran sient ther mal-hy drau lics. In other terms and in tro duc ing some simplifications, the insuf ficient knowledge of tur bulence imposes the design of roadmaps for the application of imperfect (thermal-hydraulic) models to the evaluation of design features and of safety for com plex tech no log i cal in stal la tions or sys tems like the Nu clear Power Plants and, more specif ically, the water cooled nuclear reac tors. Furthermore, the legal counter part of nu clear re ac tor safety, or the li cens ing, is con cerned: there fore the best es ti mate plus un cer - tainty must ac count for rules and reg u la tions de rived from the fun da men tal radioprotection prin ci ple which im poses the minimization of the im pact of ra di a tions upon hu mans and the en vi ron ment un der any cir cum stance. In the pres ent pa per, the key el e ments of the ap proach are iden ti fied and char ac ter ized. These shall be seen as the sup port for a con sis tent ap pli ca tion of ther mal-hy drau lics to the de sign and safety of wa ter-cooled nu clear re ac tors.