OUTLINE OF THE UNCERTAINTY METHODOLOGY BASED ON ACCURACY EXTRAPOLATION

Uncertainty methodology based on accuracy extrapolation (UMAE) is outl ined. This methodology is suitable for evaluating the uncertainty in t he prediction of transient scenarios in nuclear reactors when carried out by thermal-hydraulic system codes. It is based on the extrapolatio n of the accuracy resulting from a comparison between code results and relevant experimental data obtained in small scale facilities. A simp lified diagram of the UMAE is compared with a similar one derived for the code scaling, applicability and uncertainty evaluation (CSAU) prev iously proposed by the U.S. Nuclear Regulatory Commission. A few resul ts related to the full application of the UMAE to a small-break loss-o f-coolant accident in a pressurized water reactor, including core unco very, are also reported.