The capability of internal assessment of uncertainty

The internal assessment of uncertainty is a desirable capability for thermalhydraulic system codes. This consists in the possibility of obtaining proper uncertainty bands each time a nuclear plant transient scenario is calculated. In the present paper, a methodology suitable for introducing such a capability into a system code is discussed. At the basis of the derivation of the CIAU (Code with - the capability of - Internat Assessment of Uncertainty) there is the UMAE (Uncertainty Methodology based on the Accuracy Extrapolation), previously proposed by University of Pisa, though other uncertainty methodologies can be used for the same purpose. The idea of the CIAU is the identification and the characterization of standard plant statuses and the association of uncertainty to each status. One hypercube and one time interval identify the plant status. 'Quantity' and 'Time' uncertainties are combined for each plant status. The recently released US NRC (Nuclear Regulatory Commission) Relap5/mod3.2 system code constitutes the CIAU. This is used for showing the applicability of the proposed method. In the present paper the derivation of the methodology is discussed and reference results of PWR (Pressurized Water Reactor) plant transients are shown bounded by the CIAU calculated uncertainty bands. (authors)