Approach And Methods To Evaluate The Uncertainty In System Thermalhydraulic Calculations.

Abstract

The evaluation of uncertainty constitutes the necessary supplement of Best
Estimate (BE) calculations performed to understand accident scenarios in water cooled
nuclear reactors. The needs come from the imperfection of computational tools on the one
side and from the interest in using such tool to get more precise evaluation of safety margins.
In the present paper the approaches to uncertainty are outlined and the CIAU (Code with
capability of Internal Assessment of Uncertainty) method proposed by the University of Pisa
is described including ideas at the basis and results from applications. An activity in progress
at the International Atomic Energy Agency (IAEA) is considered.
Two approaches are distinguished that are characterized as “propagation of code input
uncertainty” and “propagation of code output errors”. For both methods, the thermalhydraulic
code is at the centre of the process of uncertainty evaluation: in the former case the
code itself is adopted to compute the error bands and to propagate the input errors, in the latter
case the errors in code application to relevant measurements are used to derive the error
bands.
The CIAU method exploits the idea of the “status approach” for identifying the
thermal-hydraulic conditions of an accident in any Nuclear Power Plant (NPP). Errors in
predicting such status are derived from the comparison between predicted and measured
quantities and, in the stage of the application of the method, are used to compute the
uncertainty.