Enlargement of the uncertainty database of the CIAU methodology

The development and the application of uncertainty methodologies constitute the final step of wide research programs carried out on a international ground in the last forty years, with the main purpose of quantifying the safety margins of existing Nuclear Power Plants (NPP). Best-estimate thermal-hydraulic system code have been developed and are currently used to simulate transients behaviour in small scale test facilities and in NPPs. While the error made in predicting plant behaviour is called uncertainty and it is unknown, the discrepancy between measured and calculated trends related to tests performed in experimental facilities is defined as the accuracy of the predictions and constitutes the known error. As a consequence, the process of application of best-estimate (or realistic) computer codes to the safety analysis of NPPs implies the evaluation of uncertainties. This is connected with the (imperfect) nature of the code and of the process of code application. In other words, ‘sources of uncertainty’ affect the prediction of the results of best-estimate codes and must be taken into account. A detailed list of uncertainty sources can be found in Ref. [1], where an attempt has been made to distinguish ‘independent’ sources of ‘basic’ uncertainty. As a consequence of the large efforts necessary to calculate the uncertainty by any of the existing methodologies, the Internal Assessment of Uncertainty (IAU) has been recommended [2] as a desirable capability for thermalhydraulic system codes. In this context, the Code with (the capability of) Internal Assessment of Uncertainty (CIAU) has been proposed by the University of Pisa to realize the integration between a qualified system code and an uncertainty methodology and to supply proper uncertainty bands each time a NPP transient scenario is calculated. At the basis of the derivation of CIAU, there is the UMAE (Uncertainty Methodology based on Accuracy Extrapolation) previously proposed by University of Pisa [3] and the RELAP5/MOD3.2 system code, though other uncertainty methodologies and qualified thermalhydraulic codes can be used for the same purpose. In a joint effort between University of Pisa and Penn State University, the CIAU method has been recently extended to evaluate the uncertainty of coupled 3-D neutronkinetics/ thermal-hydraulics calculations. The result is CIAU-TN [4]. The feasibility of the approach has been demonstrated and sample results related to the turbine trip transient in the Peach Bottom NPP [4] and to the OECD/NRC PWR MSLB Benchmark [5] have been obtained. Notwithstanding the full implementation and use of the CIAU-TN procedure requires a database of errors not available at the moment, the results give an idea of the errors expected from the present coupled 3-D neutron-kinetics/thermal-hydraulics computational tools. A comprehensive description of the CIAU process is outside of the present work. Interested readers should refer to Refs. [1, 4]. Nevertheless, an idea of the CIAU methodology and of the process to store the data is given in the following paragraph. The paper mainly discusses about the enlargement of the CIAU (uncertainty) database obtained including 11 new tests concerning different transients and facilities. The process by which new qualified tests (in the sense required by UMAE methodology) are added to the existing database constitutes the current main priority of the CIAU methodology and it is continuously ongoing. The ‘internal’ qualification process of the new database has been fulfilled and the application of the updated uncertainty matrices on the CL-18 OECD/CSNI UMS test (SBLOCA in the LSFT facility) has been also considered in order to compare the new uncertainty bands with the ones previously calculated and to demonstrate the ‘external’ (independent) qualification of the CIAU database.