• uncertainty
• RELAP
• reflood
• PREMIUM
• IPREM
• FFTBM
• BEPU

The purpose of the present doctoral research is to improve on two issues identified in the frame of use of Best Estimate Plus Uncertainty approach in nuclear safety studies: proper estimation of code input parameter uncertainties and quality assurance in qualification of best-estimate models. The application of best-estimate computer codes and models implies the evaluation of uncertainties. This is connected with the imperfect nature of the codes and of the process of codes application. While the ‘code user effect’ is already a well-known issue in the frame of the deterministic analysis and application of system thermal-hydraulic codes, the adoption of BEPU approach poses a new problem – so called uncertainty methodology user effect. Lessons learnt from the international benchmarks showed that the use of engineering judgment in identification of considered code input parameters and imperfect knowledge of the code input parameter uncertainties, greatly affects the results of performed uncertainty analysis. The application of computer codes to calculation of experimental tests provides a first step to obtain code-specific uncertainties of input parameters representative of a studied correlation. A proper methodology of treatment of calculation and experimental results has to be adopted in this case. Therefore, in order to address this issue an Input Parameter Range Evaluation Methodology (IPREM) has been developed. The IPREM adopts the mathematical apparatus of Fast Fourier Transform Based Method (FFTBM) that has been previously developed at University of Pisa and applied for quantification of accuracy of thermal-hydraulic calculations. The proper IPREM procedure, figures-of-merit and criteria have been established in order to quantify the variation ranges of an input parameter. The IPREM has been applied to evaluation of uncertainty of reflood-related input parameters and models of RELAP5 Mod3.3 and CATHARE2 codes. In this framework, the simulations of various experimental tests of FEBA, PERICLES and ACHILLES separate effect test facilities have been performed. The obtained results in the form of variation ranges of input parameters have been subjected to various “internal” qualifications and extensive validation. The validation has been carried out through uncertainty analysis of “blind” thermal-hydraulic calculations of numerous reflood tests performed at different experimental facilities and at different conditions. The values of experimentally measured peak cladding temperature have been compared with maximum of evaluated upper uncertainty ranges of predicted temperature trends. The majority of evaluated uncertainty bands encompasses the measured values of peak cladding temperature and allows to confirm the validity of the application of the IPREM methodology for the evaluation of uncertainty of code input parameters. Although the IPREM is based on rather engineering considerations than on substantial statistical basis (and therefore does not provide the probability distribution of considered parameters), it proved to be code-, geometry- and condition-independent. The methodology is cost efficient, in general does not require code modification and requires few experimental tests with time-dependent measurements in order to quantify and validate the ranges of variation of input parameters of interest.
On the other hand, a key feature of the activities performed in nuclear reactor safety technology is constituted by the necessity to demonstrate the qualification level of each tool adopted within an assigned process and of each step of the concerned process. Therefore, the qualification of best-estimate codes, models and “best modeling practices” must be considered of great importance in order to ensure the validity of performed BEPU analysis. A consistent code assessment supported by a qualified experimental database is an important step for developing a solid ground for the uncertainty evaluation in the frame of BEPU approach. Thus, a methodology has to been developed in order to address the issue of quality assurance in the process of code assessment. The solution to quality assurance problem has been proposed in a form of a Standard Consolidated Reference Experimental Database (SCRED), which includes a series of documents which goal is to demonstrate the qualification level of the achieved code results. The structure and procedure to set up reference data sets, qualification report and engineering handbook has been outlined. In the framework of application of the SCRED to validation of thermal-hydraulic code RELAP5-3D©, the RDS, model for LOBI-MOD1 and LOBI-MOD2 integral test facilities and Engineering Handbook have been developed. The calculation of large break LOCA and small break LOCA tests has been performed and calculation results have been subjected to qualification process. The obtained documents and code calculation results demonstrate the maturity level and the effectiveness of the procedures itself, which is reflected in the excellent results of the performed simulations. This confirms that the use of qualified experimental databases has the key role in providing the quality assurance to the “best nodalization practices”, which currently is the only viable approach in consideration of nodalization-related uncertainties.