THE CONTRIBUTION OF CROSS-SECTION UNCERTAINTIES TO PEBBLE BED REACTOR EIGENVALUES RESULTS: IAEA COOPERATIVE RESEARCH PROJECT PHASE I STANDALONE NEUTRONICS

摘要

The availability of more reliable and high fidelity codes and models for nuclear power plant safety and licensing analysis allows the use of best-estimate plus uncertainty analysis to replace the traditional conservative analysis. Methodologies to do this have been developed and applied for light water reactors but need to be studied for other reactor technologies, where the sources and magnitude of uncertainties are different. The evaluation of uncertainties in high temperature gas-cooled reactors analysis has been investigated within the IAEA coordinated research project. Phase I includes the quantification of the contribution of cross section uncertainties to the eigenvalue uncertainty for representative but simplified pebble bed reactor cell calculations as part of the "local neutronics" effect. The proposed test cases are defined, reference results are proposed and the cross section uncertainty is quantified for different temperatures, fuel compositions and neutron spectra. Special attention is given to the treatment of double-heterogeneity and it was shown the effect must be included (at least in an approximate way) to be able to correctly perform the uncertainty quantification and propagation. The calculated uncertainties are slightly larger but consistent with those of light water reactors. Cases at high temperature and with burned-fuel isotopic contents resulted in larger uncertainties. The nuclide reactions that make the largest contribution to the uncertainties were identified.