Importance Analysis for Uncertain Thermal-Hydraulics Transient Computations

Results of the codes simulating transients and abnormal conditions in nuclear power plants are inevitably uncertain. In application to thermal-hydraulic calculations by thermal-hydraulics codes, uncertainty importance analysis can be used to quantitatively confirm the results of qualitative phenomena identification and ranking table (PIRT). Several methodologies have been developed to address uncertainty importance assessment. Existing uncertainty importance measures which are mainly devised for the PRA applications are not suitable for tedious calculations of the complex codes like RELAP. On the other hand, for the quantification of the degree of the contribution of each phenomenon to the total uncertainty of the output, a new uncertainty importance measure that needs affordable computational cost is very promising. A new uncertainty importance measure is introduced in this article to cope with the aforementioned deficiencies of the TH uncertainty importance analysis. Important parameters are identified qualitatively by the modified PIRT approach while their uncertainty importance is quantified by the proposed index. Application of the proposed methodology is demonstrated on LOFT-LB1 test facility.