Treatment of the uncertainties in fuel rod design analysis

摘要

UJV Rez is providing an analytical support to utilities in many fields, for example core physics calculations, nuclear fuel performance evaluation and safety analyses including both design basis conditions and severe accidents In all these areas the methodology is as important as the computational tools applied So far, the fuel rod performance licensing analysis in both normal operation and anticipated operational occurrences and in the design basis accident conditions has been performed with the best estimate code models and the fuel rod parameters selected at extreme values of the fabrication tolerances according to an engineering judgement, supplemented with a limited sensitivity analysis This paper describes the results of the effort performed in order to systematically treat uncertainties in the fuel rod performance analysis with the aim to both provide the conservative results while also showing the uncertainty of the results (Best Estimate plus Uncertainty) Considered are the uncertainties in the 1.Code models and material properties 2.Fuel rod as fabricated parameters 3.Core power and power distribution determination While the first two areas may be easily covered by the statistical analysis, the last area requires good knowledge of the origin of the uncertainties Especially in tall PWR and VVER-1000 cores with the possibility of fuel assembly bowing significant portion of the uncertainty is related to the inter-assembly gap width. In general, this parameter evolves during the residence of the fuel assembly in the core with a step wise changes during the core reloads. Several methodological approaches to the core power and power distribution are compared using the TRANSURANUS code. The example of adoption of the same statistical approach in the code validation is also presented in the paper.