Application of an Improved Technique for Burnup Evaluation of WWER Leaking Fuel Based on the Data on Spiking Events

A conventional approach to evaluating burnup of the leaking fuel assemblies (LFA) at operating WWER power units is to use a burnup correlation of ¹³⁴Cs/¹³⁷Cs activity ratio in the primary coolant during spiking events. This technique was revised in an updated regulatory document, considering the differences in evolution of cesium (Cs) content in fuel pellets and in voids inside a fuel rod. However, from 2000s , analysis of spiking events and data of post-irradiation examinations (PIEs) show this technique to be rather inaccurate in some cases. Recent NPP data on activity spiking events at WWER-1000 power units and LFAs examination results have filled the data gap in the burnup range of 40-50 MWd/kgU for Cs inventory in fuel rods with initial enrichment of 4.95 wt%. This new data enables a thorough comparative analysis of systematic factors causing deviation of experimental estimates compared to the results of the conventional technique for various fuel enrichments including fuel with solid fuel pellets. The paper briefly describes an improved technique introduced previously for express calculation of build-up of Cs isotopes in fuel pellets for every fuel rod in the core. This approach employs the NPPs results of routine neutronic calculations of actual pin-wise linear heat generation rate (LHGR) for each fuel cycle with a particular core loading pattern. The activity ratios A(¹³⁴Cs)/A(¹³⁷Cs) in fuel calculated in the frame of this technique show good agreement with the available NPP data for LFAs with solid fuel pellets and are also compared to the new RIAR PIE data. It is suggested that an increased reliability in evaluation of leaking fuel burnup at operating WWERs and subsequent LFAs identification could be achieved if the key factors affecting evolution of ¹³⁴Cs/¹³⁷Cs activity ratio for each FA in the core are considered simultaneously with distinctive features of mass transfer in leaking fuel rods of different design. (author)