Advanced technique for identifying VVER fuel assemblies with leaking fuel rods by the activity of 134Cs and 137Cs during spike effects

Abstract

Background

One of the challenges of fuel integrity analysis during reactor operation is significant uncertainties when leaking fuel burnup is estimated by ¹³⁴Cs and ¹³⁷Cs activities during spiking events. For better radiation safety and lower financial losses, advanced methods of fuel integrity analysis are required.

Aim

To develop an advanced technique for detecting leaking fuel assemblies (FAs) in the core of VVER reactors using the features of ¹³⁴Cs and ¹³⁷Cs accumulation depending on the fuel type and irradiation history of each fuel rod.

Materials and methods

Axial distributions of ¹³⁴Cs and ¹³⁷Cs in each fuel rod are calculated for the entire history of fuel assembly operation, taking into account the dependence of the ¹³⁴Cs production on the neutron spectrum. The spectrum is sensitive to the fuel enrichment and burnup, gadolinia content in the Gd-fuel rods, position of the fuel rod in the fuel assembly, and characteristics of the nearest fuel assemblies. In this regard, ¹³⁴Cs/¹³⁷Cs activity ratio as a function of fuel burnup for different fuel rods differs and changes every time the fuel assembly layout in the reactor core changes from campaign to campaign. The initial data for calculations are standard output files of the KASKAD software package for each campaign. The calculated concentration of ¹³⁴Cs and ¹³⁷Cs in each fuel rod is compared with the activity measured during the spike effects. The developed software automatically selects fuel rods with coincinding calculated and measured values.

Results

A new method for identifying leaking fuel assemblies of the VVER reactor is developed. The CAESAR software is developed for automated calculation of cesium concentration in every fuel rod in the core. Validation on NPP data shows the proposed technique significantly more effective than the standard method for assessment of leaking fuel burnup.

Conclusion

The developed technique eliminates most of the uncertainties that lead to significant errors in assessing the fuel burnup of leaking fuel assemblies using the standard method specified for VVER reactors. The technique also helps to optimize the sequence of fuel testing in the casks of spent fuel pool in order to find quickly the leaking fuel assembly and reduce the total duration of the reactor outage.