Abstract
The present work continues the series of papers on the revision of the conventional technique for evaluation of leaking fuel burnup during reactor operation at nuclear power plants (NPPs). The focus was made on reduction of uncertainties in evaluation of leaking fuel burnup in modern fuel cycles at WWER-1000 power units. A set of models was proposed for express calculation of the build-up of caesium isotopes in fuel and to relate 134Cs/137Cs activity ratio with fuel burnup for each rod in the core. These models are based on routine neutronic calculations of pin-by-pin linear heat generation rates which are performed at NPPs for each particular fuel cycle with particular core loading pattern (however, these calculations do not provide data on caesium inventory in fuel). Previously, the proposed models have been validated against several practical cases. This latest validation study relied on the analysis of the most recent fuel cycles at two NPPs that reported spike-events and identified the leaking fuel assemblies (LFAs) after the reactor shutdown. The calculated 134Cs/137Cs activity ratios in the fuel of the LFAs were compared to the NPPs data on the activity measurements, and to the post-irradiation examination (PIE) data provided for one FA. A reasonable agreement between the model predictions and the experimental data on 134Cs/137Cs activity ratios in the fuel as a function of its burnup is shown for the advanced FA designs in modern fuel cycles.
Highlights
Radiation safety assurance procedures at operating WWER and PWR power units worldwide include monitoring of activity of the reference radionuclides in the primary coolant (PC)
In case of fuel failures and release of radionuclides into the PC, one of the most sensitive indicators of the leaking fuel burnup for WWER and PWR power units is the ratio of 134Cs and 137Cs activity values measured in the PC during an activity spiking event recorded at interim power drops or at reactor shutdown, [2,3,4,5]
This correlation is based on averaging of neutronic calculations for typical WWER fuel cycles applied in the early 2000-s; this technique was revised in an updated document [6], taking into account the differences in evolution of caesium (Cs) content in fuel pellets and in voids inside a fuel rod
Summary
Radiation safety assurance procedures at operating WWER and PWR power units worldwide include monitoring of activity of the reference radionuclides in the primary coolant (PC). This correlation is based on averaging of neutronic calculations for typical WWER fuel cycles applied in the early 2000-s; this technique was revised in an updated document [6], taking into account the differences in evolution of caesium (Cs) content in fuel pellets and in voids inside a fuel rod (see figure 6).
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