In-Situ Investigation of VVER Steam Generator Heat Transfer Tubes by XRF Technique

Steam generators of VVER 440 type nuclear power plant are not interchangeable economically. Knowledge of the amount of possible contaminates on the tubes’ surface is necessary to evaluate the corrosion risk. Thickness of the hidrothermally grown oxide film can influence the boiling and heat transfer. Common way of the investigation of the oxide layer on the tube is based on cut-off a tube. This allows a comprehensive study by sacrificing one working tube. A non-destructive, in-situ semi-quantitative method was developed and tested in Paks NPP. X-ray fluorescence analysis (XRF) can give an elemental composition of the outer 10-100 µm layer of the sample. Portable analyzer with appropriate shielding can be used for in-situ measurement in the opened steam generator. Diffuse gamma radiation of the activated corrosion products on the inner surface of the tubes can result in an external excitation of the detected X-ray lines over the excitation by the X-ray source of the equipment. Therefore calibration under the internal condition of the steam generator is essential. The effect of the external excitation was observed in the case of copper and lead. This effect is not significant in the case of the main elements of the steel. Information depth of the XRF technique is somewhat higher than the typical thickness of the oxide layer, so contaminates (i.e. lead, silver, copper and zinc) on the surface and in the oxide layer can also be detected. Quantitation can be done based on an assumption for the depth distribution.

Outer and thicker layer of the surface oxides on the heat transfer tube is made of crucially magnetite (Fe3O4). From the intensity of chromium and nickel K-lines can be estimate the thickness of the oxide layer based on the absorption of X-ray photons in the magnetite. The intensity of the used lines has to be measured on a sample with same bulk composition without oxide layer. All the measurement have to be carried out strictly in the same geometry. Magnetite absorbs less the Cr-K lines than the Ni-K lines. So the evaluation of the Ni lines gives more accurate results for thin layers (<20 µm). Cr lines can be used in the 20-100 µm range.

The described method is useful for non-destructive quick check of the thickness and composition of the heat transfer tube.    

 

Contact
Zsolt KERNER, MTA Center for Energy Research
zsolt.kerner@energia.mta.hu