1. Tendencies in the corrosion state of the steam generators at Paks NPP
On the basis of the results of electrochemical (voltammetric) thematic research between 2000-2008 I have concluded that there is no adverse tendency in the corrosion characteristics (Ek, ik, ip és vk) of the heat exchanger tubes of steam generators with the operation time. I have demonstrated that the stability of the oxide structure on the surface of the decontaminated heat exchanger tubes improved during the time of operation, and today all the steam generators of the reactor blocks 1-4 shows favourable erosion-corrosion conditions (removable thickness of the oxide layer d ~ 0,1 µm).
However, the fact that the major part of the corrosion product removed from heat exchanger surfaces is still given by the disperse phase (solid residue) must be emphasized. Evaluating the findings obtained by surface analytical methods (CEMS, SEM-EDX) I proved that there is a layer of spinel-structure magnetite rich in Cr and Ni in the near-surface region of the decontaminated tube samples, which enhance the passivity of oxide layer grown on the corrosion resistant steel surfaces. On the basis of the improving data of mobility as well as the favourable morphological and surface structure changes in relation to the operation time after the 2001 mass application of the AP-CITROX technology we can assume that the protective features (solubility, resistance to chemicals) of the heat exchanger tubes of decontaminated steam generators and its mechanical stability goes through a favourable change.
2. The efficiency of chemical decontamination technologies and their surface chemical and corrosion effects
2.1. The efficiency of the decontamination technologies in the knowledge of the surface characteristics of the treated steel tubes
Examining the procedures applied for the decontamination of different apparatus in the primary circle developed on the basis of the decontamination base technology elaborated in the IRR PE I realized that the efficiency of these technologies depends on the thickness, the chemical and phase composition, the morphology (particle size and specific surface area) of oxide layer and on the decontamination history of the steel tubes surface to a great extent.
I proved that
- there is a basic difference in the decontamination ability between the austenitic steel samples originating from the SGs in Block 4 and the SGs in Block 1-3
- the efficiency of the chemical treatments in both cases is primarily affected by the removability of the 60Co radionuclide
Consequently, it is vital in the case of the industrial implementation of decontamination procedures for the appropriate efficiency to be aware of the complex knowledge of the surface characteristics of the equipment to be contaminated.
2.1.1. The interpretation of the decontamination ability of the austenitic steel surfaces
By the comparative analysis of the experiment results and the literature data I concluded that the decontamination ability of the steel surfaces in Block 4 (type:
08X18H10T (GOST 5632-61) which corresponds to AISI 321 and DIN 1.4541) greatly depends on the chemical composition and morphology of the crystalline deposits covering the surfaces.
I proved that the Cr content, particle size and specific surface are determining as far as the decontamination ability of the surfaces is concerned. Knowing these parameters allows us to estimate the radioactivity of the steel surfaces and the potential order of their chemical resistance. To measure the amount of the corrosion
product radionuclides carried by the crystals I used the so-called activation and transport mechanism. At the same time, it can be stated (independently from the dominant mechanism of the decontamination of surfaces) that the crystal with higher Cr content carries more activity, however, it is more resistant chemically, so it is more difficult to remove during the decontamination treatment. This is supported by the following experimental results:
a) During the chemical decontamination of surfaces covered by crystals of small Cr content – depending on the particle size of the crystals – the removability of 60Co from the surface is 98-99% feasible.
b) During the decontamination of surfaces covered by the crystalline oxide deposits having high Cr content the efficiency of the removability of 60Co is 60-82 %. However, the efficiency can be improved by increasing the amount of the specific active agent of the decontamination solution.
2.1.2. The interpretation of the decontamination ability of austenitic steel samples decontaminated in the previous years at Paks NPP.
It has been revealed that steel tubes from the SGs of Block 1-3 (type:
08X18H10T (GOST 5632-61) which corresponds to AISI 321 and DIN 1.4541) can be more efficiently decontaminated (the removability of radionuclide 60Co is higher than 99%). This may be explained by the fact that crystalline deposit on the surface underwent a partial dissolution through the industrial decontamination in the previous years and a thick (5-11 µm) so-called hybrid oxide structure was formed.
The distribution of the radionuclides is inhomogeneous in the hybrid structure oxide layer, and the efficiency data reached show that the oxide layer has lower resistance during the decontamination than the original surface exempt from treatment.
2.2. The corrosion and surface chemical effect analysis of the procedures
a) On the basis of the electrochemical (voltammetry) and surface analytical (SEM-EDX) results I have concluded that after the application of the improved versions of base technology the surfaces display no selective corrosion changes (pitting, crevice corrosion and intergranular stress
corrosion cracking). The passive condition of the surfaces treated did not deteriorate compared to the original surface. No adverse tendency in the corrosion characteristics of the sample surfaces has been exhibited. After the decontamination treatment the average corrosion rate of the surfaces is less than 0,45 µm⋅year-1.
b) As a result of the decontamination treatment the morphology of the sample surfaces favourably changed. In the case of the samples where the whole oxide layer was removed the hoarseness of the surfaces decreased, and the chemical composition of the surfaces treated matches that of the bulk steel phase. After the decontamination procedure the Cr content of the oxide residue on the sample surfaces is significant, so these are chemically excellently resistant. The ICP-OES results prove that the amount of metal removed in the last steps of the technology are in all cases at least two times less in magnitude than the concentrations of the alloy components which got in the solution phase in the pre-oxidation period. On the basis of these it is to be presumed that the mobility of the oxide residue on the surfaces treated is low. No significant amount of chemical residues (manganese-dioxide and organic carbon) of the decontamination solution are detected on the surfaces treated.