Chemical treatment of deposits of junctions 'collector-tube' of horizontal steam generators

A method of chemical treatment of deposits of junctions 'collector - tube' of horizontal steam generators of NPP with reactor VVER has been developed at the department of NPP of the Moscow Power Engineering Institute (Russia). The underexpanding zones of heat-exchanger tubes (HET) in the collector casing, forming fricative gaps with mass-transfer decrease plays a special role in corrosion damage of steam generator collectors. Moreover, if they are filled with porous deposit, then it can become an ideal place of potential concentration of aggressive impurity in the least thermal loading zone: just an accumulation of slime takes place in this zone most intensively. At present, there are series of methods for treating the deposits and for fighting against their formulation. One of the most effective widely used treatment methods is the chemical dissolution. Morpholine or Trilon-B can be used as reagents. The artificially created protective film of ceramic structures made of lithium ferrite on the surfaces of HETs reduces the corrosion-fatigue cracking process rate in the water with the parameters of the second contour. The stability of this film towards dissolution in contact with morpholine was experimentally tested and also a sufficiently durational presence of the film on the pipes' surfaces HETs and ring cracks has been verified by a repeated test. For reinforcing the protective effect, it is needed to maintain the film uniformity in the working process (microdosage of lithium hydroxide-LiOH). On the surface oxidized or polluted with deposits, first of all the LiOH is utilized for interaction with magnetite, then a formation of a coating of a mixed structure of lithium ferrite plus magnetite takes place. Since the combination of magnetite and lithium ferrite is insoluble in water then there is also no transfer of corrosion products in water from the protected surface. This circumstance strongly slows down the deposit formation process. Thus, iron-oxide deposits in ring gaps of underexpander can allow under specific conditions, created by lithium hydroxide, to provide a vapor lock and dry salt in the top of a crack. It results are of the density of corrosion current and the hydrogenation rate of coffer-dam perforated part of collector made from steel 08Ch18N10T. From these viewpoints the deposits are the important factor of prolongation the resource of coffer-dam. Complexing combination agents during their simultaneous presence in water, tends to form more than complex, mixed complexes with increased solubility for example, Ttrilon-B (EDTA) and its sodium salts. So if representing Complexing agents of iron represent in the form of [Fe-EDTA]2 and [Fe-EDTA]- or hydroxocomplexing agents of iron [FeOH EDTA]- [FeOH EDTA]2[FeOH EDTA]3[Fe-EDTA]4 a results of research EDTA solutions complexing agents indicate especially to a higher strength FE - EDTA and FEOH - EDTA (a great value of pK). The following conclusions are formulated in the paper: Optimization of water-chemical mode of the second contour remaining, the importance factor, for reducing the corrosion damage of steam generators and should be carried out according to the criteria of increasing the technical resource of metal as junctions, so as a whole of steam generator. Optimization processes of chemical washing should be come with reconstruction facilities of steam generator and its strapping (systems, blow-down, level measuring). Deactivation and chemical washing processes should be completed step of a passive protective film. (author)