The phenomenon of the emission and absorption of a y-ray photon without energy losses due to recoil of the nucleus and without thermal broadening is known as the Mossbauer

effect. Its unique feature is in the production of monochromatic electromagnetic radiation with a very narrowly defined energy spectrum that allows resolving minute energy differences [2,3].

Mossbauer spectroscopy (MS) is a powerful analytical technique because of its specificity for one single element and because of its extremely high sensitivity to changes in the atomic configuration in the near vicinity of the probe isotopes (in this case 57Fe). MS measures hyperfine interactions and these provide valuable and often unique information about the magnetic and electronic state of the iron samples, their chemical bonding to co-ordinating ligands, the local crystal symmetry at the iron sites, structural defects, lattice-dynamical properties, elastic stresses, etc. [1,4]. Hyperfine interactions include the electric monopole interaction, i. e., the isomer shift, the electric quadrupole interaction, i. e., the quadrupole splitting, and the magnetic dipole (or nuclear Zeeman) interaction, i. e., hyperfine magnetic splitting. These interactions often enable us detailed insight into the structural and magnetic environment of the Mossbauer isotope. Indeed, more than four decades after its discovery (1958), Mossbauer spectroscopy still continues to develop as a sophistical scientific technique and it is often the most effective way of characterizing the range of structures, phases, and metastable states.

In general, a Mossbauer spectrum shows different components if the probe atoms are located at lattice positions, which are chemically or crystalographically unequivalent. From the parameters that characterise a particular Mossbauer sub-spectrum it can, for instance, be established whether the corresponding probe atoms reside in sites which are not affected by structural lattice defects, or whether they are located at defect-correlated positions. Each compound or phase, which contains iron, has typical parameters of its Mossabuer spectrum. It means, the method is suitable for quantitative as well as qualitative analysis. Mossbauer spectroscopy is non-destructive and requires relative small quantities of samples (~100 mg) [5-8].

Application of Mossbauer spectroscopy for precise analysis of phase composition of corrosion products was performed from selected areas of primary and secondary circuit and SG. Interpretation of measured results, having in vision the long-term operation and nuclear safety, is not easy, nor straightforward. Thanks to our more than 25 years of experiences in this area, there exists already a base for the relevant evaluation of results. Optimisation of operating chemical regimes as well as regimes at decontamination and passivation seems to be an excellent output.