Six samples for Mossbauer effect experiments collected from different parts of NPP Bohunice unit were prepared by crushing to powder pieces (Table 9). These samples consisted of corrosion products taken from small coolant circuit of pumps (sample No. 3.1), deposits scraped from filters after filtration of SG — feed water during operation (sample No. 3.2), corrosion products taken from SG42 pipelines — low level (sample No. 3.3), mixture of corrosion products, ionex, sand taken from filter of condenser to TG 42 (sample No. 3.4), deposit from filters after refiltering 340 l of feed water of SG S3-09 during passivation 27. and 28. 5. 08 (sample No. 3.5) and finally deposit from filters after 367 l of feed water of SG S4-09 during passivation 27. and 28. 5. 08 (sample No. 3.6). All samples were measured at room temperature in transmission geometry using a 57Co(Rh) source. Calibration was performed with a-Fe. Hyperfine parameters of the spectra including spectral area (Arei), isomer shift (IS), quadrupole splitting (QS), as well as hyperfine magnetic field (Bhf), were refined using the CONFIT fitting software [27], the accuracy in their determination are of +0.5 % for relative area Are], +0.04 mm/ s for Isomer Shift and Quadrupole splitting and ±0.5 T for hyperfine field correspondingly. Hyperfine parameters of identified components (hematite, magnetite, goethite, lepidocrocite, feroxyhyte) were taken from [28].

All measured spectra contained iron in magnetic and many times also in paramagnetic phases. Magnetic phases contained iron in nonstoichiometric magnetite Fe3-xMxC>4 where Mx are impurities and vacancies which substitute iron in octahedral (B) sites. Another magnetic fraction is hematite, a-Fe2C3. In one sample also the magnetic hydroxide (goethite a — FeCCH) was identified.

Paramagnetic fractions are presented in the spectra by quadrupole doublets (QS). Their parameters are close to those of hydroxides e. g. lepidocrocite у — FeOOH or to small, so called superparamagnetic particles of iron oxides or hydroxides with the mean diameter of about 10 nm. It should be noted that there is no problem to distinguish among different magnetically ordered phases when they are present in a well crystalline form with low degree (or without) substitution. Both the substitutions and the presence of small superparamagnetic particles make the situation more complicated [29]. In such cases, it is necessary to perform other supplementary measurements at different temperatures down to liquid nitrogen or liquid helium temperatures without and with external magnetic field [30].

Mossbauer spectrum (Fig. 10) of sample no. 3.1 (corrosion products taken from small coolant circuit of pumps) consist of three magnetically split components, where the component with hyperfine field Bhf = 35.8 T was identified as goethite (a-FeOOH). Hyperfine parameters of remaining two magnetically split components are assigned to A — sites and B — sites of magnetite (Fe3C4). Cne paramagnetic spectral component has appeared. According to water environment and pH [31], this component should be assigned to hydrooxide (feroxyhyte 8-FeOOH).

Fig. 10. Mossbauer spectrum of sample no. 3.1. A-site (red), B-site (dark red) magnetite, goethite (pink) and hydroxide (green) was identified

The sample No. 3.2 (deposits scraped from filters after filtration of SG — feed water during operation) also consists of three magnetically split components, where two of them were assigned to magnetite (Fe3C4) as in previous spectra, and the remaining magnetically split component was identified as hematite (a-Fe2O3). Paramagnetic part of the spectra was formed by one doublet, whose hyperfine parameters were assigned to hydroxide (lepidocrocite, y-FeOOH). The spectrum is shown in Fig. 11.

Fig. 11. Mossbauer spectrum of sample no.3. 2. A-site (red), B-site (dark red) magnetite, hematite (blue) and hydroxide (green) was identified

The spectrum (Fig. 12) of the sample No. 3.3 (corrosion products taken from SG42 pipelines — low level) consists only of two magnetically split components with hyperfine parameters assigned to A — sites and B — sites of nearly stoichiometric magnetite (Fe3O4) with a relative area ratio в = 1.85.

Fig. 12. Mossbauer spectrum of sample no. 3.3. A-site (red) , B-site (dark red) magnetite was identified

The sample No. 3.4 (mixture of corrosion products, ionex, sand taken from filter of condenser to TG 42) also consists of a magnetically split component which corresponds to hematite (a-Fe2O3) and two magnetically split components were assigned to magnetite (Fe3O4) as in previous spectra, and the remaining paramagnetic component was identified as hydroxide. The spectrum of the sample No. 3.4 is shown in Fig. 13.

velocity (mm/s)

Fig. 13. Mossbauer spectrum of sample no. 3.4. Haematite (blue), A-site (red) , B-site (dark red) magnetite and hydroxide (green) was identified

Both the sample No. 3.5 (deposit from filters after 340 l of feed water of SG S3-09 during passivation 27. and 28. 5. 08) and the sample No. 3.6 (deposit from filters after 367 l of feed water of SG S4-09 during passivation 27. and 28. 5. 08) consist of three magnetically split components, identified as hematite (a-Fe2O3) and magnetite (Fe3O4) and the remaining paramagnetic component in both spectra was assigned to hydrooxide (lepidocrocite y — FeOOH). The spectra of the samples No. 3.5 and 3.6 are shown in Figs. 14 and 15. Based on comparison of results from samples 3.5 and 3.6 it can be concluded that the longer passivation leads more to magnetite fraction (from 88% to 91%) in the corrosion products composition.

As it was mentioned, above all hydroxides could be also small superparamagnetic particles.

The refined spectral parameters of individual components including spectral area (Arei), isomer shift (IS), quadrupole splitting (QS), as well as hyperfine magnetic field (Bf) are listed in Table 9 for room (300 K) temperature Mossbauer effect experiments. The hyperfine parameters for identified components (hematite, magnetite, goethite, lepidocrocite, feroxyhyte) are listed in [28].

Major fraction in all samples consists of magnetically ordered iron oxides, mainly magnetite (apart from the sample No. 3.1 and 3.2, where also goethite and hematite has appeared, respectively). Magnetite crystallizes in the cubic inverse spinel structure. The oxygen ions form

velocity (mm/s)

s

velocity (mm/s)

a closed packed cubic structure with Fe ions localized in two different sites, octahedral and tetrahedral. The tetrahedral sites (A) are occupied by trivalent Fe ions. Tri- and divalent Fe ions occupying the octahedral sites (B) are randomly arranged at room temperature because of electron hopping. At room temperature, when the electron hopping process is fast, the Mossbauer spectrum is characterized by two sextets. The one with the hyperfine magnetic field Bhf = 48.8 T and the isomer shift IS = 0.27 mm/ s relative to a-Fe corresponds to the Fe3+

sample	Component	Area [%]	Isomer shift [mm/s]	Quadrupole shift/splitti ng [mm/s]	Hyperfine field [T]
	magnetite A-site	36.3	0.28	0.00	48.90
Sample no. 3.1 Small coolant circuit of	magnetite B-site	37.2	0.64	0.00	45.60
pumps 17. 10. 2007	goethite	14.4	0.36	-0.25	35.80
	hydrooxide	12.1	0.36	0.70	—
Sample no. 3.2.	hematite	15.8	0.38	-0.23	51.56
Deposites scraped from filters after filtration of	magnetite A-site	32.6	0.28	0.00	49.14
SG — feed water during	magnetite B-site	41.8	0.65	0.00	45.91
operation	hydrooxide	9.7	0.38	0.56	—
Sample no. 3.3. SG42 pipelines — low	magnetite A-site	34.6	0.28	0.00	49.14
level	magnetite B-site	65.4	0.65	0.00	45.83
	hematite	9.2	0.38	-0.22	51.29
Sample no. 3.4. Mixture of corrosion products, ionex, sand	magnetite A-site	45.4	0.28	0.00	49.20
taken from filter of	magnetite B-site	40.7	0.66	0.00	45.87
condenser to TG 42	hydrooxide	4.7	0.37	0.56	—
	hematite	8.3	0.36	-0.22	51.33
Sample no. 3.5. Deposit from filters after 340 l of feed water of SG	magnetite A-site	49.3	0.30	0.00	49.11
S3-09 during pasivation	magnetite B-site	38.5	0.61	0.00	45.51
27. and 28. 5. 08	hydrooxide	3.9	0.37	0.55	—
Sample no. 3.6.	hematite	6.4	0.38	-0.25	51.26
Deposit from filters after 367 l of feed water of SG	magnetite A-site	50.3	0.29	0.00	49.14
S4-09 during pasivation	magnetite B-site	40.7	0.66	0.00	45.61
27. and 28. 5. 08	hydrooxide	2.6	0.37	0.54	—

Table 9. Spectral parameters of individual components including spectral area (Arei), isomer shift (IS), quadrupole splitting (QS), as well as hyperfine magnetic field (Bhf) for each sample with according components

ions at the tetrahedral A — sites. The second one with Бы = 45.7 T and IS = 0.65 mm/ s is the pe2.5+ — like average signal from the cations at octahedral B sites. Fe2+ and Fe3+ are indistinguishable due to fast electron transfer (electron hopping), which is faster (~1 ns) than the 57Fe excited state lifetime (98 ns). The magnetite unit cell contains eight Fe3+ ions and eight Fe2+ and Fe3+ ions, 16 in total at the B sites, therefore, the intensity ratio в = I(B)/I(A) of the two spectral components is a sensitive measure of the stoichiometry. Assuming that the room temperature ratio of the recoil-free fractions fB/ fA for the B and A sites is 0.97 [32], the intensity ratio в for a perfect stoichiometry should be 1.94. In non-stoichiometric magnetite, under an excess of oxygen, cation vacancies and substitutions at the B sites are created. The vacancies screen the charge transfer and isolate the hopping process. For each vacancy, five Fe3+ ions in octahedral sites become trapped. In the Mossbauer spectrum these trapped Fe3+ ions at the octahedral sites and Fe3+ ions at tetrahedral sites are indistinguishable without applying an external magnetic field. Therefore, in the spectrum of non-stoichiometric magnetite, intensity transfer from the Fe2.5+ to Fe3+-like components is observed. Therefore, the intensity ratio в decreases markedly with the oxidation process, until the stoichiometry reaches the y-Fe2O3 phase. It should be noted that in our samples the intensity ratio в is far from 1.94 (for perfect stoichiometry), varies from 0.97 up to 1.85.

2. Conclusions

Material degradation and corrosion are serious risks for long-term and reliable operation of NPP. The paper summarises results of long-term measurements (1984-2008) of corrosion products phase composition using Mossbauer spectroscopy.

The first period (mostly results achieved in 80-ties) was important for improving proper Mossbauer technique [5]. The benefit from this period came via experience collection, optimization of measurement condition and evaluation programs improvement. Unfortunately, the samples were not well defined and having in mind also different level of technique and evaluation procedures, it would be not serious to compare results from this period to results obtained from measurement after 1998.

The replacement of STN 12022 steel (in Russian NPP marked as GOST 20K) used in the steam generator feed water systems is necessary and very important from the operational as well as nuclear safety point of view. Steel STN 17 247 proved 5 years in operation at SG35 seems to be optimal solution of this problem. Nevertheless, periodical inspection of the feed water tubes corrosion (after 10, 15 and 20 years) was recommended.

Based on results of visual inspection performed at April 19, 2002 at SG16 (NPP V1) it was confirmed, that the steam generator was in good condition also after 23 years of operation. Samples taken from the internal body surface of PG16 confirmed that the hematite concentration increases in the vertical direction (from bottom part to the top).

The newest results from 2008 confirm good operational experiences and suitable chemical regimes (reduction environment) which results mostly in creation of magnetite (on the level 70% or higher) and small portions of hematite, goethite or hydrooxides.

Regular observation of corrosion/ erosion processes is essential for keeping NPP operation on high safety level. The output from performed material analyses influences the optimisation of operating chemical regimes and it can be used in optimisation of regimes at

HEMATITE

MAGNETITE

Period 1998-99 1-13

Period 2002-03 2.11-2.15

Period 2006-08 3.1-3.6

Fig. 16. Summarized figure of corrosion products phase composition at NPP V-2 Bohunice (Slovakia) performed according to results from period 1998-2008

decontamination and passivation of pipelines or secondary circuit components. It can be concluded that a longer passivation time leads more to magnetite fraction in the corrosion products composition.

Differences in hematite and magnetite content in corrosion layers taken from hot and cold collectors at SG11 in 2004 show, that there is a significantly lower presence of magnetite in case of hot collector. This fact can be derived from 2 parallel factors: (i) difference in temperature (about 298°C — HC) and (about 223°C — CC) and mostly due to (ii) higher dynamic of secondary water flowing in the vicinity of hot collector, which high probably removes the corrosion layer away from the collector surface.

With the aim to summarize our results in the form suitable for daily use in the operational conditions a summarized figure was created (see Fig. 16). Corrosion products phase composition (limited on magnetite and hematite only) is presented in form of circular diagrams.

Basically, the corrosion of new feed water pipelines system (from austenitic steel) in combination with operation regimes (as it was at SG35 since 1998) goes to magnetite. In samples taken from positions 5 to 14 (see Fig. 16 — right corner). The hematite presence is mostly on the internal surface of SG body (constructed from "carbon steel" according to GOST20K). Its concentration increases towards the top of the body and is much significant in the seam part of SG where flowing water removes the corrosion layer via erosion better than from the dry part of the internal surface or upper part of pipeline.

The long-term study of phase composition of corrosion products at VVER reactors is one of precondition to the safe operation over the projected NPP lifetime. The long-term observation of corrosion situation by Mossbauer spectroscopy is in favour of utility and is not costly. Based on the achieved results, the following points could be established as an outlook for the next period:

1. In collaboration with NPP-Bohunice experts for operation as well as for chemical regimes, several new additional samples from not studied places should be extracted and measured by Mossbauer spectroscopy with the aim to complete the existing results database.

2. Optimisation of chemical regimes (having in mind the measured phase composition of measured corrosion specimens from past) could be discussed and perhaps improved.

3. Optimisation and re-evaluation of chemical solutions used in cleaning and/or decommissioning processes during NPP operation can be considered.

In connection to the planned NPP Mochovce 3, 4 commissioning (announced officially at 3.10.2008) it is recommended that all feed water pipelines and water distribution systems in steam generators should be replaced immediately before putting in operation by new ones constructed from austenitic steels. The Bohunice design with feed water distribution boxes is highly recommended and it seems to be accepted from the utility side.

3. Acknowledgement

This work was supported by company ENEL Produzione, Pisa and by VEGA 1/0129/09.

[1] Some countries (e. g. France) does not consider the fuel pellet as a safety barrier

[2] The analysis of GDH guillotine break at complete ECCS failure and at operation of one, two or three ECCS pumps demonstrated that for reliable cooling of the core at a long stage two ECCS pumps are enough. With such number of ECCS equipment, after one hour from the beginning of the accident, the ECCS water flow rate starts to exceed water discharge through the break. However, short-term increase in temperatures of fuel rod cladding and FC walls at the initial stage of accident in FCs, connected to the broken GDH, is inevitable in any case. The excess of acceptance criterion for fuel rod cladding (700 °С) is probable in 12 FCs. A more detailed analysis (using RELAP/SCDAPSIM model presented in Fig. 7) shows that this short temperature peak does not lead to the failure of any fuel rods.

• In the case of coolant flow rate stagnation in channels, multiple FCs breaks are probable after approximately 20 s from the beginning of the accident if the reactor is not shutdown on time. On the contrary, if a reactor is shutdown quickly (until the FCs heat up), the acceptance criterion for channels walls will not be violated and the channels

[3] The results of the analysis showed that at LOCA in Zone 4 the operation of ECCS short­term subsystem is not necessary, i. e. the temperature of fuel rod cladding and FC walls is much lower than acceptance criteria without operation of this subsystem.

• For reliable cooling of the reactor core in long-term post-accidental period, it is necessary to have not less than two ECCS pumps in operation in the case of two steamlines break, and not less than one pump in the case of one steamline break.

• In case of breaks in Zone 4 without the reactor shutdown (break of steamlines), the temperature rises much more slowly (especially the temperature of FCs walls). This specifies that such breaks in Zone 4 are less dangerous, than breaks in Zone 1 (Fig. 19). In the case without the reactor shutdown, the melting of the core at low pressure in RCS is probable, but does not result in the immediate damage of the reactor cavity.

[4] loss of intermediate cooling circuit;

• loss of service water;

• station blackout.

The station blackout (the most likely initiating event) is the loss of normal electrical power supply for local needs with an additional failure on start-up of all diesel generators. In the case of loss of electrical power supply MCPs, the circulating pumps of the service water system and feedwater supply pumps are switched-off. The failure of diesel generators leads to the non-operability of the emergency long-term core cooling subsystem. It means the

[5] For the first group of accidents (accidents with no severe damage of the core) it was showed: (1) In the case of erroneously withdrawn of a group of control rods, the local power increase appears in the adjacent fuel channels, but this do not lead to overheating of the fuel in these channels. The operators have possibility to compensate this local power increase by inserting remaining control rods. In the case the local power exceeds limits — the reactor will be shutdown automatically by activation of emergency shutdown system. (2) In the case of loss of long-term cooling,

[6] Turbulent buffeting

• Vorticity excitation

• Fluid-elastic excitation

• Acoustic resonance

Turbulent buffeting cannot be avoided in heat exchangers, as significant turbulence levels are always present. Vibration at or near shedding frequency has a strong organizing effect on the wake. Vorticity or vortex shedding or periodic wake shedding is a discrete, periodic, and a constant Strouhal number phenomenon. Strouhal number is the proportionality constant between the frequency of vortex shedding and free stream velocity divided by

[7] First, information about the present status of the waste was gathered. Attention was paid to the variability and accuracy of data on quantities, the radionuclide inventory and the activity of different types of waste.

• Then, based on this information, what may most likely be expected (with regard with these waste characteristics) by the end of the anticipated operational period of Krsko NPP, i. e. 2023, was estimated. The ORIGEN2 computer code was used for calculating isotope generation, activity build-up and depletion, and the decay heat of spent fuel (Croff, 1983), while a specific code was developed for calculations associated with LILW.

• The total activity, its time dependent change and the identification of radionuclides that mainly contribute to the activity in long timeframes, were applied as key information for discussing waste-disposal options for spent fuel (HLW). Changes (variations, uncertainty) in these characteristics were evaluated based on the technical specifications that are in place after the replacement of steam generators at the plant in the 17th fuel cycle in 2002. The variations considered were 3-5 % of U-235 in the fuel, and an operational period of the plant of five years more or five years less than that envisaged. The basic estimate was that Krsko NPP uses fuel with 4% U-235 in all future cycles and that it operates for 35 years.

The key input data for calculating burn-up and fuel characteristics in future cycles is not available at the moment. Consequently, certain assumptions had to be made. These were:

• 35 fuel cycles are assumed for the operational period of Krsko NPP.

• The average cycle burn-up is 12,000 MWd/tU. This value was adopted based on the following: The average number of effective days of full power operation per cycle is

[8] Annular flow: Heat flux continues increasing, and the dryness fraction in the channel increases too. When the vapor content is higher than that of churn flow, the liquid block is smashed and the vapor unites to be a continuous axle center in the core of the tube-bundle channel. The liquid film goes upward along the wall of PMMA pipe. Thus, annular flow occurs. The liquid film might be broken due to the effect of the vapor wave, as shown in Fig. 4 and Fig. 13 (d).

[9] Kliuchnikov Olexander, Seniuk Olga, Gorovyy Leontiy, Zhidkov Alexander, Ribalka Valeriy, Berezhna Valentyna, Bilko Nadiya, Sakada Volodimir, Bilko Denis, Borbuliak Irina, Kovalev Vasiliy, Krul Mikola, Petelin Grigoriy

Institute Cell Biology & Genetic Engineering of NAS of Ukraine, Ukraine Institute for Safety Problems of Nuclear Power Plants NAS of Ukraine, Ukraine National University "Kyievo-Mogiljanskaja Academy", Ukraine

[10] Samples m006, m008, m010 were taken from outside surface, samples M007, M009, M012 from inside surface of the feed water pipeline according to the same positions 1, 2 and 3, respectively. Sample M15 — see Fig. 7, position 7).

[11] Samples l754-l757 were taken from the feed water pipelines in situ during the reactor shut down. Samples l758-l790 were taken from the same steam generator from selected parts of feed water dispersion box (see Table 3 and Fig. 6, positions 1-14)