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A study of the pressure vessel steel of the WWER-440 unit 1 of the Kozloduy nuclear power plant

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Abstract

A comparison between highly neutron irradiated samples from the region of weld № 4 and low irradiated samples from weld № 1 taken from the pressure vessel of the WWER-440 Unit № 1 of the Kozloduy NPP has been performed. Measurements of the residual activity of samples from the outer surface of the reactor pressure vessel bottom corpus reveal very low activity of 60Co. Insofar as there the base and weld metal appear to be exposed to a very low neutron fluence, the samples from these locations can be considered as practically not affected and may serve as a reference basis for comparison with highly irradiated pressure vessel regions. The Mössbauer parameters isomer shift (IS) and quadrupole splitting (QS) were found to be absolutely irradiation insensitive. A stepwise reduction of the internal hyperfine magnetic field Bhf, each by about 2.6 T, was observed. This can be attributed to the replacement of one or two surrounding iron atoms as first nearest neighbors by non-iron alloying atoms. The Mössbauer experimental line widths for irradiated and non-irradiated samples are practically the same, which is a quite unexpected result. The area fraction ratio for the three main Zeeman sextet subspectra S1:S2:S3 shows very high irradiation sensitivity. For the bottom low irradiated region of the reactor vessel the values are S1:S2:S3 = 50.1:40.0:9.4. After seven years of operation between the pressure vessel annealing in 1989 and the autumn of 1996 when the samples from weld № 4 were taken the ratio changes strongly to S1:S2:S3 = 56.4:34.7:8.5. A possible explanation of this result is that neutron irradiation gives rise to a precipitation process involving predominantly alloying atoms as Ni, Mn, Cr, Mo and V which become mobile and precipitate in the form of carbides and/or P-rich phases and alloying atom aggregates. This “refinement” process lowers the partial area of subspectra S2 and S3 where alloying atoms are involved and leads to a higher area fraction of the pure iron component S1, which is the major experimental result. For a more complete Mössbauer investigation on the processes of generation of structure defects caused by the neutron fluence, a new series of measurements will be performed by using a set of so-called surveillance specimens with different irradiation histories which are available only for the WWER-1000 reactors of the Kozloduy NPP.

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Acknowledgements

We would like to thank Dr. Ivaylo Christoskov for the neutron 'uence calculations, the helpful discussions and his standing interest in this work. Special thanks are due to Mr. Momchil Kazakov for his help in the recent sampling at the Kozloduy NPP. The partial financial support of the Research Fund of the University of Sofia (Contract No. 80-10-125/2017) is highly appreciated.

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Authors and Affiliations

  1. Department of Nuclear Engineering, University of Sofia, 5 James Bourchier Blvd., 1164, Sofia, Bulgaria

    E. Kostadinova & T. Avdjieva

  2. Institute of Catalysis, BAS, Acad. G. Bonchev St., Bldg. 11, 1113, Sofia, Bulgaria

    N. Velinov & I. Mitov

  3. Department of Atomic Physics, University of Sofia, 5 James Bourchier Blvd., 1164, Sofia, Bulgaria

    V. Rusanov

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  1. E. Kostadinova
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  2. N. Velinov
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  3. T. Avdjieva
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  4. I. Mitov
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  5. V. Rusanov
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Correspondence to V. Rusanov.

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This article is part of the Topical Collection on Proceedings of the International Conference on the Applications of the Mössbauer Effect (ICAME 2017), Saint-Petersburg, Russia, 3-8 September 2017 Edited by Valentin Semenov

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Kostadinova, E., Velinov, N., Avdjieva, T. et al. A study of the pressure vessel steel of the WWER-440 unit 1 of the Kozloduy nuclear power plant. Hyperfine Interact 238, 94 (2017). https://doi.org/10.1007/s10751-017-1467-x

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  • DOI: https://doi.org/10.1007/s10751-017-1467-x

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