M.V. Zheltonozhskaya1, V.A. Zheltonozhsky2, N.V. Kuzmenkova3, A.N. Nikitin4,  V.V. Rozanov5, A.P. Chernyaev6

1,3, 5, 6 Lomonosov Moscow State University (Moscow, Russia)

2 Lomonosov Moscow State University, Skobeltsyn Institute of Nuclear Physics

4 Institute of Radiobiology of the National Academy of Sciences of Belarus (Gomel, Belarus)

The paper presents a study of bottom sediment samples in the Chernobyl nuclear power plant (ChNPP) cooling pond. During the accident at the 4th power unit of the ChNPP, a large amount of finely dispersed nuclear fuel in the form of particles, mainly 1-10 microns in size, got into the environment. One of the most important reservoirs of the fuel component of fallout in the Chernobyl exclusion zone is the cooling pond. Over the past three years, the reservoir's water level has dropped sharply, and its gradual decrease continues. As a result, a large number of hot particles, which were previously preserved in the cooling pond oxygen-free organogenic silts, are now exposed to atmospheric oxygen and are subject to weathering and chemical changes. The work investigates the isotopic composition and vertical distribution of radionuclides in the bottom sediments of the ChNPP cooling pond. Samples were taken from different points of the cooling pond in the form of cores to a depth of 30 cm. The gamma spectra of the samples were analyzed using an anti-Compton HPGe spectrometer with a beryllium window and an energy resolution of 1.9 keV for gamma rays 60Co 1173 keV and 1333 keV and 350 eV for gamma radiation for the gamma ray 241Am 59 keV. As a result, the activity of 60Co, 94Nb, 134Cs, 137Cs, 154Eu, 155Eu, 241Am, 243Am, 243Cm in the studied samples of bottom sediments were obtained. In addition, the 90Sr activity was obtained by analyzing the electron spectra and characteristic radiation. We used direct alpha spectrometry with minor changes to assess the isotopic composition of alpha emitters and calculate the specific activity of alpha-emitting radionuclides. Data were obtained on the abundances of 234U, 236U, 238U, 238Pu, 239+240Pu, 242Pu, 241Am, 243Am, 242Cm, 243+244Cm in the studied samples.

As a result, we identified core sections where the main activity was concentrated at a 15-20 cm depth. Precipitation down to a depth of 10 cm is mainly associated with the migration of aerosol fallout in subsequent years. In the fallout of bottom sediments of the cooling pond, the same component is released as in the soils of the contaminated areas adjacent to the ChNPP 4th unit, which was well studied in the post-accident period. Analysis of the ratios of 60Co and 94Nb activation radionuclides at a depth of 15-20 cm and comparison with such data on structural materials of the ChNPP 2nd unit showed that most likely we observed the fallout from the first explosion of the ChNPP 4th unit, which occurred on April 26, 1986. Furthermore, obtained data of the composition of alpha emitters for these precipitations demonstrate that the isotopic ratios of transuranium nuclides in the bottom sediment samples at a depth of 15-20 cm are close to the isotopic ratios in the fuel elements of the 4th power unit of the ChNPP.

Zheltonozhskaya M.V., Zheltonozhsky V.A., Kuzmenkova N.V., Nikitin A.N., Rozanov V.V., Chernyaev A.P. Chernobyl nuclear power plant cooling pond fuel fallout research. Science Intensive Technologies. 2021. V. 22. № 7. P. 60−67. DOI: https://doi.org/10.18127/j19998465-202107-08 (in Russian)

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