DISTRIBUTION OF ¹³⁷CS AND ⁹⁰SR ACTIVITY CONCENTRATION IN STEM WOOD OF SCOTS PINE (PINUS SYLVESTRIS L.) TREE NEAR CHERNOBYL NUCLEAR POWER PLANT

D. M. Holiaka¹, V. I. Yoschenko², S. E. Levchuk¹, L. V. Yoschenko¹, V. O. Kashparov¹

¹ Ukrainian Institute of Agricultural Radiology of National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine

² Institute of Environmental Radioactivity of Fukushima University, Fukushima, Japan

One of the world's tightest hygiene standards for the activity concentration of radionuclides in wood and wood products has been developed and implemented in Ukraine. It has defined the requirements for ¹³⁷Cs content in fuel wood, timber products and wood products, and for ⁹⁰Sr – only in fuel wood, but activity concentration in different organs or their parts often has significant variability.

The collection of experimental material on an experimental site done before the beginning of the growing season in 2018. The forest stand is located 5 km north of the fourth power unit of the Chernobyl Nuclear Power Plant (coordinates WGS84: N°51.4342, E°31.1086). The contamination density soil at forest site for ¹³⁷Cs is (GM×GSD^±1) 2510×1.42^±1 kBq·m^-2, and for ⁹⁰Sr – 1120×1.44^±1 kBq·m^-2. The selection of wood samples from model trees for determining the radionuclides activity concentration was done on the basis of cut out wood discs in the thickness of 4-5 cm at relative heights: 0.00H (at the base of the cut tree), 0.10H (10% height of the barrel, etc.), 0.25H, 0.50H, 0.75H, 0.90H, and also at a height of 1.3 m from ground surface. The detailed radial and profile distribution of ⁹⁰Sr and ¹³⁷Cs in stem wood is studied on the basis of woody discs of 4 model trees. The total number of stem wood samples for measurement of activity concentration of radionuclides was 156 psc.

Previous exploratory analysis using the nonparametric Spearman`s correlation coefficient of the input data for wood samples from cut out disks at a height of 1.3 m found a tight monotonic dependence between ¹³⁷Cs and ⁹⁰Sr activity concentration and next parameters: middle diameter
(r_s is 0.66 and –0.77), years of formation (r_s is 0.54 and –0.88), anatomical parts stem of wood (r_s is
–0.44 and 0.83 respectively). All correlation coefficients are significant at p<5%. The asynchrony between the change contents of ¹³⁷Cs and ⁹⁰Sr in the radial direction is clearly traced. If the peak of the ¹³⁷Cs activity concentration is observed near the periphery of the wood stem (that adjacent to the bark), then the maximum for ⁹⁰Sr is detected in the center of the stem.

As already noted in the paper above, we can assert that ¹³⁷Cs activity concentration in the peripheral wood layers of the pine stem is the largest. This generalization completely coincides with the results of previous studies of pine plantations of exposed to radioactive contamination as a result of the Chernobyl disaster. Each model trees at an altitude of 1.3 m have noticeable decrease in the content of ¹³⁷Cs, which is observed with a decrease in the relative diameter of the range 0.4-0.6. In these studies, it coincides with the radial transition of sapwood into the heartwood of stems.

The ⁹⁰Sr radial distribution at a tree height of 1.3 m for the collected experimental material has a monotonous decrease with an increase of the relative diameter from 0.30 to 0.95. A slight increase of the radioisotope content at the periphery of the stem wood was detected, which is caused by the impossibility to fully clean the lateral surface of the wood disk from the inner bark (it has more heighted ⁹⁰Sr content then sapwood). The layers of stem wood are characterized by the highest contamination of ⁹⁰Sr in the range of relative diameters of 0.0-0.3.

The analysis of the ¹³⁷Cs activity concentration distribution in wood indicates about gradual its decrease in peripheral layers with increasing height of stem, but the pollution of the central layers increases from 5.3 kBq·kg^-1 to about 10.0 kBq·kg^-1, because upper part of trees forms more late annual rings.

The content of ⁹⁰Sr in annual rings of wood grows when an approach to the piths of the tree stems. This caused a gradual reduction of the "average" pollution with increasing height of the tree. In the peripheral layers of the stem wood are characterized by the lowest concentration of ⁹⁰Sr at all heights from which had cutted up wood disks.

This work was supported research project by the National University of Life and Environmental Sciences of Ukraine N^o 110/90-f “Experimentally study and estimation biogenic flows of ⁹⁰Sr in forest ecosystems”.