LT5897B - METHOD FOR DETERMINING OF 137Cs RADIOCONTAMINATION IN PINE RINGS - Google Patents

Abstract

The invention relates to the methods of analysis for determining specific activities of gamma radiation emitting radionuclide in wood, without crossing the trees. The specific activity of 137Cs is determined by the distribution of the nuclide in the soil under the tree crown. To determine the distribution of 137Cs in soil under the tree crown, the collected soil samples at a distance of 1 m from tree is dried to constant weight at temperature 105 oC , prepared to analysis and a gamma spectrometry carried out. Specific activity of the last formed wood ring is determined from coefficients of the competitiveness of chemical analogues, 40K relative distribution and gamma spectroscopy data – relative percentage difference between the results obtained by the method and the specific results of the experiment is up to 16,7 percent. Assessing the distribution of the specific activities of 137Cs in the annual rings the average relative percentage difference between the results obtained by the method and the specific results of the experiment is obtained: it is 27,0 percent.

Description

Technical field

The present invention relates to methods of analyzing environmental contamination by determining the specific activity of a radionuclide ( ^l37 Cs) emitting gamma radiation in wood.

State of the art

A method for assessing environmental contamination by radionuclides is described in patent RU 2265869 Cl. This method involves the selection of wood specimens, their preparation and the analysis of the distribution of radionuclides in the annual groove by the use of splitting radiography. This method can be used to estimate the maximum release of radionuclides into the environment. However, this method does not determine the specific value of radioactive contamination and the specific activity of a particular radionuclide in the annual groove.

Currently, one of the methods used for retrospective analysis of radioactive contamination is to ^{determine the} specific activity of ^I37 Cs in the annual ^{bark of} common pine by gamma spectrometric analysis. This method involves harvesting and primary collection of wood specimens, followed by the preparation of wood specimens for gamma spectrometric analysis, ie splitting, drying, burning and shredding, and the gamma spectral analysis of the wood specimens itself, which detects radioactive contamination of annual wood bark (Buziny, M. ; Los, L.; Shepelevich, K. 2000. The distribution of ^l37 Cs and ⁹⁰ Sr in the biomass of pine trees planted from 1987 to 1988 in the near-zone Chemobyl nuclear power plant Radiation and Isotopes 52: 905-910. Kagawa, A; Aoki, T; Okada, N; Katayama, Y. 2002. Tree Ring Strontium - 90 and Cesium - 137 as Potential Indicators of Radioactive Pollution, Journal of Envoronmental Quality 31: 2002-2007; Malek , MA; Hinton, TG; Webb, SB 2000. A comparison of ⁹⁰ Sr and ¹³⁷ Cs uptake in plants via three pathways at two Chernobyl - contaminated site. Journal of Envoronmental Radioactivity 58: 129-141).

The main disadvantage of this method is tree felling.

The essence of the invention

The object of the invention is to provide an opportunity to evaluate retrospective radioactive contamination in tree trunks by preserving the tree.

The method of detecting radioactive contamination in tree trunks, like the prototype, involves specimen collection and preparation for spectrometric analysis and spectrometric analysis. New is that samples are formed from soil under tree crown, radionuclide content in tree trunks is estimated by spectrometric analysis of specific activity of soil radionuclide using coefficients of ⁴⁰ K relative distribution and chemical analogues. In addition, soil samples shall be taken at a distance of 0.5-1.3 m from the tree every 2 cm to 10 cm deep and in deeper layers every 5 cm.

Environmental contamination of ¹³⁷ Cs is reflected in dendrochronological sequences. Thus, the present invention provides a method for investigating the accumulation of ¹³⁷ Cs in annual ridges based on analysis of its distribution in the soil under tree crown.

Description of the drawing

The invention is illustrated by a graph showing the relationship of the age of the tree to the coefficient describing the relationship between the specific activity Cs of the last groove (A _r ) and the preformed grooves (y4 ( _r . _N ), where n = l ... n).

DETAILED DESCRIPTION OF THE INVENTION The specific activity of ^I37 Cs in the annual groove is determined by the distribution of this radionuclide in the soil beneath the canopy. ^The distribution of ¹³⁷ Cs in the soil under the tree crown was determined by collecting soil samples at a distance of 1 m from the tree, every 2 cm at a depth of 0-10 cm, and every 5 cm at a depth of 10-25 cm. The soil samples are dried to constant weight at 105 ° C, prepared for analysis and the specific activity of ¹³⁷ Cs in the soil is determined.

In order to evaluate the distribution of Cs in the annual bark of Pinus sylvestris L. it is assumed that:

1. ⁴⁰ K ( ¹³⁷ Cs chemical-biological analogue) is homogeneously distributed in soil.

2. Cs is evenly distributed in the soil under tree canopy across the world.

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3. The vertical distribution of Cs in soil shall be determined experimentally in the reference year.

4. The vertical distribution of Cs in soil in previous years is estimated theoretically by the solution of ¹³⁷ Cs diffusion and directional transfer equation.

5. It is taken into account that the root system of ¹³⁷ Cs enters the tree about 80%, and the most effective ¹³⁷ Cs uptake about 9 to 15 years after pollination. In addition, from 0 to 5 cm soil depth ¹³⁷ Cs is slightly released into growing pine wood, whereas the most active Cs transport is from 15-20 cm soil depth. Distribution of pine root biomass in soil vertical is as follows: 0 - 5 cm depth 5%; 5-10 cm - 16.7%; 10-15 cm by 21.7%; 15-20 cm - 28.3%; 20-25 cm - 21.7%; 25 30 cm - 10.0%.

6. An evaluation of the specific activity ratios of ¹³ 'Cs and ^4U K from experimental data for soil and wood results in a coefficient of competition G for the chemical analogues, which is equal to:

4 (Cs))

Specific activity of soil in a given year, Bq / kg 4 (K) J _d {D hereT (Cs) d - ¹³⁷ Cs;

T (Cs) _m - specific activity of ¹³⁷ Cs in the annual groove, after radioactive decay, Bq / kg;

> l (K) d - ⁴⁰ K specific activity in soil, Bq / kg;

T (K) _m - Specific activity of ⁴⁰ K in the annual groove, Bq / kg.

7. Determine ⁴⁰ K relative partition coefficient L, averaging ⁴⁰ K specific activity in soil and wood:

r _ 4 (K _d ) AKJ '(2)

Knowing the competitiveness coefficient of chemical analogues G and the L 37 coefficient of distribution of ⁴⁰ K, the specific activity of Cs in the last formed tree bark is determined according to the formula:

^ (Cs) _m

4 (Cs) _di ^ i ^ d (3) where Χ, - is a coefficient that takes account of the vertical arrangement of the roots of the tree;

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T (Cs) d, i - Specific activity of Cs at given soil depths during the study year, Bq / kg;

Zvid is a factor expressing the average ratio of ⁴⁰ K of specific activity in soil and wood;

Gi is a coefficient that estimates the specific activity relationships of ¹³⁷ Cs and ⁴⁰ K in soil and wood for a given soil depth.

Formula (3), which estimates the amount of ¹³⁷ Cs in wood, disregards components that describe Cs uptake from deeper than 15 cm layers of soil, since these layers contain small amounts of the latter radionuclide, an average of less than 8% according to experimental data. Therefore, the specific activity of ¹³⁷ Cs in the last formed tree trunk is determined by the formula:

The (Cs) dj

G L! Vld

X ₀ _ ₂ -A (Cs) ₀ . ₂ , X ₂ _ ₄ -A (Cs) ₂ _ ₄ , X ₄ _ ₆ -A (Cs) ₄ _ ₆ ,

- -t b --'---- 1_ ~ * vid ⁷ 0-2

2-4 '4-6 ₊ - ^ 6-8' ^ (^) 0-8 ₊ - ^ 8-10 '4 (C ^) _g _iQ + Y10-15'4 (C5) _{| Q} _ _{| 5} ^ 6-8 ^ 8-10 ^ 10-15 / Here Λ0-2,2-4,4-6,6-8,8-10,10-15 - Factor for the vertical arrangement of the roots of the tree 0-2, 2- 4.4-6, 6-8, 8-10.10-10-15 cm at soil depths;

G0-2,2-4,4-6,6-8,8-10, 10-15 - coefficient evaluating ^l37 Cs and ⁴⁰ K specific activity ratios in soil and wood, 0-2, 2-4,4-6 , 6-8, 8-10, 10-15 cm at soil depths.

^{The 40} k relative partition coefficient f _v id, which describes the ratio of ⁴⁰ K in soil to wood, depending on soil characteristics, is given in Table 1.

In Table 1. Values of Lyg for sandy loam

.Lvid 1.72 2.90 0.70 1.76 2.98 2.53 Soil pH 4.7 4.0 5.2 4.4 4.2 4.2 Sand,% 95 91 91 94 90 90 Clay,% 2 2 4 3 2 2

The competitive factor G of the chemical analogues, which estimates the ratio of ^l37 Cs to ⁴⁰ K in soil-to-tree ratios based on soil depths for the last tree-formed groove, is shown in Table 2.

table. G-values of the competitive factor of chemical analogues

Depth, cm 0-2 2-4 4-6 6-8 8-10 10-15 15-20 G 6.01 7.05 4.56 1.85 0.59 0.33 0.03

By evaluating the specific activity of ¹³⁷ Cs in the last formed tree bark according to formula (4), it is determined what specific activity is present in the formed tree bark of previous years. Figure 1 gives the dependence of the age of the tree on the coefficient describing the ratio of ¹³⁷ Cs specific activity in the last groove (A _T ) and in the preformed grooves (A ( _r .

n), chian = l ... n).

For slabs formed before 2005, the coefficient describing the ratio of ^I37 Cs in the last slab (A _T ) of the specific activity to the preformed slabs (A ^ n) is estimated by the following equation obtained experimentally:

Ar-n / Ab = 7 · 10 ’ ⁷ · r ⁶ - 6 · 10’ ⁵ -r ⁵ + 0.0022 -r ⁴ - 0.0381 · r ³ + 0.2971 · r ² - 0.7523 -r + 0.8522, (5) hereA _r - ¹³⁷ Cs specific activity in the last groove, Bq / kg;

yl (r.n) - specific activity of Cs in preformed grooves, where n = 1 ... n, Bq / kg; r is the year of growth of the tree.

For rows formed after 2005 (in this case corresponds to 70 years of tree growth), the dependence of the age of the tree on the coefficient describing the ratio of ¹³⁷ Cs specific activity in the last ridge (A _r ) and pre-formed rails G4 _tr _ _n ), where n = l. ..n) determined by the formula:

Ar-n / Ad = 0.0428 · r + 0.9338, (6) where A - ^l37 Cs specific activity in the last groove, Bq / kg;

Ar-n) - ¹³⁷ Cs specific activity of preformed grooves, where n = 1 ... n, Bq / kg; r is the year of growth of the tree.

Knowing the ratio of the coefficient describing the ¹³⁷ Cs specific activity in the last groove (A) to the preformed grooves (Ar-n), where n = l ... n), the ¹³⁷ Cs specific activity values in the groove are given by the formula:

ACs) _r = ACs) '(7) where X (Cs.A - ^l37 Specific activity of Cs in any groove, Bq / kg;

Specific activity of A (Cs) _m - ^l37 Cs in the last formed groove, evaluated according to formula (4), Bq / kg;

⁽ A ^{λ A} rn \)

The factor XI, which describes the ratio of the specific activity of Cs in the last groove (A,) to the preformed grooves (Ar-n), where n = l ... n), which is derived from Figure 1.

Evaluating the dependence of the age of the tree on the coefficient describing the ratio of ¹³⁷ Cs specific activity in the last groove (A) and previously formed grooves (Ar-n), where n =

l ... n), a breakdown of the ridges with respect to the period appeared. Since 2005 (70 years of growth), the nature of the dependence has been determined by factors such as past radioactive atmospheric pollution due to nuclear testing and nuclear plant accidents, vertical ^{uptake of 137} Cs in the soil and influence on the growing plant. Figure 1 dependence of the age of the tree on the coefficient describing the specific activity of Cs in the last groove (Ar) and in preformed grooves (Ar-n here n = l ... n) when the grooves were formed after 2005 is of slightly rising line nature. This change is consistent with the assumption that after 2005 there is no additional radioactive contamination in the atmosphere and the main source of radioactive contamination of the tree is the radionuclides present in the soil. At present, ¹³⁷ Cs has the highest specific activity at 5 cm soil layer. After 40 years, this radionuclide is expected to reach the root biomass of the tree. Knowing that ¹³⁷ Cs T0.5 = 30 years, the specific activity of this radionuclide will be reduced by more than half. Not only vertical downward migration, but also horizontal redistribution and soil remediation by other lower-class plants are also occurring in the soil.

Using the test method for determining the specific activity of ¹³⁷ Cs in the annual bark of common pine, based on the analysis of its distribution in the soil under the canopy, the average relative error (%) between the results of the test method and 7%, and 27.0% for the distribution of specific activity across all cortex.

Claims (3)

Definition of the Invention 1 T7 1. Method for the determination of radioactive contamination in Cs tree bark, comprising specimen collection and preparation for spectrometric analysis and spectrometric analysis, 5, characterized in that specimens are formed from soil under tree canopy, the radionuclide content in tree trunks is estimated by spectrometric analysis of the specific activity of this radionuclide in the god's soil, using coefficients of ⁴⁰ K relative distribution and chemical analogues. 10th 2. A method for detecting radioactive contamination of ¹³⁷ Cs in tree trunks according to claim 1, characterized in that the soil samples are selected at a distance of 0.5-1.3 m from the tree. Method for detecting radioactive contamination in tree trunks according to claims 1-2 15 characterized in that soil samples are collected every 2 cm to 10 cm deep and in deeper layers every 5 cm.