Problem analysis and assessment of the prospects of fuel wood use obtained in the territories of the pollution by Chernobyl accident

Abstract

The Chernobyl accident led to radioactive contamination of large areas of Ukraine. The maximum number of radionuclides fell in the Ukrainian Polissya Zone. The areas of Zhytomyr, Rivne, Kyiv, Chernihiv and Volyn regions were especially polluted, with almost 40% of our country's forest areas concentrated, where significant volumes of timber harvesting occur. The forest, which is most contaminated with radioactive isotopes, grows on peat and sod-podzolic soils. These soils have varying degrees of moisture, high acidity (pH up to 3.2), low humus content, particles, exchangeable cations and clay minerals. Under these conditions, the ¹³⁷Cs and ⁹⁰Sr isotopes have high bioavailability.

The research purpose is substantiating the possibility of using wood for fuel power plants from the territories of the forest that have been exposed to radioactive contamination due to the Chernobyl accident.

The studies were conducted on the territory of forest areas in the northern parts of Zhytomyr (Naroditsky, Ovrutsky, Olevsky, Lugin and Malinsky areas) and Rivne regions (Sarninsky and Rokytnovsky areas). The levels of contamination of fuel wood were assessed by sampling soil and wood of the most common deciduous and coniferous tree species in these forests.

At each sampling point, the GPSmap 78s (Garmin, USA) set geographic coordinates on the WGS84 and mapped them.

The ¹³⁷Cs and ⁹⁰Sr activity were measured in the soil and wood samples selected. The ¹³⁷Cs content was determined using a gamma spectrometer with a semiconductor detector made of high purity germanium "GEM-30185" by "EG & ORTEC" (USA). The content of ⁹⁰Sr in the samples was determined after its radiochemical isolation according to the activity of its ⁹⁰Y daughter radionuclide on the SEB-01 beta spectrometer (ACP, Ukraine).

The specific activity of ¹³⁷Cs and ⁹⁰Sr in forest soils was 33¸675 Bq / kg and 1.5¸51 Bq / kg, respectively, which corresponds to a contamination density of 20-100 kBq / m² at ¹³⁷Cs and 0.2–4 kBq / m² at ⁹⁰Sr. Most samples showed a significant decrease in ⁹⁰Sr content relative to ¹³⁷Cs in the 20 cm layer of forest soil.  On average, the content of ¹³⁷Cs in soil was 22 times the content of ⁹⁰Sr.

The acidity of the soil extract varied within the pH = 4.0–7.4 and averaged pH = 6.4. The calcium content is very low and averages 0.7 mg-eq / 100 g of soil.

The ¹³⁷Cs content in the wood was found to be ˂2.0 to 690 ± 83 Bq / kg. About 98% of the analyzed samples answered GNPAR-2005 with respect to ¹³⁷Cs content in the fuel wood (600 Bq / kg). The ⁹⁰Sr content in the samples ranged from 3 ± 0.3 to 473 ± 37 Bq / kg. The overdose of its acceptable level was recorded in 14.7% of samples.

In the hardwood samples, the ratio of ¹³⁷Cs / ⁹⁰Sr was 1.4 ± 0.6. In conifer samples, this ratio has a wide amplitude of oscillation and averages 4.82 ± 2.

According to the breed composition, the ratio ¹³⁷Cs / ⁹⁰Sr in wood is as follows: common pine˃ birch˃ oak˃ alder˃ hornbeam˃ aspen.

A number of wood samples had a specific activity at ⁹⁰Sr over 100 Bq / kg. When burning this wood, the radioactivity of the ash will be ≥ 10 kBq / kg, which, according to the OSPU-2005, is radioactive waste and requires appropriate management of such ash.

The analysis of the results revealed that even from radioactively contaminated forests with a contamination density of ⁹⁰Sr in the area below 4 kBq / m², it is possible to obtain wood with a content of ⁹⁰Sr above the hygienic standard for fuel wood (GNPAR-2005), which requires a restriction onsite of  fuel and wood from these territories.

Hardwoods have been found to accumulate 90Sr much more intensively than pine.

It was determined that 14.7% of wood samples with ⁹⁰Sr content and 1.1%

with ¹³⁷Cs content exceed the hygienic standard for fuel and fuel wood firewood according to GNPAR-2005 (60 Bq / kg and 600 Bq / kg respectively).

Contamination of the ⁹⁰Sr forest area with a density of 4 kBq / m² is possible to obtain wood with a content of ⁹⁰Sr above the hygienic standard for fuel and fuel beams (GNPAR-2005), which requires restrictions on the use of fuel and small wood from these territories.

The specific radioactivity of the ash obtained by burning such wood exceeds 10 kBq / kg. The use of wood for radionuclide contaminated forests requires careful radiological monitoring and the introduction of appropriate permissible levels for fuel power plants.