Isolation and Microscopic Characterization of Nuclear Fuel Particles from Two Contaminated Soils of Chernobyl
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STRACT Nuclear fuel particles were separated, from two different Chernobyl contaminated soils, by sedimentation in bromoform. Their physico-chemical characteristics were studied by scanning electron microscopy. The two soils show the same types of fuel particles: one part of these particles contains U and O and the other part contains U, Zr and O. The structure and the surface morphology of the studied particles are variable and are characteristic of the conditions of their formation. The source term is thus heterogeneous. This heterogeneity has direct consequences on the dissolution of the fuel particles and should be taken into account in mathematical models. INTRODUCTION As a result of the explosion at unit 4 of the Chernobyl nuclear power plant, about 3.5% of its fuel was released into the atmosphere [1] in the form of fuel particles and highly mobile volatile radionuclides (Xe, Kr, I, Te) and part of the Cs. Some of these volatile radionuclides were condensed on inert carriers, thus forming condensation particles [2,3,4]. Within the 30 km restriction zone around the reactor, fuel particles were estimated to account for more than 75% of the total radioactive contamination on the ground [5,6]. Konoplev [7] reported that within the 30 km zone, 80% of the 90Sr were associated with fuel particles compared with 30-40% of 137Cs. In order to quantify the source term which serves as input into dissolution and transport models for radionuclide migration, it is necessary to determine the amount of fuel particles occurring in contaminated soils and their dissolution rates which depend on the physico-chemical characteristics of the soils which have received these particles. To estimate the influence of the soil type on the release of radionuclides, 2 different soils (peaty and soddy-podzolic, closed to Chernobyl NPP) were collected in 1996. These soils are supposed to have received the same kind of particles because of their close proximity. Therefore the study of the particles 10 years after their deposition can bring some information on the influence of the soils composition on the degradation of the particles. A separation procedure has been developed and applied to these soils and the particles were then examined by scanning electronic microscopy.
SAMPLE AND METHODS Two soil samples were collected from Chistogalovka site (Table I) to a depth of 5 cm where fuel particles (mainly as non oxidized fuel particles) are concentrated [8,9,10]. These soils have different chemical composition. Chistogalovka peaty (CHT “P”) soil is saturated and shows a high organic matter content, while Chistogalovka soddy-podzolic (CHT “S”) soil is unsaturated and shows a high silica content (Table I). Table I. Characteristics of the 2 soils sampled in Chistogalovka locate at 4.4 km WSW from the Chernobyl Nuclear Power plant Site
Surface activity [1] 127
Cs MBq/m²
CHT “P”
28
CHT “S”
20
Soil type
Chemical composition (%)
pH H2O
SiO2
Al2O3
FeT
OM.*
LOI.**
14
1.1
1.6
60
80
5
96
1
0.16
0.84
1.9
4.5
Peaty Soddypodzolic
*
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