Recent developments of the radiography method for characterisation of Fuel particles in Surface water bodies of Chornobyl Exclusion zone

The Chornobyl accident in 1986 led to radioactive contamination of surface water bodies (SWB) in the Chornobyl Exclusion Zone (ChEZ), including lakes (Azbuchyn, Glyboke, Yanov crawl etc) and the Cooling Pond (CP). An abundance in fallout the dispersed fuel particles (FPs) was a specific feature of the accident, comprising the debris of irradiated nuclear fuel in different states of uranium oxidation mixed with construction materials.

Contamination of SWB by 90Sr and transuranic isotopes was mainly because of FPs. Experimental studies on the behaviour of FPs in soils and aquatic systems have shown that main factors controlling release of radionuclides outside FPs are: composition of the matrix, state of initial oxidation and oxidation properties of the environment.  

FPs behavior in SWB has not been sufficiently studied, though limited data suggest contrasting differences to terrestrial environments. Mainly is because of the different oxidation properties of soil and bottom sediment, creating better conservation conditions for FPs in sediment. In case when in SWB bed become dry and exposed, as is the case in the CP after decommissioning, an intensive processes of primary soil formation begin on exposed areas. This later forced dissolution of FPs, and hence radionuclides transition into more mobile forms followed by release to groundwater and surface runoff. 

We have developed convenient method for identifying Chornobyl FPs based on radiography, which comprises the exposure of X-ray film by spreading over a thin-layered dry solid sample. Processing X-ray films and image analysis makes it possible to estimate the size of FPs, as well as dispersion and distribution of radionuclide activity within the FPs of different size fractions. This also facilitates picking up single FPs to carry out extended individual analysis by EDXRF, SEMs etc. The radiography method was used to estimate the chemical resistance of FPs after sequential leaching to predict the behaviour of radionuclides in natural field conditions.

Overall, more than 120 samples were collected from bottom sediments cores taken from different SWB in ChEZ and analyzed by radiography. The results obtained show that from 70 to 90% of activity of radionuclides in sediments are associated with FPs. One gram of sediment contains several 10s to several 100s of individual FPs, while there is significant spatial heterogeneity of FPs density over the territory of ChEZ. The majority of FPs are less than 3 microns and their contribution to total activity was estimated as minor.

The main contribution to activity (>70%) comes from particles with a diameter of more than 10 microns and, accordingly, mobility of radionuclides will be determined by processes of destruction and leaching of radionuclides from particles of these size. Chemically resistant FPs are of 3-5 microns, and the highest concentration of fuel particles is typical for SWB located in close proximity to the ChNPP.

Up to 7% of the activity remains associated with FPs after being treated with strong 8M nitric acid indicating that 90Sr, transuranic and, partly, 137Cs are confined in chemically very stable particles and may not be mobilized under natural conditions for many decades.