E. M. Pazukhin

Forest Fire as a Factor of Environmental Redistribution of Radionuclides Originating from Chernobyl Accident

Radionuclide composition in particles of the smoke plume from the forest fire in the 30-km zone around the Chernobyl NPP was studied. The activity ratios of the radioactive isotopes of cesium, cerium, and plutonium in the forest combustible materials and in the aerosols yielded by the fire showed that these aerosols are enriched in radioactive cesium occurring predominantly on submicron particles transported to long distance. The fire area gets depleted in cesium radioisotopes and, to a lesser extent, in other radioactive products generated by the Chernobyl accident.

Formation and spread of Chernobyl lavas

The results of implementation of the ISTC Project “Development of a Model of the Behavior of Nuclear Fuel in the Active Stage of the Chernobyl Accident” are summarized. The project work was jointly undertaken by the Russian Research Centre Kurchatov Institute (RNTs KI) and Nuclear Safety Institute, Russian Academy of Sciences (IBRAE RAN). The aim of the project was to systematize a huge amount of data on lava-like fuel-containing materials (LFCM), collected over the 20 years of research work at the Shelter, into database and to develop a model of LFCM formation and spread during early post-accident days. The model will be helpful in producing recommendations on accident prevention and promote development of the best technologies for lava removal, thereby reducing the financial expenses and dose burden. Also, with the model developed, the results of the virtually unique “experiment” with the nuclear fuel of the Unit 4 reactor, organized on a huge scale, can be used for finding solutions to the general nuclear safety problems.

Radionuclides in ground waters from observation holes in the Shelter local area

The volume activity of 3H, 90Sr, 137Cs, 234U, 235U, 238U, 238Pu, 239+240Pu, and 241Am in ground waters from observation holes 1-G-6-G in the north section of the Shelter local area of the Chernobyl Nuclear Power Plant (CNPP) was measured. The distribution of radionuclides in the suspension fractions of the ground waters was evaluated. The main contribution to the pollution of ground waters with uranium is due to natural uranium isotopes: 234,235,238U. The activity ratios of 238Pu, 239+240Pu, and 241Am in ground waters are similar to those in the spent fuel of 4th CNPP block.

Nuclear-hazardous accumulations of fuel-containing materials in the destroyed fourth Unit of the Chernobyl NPP

The neutron incident that happened in June 1990 in the 4th Unit of the Chernobyl NPP, destroyed in the accident, at the boundary of rooms 304/3 and 305/2 is analyzed. One of possible scenarios of the incident, consistently accounting for the dynamics and self-extinguishment of the detected neutron anomaly, is suggested.

Fuel at upper levels of the destroyed fourth block of Chernobyl NPP. Refining the formation scenario of the polychromatic ceramics

Polychromatic ceramics samples taken from the lava formation site in subreactor room no. 305/2 were examined. Polychromatic ceramics is nonuniform; its chemical and radiochemical compositions vary with the color of the inclusions; it was formed at upper levels of the destroyed Fourth Block of the Chernobyl NPP by a peculiar mechanism. The U content in the samples varies from 5 to 8%. Possible sites of cooking of the lavas which are the sources of formation of the polychromatic ceramics were identified at upper levels of the destroyed Fourth Block. A model of formation of the lavas at upper levels of the Fourth Block was suggested. Based on the elucidated chemical composition of the polychromatic lava-like fuel-containing materials, the lava formation process was simulated. The calculations show that the lava formation during the active stage of the Chernobyl accident required ca. 1 t of the fuel.

Separation of Am(III) and Pu(IV) at Their Joint Oxalate Precipitation

A procedure was developed for purification of plutonium to remove americium by oxalate precipitation using diethyl oxalate as a precipitant. The developed procedure was tested under laboratory conditions and on an enlarged installation. It was shown that the decontamination factor of plutonium from americium can exceed 2 ×103. The plutonium oxalate precipitate can be dissolved by heating with HNO3 in the presence of V(V) catalyst and also by electrochemical dissolution at alternating current.

Transuranium elements in liquid radioactive wastes from the Shelter

The volume activity of 90Sr, 137Cs, 238Pu, 239+240Pu, 241Am, and 244Cm in water taken from unorganized water accumulations on lower marks of the Shelter was determined. Separate water accumulations are characterized by their specific radionuclide activity ratios 238Pu/239+240Pu, 241Am/239+240Pu, and 244Cm/241Am. The activity ratios 241Am/239+240Pu and 244Cm/239+240Pu in the water accumulations are 5–10 times higher than in the irradiated fuel of the 4th Unit of the Chernobyl NPP and in lava-like fuel-containing materials.

220Rn as an additional factor of radiation hazard in the Shelter

In 2003–2007, the content of 212Pb (220Rn daughter product) inside the Shelter was, as a rule, in the range 0.5–5 Bq m−3. No seasonal and time dynamics of the 212Pb concentration and no relationship with the technical activity were revealed. At simultaneous sampling of aerosols, the volume activities of 212Pb in the bypass were higher by 1–2 orders of magnitude than in the surrounding medium (local zone). Hence, the sources of 220Rn emanation, in particular, 232U are inside the Shelter. Among 220 samples taken in the bypass, 94% had the activity median aerodynamic diameter of aerosol particles bearing 222Rn and 220Rn daughter products in the range from 0.05 to 0.4 μm. For the Shelter staff members, 222Rn and 220Rn daughter products, and also submicron size of their carriers are negative factors which were not taken into account previously when determining ionizing irradiation doses. The additional inhalation doses can reach tens of percents of the permissible annual dose.

Interaction of lava-like fuel-containing masses with structural concrete in the active phase of the Chernobyl accident

Interaction of lava-like fuel-containing masses of Unit 4 at the Chernobyl NPP with structural concrete in the active phase of the accident of April 26, 1986, was studied. It was found that the fission fragments and the accumulated products contained in spent nuclear fuel penetrated the concrete from lava with different fractionation coefficients Kfr. The absolute values of Kfr were calculated, and this phenomenon was identified as a factor to be taken into account when estimating the total amount of radioactive waste for the Shelter.

Impact of open accumulations of lava-like fuel-containing materials in the CNPP shelter object on the personnel and environment

The impact of lava-like fuel-containing materials (LFCMs) of the Shelter object (SO) of the Chernobyl NPP on the environment components is evaluated. The evaluation is based on modern data on the surface area and dust-generating ability of SO LFCMs. It is shown that today SO LFCMs make a negligible contribution to the contamination of environment components in the CNPP exclusion zone by radioactive substances.