Olga K. Karlina, Arkady G. Petrov, Viktor M. Tivansky, Michael I. Ojovan, Igor A. Sobolev
Scientific and Industrial Association "Radon", The 7-th Rostovsky Lane, 2/14,
Moscow, 119121, Russia,
Tel. (095)248 1680, Fax ( 095 )248 1941, E-mail: Oj@nporadon.msk.ru

Exothermic powder-like metallic mixtures as applied to decontamination of concrete through thermal destruction and removal of its upper contaminated layer are optimized. Experiments on simulated and actually contaminated concrete objects are carried out. These compositions demonstrated are highly efficient for decontamination of concrete in the laboratory experiments. Upon completion of decontamination of actually contaminated plate the level of residual contamination did not exceed the background one.

Development of new technique for asphalt and metal surfaces decontamination through the use of exothermic powder - like metallic mixtures was previously reported [1 - 3]. Development and optimization of new mixture compositions made possible their expanding use for decontamination of other non - combustible surfaces, for example, concrete ones.

Method for concrete decontamination by the thermal destruction and removing its upper contaminated layer was studied. This method uses exothermic metallic mixtures which can release heat in the amount of up to 15 - 20 MJ/kg during oxidation. The oxidation method classifies the mixtures into ones with diffusion oxidation by the atmospheric oxygen and mixtures containing their own oxidizer ( reduction - oxidation mixtures ).

Various compositions of the exothermic mixtures consist of three basic components: highly dispersed metal fuel powders, combustion activators ( atmospheric oxygen or oxygen - containing oxidizers ) and technological additives ( blowing agents, combustion stabilizers, slag - forming additives etc.).

The decontamination process is realized in a following way: powder - like metallic mixture with a thickness of ~ 1 cm is applied on the surface to be decontaminated. Then the composition is initiated by local ignition with the use of electric igniter or fuse.

During mixture flameless combustion due to the heat released from redox reaction the temperature on the concrete surface increases from 600 to 1200 and higher ( depending on mixture composition ) within some minutes. Reacted mixture produces liquid glass - like slag melt. This melt agglomerates with upper concrete layer and on cooling scales off with the layer as glass - like monolith. The linear rate of mixture combustion wave on surface is 1,0 cm/c, slag cooling time is 5 - 10 minutes.

Experiments on concrete objects with simulated and actual contamination were carried out. Simulated samples were prepared by covering the aqueous solutions of Cs - 137 and Sr - 90 nitrates on concrete surface. Then the samples were cured to complete soaking of the solutions and dried. Contamination depth for simulated samples was not over 0,5 cm.

Actually contaminated concrete plate ( overall dimensions 1,5*2,0*0,5 m ) was contaminated by radionuclides Cs - 137 and Sr - 90 at a depth up to 1 cm. Activity was of the order of 2 - 4*10⁶ Bq/m².

The temperature on the surface to be decontaminated was registered by platinum-rhodium thermocouple.

Radionuclide and aerosol volatilization was studied using special exhaust plant for capture of the overall gas-aerosol phase generated from the decontamination process.

More detailed description of the method of measurements and samples preparation is given in the report[4 ].

In an effort to implement decontamination process on actual objects prototype of transportable decontamination facility including unit for collection and purification of gas - aerosol phase ( ventilator, hood - gas collector, coarse cyclone and fine filter ), attachment and container for slag collection was developed.

Efficiency of decontamination of concrete surface (without destruction and removing of its upper layer) only due to volatilization of radionuclides was 30 % at the surface temperature 800 ⁰C, 40 - 45 % at the surface temperature 1000 ⁰C, 60 % at 1200 ⁰C. Maximum decontamination factor 60 % for concrete ( part of activity removed) is achieved by heating of concrete surface to 1100 ^oC for Sr - 90 and to 1250 ^oC for Cs - 137 ( within 3 - 5 minutes ) ( see Fig. 2 ). The effectiveness of decontamination K (%) is determined by the formula:

where Ao, Ak - the activities of the concrete surface in Bq before and after the decontamination, respectively.

Increase of decontamination efficiency was achieved by development of mixture compositions which could provide formation of liquid glass - like slag agglomerating with upper concrete layer. This slag scales this layer off during cooling due to thermal stresses arising in it. When we use these compositions the decontamination efficiency depends on contamination depth. Complete decontamination is achieved in one decontamination act at contamination depth to 4 - 6 mm. When contamination is deeper, another act of decontamination of surface is required for it complete decontamination.

Concentration and particle sizes of resulting aerosols as well as radionuclide releases define ecological purity of decontamination process to a large extent. Study of gas - aerosol releases has shown that the process is safe from ecological point of view. Amount of aerosols released is dictated essentially by the amount and sort of technological additives in the mixtures and ranges within 0,3 to 0,5% of the initial mass of mixture.

Volatilization of radionuclides with aerosol phase is insignificant and amounts the order of 0,1 - 0,2% of initial activity for Cs - 137 evidently due to short time of surface heating as well as to properties of slag which serves as protective screen. Volatilization of radionuclides with gas phase is not found.

The above-mentioned compositions are used for decontamination of actually contaminated concrete plate. On completion of the decontamination process the activity level did not exceed the background one.

1. Compositions of exothermic metallic mixtures of diffusion combustion and glass-forming compositions for decontamination of concrete are developed.
2. High efficiency of the mentioned compositions for decontamination of concrete in the laboratory experiments as well as for decontamination of actually contaminated concrete plate is demonstrated.

1. O.K. KARLINA et al., "Investigations of the Possibility of Decontamination of Asphalt Concrete Surfaces Contaminated by Radionuclides," Atomic Energy (in Russian), 1995, v. 78, No. 4, p. 270 - 274.
2. A.S. BARINOV et al., "Removal of Contaminated Asphalt Layers by Using Heat Generating Powder Metallic Systems," Proc. Int. Conf. Nuclear and Hazardous Waste Management Spectrum’96, Seattle, Washington, August 18 - 23, 1996, v. 1, p. 104 - 106.
3. O.K. KARLINA et al., "Decontamination of Different Materials by Using Exothermic Metallic Compositions," Proc. 3rd Symp. Conditioning of Radioactive Operational & Decommissioning Wastes, Hamburg, Germany, March 19-21, 1997, p. 522-526.
4. I.A. SOBOLEV et al., "Thermochemical Decontamination of Metallic Surfaces," Proc. Symp. Waste Management’97 (to be published).

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