Perfecting the technology for radioactive waste cementing

Microbiological protection of cement compounds is very important. This is and urgent issue not only for the cemented radioactive waste, but also for the containers and storage facilities made of concrete.

Polyfunctional additive contains components that increase strength, frost resistance and waterproof properties of cast stone, its stability against cracking and biostability. The additive also increases penetrability and stability of cement mortars, modifies cement slurry thickening time and stabilises its consistency, essentially decreases the rate of radionuclide leaching, and prevents foaming during preparation of the cement mortar.

Total amount of the radioactive waste (RAW) and spent nuclear fuel has reached a considerable quantity and continues to grow. According to the IAEA estimates, the amount of spent nuclear fuel worldwide will reach 200 000 tons by year 2010.

The total activity of radioactive waste and spent nuclear fuel in Russia in 2001 was more than 6 000 million Curie (which is almost 120 times more than the total activity of the radionuclides released as a result of Chernobyl accident!!!). The “military” RAW and the waste of nuclear power production facilities contribute most to the activity this waste.

However, low- and medium activity RAW produced due to the application of radioactive isotopes and sources of ionising radiation in industry constitute a greater hazard for general public. The reason is that such waste can be often met near dense-populated cities or even within city limits and thus might get into the hands of people who do not realise the hazard of handling radioactive objects.

The problem of RAW produced outside the nuclear fuel cycle facilities is very urgent. Thus, the number of detected locations contaminated with radioactive isotopes and the growing quantity of radioactive waste requiring immediate treatment in the Central region of Russia increased by an order of magnitude over the last decade. In the city limits of Moscow alone 792 cubic meters of radioactive waste were removed during past three years.

Radioactive waste originating from outside the nuclear fuel cycle facilities should be localised in order to ensure the radiation safety of people. The reliability of localisation depends mostly on the safety of technology applied for treatment of radioactive waste.

The process of radioactive waste localisation is aimed at converting the waste into stable forms, which decreases its potential danger during temporal storage, transport, and final disposal.

Matrices made of various materials (cement, bitumen, organic polymers, etc.) are used for localisation of radioactive waste. The choice of materials depends on the type of radioactive components of the waste (radionuclide type, specific radioactivity, and total activity level), as well as on its chemical and physical properties.

Besides, the processing technology for matrix material should be relatively simple, and using matrices should not lead to an essential increase of the volume of the final product compared to the initial volume of the waste. Such issues as the availability of the material in industrial production and connected economical issues are also taken into account in choosing the materials for the matrix.

Materials used in matrices should ensure the homogeneity of the localised forms of the waste, stability against the leaching action of water, and impermeability to water. They should possess mechanical strength, stability against the external factors (chemical, biological, etc.), and also thermal, radiation, and long-term storage stability.

Cementing is the first and most widespread way for conditioning radioactive waste with low and medium activity. The reasons for the wide use of cementing are the incombustibility, absence of plasticity of the conditioned product, relative simplicity of the technological process. The ability of cement to bind water is an important feature for conditioning of liquid radioactive waste.

Development of technology, accumulation of new types of radioactive waste, increasing volume of the waste on the one hand, and increasingly strict regulations on safety of handling and storing radioactive materials, on the other, require further development of the radioactive waste cementation process.

Further development of radioactive waste cementing technology in State-owned Research and Production Company “Radon” in co-operation with the Institute of Ecotechnologies is conducted in several directions.

New equipment for waste cementing is developed, new technological processes are introduced, and the quality of cement matrices and containers for radioactive waste localisation is perfected.

Prevention of biocorrosion of cement-based construction materials is a very urgent problem. Microbiological factor of corrosion is due to the settling and development of bacteria and other microorganisms on the surfaces of concrete-made structures. The microorganism metabolism leads to excretion of methane, carbon dioxide, sulphuric anhydride, hydrogen, volatile chlorine-containing components, sulphuric acids, and other chemical substances.

These substances attack the construction mortars, concrete, and reinforcement bars. This, in turn, leads to an essential degradation of the technical characteristics of the cast stone.

Formation of various biocenoses using organic substances as nutrient medium occurs when concrete surfaces come in contact with air, water, and soil. This is usually accompanied by decomposition of protein and cellulose-containing substances accidentally admixed to the cement compound.

Decay of concrete constructions is known to be due mostly to the dissolving of hydrosilicates when acids come in contact with the binding material. Though the alkaline hydrates neutralise the initial quantity of acid, fermentation and metabolic activity continues, because such factors as nutrient medium, bacteria, and humidity remain. This leads to the loss of strength of the affected concrete.

The situation is still aggravated by the season temperature variations, which promote the increase of the cement compound porosity, formation of cavities inside the cement monolith and thus stimulate the permeation of the compound with surface and subterranean waters.

After a comprehensive study of a wide range of Russian- and foreign-made chemicals, biocidal polymers of the polyalkylene guanidine (PAG) class have been chosen as the most perspective for protecting cement matrices in the technology of radioactive waste cementing.

The main advantages of these chemicals are high biocidal activity, good solubility in water, very low toxicity, and long shelf life without loss of functional properties. Last, but not least of the advantages is that these biocidal chemicals are developed and produced in Russia by the Institute of Ecotechnologies.

Further tests have shown that PAG chemicals do not degrade the properties of cement compositions as most biocides do. On the opposite, PAGs can essentially enhance the following characteristics:

* mechanical strength of cement compounds and their stability in aggressive media;

* rheological properties and penetrability of the cement slurries for permeation of radioactive waste;

* technological and construction properties of cement-sand mixes for production of concrete containers for long-term storage of conditioned radioactive waste.

The results of these tests allow classifying PAGs as polyfunctional additive that has biocide, plastifying, stabilising, and strengthening action. It is recommended for application in various technologies for radioactive waste cementing.

Further work in this direction resulted in development of PAG-containing polyfunctional additives (PFAs) for modification of cement compound properties. The PFAs developed by the Institute of Ecotechnologies and State-owned Research and Production Company “Radon” should be added to the normal cement in ratio of 5-20 %.

These PFAs contain 3-5 macro- and microcomponents that do not react with each other and do not change their properties in a multicomponent mix. The PFAs enhance a number of properties of the cement compound.

Variation of the PFA ratio in the mix with traditional Portland cement allows gradual changing of various properties of the cement compound. This is necessary to adapt it to different technologies for cementing liquid and solid radioactive waste. The PFAs also help enhancing the quality of the cement compound and increasing the ratio of radioactive waste in the final product thus decreasing its quantity.

The additive is used in cementing in a ready dry form, which is important. It can be added directly into the concrete mixer with the cement. The ready multicomponent additive does not require any complication of the process of radioactive waste cementing. No expensive and fragile dosing equipment is necessary.

Moreover, homogeneous distribution of macro- and microcomponents in the additive make the cementing process even more simple. The productivity increase is reached through the decrease of time necessary for thorough mixing of the cement compound.

The “Radon” Company in co-operation with the Institute of Ecotechnologies actively pursue further development of technological processes, equipment, and cement compositions that can find application in cementing of radioactive waste of nuclear power plants and various radiochemical technologies.

Alexandr BARINOV

Head engineer of the

State-owned Research and

Production Company “Radon”

Andrei VARLAKOV

Machine shop manager of the

State-owned Research and

Production Company “Radon”

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