[en] In the nuclear field, bitumen is used as a conditioning material for radioactive wastes. Characteristics of bituminized packages may be modified under internal self-irradiation due to beta, gamma, and alpha embedded emitters. Interactions between radiations and materials induce structural modifications which must be well-known within the frame of safety studies related to final storage. In the case of real drums, these changes may occur during a period as long as 300 years. It is therefore necessary to simulate this aging by laboratory trials on a shorter period (600 times faster), using scaled down samples subjected to an external gamma-radiation flux (under no activity restraint). An operating process has been developed. Results show: a radiolytic gas production may be accompanied by swelling when accumulating in the form of bubbles; a surface oxidation which can be followed-up by carbon dioxide evolving and oxygen consumption; changes in physical properties: hardening that results in viscosity and softening point increase and in needle-penetrability decrease; no significant changes in bitumen chemical composition. From these observations, a mathematical model is being elaborated, enable to predict the behavior of full scale drums. The validation of this model is in progress on inactive samples of various sizes (80, 200, 600mm in diameter) and on real full size drums doped with radioelements

[en] Inactive lab samples of pure bitumen, cement binders, or thermo-setting resins and embedded waste products are submitted to an external γ -dose-rate varying from 10 to 2000 Gy. h^-1 in the IRCA irradiator at Cadarache Depending on the number and the size of the samples, dose-rate may be either measured with a ionization chamber or calculated with a computer program. Samples are conditioned into gas-tight vessels. This allows simultaneous analysis of radiolysis gas and possible swelling measure. Gas (mainly hydrogen) production rate is obtained by adding released and trapped gas. Gas transfer mode my be diffusion or permeation. It is determined as well as sorption capacity from experiments on a described diffusion cell. Results are the input data of a theoretical model used to forecast the behaviour of industrial drums

[en] The gamma dose-rate is calculated in large samples of bitumen and concrete using the code Mercure IV. The influence of the diameter, of the composition and of the number of sources are investigated. Experimental dose-rate measured in air can be directly applied to bitumen and bituminisates but not to denser materials like binder and hematite-concrete. The dose-rate homogeneity is correct for diameters below 15 cm for bitumen, 14 cm for bituminisates, 12 cm for binder and 9 cm for hematite-concrete. (Authors). 5 refs., 2 figs., 1 tab

[en] Studies are carried out by the CEA in order to predict the behaviour of bituminized radioactive wastes under self irradiation. Bitumen radiolysis produces gas (mainly H₂) which diffuses in the organic matrix. If the hydrogen yield is higher than the diffusion flux through the free surface, hydrogen concentration increases and exceeds its solubility in bitumen. Beyond saturation, bubbles are formed and gas is also evacuated by bubbles drift. The aim of these studies is to evaluate the evacuation capacity of radiolytic gas produced in function of initial bituminized wasteform characteristics. A model was developed to achieve this purpose, by calculating the evolution of bubbles population considering all elementary mechanisms of gas evacuation. (authors)

[en] The bituminization process has been used for conditioning low and medium level (LML) radioactive waste, particularly to immobilize coprecipitation slurries and evaporation concentrates generated by effluent treatment. The process consists in mixing bitumen matrix with inactive soluble and slightly soluble salts added to insolubilize the radionuclides or resulting from the neutralization of acid effluents. This operation is performed at a sufficient temperature - depending on waste composition and bitumen grade to ensure the flow of the resulting mixture into metal containers. Exothermicity due to salts/salts or salts/bitumen reactions depending on the type of waste can be induced during or after the mixing step. This could produce an additional heat emission that the drum must be able to release to avoid a potentially incidental pattern with ignition risk, explaining why the CEA has been involved in evaluating the thermal reactivity of bituminized waste and its repercussions on the bituminization process. Given the difficulty of discriminating each exothermal reaction, the characterization of a global reactivity appears as a further precautionary measure, in addition to the definition of a working safety margin. The CEA has accordingly developed studies on this aspect. The article discusses the experimental methodology developed for the determination of the global reactivity. (authors)

[en] Although very few incidents have to be deplored all over the world in radioactive waste bituminization process, the bitumen inflammation risk has to be brought under control. In order to prevent such a risk, a zero thermal reactivity has been searched up to now to authorize an operationally waste embedding. Elsewhere a model has been developed to predict the thermal behaviour of a drum during the cooling phase in order to precise how reasonable could be a non zero reactivity. One of the necessary input data is the evolution of the thermal power versus temperature. This document describes the experimental method proposed by the CEA to the plant operators to measure the potential reactivity of a waste destined to be conditioned in bitumen. Micro-calorimetry has proved to be the most efficient technique. The application of this procedure is in progress at Marcoule Cogema plant and Saclay CEA center before all set up of bituminization operation. (authors)

[en] The effect of many parameters need to be studied to characterize the long term behavior of nuclear waste in a deep repository. These parameters concern the chemical effects, radiolytic effects, mechanical properties, water composition, and microbiological activity. To evaluate microbial activity in such an environment, work was focused on an inventory of key nutrients (C, H, 0, N, P, S) and energy sources required for bacterial growth. The production of hydrogen in the nuclear waste environment leads to the growth of hydrogen oxidizing bacteria, which modify the gas production balance. A deep repository containing bituminized waste drums implies several sources of hydrogen: - water radiolysis; -corrosion of metal containers; - radiolysis of the embedding matrix (bitumen). Two deep geological disposal conditions leading to H₂ production in a bituminized nuclear waste environment were simulated in the present study: - H₂ production by iron corrosion under anaerobic conditions was simulated by adding 10% of H₂ in the atmosphere; - H₂ production by radiolysis of bitumen matrix was approached by subjecting this material to external gamma irradiation with a dose rate near real conditions (6 Gy/h). The presence of dissolved H₂ in water allows the growth of hydrogen oxidizing bacteria leading to: - CO₂ and N₂ production; - H₂ consumption; - lower NO₃^- concentration caused by reduction to nitrogen. In the first case, hydrogen consumption is limited by the NO₃^- release rate from the bitumen matrix. In the second case, however, under gamma radiation at a low dose rate, hydrogen production is weak, and the hydrogen is completely consumed by microorganisms. Knowledge about these hydrogen oxidizing bacteria is just beginning to emerge. Heterotrophic denitrifying bacteria adapt well to hydrogen metabolism (autotrophic metabolism) by oxidizing H₂ instead of hydrocarbons. (authors)

[en] During interim storage, the main evolution factor of the bituminized wastes is radiolysis under auto-irradiation, while in deep disposal conditions, after radioactive decrease, site resaturation with water, and container corrosion, the main factor of evolution is leaching by water. This paper presents two models, designed to describe respectively radiolytic gas evacuation, and bituminized waste alteration under leaching. The first model is based on the integration of simple descriptions of gas production by radiolysis, gas diffusive transport, bubble nucleation and ripening, and bubble transport under buoyancy and viscosity. The second model is based on simple descriptions of water adsorption and solubilization at the surface, water diffusion into the matrix, water uptake by the most soluble salts, salt grain swelling and alteration of matrix waterproofing, diffusive transport of salts out of the waste. Copyright (2001) Material Research Society

[en] The behaviour of bitumen matrices for the storage of nuclear wastes may be studied through the comparison with natural analogues. In several sedimentary uranium deposits, high grade uranium mineralizations are closely related to migrated bitumens that display a large range of chemical composition. Different analytical and micro-spectroscopic techniques were used in order to discriminate the different types of bitumens and to show evidence for the effects of various alteration processes including radiolytic degradation. The methodology including the chemical and structural parameters as well as the data derived from the analysis of natural bitumens can be used for the evaluation of the bitumen long term reactivity in a nuclear waste storage matrix

[en] In France, the reference scenario for long term management of intermediate activity bituminized wastes takes into account the presence of water in contact with waste forms. In bituminized waste, each salt grain may be considered as surrounded by a thin bitumen film which constitutes a water proof matrix. Radionuclides release is able to begin only after an alteration of this confinement linked both to diffusion of water and presence of soluble salts. The aim of our studies is to identify and evaluate risks linked to radioactivity dispersion with time and its evolution under various environment factors. Risks of inactive salts and potentially complexing organic matter release are studied too. In order to predict the behaviour of all the bituminized drums, the selected approach lies on modelling of mechanisms. Parametric studies will then allow to closely connect behaviour and initial characteristics. Our communication presents the state of our work concerning the phenomenology of solubilized species release in presence of water and associated modelling. (authors)