Storage stability of bituminized reactor wastes

The aim of the research was to study the protection given by bitumen for nuclear waste in conditions of ground water storage during a time equivalent to the halflife of low and medium level radionuclides. The ageing of bitumen in road conditions and different kinds of water contacts was looked upon in the light of information obtained from background literature. The specimens of standard shape were kept in laboratory for six months in ground water conditions simulating the storage of bituminized resin wastes in steel drums deep in bedrock. The quality changes of bitumen and resin and bitumen mixtures were analysed by means of standard tests both before and after storage. In part dealing with background literature, it was estimated that the waste container would, in theory, remain undamaged for 200 years at most under the influence of internal pressure, swelling and corrosion. After this, gases will gain access through the first corroded spot to the ground water, and water-soluble salts will find their way through the embrittled bitumen film. The results obtained from lime water storage of bitumen (in literature) indicate that bitumen films and the surface layer of the structure cannot withstand unembrittled. The proportional distribution of bitumen components underwent a change already in a short-term storage: the number of asphaltenes rose and the resin and asphaltene ratio decreased. This, the rise in stiffness and growing compressibility indicate a slight embrittlement of bitumen. The structure of the are resin and bitumen mixture did not prevent the absorption of ground water through hair cracks and flaws - on the contrary, the specimens swelled by approximately 50 vol.%. The mixtures which contained ARA or powdered resin and bitumen swelled noticeably less, 3.0 and 0.6 vol.%, respectively. Bitumen repels water but diffusion of water may take place through a damaged bitumen film. Water penetrating under the films on the resin surface weakens adhesion. The considerable swelling of resins and internal gas formation weaken adhesion and also promote leaching. These phenomena noticeably weaken the effect of the otherwise excellent waterproofing properties of bitumen. To eliminate the risk of leaching the research report recommends that the steel drums be made thicker and that the amount of bitumen be increased depending on the degree of fineness of the resin type in question. It means that the fine resin type needs more bitumen than the granular resin type. The resin waste should, moreover, be surrounded by a sufficiently thick waterproofing layer of pure bitumen. For a follow-up research, it is suggested that a bituminization capsule which would contain bitumen and resin waste mixture and which would be immersed in bitumen be developed.