Editing Bitumen (section)

=== Radioactive waste encapsulation matrix ===
Bitumen was used starting in the 1960s as a [[hydrophobic]] matrix aiming to encapsulate [[radioactive waste]] such as medium-activity salts (mainly soluble [[sodium nitrate]] and [[sodium sulfate]]) produced by the reprocessing of [[spent nuclear fuel]]s or radioactive [[sludge]]s from sedimentation ponds.<ref>Rodier, J., Scheidhauer, J., & Malabre, M. (1961). The conditioning of radioactive waste by bitumen (No. CEA-R{{snd}}1992). CEA Marcoule.</ref><ref>Lefillatre, G., Rodier, J., Hullo, R., Cudel, Y., & Rodi, L. (1969). Use of a thin-film evaporator for bitumen coating of radioactive concentrates (No. CEA-R{{snd}}3742). CEA Marcoule.</ref> Bituminised radioactive waste containing highly [[radiotoxic]] [[ionizing radiation#Alpha particles|alpha-emitting]] [[Transuranium element|transuranic element]]s from nuclear reprocessing plants have been produced at industrial scale in France, Belgium and Japan, but this type of waste conditioning has been abandoned because operational safety issues (risks of fire, as occurred in a bituminisation plant at Tokai Works in Japan)<ref>Sato, Y., Miura, A., Kato, Y., Suzuki, H., Shigetome, Y., Koyama, T., ... & Yamanouchi, T. (2000). Study on the cause of the fire and explosion incident at Bituminization Demonstration Facility of PNC Tokai Works. In Nuclear waste: from research to industrial maturity. International conference (pp. 179–190).</ref><ref>Okada, K., Nur, R. M., & Fujii, Y. (1999). The formation of explosive compounds in bitumen/nitrate mixtures. Journal of hazardous materials, 69(3), 245–256.</ref> and long-term stability problems related to their [[Deep geological repository|geological disposal]] in deep rock formations. One of the main problems is the swelling of bitumen exposed to radiation and to water. Bitumen swelling is first induced by radiation because of the presence of [[hydrogen]] gas bubbles generated by alpha and gamma [[radiolysis]].<ref>Johnson, D.I., Hitchon, J.W., & Phillips, D.C. (1986). Further observations of the swelling of bitumens and simulated bitumen wasteforms during γ-irradiation (No. AERE-R{{snd}}12292). UKAEA Harwell Lab. Materials Development Division.</ref><ref>Phillips, D. C., Hitchon, J. W., Johnson, D. I., & Matthews, J. R. (1984). The radiation swelling of bitumens and bitumenised wastes. Journal of nuclear materials, 125(2), 202–218.</ref> A second mechanism is the matrix swelling when the encapsulated [[hygroscopic]] salts exposed to water or moisture start to rehydrate and to dissolve. The high concentration of salt in the pore solution inside the bituminised matrix is then responsible for [[osmosis|osmotic]] effects inside the bituminised matrix. The water moves in the direction of the concentrated salts, the bitumen acting as a [[Semipermeable membrane|semi-permeable membrane]]. This also causes the matrix to swell. The swelling pressure due to osmotic effect under constant volume can be as high as 200 bar. If not properly managed, this high pressure can cause fractures in the near field of a disposal gallery of bituminised medium-level waste. When the bituminised matrix has been altered by swelling, encapsulated radionuclides are easily leached by the contact of ground water and released in the geosphere. The high [[ionic strength]] of the concentrated saline solution also favours the migration of radionuclides in clay host rocks. The presence of chemically reactive nitrate can also affect the [[redox]] conditions prevailing in the host rock by establishing oxidizing conditions, preventing the reduction of redox-sensitive radionuclides. Under their higher valences, radionuclides of elements such as [[selenium]], [[technetium]], [[uranium]], [[neptunium]] and [[plutonium]] have a higher solubility and are also often present in water as non-retarded [[anion]]s. This makes the disposal of medium-level bituminised waste very challenging.

Different types of bitumen have been used: blown bitumen (partly oxidized with air oxygen at high temperature after distillation, and harder) and direct distillation bitumen (softer). Blown bitumens like Mexphalte, with a high content of saturated hydrocarbons, are more easily biodegraded by microorganisms than direct distillation bitumen, with a low content of saturated hydrocarbons and a high content of aromatic hydrocarbons.<ref>Ait-Langomazino, N., Sellier, R., Jouquet, G., & Trescinski, M. (1991). Microbial degradation of bitumen. Experientia, 47(6), 533–539.</ref>

Concrete encapsulation of radwaste is presently considered a safer alternative by the [[nuclear industry]] and the waste management organisations.