Vitrification of radioactive waste

In the late 1950s, the development of nuclear fuel processing programs for electricity-generating reactors led to the need to find a scientific and industrial solution for the containment of high-level, long-lived liquid final waste (nitric solutions of PF fission products). Research programs launched at the time, initially in Canada and France, led to the following results, after an examination of various crystalline solid compounds such as phlogopite micas K(Si ₃ Al)(Mg ₃ )O ₁₀ (OH) ₂ and feldspars (Na,K)AlSi ₃ O ₈ , on the choice of borosilicate glass to solidify the liquid waste. Glass is capable of confining the thirty or so radionuclides present in PF solutions, not by coating, but thanks to the existence of chemical bonds with glassy network-forming oxide constituents. Compared with crystalline structures, which are much more specific to the insertion of certain chemical elements by substitution of atoms in their structure, glass material offers great chemical flexibility.

In parallel with studies into the formulation of nuclear glass, a technological process for the continuous vitrification of PF solutions, capable of manufacturing glass in a highly radioactive environment, was developed. The first active demonstration of the feasibility of the glass and its process was carried out on the PIVER vitrification pilot plant at Marcoule in 1969. In 1978, the cylindrical metal pot vitrification line at the Atelier Industriel de Vitrification de Marcoule (AVM) was commissioned to contain waste from the processing of spent fuel from natural uranium reactors. Following further development, this process was implemented in six vitrification lines in the R7 and T7 workshops at the La Hague site (AVH) in 1989 and 1992 respectively, where enriched uranium oxide spent fuel is processed. The current process at La Hague is characterized by two stages:

• a calcination step for liquid PF solutions, at around 400°C;

• then a melting stage at 1,100°C in an ovoid metal pot heated by electromagnetic induction. It is in this melting pot that the mixture of vitrification additives and PF cullet is melted. After refining, the glassmaking cast iron is cast and solidified in 400 kg metal containers. At the time of their start-up, the AVM, R7 and T7 (or AVH) plants had to process the accumulated stocks of fission product solutions from earlier reprocessing operations. Since then, high-level fission product solutions from the reprocessing of LEU fuel have been vitrified...