Architecture and management of a geological repository - Andra

4 – Waste disposal PackagesHowever, during the first few hundred years, the decay of fission products and actinids having averagehalf-lives engenders significant local heating. The mastery of phenomena during this so-called"thermal phase" begs the question of primary waste package disposal in their existing condition or ofthe advantage of placing them in an over-pack prior to disposal. Based on the current state ofknowledge, Andra has chosen the second option.This chapter describes the scientific considerations which led to this choice, the different technicalresponses envisaged and the option preferred at this stage of study.With the aim of evaluating feasibility, it also includes the dimensions of the over-pack andmanufacturing techniques based on tried and tested industrial practices.4.2.1 Description of the main questions4.2.1.1 Need for an over-packTo understand the justification of studying an over-pack, we should first remember that the primarypackage consists of a glass matrix inside a stainless steel envelope. The need for an over-pack resultsfrom phenomenological aspects of the so-called "thermal" phase during which the temperature of thepackage falls gradually after rising to a maximum or "thermal peak" 32 . The phenomena characterisingthis phase increase the alterability of the glass matrix, influence the behaviour of the radionuclides itcontains and weaken the stainless steel envelope of the primary packages with respect to corrosion[37].Concerning the alterability of the matrix, glass dissolution models take into account (i) a control of thealteration process kinetics by silica dissolution, main constituent of glass and (ii) radionuclide releasegoverned by dissolution congruent 33 with that of the silica. For C1 to C4 type packages, the modelpreferred for its robustness given strength at the current state of knowledge, consists of two phases.During the first phase, the initial dissolution speed [V 0 ] results from geochemical imbalance betweenthe glass and its environment. During the second phase, silica saturation of the waste environment isreflected in a slowing of dissolution towards a residual speed [V R ]. For C0 type packages, thedissolution regime according to the initial speed is assumed to continue for a long time. These twospeeds, V 0 et V R , depend on temperature: between 50°C and 90°C, the initial speed of dissolutionincreases by a factor of 30, the residual speed by a factor of about 15 34 [40].Concerning the behaviour of radionuclides likely to have been released, the question of mastering thethermodynamic phenomena governing chemical equilibria must be considered. Indeed, thephysicochemical phenomena involved are temperature-sensitive. Element solubility may be increasedor decreased when the temperature increases. The physical data which determine the speciation 35 of anelement in solution are known in a temperature range which is currently theoretically limited to 50-60°C. In practice, it is considered that uncertainty in extrapolating speciation evaluated on the basis ofequilibrium constants acquired at 25°C, is understood up to 80°C. The temperature also influencessorption mechanisms at solid phase interfaces, as well as the transport of chemical species in water bydiffusion (in particular via the "Soret" effect); the speed of diffusion increases with temperature.32333435For information, this peak is 100°C at most on the surface of the package.Congruence means that the same dissolution speed is used for all the species in the glass.Note also that the description of the passage from V0 to VR may show increasing uncertainty with temperature, because of theextrapolation of thermodynamic data available governing the behaviour of chemical species in solution and at the interface between theliquid and solid phases (sorption mechanisms).Radionuclide speciation in solution is evaluated on the basis of mass action, enthalpic and heat capacity constants. The last twoparameters are particularly difficult to measure directly in temperature; this leads to considering for the speciation models a reactionalenthalpy that is independent of temperature and a heat capacity of zero; as a corollary to this, this approximation limits the range ofvalidity of these models..DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM129/495