2. Glass

Currently glasses are the main waste forms for the immobilisation of non separated HLW. Borosilicate glasses which mainly consist of SiO₂, B₂O₃ and Al₂O₃ are preferably chosen because of the good chemical durability and the flexibility to incorporate a variety of chemical elements in HLW. The generation process of the glass waste form is simply melting the wastes with the glass frit in high temperature (1100°C) and cooling them in the canisters. Joule heated ceramic melter (JHCM) which is used in USA, Russia, China and Japan directly heats the melt by Joule effect with the submerged electrodes. The AVM process used in France and UK firstly evaporates and calcines the HLW solution in a rotating kiln (500°C) and then puts the calcined wastes with glass frit into an induction-heated metallic melter (1150°C). (Caurant, Daniel and Majérus, Odile [5])

In the room temperature glass is an amorphous solid which does not posess a long-range order of atomic organization. The structure of the pure silica glass is a continuous random network of SiO₄ tetrahedra. Each tetrahedron is linked to four tetrahedra and each oxygen atom is shared between two Si atoms which form strong directional bonds. The Addition of sodium oxide Na₂O to the silica glass network decreases the liquidus temperature and the viscosity. The reason is that it transforms two strong Si-O bonds into two weak Na-O ionic bonds. (Figure 4) The further addition of boron oxide B₂O₃ results in the reinforcement of the glass network. B₂O₃ participates in the network and generally reduces the weak ionic bonds in the network. Na₂O is called a network-modifying oxide and B₂O₃ is a network-forming oxide. To predict those two different behaviors, the field strength F of a cation M in an oxide is defined as F = Z/d² where Z is the cation charge of M and d is the mean distance between the cation M and oxygen (in Å (10^-10m) unit).

Dietzel Theorem: If F >= 1.5, it is the network-forming oxide and if F <= 0.4, it is the network-modifying oxide. (Caurant, Daniel et al. [4])

If the oxide has high field strength (F >= 1.5) and cannot participate in the network, it tends to create its own network (phase). In the following sentences the glasses mean the borosilicate glasses. 1. alkalis and alkaline-earths
The alkalis and alkalis-earth cations such as Cs⁺, Rb⁺, Ba²⁺, and Sr²⁺ have low field strength and play the role of network modifier or charge compensator. They are easily dissolved in the glass structure. However, they are also dissolved well in water. When there are poorly soluble anionic entities such as orthophosphate (PO₄^3-) and molybdate (MoO₄^2-), they form the separate phases and deteriorate the overall durability of the glass wasteforms. (Caurant, Daniel and Majérus, Odile [5])

2. Transition metals
2.1. Zirconium
Zirconium Zr (20% of which is ⁹³Zr with half-life of 1.5 10⁶ years) has the high field strength and can participate in the glass network as ZrO₆ octahedra. Zirconium can form separate phases such as ZrO₂ and ZrSiO₄ which are well known for their good chemical durability. (Caurant, Daniel and Majérus, Odile [5])

2.2. Molybdate
Molybdate Mo⁶⁺ is a stable fission product and has high field strength. Molybdate is poorly soluble in the glass structure and forms the separate network as MoO₄^2- tetrahedra, called “yellow phase”, which deteriorates the durability of the wasteform. (Caurant, Daniel and Majérus, Odile [5])

3. Platinoids
Platinoids are almost insoluble in the melt and exist as crystalline phase RuO₂ and polymetallic (Pd, Rh, Te) alloy. (Caurant, Daniel and Majérus, Odile [5]) They increase the viscosity of the melt and the electrical conductivity. Because of the high density (d_Pd = 12, d_Ru02 = 7 compared to d_glass = 2.5), they tend to segregate at the bottom of the furnaces. The increased viscosity may block the flow of the melt to the canisters. In JHCM (Joule heated ceramic melter), the increased local electrical conductivity deprives the other part of the Joule heat which is necessary for the vitrification process. It may also cause the risk of short circuit. (Caurant, Daniel et al. [4])

4. Rare earths and actinides
4.1. Lanthanides and actinoides (Am, Cm)
(La, Ce, Pr, Nd) represent more than 90 wt% of lanthanides. Lanthanides (La, Ce, Pr, Nd) and actinoides (Am, Cm) possess similar chemical properties and Lanthanides (La, Ce, Pr, Nd) are used as the MA surrogates in the experiments in the laboratory. Let’s use the notation Ln to represent those elements. Ln³⁺ has high field strength and is generally well dissolved in the glass network. They form strong Ln-O-Si bonds. (Caurant, Daniel and Majérus, Odile [5])

4.2. Actinoids (U, Pu, Np)
Lighter actinoids (U, Pu, Np) behave differently from the heavier actinoids (Am, Cm). Pu⁴⁺ exists in the normal and the oxidizing conditions. Pu³⁺ occurs only in the reducing conditions. Pu³⁺ is more soluble in the glass network than Pu⁴⁺.
A better method to immobilise plutonium is to use the ceramic wasteform, which will be explained in the ceramic section.