Undissolved solids and technetium

The solid component of the irradiated fuel that does not dissolve in hot nitric acid is rich in Pd, Ru, Rh, Tc, Mo, Zr, and O. It also may contain sig­nificant concentrations of Te. The noble metals, Mo and Tc, are often in the form of the epsilon metal phase (or 5-metal phase). Molybdenum, Zr, Tc, and O are often found to vary significantly in the undissolved solids (UDS) depending on the dissolution process parameters if voloxidation is performed.

Roughly half of the technetium (depending on parameters) is dissolved. The dissolved fraction may follow uranium or the HLW raffinate depending on process parameters. The primary radionuclide to immobilize in the UDS

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Iodine immobilization

Chemical composition

Conceptual diagrams

Iodine release

techniques

of iodine confinement/

parameter

Raw material

Waste form

immobilization

 

Подпись: Published by Woodhead Publishing Limited, 2013

Crystalline. Rock matrix

 

Spent silver-sorbent (= silica gel, Agl)

 

Si02(quartz), Agl (I content: 14wt%)

 

Diffusion coefficient: 1.0 x 10-20 (m2/s)

 

Fluor-apatite (Ca10(PO4)6F2): Zeolite (I sorption) = 85 : 15 (weight ratio)

 

Fluor-apatite (Ca10(PO4)6F2) : Zeolite (I sorption) (I content: 2wt%)

Cu powder: Spent silver-sorbent (I content: 0.7wt%)

 

Apatite

 

Cu powder: Spent silver — sorbent = 50 : 50 (vol%)

 

Leach rate: 90 x 10s (Bq/y)

 

Copper

matrix

 

Leach flux: 1.0 x

10s — 10-7 (g/ cm2/d)

 

Glass Agl glass

 

Agl : Ag4P207 = 3 : 1(mol ratio) 3Agl-2Ag20-P205 glass

(I content: 30wt%)

 

Continued

 

image56"image57"image58"

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Подпись: Published by Woodhead Publishing Limited, 2013

•Alumina cement/ CaS042H20 =100/15.5

• Nal03 cone. 0.4mol/dm3

• Nal03 sol./cement = 0.56

 

AFm: 10wt%

AFt: 46wt% AI(OH)3:44wt%

(I content: 1.85wt%)

 

Distribution

coefficient:

>100ml/g

 

Cement

 

Hardened

cememt

 

Synthetic NaAI02 : Si02 : Nal = 3:3:1

sodalite (mol ratio)

 

Na8(AISi04)6l2 (I content: 11 wt%) (measured value)

 

Synthetic

mineral

 

Synthetic PbO : V205 : Pbl2 = 9:3:1

lead (mol ratio)

apatite

 

Pb10(VO4)6l2 (I content pb

7.2wt%) (measured v

value) АЦр — t

 

Source: JAEA, 2007.

 

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is 99Tc and some transuranic (TRU) elements. With a half-life of 0.21 x 106 years, this waste must be immobilized for hundreds of thousands of years. In an oxidizing environment, Tc is most often found as pertechnetate (TcO4-), which interacts weakly with minerals and typically migrates freely in the subsurface. Two primary options are considered for these Tc-bearing wastes: (1) combine with the HLW raffinate for immobilization (e. g., in glass) or (2) form a separate waste form specifically aimed at immobilizing Tc. In the first option the noble metals (Pd, Ru, Rh) may significantly limit the loading of HLW in glass. However, addition to the HLW stream reduces the number of processes required to treat the waste and number of waste forms with very long-lived radionuclides requiring qualification.

Many waste forms have been developed for immobilization of these Tc — bearing materials:

1. An iron-based alloy that contains the UDS, reduced soluble fraction of the technetium, and potentially the soluble fraction of the noble metals and stainless steel fuel assembly hardware. This waste form strives to maintain Tc in the immobile metallic state (Tc0). Relatively low process­ing temperatures (<1600°C) are required to form this alloy (Ebert,

2005) .

2. The epsilon-metal phase, which makes up a large fraction of the undis­solved solids, would make an outstanding waste form. This waste form incorporates the UDS, the soluble fraction of Tc, and the soluble fractions of the noble metals (Strachan et al., 2010b). There are natural analogs that attest to the durability of this phase. Natural reactors operating in Gabon, Africa produced epsilon metals. The decayed Tc-99 was found to migrate less than one meter in roughly 2 billion years (Utsunomiya and Ewing 2006). That fact combined with the fact that they survive the boiling nitric acid solution despite the very high surface area attest to the high resistance to strongly oxidizing environments, despite the more extreme environment than any repository.

3. The third waste form developed for immobilization of UDS, soluble Tc, soluble noble metals, and potentially lanthanide fission products in oxide ceramics such as the mixed pyrochlore (Zr/Nb/Mo/Tc/Ru/Rh/ Pd)2Ln2O7 (Hartmann et al., 2011), magnesium titanate (den Exter et al.,

2006) , phosphates (Singh et al., 2006), iron oxides/oxyhydroxides (Um et al., 2011), etc. These waste forms immobilize Tc in the immobile Tc4+ state and display relatively high chemical durability that is independent of disposal environment.

4. Alkali-alumino silicates generated by a fluidized bed steam reforming process was studied extensively for the immobilization of Tc and I bearing wastes at Hanford (Jantzen 2008).

However, in current practice, the Tc is often vitrified with the HLW raffinate to form a borosilicate glass (e. g., at LaHague, Rokkasho, and currently planned for Hanford).

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