Advanced Hydrometallurgical Separation of Actinides and Rare Metals

in Nuclear Fuel Cycle

Masaki OZAWA1*,2, Shinichi SUZUKI1 and Kenji TAKESHITA2 1Japan Atomic Energy Agency, 2-4 Shirane, Shirakata Tokai-mura 319-1195, Japan,

2Tokyo Institute of Technology, O-okayama, Meguro, Tokyo, 152-8550, Japan

(Received March 16, 2010; Accepted March 30, 2010)

A hydrometallurgical separation technologies by novel solvent extraction (SX), ion exchange

chromatography (IXC) and electrolytic extraction techniques are reviewed as separation tools

for light PGM (Ru, Rh, Pd), Tc and f-elements in high level liquid wastes of the nuclear fuel

cycle. The SX process using N,N-dialkylamide can isolate U(VI) from fission products

without Pu(IV) valence control, and extractants with soft-hard hybrid donors (PTA and PDA)

and those containing six soft donors (TPEN) show good separation of actinides (III) from

lanthanides (III). The IXC process utilizing a tertiary pyridine resin (TPR) provides a very

high degree of separation of the f-elements in spent nuclear fuel and the recovery of pure Am

and Cm products. The catalytic electrolytic extraction (CEE) process utilizing Pdadatom or

Rhadatom can effectively separate platinum group metals (PGM), Tc and Re by means of

controlled under potential deposition (UPD). Some of the basic work on the

hydrometallurgical separation of the elements of interest has been carried out through the

strategic Advanced (Adv.-) ORIENT Cycle research in Japan. The Adv.-ORIENT Cycle process

cannot only improve the radioactive waste problem, but can also provide useful rare metals to

leading industries as from this secondary resource.

1. Introduction

Resources of natural energy (oil, gas, 235U) and most of rare metals will run out within 200 years. In

particular at fiscal year 2004, the R (resource) / P (production) ratio (year) for oil was 41 years, 67 years for

natural gas, 192 years for coal for and 85 years for uranium. Despite the rather long R/P ratio of ca.150

years for the platinum group metals (PGM), the current price increases for Ru, Rh, and Pd in the market

have been significant, and it should be noted that the production of PGM is limited to mainly those two

countries namely South Africa (75 %) and Russia (17 %) in the year 2008[1]. On the other hand, the R/P

ratio for rare earths is not so limiting, but 93 % of rare earth production is monopolized by one country,

Solvent Extraction Research and Development, Japan, Vol. 17, 19 – 34 (2010) – Reviews –

- 19 -

China. In this context, nuclear fission is said to be able to counter such a natural energy crisis issue if 238U

(239Pu) can be utilized in fast breeder reactors (FBR) in future.

Fission reaction of 235U and 239Pu currently is creating more than 40 elements and 400 nuclides as

fission products (FP) in the spent fuel, while generating enormous amounts of energy, approximately two

million times greater than that from chemical reaction per gram of fuel. Among them, 31 elements are

categorized as rare metals, and particularly Zr, Mo, Ru, Pd, Cs, Ce, Nd are highly enriched in FBR spent

fuel. Because of their individual radiochemical properties, these should be recognized as not only the

radioactive wastes but a second source of nuclear rare metals (NRM). Separation and utilization

(stock-pile) technologies should be at once developed for the next generation, and hence in the nuclear fuel

cycle, a policy change such as a Copernican Revolution is necessarily. This paper will review the state of

the art of the hydrometallurgical, e.g., solvent extraction (SX), ion exchange chromatography (IXC) and

electrolytic extraction (EE), technologies for the separation and recovery of NRM as well as actinides

present in the radioactive wastes.

2. Nuclear Rare Metals, a New Product from a Copernican Revolution of the Nuclear Fuel Cycle

Typical yields for Pd, Ru, Rh (light PGMs) and Tc will reach to around 11kg, 13kg, 4kg and 3kg,

respectively per metric ton of the reference FBR spent fuel (150 GWd/t, cooled for 5 years). The quantity

of NRM is shown in Figure 1. Since

such yields are proportional to the

degree of burn-up, those in common

light water reactors (LWR) will be

aproximately one third of FBR. It is

notable that, Mo and some heavy

lanthanides (Ln) (Dy, Er, Yb) are

already non-radioactive and

non-exothermic on reprocessing

after 5 years cooling. Also, Nd and

La are no longer radioactive beyond

the natural one’s level. Furthermore,

after cooling for more than 50 years in a stock-pile, the specific radioactivity of Ru, In, some of Ln like Pr,

Gd and Tb will be less than 0.1 Bq/g. The quality (isotopic composition) of some of NRM is shown in

Figure 2. The radiochemical properties are summarized as follows [2],

(a) After 40 years in a FP stock-pile the radioactivity of Ru will decrease to be below the exemption level

(BSS*) of 106Ru. Its isotopic abundance will become to stable Ru (99Ru, 100Ru, 101Ru, 102Ru, 104Ru) and 106Pd only. *Note BSS; International Basic Safety Standards for Protection against Ionizing Radiation

and for the Safety of Radiation Sources, Safety Series No.115, IAEA, Vienna (1996).

10

100

1000

10000

100000

35 40 45 50 55 60 65

Atomic No.

Wei

ght (

g/t

HM

) .

Sr

Y

Zr Mo

Tc

Ru

Rh

Pd

Ag Cd Sn

Sb

Te

I

XeCs

BaLa

Ce

Pr

Nd

Pm

Sm

Eu Gd

InNb

RbKr

BrTbDy

10

100

1000

10000

100000

35 40 45 50 55 60 65

Atomic No.

Wei

ght (

g/t

HM

) .

Sr

Y

Zr Mo

Tc

Ru

Rh

Pd

Ag Cd Sn

Sb

Te

I

XeCs

BaLa

Ce

Pr

Nd

Pm

Sm

Eu Gd

InNb

RbKr

BrTbDy

Figure 1 Typical yields for Pd, Ru, Rh (light PGM) and Tc in the reference FBR spent fuel (150 GWd/t, cooled for 5 years)

- 20 -

(b) After 80 years in a FP stockpile the radioactivity of Rh will decrease to below the exemption level

(NRPB*) of 102Rh. Its isotopic abundance will become stable at 103Rh and 102Ru. *Note NRPB; National

Radiological Protection Board-R306 (1999).

(c)Only 107Pd only is radioactive (long-lived) in isotopic abundance in FP Pd. Its ratio is ca.16 wt %, and 107Ag will be gradually generated. The radio toxicity of FP Pd is very low, just ca. 30 times as high as 107Pd’s BSS level (105 Bq/g).

(d) 99Tc is the only major radioactive (long-lived) nuclide in isotopic abundance of FP Tc. Stable 99Ru will

be gradually generated.

From close investigation, a possible＂exit strategy＂can be drawn up for individual NRM with

regard to utilization. Namely, (i) Material/Chemical use; Ru, Rh, Pd, Mo, Ln (La, Nd, Dy, etc) and Tc. It

is particularly noted that the isotopic abundance of Mo in stable FP will be composed of mainly higher

order nuclides like 97Mo (22.1 wt %) and 98Mo (26.8 wt %). Such abundances might be advantageous for

the production of 99Mo and 99mTc. (ii) Radiochemical use; 137Cs (e.g., radiation source as an alternative to 60Co), 90Sr, 238Pu, 241Am and 242,244Cm, (iii) Additional nuclear fuel; 237Np, 241Am and Cm (as 238,240Pu, by

α decay of 242,244Cm), (iv) Sale of stable isotopes on the market; 99Ru, 102Ru, 103Rh, 106Pd and 107Ag. These

stable nuclides will be obtained after long-term stock-piling of FP Ru, Rh, Pd and Tc. 100Ru can be also

Figure 2 Time dependence of isotopic abundance of fission products, PMGs and Tc, after the separation (for a fast reactor, 150,000MWd/t, cooled for 5 years)

- 21 -

obtained as a transmutation product of 99Tc. The exit strategy for PGM will depend on the ability to cool

for several decades.

Prior to the industrial utilization of NRM, a radiochemically precise separation of them and the

actinides in the spent fuel is required. Such separation technologies should be integrated to be well

compatible with each other in the fuel cycle where the

reprocessing function must also be changed to meet

environment-friendly requirements.

3. Advanced Reprocessing

Spent nuclear fuel reprocessing recovers more than

95.5 % uranium and plutonium from irradiated nuclear fuels in

nuclear power plants. Solvent extraction technique is used in

current spent nuclear fuel reprocessing for the separation of

uranium and plutonium efficiently from the fission products.

This reprocessing process is called the PUREX process, and tri

butyl phosphate (TBP) used as the extractant.

Around 1950, new extractant design for reprocessing was

developed energetically, and TBP was developed through

methyl isobutyl ketone and di-butyl carbitol (Figure 3).

Uranium and plutonium, which are the separation targets in the

PUREX process, are strong acids as defined by the HSAB

principle. Furthermore, uranium and plutonium are hard donor

atoms, and thus uranium and plutonium have affinities for hard

oxygen elements. Therefore, ketones, ethers, and phosphates

containing oxygen atoms have been tried as an extractants for

the separation of uranium or plutonium.

For new extractant design for reprocessing, the

requirements for the extractant are as follows, 1) extraction

capacity for high-concentrated uranium, 2) high selectivity for

uranium and plutonium, 3) high solubility in non-polar solvents,

4) high stability towards hydrolysis and radiolysis, 5) small

influence of degradation products, 6) complete incineration, etc.

The difference in these requirements from non-radioactive solvent extraction is the stability against

radiation. In the FBR under present development, the degree of burn-up is much higher than for LWR fuels,

and thereby the demand for radiation stability is a very important factor. Moreover, the influence of

degradation products must be evaluated and these degradation products also need to be removed.

O

Methyl isobutyl ketone (MIBK)

OO

O

Di butyl carbitol

P

O

OOO

Tri butyl phosphate (TBP)

N

O

N,N-dihexyloctanamide (DOHA)

N

O

N,N-di(2-ethyl)hexyl-(2,2-dimethyl) propanamide (D2EHDMPA)

Figure 3 Structure of oxygen donor

ligands.

- 22 -

The development of an N,N-dialkylamide as a new, more highly efficient extractant than TBP began

in the 1960’s in the USA [3,4] and, up to the present, work is also reported from Italy [5,6], France [7-9] ,

Russia [10,11], and India [12,13]. Also in the Japan Atomic Energy Agency (JAEA), development of new

N,N-dialkyamides as alternative extractants to TBP began in

1992 [14,15]. N,N-dialkylamides have some advantages,

namely, their complete incinerability (salt-free property) and

high stability against hydrolysis and radiolysis. Their main

degradation products are carboxylic acids and secondary

amines which hardly affect the separation of U (VI) and Pu (IV)

from FP. Further, the synthesis of N,N-dialkylamides is

relatively easy, and various N,N-dialkylamides can be used for

extraction. The most important advantage of solvent extraction

technique using an extractant like TBP is the recovery of the

objective metal cations by back-extraction. Therefore, it is

difficult to develop a new extractant with moderate

complexation capability between the extractant and the metal

cations.

N,N-dialkylamides have been obtained by reacting an

alkyl carboxylic chloride with a secondary amine, according to

the reaction [1].

R1-COCl + HN R2R3 R1 CO NR2R3 + HCl [1]

R1, R2 and R3 in this reaction are alkyl radicals. Purification involves a distillation under low pressure (1

mm of Hg).

Typical extraction profiles for U (VI), Pu (IV), Np (IV, VI), and several types of simulated fission

products from nitric acid media by N,N-dioctyl hexanamide (DOHA) are shown in Figure 4. From these

results, U (VI), Pu (IV), Np (IV) and Np (VI) can be recovered by DOHA. These R1, R2, and R3

substituents played an important role with regard to the coordination properties for the metal cation and the

lipophilicity of the extractant. Furthermore, steric hindrance occurs with long alkyl chains and branched

alkyl chains, which can be exploited in the separation of U (VI). Figure 5 shows extraction profiles for U

(VI) and Pu (IV) which were obtained with the linear alkyl typed amide; DOHA and the branched alkyl

type; N,N-di-(2-ethyl)hexyl-(2,2-dimethyl)propanamide (D2EHDMPA). Bulky N,N-dialkylamides like

D2EHDMPA have selectivity only for U (VI). For this selectivity, branched N,N-dialkylamides are

promising new extractants for reprocessing in the transition period from LWR to FBR (L-F transition

periods). However, although D2EHDMPA was highly selective for U (VI), D2EHDMPA had a poor

extraction capacity for highly concentrated U (VI). Therefore, a new amide compound in which only R1

was branched was developed. The new N,N-dialkylamide maintains the selectivity for U (VI) and has a

10-4

10-3

10-2

10-1

100

101

102

103

0 2 4 6 8 10

U(VI)Pu(IV)Np(VI)Np(IV)PdRuRhZrTc

Dis

trib

utio

n ra

tio:

D

HNO3 (M)

Figure 4 Nitric acid dependency of

DM for 1.0 M (mol dm-3)

DOHA-dodecane.

- 23 -

high loading performance for highly concentrated U (VI). On

the other hand, a disadvantage of N,N-dialkylamides is the

production of a secondary amine as the degradation product

from hydrolysis and/or radiolysis. This secondary amine with a

long alkyl chain exists in the organic phase, and cannot be

removed by alkali scrubbing. Now, an evaluation of secondary

amines in the reprocessing and removal techniques is being

carried out.

In nuclear fuel reprocessing, the development of the

isolation technology for U (VI) is a research objective. As seen

from an industrial point of view, the next step in

N,N-dialkylamides development should be tests with real spent

fuels.

4. Separation of Minor Actinides

In the past three decades, partitioning and transmutation (P&T) of long-lived nuclides has been

studied world-wide with the objective of creating an environmentally friendly nuclear fuel cycle. In this

context, the TRUEX (TRansUranium EXtraction) solvent extraction process, using a bifunctional

extractant OØD[IB]CMPO (n-octyl(phenyl)- N,N-diisobutylcarbamoylmethylphosphine oxide) with TBP in

n-dodecane as the solvent, was successfully found to be a vital method for recycling trivalent actinides

(An(III)) from high level liquid wastes (HLW) of spent fuel reprocessing [16]. The separation of trivalent

actinides and lanthanides from HLW was investigated by not only CMPO but also malonamide in the

European framework projects [17]. These extractants are in the development stage for practical use. The

process flow-sheets using these extractants have been successfully tested. One of the technological issues

was inter-group separation of trivalent Ln (III)) from An(III) for efficient actinide recycling into the FBR.

As these are reviewed in other papers [18], the most topical two technologies for minor actinide (MA) / Ln

separation and americium (Am) / curium (Cm) separation by P&T are reviewed in this section.

4.1 Separation of Minor Actinides from Lanthanides

As mentioned above, tri-valent An (III) existing in spent nuclear fuels, have been treated as high

level wastes in the nuclear cycle. At present, transmutation of MA like An (III) by fast neutrons with an

Accelerator Driven System (ADS) and/or a FBR has been studied. These transmutation technologies use a

neutron source. Therefore, if a burnable poison like Ln is present in this target, the transmutation efficiency

of Am (III) and Cm (III) is decreased. In order to achieve a high transmutation efficiency of Am (III) and

Cm (III), separation of An (III) from Ln (III) is very important. On the other hand, the 5f-electrons on An

(III) are of a largely relativistic, itinerant nature, providing a degree of covalency, which makes the

behavior of An (III) ions slightly softer than Ln (III) ions. Consequently, most separations of An (III) from

10-4

10-3

10-2

10-1

100

101

102

10-2 10-1 100 101

DOHA-U(VI)DOHA-Pu(IV)D2EHDMPA-U(VI)D2EHDMPA-Pu(IV)

Dis

trib

utio

n ra

tio :

DM

HNO3 (M)

Figure 5 Nitric acid dependency of DU and DPu by 1.0 M DOHA-dodecaneand 1.0 M D2EHDMPA-dodecane.

- 24 -

Ln (III) were achieved with ligands using soft donors like sulfur (S) or nitrogen (N) atoms.

For this purpose, new MA/Ln separation ligands were developed in several countries. These ligand

structures are shown in Figure 6.

P

S

HS

N

N

N

N N

N

Bis(2,4,4-trimethylpentyl)di 2,4,6-tri(2-pyridyl)-1,3,5-triazines

-thiophosphinic acid (CYANEX 301) (TPTZ)

NN

NN N

N

N

N N

N

N

N N

N

N

2,6-bis-(5,6-di-n-propyl- 1,2,4-triazin 6,6’-bis(5,6-diphenyl-1,2,4-triazin-3-yl)-

-3-yl)pyridine (nPrBTP) 2,2’- bipyridine (C5-BTBP)

NN

R1

NR1

O O

NN

R1

O N

N,N’-dialkyl-N,N’-diphenyl pyridine N,N’-dialkyl-1,10-phenathroline

-2,6-dicarboxyamide (PDA) -2-carboxyamide (PTA)

N

NN

N

NN

R

R

R

R

N,N,N’,N’-tetrakis((5-alkyloxypyridin-2-yl)methyl)

-ethylenediamine（TPEN）

Figure 6 Structure of sulfur and nitrogen donor ligands.

- 25 -

The purified bis(2,4,4-trimethylpentyl) di-thio phosphinic acid (CYANEX 301) gave a high

separation factor ; SF [Am (III) / Eu (III) ] = 4900 [19,20]. Process flowsheets were developed and tested in

China with real HLW and the radiolysis of CYANEX 301 was also investigated [21]. In the EUROPART

project for international cooperation in Europe, many types of new N-donors have been synthesized and

tested for the separation of An (III) from Ln (III). Typical N-donors are 2-pyridinecarboxyamide

(picolinamides) [22], 2,4,6-tri- (2-pyridyl)-1,3,5-triazines (TPTZ) [23], bis-triazinyl-1,2,4-pyridines (BTP)

[24,25], and 6,6'-bis(5,6- dialkyl-1,2,4-triazin-3-yl)-2,2'-bipyridines (C5-BTBP) [26-28].

2,6-bis-(5,6-di-n-propyl-1,2,4- triazin-3-yl)pyridine (nPrBTP) exhibited good selectivity, with a SF[Am

(III) / Eu (III)] of > 100. This high SF was obtained from 1.0 M (mol dm-3) HNO3 acid solution with 0.04

M nPr-BTP in TPH / n-octanol (70/30 vol %). However degradation of nPr-BTP by radiolysis was very fast.

In the ACSEPT (Actinide reCycling by SEParation and Transmutation) project in EU (which runs from

2008 to 2012), the development of a new BTBP and a new N-donor is under way.

Recently, N,N’-dialkyl-N,N’-diphenylpyridine-2,6-dicarboxyamide (PDA) [29,30], N,N’-

dialkyl-1,10-phenathroline-2-carboxyamide (PTA) [31] and N,N,N’,N’-tetrakis(2-pyridylmethyl)ethylene-

diamine (TPEN) [32-34] were synthesized and investigated for MA / Ln separation. These two ligands,

PDA and PTA, have soft (N)-hard (O) hybrid donors. In particular PTA exhibited a SF [Am (III) / Eu (III)]

value of ca. 20 at an aqueous concentration of 1.0 M HNO3. However, although PDA and PTA had good

SF [An / Ln] values, the behavior of FPs with PDA and PTA was not clear. In Japan and in the ACSEPT

project in the EU, the above separation of minor actinides from lanthanides is progressing and has a strong

connection with national strategy (especially fuel fabrication).

TPEN is a hexadentate ligand with 6 nitrogen-donors and complexes with softer metal ions selectively.

Trivalent MA ions, such as Am (III) and Cm (III), which are softer than lanthanide ions, are extracted to

the organic phase containing TPEN selectively. A SF [Am (III) / Eu (III)] value of ca. 100 was observed in

the pH range > 4 [32,33]. From the viewpoint of practical use, some hydrophobic TPEN analogs wherein

long alkyloxy groups were introduced to four pyridine rings were synthesized and the effect of the

introduction of the alkyloxy groups on the separation of Am (III) and Eu (III) was tested. In an acidic

solution at pH 3, the extractability of TPEN was improved by the introduction of butoxy groups, and a SF

[Am (III) / Eu (III)] value of ca. 90 was observed at pH 3 [33].

In the meantime, the simultaneous recovering of all of the

actinides with one type of solvent, bifunctional organophosphorus

extractants dissolved in highly polar fluorinated diluents, was

studied. 0.4M OØD[iB]CMPO with 30% TBP dissolved in

metanitrobenzotrifluoride (Fluoropole-732) was found to achieve

simultaneous extraction of all of the f-elements from the dissolver

solution of spent nuclear fuel without splitting a third phases [35].

This new solvent system was named the ORGA process Figure 7 Tertiary Pyridine Resin (TPR)

- 26 -

(*abbreviation of Organophosphoryl-fluoropole solvent for Recovery of Group of Actinides) [36].

4.2 Mutual Separation of Americium and Curium

The properties of a tertiary pyridine resin (TPR), as shown in Figure 7, with a nitrogen atom in its

six-membered ring, are characterized by simultaneously having two functions as a weakly basic

anion-exchanger and a soft donor ligand. The TPR is synthesized by co-polymerization of 4-vinylpyridine

and m/p-divinylbenzene on a high porous silica carrier (φ 60μm) at room temperature. The total exchange

capacity is ca. 5 meq / g (dry) in the Cl- form as an actual measurement value [37]. The TPR was hardly

damaged by up to 3MGy irradiation from a 60Co source, with an ensuing particularly high resistance to

radiation (～10MGy) in HCl media. An original idea by T.Suzuki, et al. [38] of combining the use of

conc.HCl and conc. HNO3 media for inter- and intra-group

separation of the f-elements was based on such facts, as

shown in Figure 8, as the recognition of softer ions like Am

(III) and Cm (III) (5f-elements) was particularly improved

(e.g., higher distribution ratios) compared to Ln (III) (4f

-elements) in HCl media, and, in HNO3 media, the

distribution ratios would depend not on the difference of the

softness between 4f- and 5f-elements but just on the

difference in ionic radius. The presence of a high conc. of

methanol in acidic media, improved the separation of

intra-group f-elements by enhancing the complexation of

the dehydrated f-element ions with anions [39]. Using these

facts, a HCl / HNO3 hybrid scheme was demonstrated in the

reprocessing of a trace amount of MOX spent fuel irradiated

(143.8 GWd/t) in the fast experimental reactor “Joyo” [38].

Individual recoveries of pure Am (III) product ((106Ru, 125Sb, 137Cs, 144Ce, 155Eu, 243Cm) / 241Am: < 10ppm) and Cm (III)

product (241Am / 243Cm: 0.78 %) were attained, respectively.

These separations will satisfy any separation degrees set up

for MA product such as Ln / MAproduct < 5 %mass.

5. Separation of Nuclear Rare Metals

In the PUREX process, SX technology was historically

been developed for the separation of An (U, Pu) from FP

while the recovery of valuable FP has been an issue in its

nuclear application. Furthermore, since their valences are

101

102

0.09 0.1

101

102

Dis

trib

utio

n co

effic

inet

, Kd

Ionic radii / nm

Am

Cm

LaCePrNdSmEuGdTb

DyHo

YEr

TmLu

Yb

LaCePr

Nd

AmCm

SmEu

GdYLu

conc.HNO3: MeOH=7:3

conc.HCl: MeOH=7:3

Figure 8 Distribution Ratios of f-elements for the Tertiary Pyridine Resin in HCl and HNO3 media (TPR)

0 2 4 6 8 10100

102

104

106

[HCl] / mol dm-3

Dis

trib

utio

n co

effic

ient

Pd(II)

Rh(III)

Ru(III)

Tc(VII)

Figure 9 Distribution Coefficients of PGM and Tc on the TPR in HCl media.

- 27 -

widely distributed from I to VII, group separation as for the f-elements seems to be impossible in the

PUREX nitric acid system. Therefore non-SX methods, like IXC, solid adsorption and EE, have been

studied for their individual separation.

5.1 Ion Exchange Chromatography of PGM and Tc

The acid dependency of the distribution coefficients of PGM and Tc on the TPR is shown in Figure 9.

The distribution coefficients were evaluated by the batch experiments carried out at room temperature. The

data suggest that all light PGM and TcO4- are strongly adsorbed by the TPR over a wide HCl concentration

range. If no adsorption of actinides (U (VI), Pu (IV), Am (III)) occurs, separation of Pd2+ and TcO4- in

highly dilute HCl conditions will be possible. These predictions have been definitely supported by the

strong adsorption of 106Ru and 125Sb in the previous hot test results.

As Figure 9 shows, it will be necessary to develop viable, salt-free, complexing reagents for efficient

elution of PGM and Tc from the resin as the next step.

On the separation of cesium and strontium, the selective separation of Cs+ by highly functional

adsorbents (silica gel loaded with ammonium molybdophosphate (AMP-SG(D)) and Sr2+ by hybrid

microcapsules containing D18C6 (D18C6-MC) are discussed in the other papers [40,41].

5.2 Catalytic Electrolytic Extraction of PGM and Tc

Catalytic electrolytic extraction (CEE) [42] can effectively separate Ru, Tc and Re (Tc simulator)

from either HNO3 or HCl solution under controlled potential deposition (UPD) with the formation of

insoluble metal (Ru, Rh, Pd) / oxide (ReO3, TcO2) solid solutions in acidic media. In the case of Re oxide,

an island type deposit was sometimes observed. In the UPD concept as shown in Figure 10, Pd or Rh may

act as a promoter at the surface of the electrode (i.e., Pdadatom, Rhadatom) and a mediator in the bulk solution

(i.e., redox ion pair).

The adatom, is a

contraction of

“adsorbed (single)

atom”, lying on

surfaces and surface

roughness, will act

as a deposition

catalyst. Pd and Rh

were easy to deposit

from the both media.

Characteristics of the

deposits are also

described in the Figure 10 Concept on Mediator and Promoter in the Under Potential Deposition

- 28 -

figure. In dilute hydrochloric acid (HCl) media in particular, such CEE occurred more readily. For

instance, the addition of Rh3+ significantly accelerated the deposition of Tc and Re as shown in Figure 11.

As typical deposition characteristics in the one to four NRM metal ion mixed solutions (corresponding to

FBR spent fuel composition), the observed deposition yields (average) were typically high in the following

order;

Pd, Rh ( > 99 %) > Re (91 %) > Ru (85 %) > Tc (69 %),

where the CEE was galvanostatically carried out with typically changing cathode current density like 2.5

mA / cm2 (1 hr) → 75 mA / cm2 (2 hr) → 100 mA / cm2 (4 hr) in 0.5M HCl at 50℃. At the initial stage,

setting a lower cathode

current density with

compulsory stirring was

necessary to form Pd

cores and the adatom

layer. This procedure was

essential to obtain higher

amounts of co-deposits

of Ru and Tc (Re) ions.

During the CEE,

the reduction process for

Rh was similar to Pd, i.e.,

through direct reduction

of Rh3+ to Rh metal.

However, reduction of Ru might proceed at least by two steps via following reactions,

RuNO3+ + e → Ru 2+ + NO E0 = 0.044V (vs. Ag/AgCl)

Ru2+ + 2e → Ru(s) E0 = 0.255 V (vs. Ag/AgCl)

A H-type electrochemical cell, where a nafion-117 cation-exchange membrane separated the catholyte and

the anolyte, was employed in both the CEE experiments and for hydrogen evolution by the NRM deposited

electrodes after changing the catholyte.

Recently, preliminary experiment on EE using a simulated high level liquid waste (S-HLW; 0.5M

HNO3, containing 26 metal elements corresponding to real HLW by LWR-reprocessing) has been carried

out [39]. The CEE deposits were apparently the same as shown in Figure 10 (right picture) with fine

coagulated spherical particles as were observed in the case of the Pd2+ involved deposits. The reduction

ratio of Pd was as high as 89 % as expected, while other NRM as Ru, Re and Mo were limited to around

20 % and less than 10 % for Rh, Te and Se. Such lower deposition ratios were probably attributed to the

significant consumption of the electricity by the large amount of the corrosion product iron contained in

S-HLW through the reduction of Fe3+ to Fe2+. In addition, the PGM content itself was lower compared to

Figure 11 Acceleration effect on the deposition of Tc and Re by adding Rh3+ ;

Electrolysis Condition Electrodes: Smooth Pt , Cathode (2cm2) , Anode (8cm2) , RE:

Ag/AgCl, Catholyte: 0.5MHCl, Tcconc.: 50ppm, Reconc.: 200ppm, Temp.: 50℃

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the previous experiments with FBR-HLW, and neither Pd2+ nor Rh3+ was added in this experiment. For

higher deposition yields, the CEE utilizing UPD by Pdadatom or Rhadatom is highly recommended with prior

separation of actinides and the other elements with nobler redox potentials by IXC. The reduction behaviors

of Mo, etc, require to be investigated in details.

The CEE method is advantageous for radiochemical separation of NRM, because of minimal

irradiation with no simultaneous deposition of actinides, but also because they are recovered in the solid

state at an electrode surface. The sphere deposits were dense and mechanically more stable than dendritic

deposits, while showing electrochemically high catalytic reactivity on electrolytic hydrogen production

[43]. In particular, the catalytic reactivity of the quaternary, Pd-Ru-Rh-Re (3.5:4:1:1, corresponding to the

composition of FBR spent fuel) deposit on a Pt electrode for electrolytic production of H2 was the highest,

exceeding that of a smooth Pt electrode by ca. twice both in alkaline solution and artificial seawater. Such

higher reactivity will attribute to the higher numbers of adsorption sites on the surface of the deposit for

protons.

Ru has been confirmed as the dominant element responsible for high reactivity, while Pd behaved as

just a ＂starter＂ nucleus and a ＂binder＂ among the involved elements during the deposition. It is

noticeable that Tc seems to have a higher or the same catalytic ability as Re. The high reactivity of Tc

suggests that it may be a possible alternative to Re in the field of catalysts.

6. A New Back-end Strategy as Copernican Revolution

Aiming at simultaneous realization of the utilization of elements/nuclides and ultimate minimization

of radioactive wastes, a new fuel cycle concept, Adv.-ORIENT (Advanced Optimization by Recycling

Instructive Elements) Cycle [2, 41, 43, 44], is proposed under the following strategies as shown in Figure

12；

1/ Trinitarian research on separation, transmutation and utilization (S&T, U) of nuclides and elements,

based on FBR fuel cycle.

2/ Significant reduction of radioactive wastes and eventual ecological risks: Within a few hundred years,

achieve a radiotoxic inventory decrease to the level of natural U tons corresponding to one ton of

vitrified HLW.

3/ Cascade separation of all actinides, NRM, Cs and Sr by a multi-functional and compact reprocessing

process and plant.

4/ Challenge on isotope separation of long lived radio nuclide 135Cs from FP Cs for advanced

transmutation.

5/ Accept and separate natural radioactive materials (U, Th) to burn, on demand of the RE industry.

The most important policy change is that NRM shall not be just the waste constituents but be the main

product in the fuel cycle. Actinides will no longer be the products, but will just be the material burned in

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the reactors. To realize this concept at both scientific and industrial levels, several separation tactics are

proposed as follows;

1/ Higher purity of NRM for utilization, while lower decontaminated actinides are permitted for burning in

FBR. Separation factors of 90-99.9 % will be chosen for the individual impact of radio nuclides.

2/ Adopt soft hydrometallurgical separation processes with salt-free reagents to reduce the secondary

radioactive wastes. Ultimately, low greenhouse gas emission technology is required.

3/ A high degree of separation of all actinides into 3-4 groups, U, Pu/U/Np, Am and Cm, directly from the

spent fuel by an IXC method.

4/ As non-SX methods, CEE and adsorptive separation methods for NRM are chosen to alleviate the

radiation effect. Solid state will be preferable as the end product for utilization and/or stockpiling.

5/ HCl media is allowed in combination with HNO3 media to improve the separability.

6/ Identification of anti-corrosive materials in both conc. HCl and HNO3 are indispensable from an

industrialization point of view. Verification of thermo- and radio-chemical stability of the novel IX resin

is also required.

7/ Preliminary separation of actinides from the NRM is advantageous because Zr, Mo, Pd, 99Tc, 106Ru and 125Sb, etc will disturb the operation of both reprocessing and vitrification of HLW.

Figure 12 Adv.-ORIENT Cycle Concept

LLFP Removalby Inorganic

Ion Exchanger

Spent fuel

Main AnSeparation/ Purification

ProcessbyIXC

NRM Recovery by CEE

Am, CmMutual

Separation

90Sr -Zeolite

Cs,Se,Sn

Sr

Other FPs(Near Surface and/or Deep Repository)

Ln

Am, Cm Am

Pu (←Cm)

U, Pu, Np

Reuse of Filter

Pd, Ru, Rh,Tc, Sb, etc

Ru,Rh,Pd,Tc,etc

Tc

Ap

plicatio

n for C

atalysts fo

r H

ydro

gen G

ene

ration a

nd/orF

uel C

ell

Application for Heat (Sr battery）and/or Radiation Source

Application forHeat and/or Radiation Source

Application as long-lived TcbatteryCatalyst after Transmutation99Tc(n,β-)100Ru

Isotope SeparationElution

133Cs

137Cs

Pd

Pd

107Pd135Cs

(Transmutation)

MoApplication for99mTc Source

Isotope

Sep

aration

Suzuki/Ozawa

NRM Separationby IXC Filter

Electro-Catalyst

137Cs-Zeolite

StrategicMaterials

Cm

Adv.-ORIENT Cycle since 2006

Multi-functionalReprocessing

Advanced Optimization by Recycling Instructive Elements Cycle

Fast BreederReactor

Fuel Fab.

EX-Cycle

IN-Cycle

Natural RE

U,Th

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The time to reduce the radio-toxicity Sv (Sievert) of 1 ton of vitrified HLLW below the level of equivalent

tons of natural raw uranium is one of the indexes for environmental impact. In the Adv.-ORIENT Cycle, by

putting the separation factors at 99.9 % for all actinides, 99 % for 137Cs, 90Sr and the other NRM, and 90 %

for Ln, such a period can dramatically be reduced to around 102 years.

7. Conclusion

The isotopic composition and radiochemical properties of nuclear rare metals have been reviewed.

Hydrometallurgical separation technologies using solvent extraction (SX), ion exchange chromatography

(IXC) and catalytic electrolytic extraction (CEE) techniques were developed as vital separation tools for

light PGM (Ru, Rh, Pd), Tc and f-elements present in high level liquid wastes of the nuclear fuel cycle. The

SX process using N,N-dialkylamide can isolate U(VI) from fission products without Pu(IV) valence

control, and extractants utilizing soft-hard hybrid donors (PDA and PTA) and those containing six soft

donor atoms (TPEN), which have been developed mainly in Japan, show good separation of An(III) from

Ln(III). The IXC process utilizing a tertiary pyridine resin (TPR) gives a high separation of the

f-elements in spent nuclear fuel and produce pure Am and Cm products. The CEE process utilizing Pdadatom

or Rhadatom can effectively separate PGM, Tc and Re by utilizing under potential deposition (UPD)

phenomena.

Some of the basic work on hydrometallurgical separation of concerned elements is being progressed

in the flame of strategic Advanced (Adv.-) ORIENT Cycle research program in Japan. The Adv.-ORIENT

Cycle process can not only improve the rad. waste problem, but also offer useful rare metals to leading

industries from this secondary resource.

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