Advanced Hydrometallurgical Separation of Actinides and Rare Metals in Nuclear Fuel Cycle Masaki OZAWA 1*,2 , Shinichi SUZUKI 1 and Kenji TAKESHITA 2 1 Japan Atomic Energy Agency, 2-4 Shirane, Shirakata Tokai-mura 319-1195, Japan, 2 Tokyo 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 Pd adatom or Rh adatom 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, 235 U) 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 -
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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.
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
- 30 -
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
- 31 -
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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