Research committee on ruthenium and technetium chemistry in PUREX system, organized by the atomic energy society of Japan

Article abstract of DOI:10.1023/A:1026047722973

The paper summarizes the activity of the committee "Ruthenium and Technetium Chemistry in the PUREX System," with a focus on basic information on technetium behavior in the PUREX process, the principles of plant design, and the behavior during the final w

Full text of DOI:10.1023/A:1026047722973

Radiochemistry, Vol. 45, No. 3, 2003, pp. 219 224. From Radiokhimiya, Vol. 45, No. 3, 2003, pp. 200 204.
Original English Text Copyright 2003 by Matsumoto, Uchiyama, Ozawa, Kobayashi, Shirato.
Research Committee on Ruthenium and Technetium Chemistry
in PUREX System, Organized by the Atomic Energy Society
of Japan1, 2
S. Matsumoto*, G. Uchiyama**, M. Ozawa***, Y. Kobayashi****, and K. Shirato*****
* Saitama University, Japan
** Japan Atomic Energy Research Institute, Japan
*** Japan Nuclear Cycle Development Institute, Japan
**** Japan Nuclear Fuel Limited, Japan
***** Ishikawajima-Harima Heavy Industries Co., Ltd., Japan
Received December 26, 2002
Abstract The paper summarizes the activity of the committee Ruthenium and Technetium Chemistry in
the PUREX System, with a focus on basic information on technetium behavior in the PUREX process,
the principles of plant design, and the behavior during the final waste treatment.
ACTIVITIES OF RESEARCH COMMITTEE
nuclide for future transmutation using a fast reactor or
99
100
accelerator [ Tc + n
Ru (stable)]. On the other
In advancement of the PUREX reprocessing sys-
tem, improving the economy with ensuring the safety,
becomes an urgent issue. In the initial developing
stage of the reprocessing technology, major studies
were aimed to attain high recovery ratios and purities
of uranium and plutonium products. Some data bases
were generated during these studies. Further, as sep-
aration of long-lived minor actinides also became
a current topic, much attention was given to the
chemical behavior of minor actinides, not only in
the PUREX process but also in novel extraction proc-
esses. However, the behavior of fission products was
studied insufficiently as compared to actinides. Ac-
tually, the chemical properties and extraction behavior
of the fission products in the PUREX process are still
insufficiently understood.
hand, ruthenium is one of the rare metal fission prod-
106
ucts to be potentially utilized in future, because
Ru
has a short half-life (T = 368 days), and its specific
1/2
radioactivity will be negligible after cooling down
for several decades.
Members of the Atomic Energy Society of Japan
have organized a scientific committee on this problem
in October 1999. The committee, entitled Ruthenium
and Technetium Chemistry in the PUREX System,
consists of more than 30 members. At present, the
committee’s activities are aimed at deeper understand-
ing of the PUREX-type LWR and FBR reprocessing
systems with respect to the design, construction,
operation, and safety evaluation.
The scope of the work of the committee includes
the following major topics: (1) basic solution and
solid-state chemistry; (2) basic solution and solid-state
chemistry of minor actinides (in particular, Np);
(3) partitioning chemistry in the PUREX system and
environmental behavior of the components; (4) proc-
esses of recovery, purification, and utilization of rare
metal fission products; (5) field data on plant design,
operation, decontamination, and decommissioning;
(6) numerical process simulations and process control
technology; (7) compilation of a data base for process
chemistry and plant engineering.
Among the fission products, ruthenium and techne-
tium show a particular tendency to form extractable
species with TBP and other organic extractants. There-
fore, they are distributed throughout the reprocessing
flowsheet and get into the environment. In the modern
106
reprocessing plants, these two nuclides ( Ru and
99
Tc) are the dominant nuclides determining the over-
all decontamination factors of the uranium and plu-
tonium products. In other words, removal of these two
nuclides will determine the design of the main separa-
99
tion process. Furthermore, Tc is long-lived (T
=
1/2
5
2.12 10 years), and it will be a possible target
Tc CHEMISTRY IN PUREX SYSTEM
1
2
This article was submitted by the authors in English.
Reported at the Third Russian Japanese Seminar on Techne-
In the step of spent fuel dissolution in the PUREX
process, some part of Tc does not pass into solution.
tium (Dubna, June 23 July 1, 2002).
1066-3622/03/4503-0219$25.00 2003 MAIK Nauka/Interperiodica

220
MATSUMOTO et al.
Pu4+ + 3NO3 + TcO4 + TBP
Pu(NO3)3(TcO4) TBP,
(3)
Zr4+ + 3NO3 + TcO4 + TBP
Zr(NO3)3(TcO4) TBP. (4)
In the absence of U, Pu and Zr, at low HNO con-
3
centration, Tc is extracted with TBP in the form of
HTcO 3TBP according to Eq. (1). In the presence of
4
these elements, Tc is coextracted as mixed complexes
UO (NO ) (TcO ) 2TBP,
Pu(NO ) (TcO ) TBP,
2
3 2
4
3 3 4
and Zr(NO ) (TcO ) TBP according to Eqs. (2) (4).
3 3
4
The Tc distribution ratio increases with an increase
in the nitric acid concentration in the range of low
[HNO ]. At higher nitric acid concentrations, the per-
3
Fig. 1. Tc distribution ratio D
: (a) in the absence of
TcO
technetate ion in mixed complexes is substituted by
nitric acid and displaced into the aqueous phase, and
the Tc distribution ratio decreases with an increase
4
U and Pu, as a function of the HNO concentration, and
(b) in the presence of U and Pu, as a function of the initial
U concentration, at Pu concentration, g l : (1) 0.5, (2) 20,
and (3) 50.
3
1
in [HNO ]. The extraction reactions proceed accord-
3
ing to the following equations (from the right side
to the left):
This fraction, estimated at 10 to 20% of the total Tc
for a UO fuel, accompanies the insoluble residues
2
TcO4 + UO2(NO3)2 2TBP
+ NO3,
UO2(NO3)(TcO4) 2TBP
(5)
essentially consisting of noble metals (Ru, Rh, Pd).
These insoluble residues are incorporated in the vitri-
fied wastes [1]).
TcO4 + Pu(NO3)4 TBP
Pu(NO3)3(TcO4) TBP + NO3,
(6)
In nitric acid medium in the absence of reducing
agent, the dissolved Tc is in its highest valence (VII),
which is the most stable. It occurs in the form of per-
technetate ion, TcO . This species can be coextracted
TcO4 + Zr(NO3)4 TBP
Zr(NO3)3(TcO4) TBP + NO3.
(7)
4
with a metal cation by substitution of the pertechne-
tate group for the nitrate group in the neutral complex
extractable with TBP.
Figure 1a shows the experimental data on the dis-
tribution ratio for equilibrium HNO concentration in
Technetium is coextracted with the main metallic
cations extracted by TBP, and particularly with the
ZrO cation, which is present in large amounts in
the solution fed to the extraction cycle. This extraction
of Tc seriously interferes with the U/Pu partitioning.
Correspondingly, the PUREX process should be
adapted to limit the extraction of Tc.
3
the aqueous phase of 0 to 5 M. In Fig. 1b, the Tc dis-
tribution ratio is plotted vs. initial uranium concentra-
tion in the aqueous phase in the system containing
2+
1
Pu and U. At 0.5 g l Pu and traces of U, the Tc
distribution ratio is similar to that in the system with
straight nitric acid. As the U concentration in the
system is increased, the Tc distribution ratio grows,
reaching a plateau. This is in line with the forma-
tion of an extractable uranium nitrate pertechnetate
species.
Tc FUNDAMENTAL CHEMISTRY
In spent fuel, Tc is mainly in the metal form. In the
dissolution step, Tc is in the state of pertechnetate ion.
Some of Tc is not dissolved in nitric acid and occurs
in white inclusions, in the mixed alloy form.
At a high Pu concentration, the Tc extraction is
greatly enhanced. Pu(IV) nitrate forms an extractable
species with Tc, similar to Zr(IV) nitrate. Like the
Pu(NO ) 2TBP species, pertechnetate-containing
3 4
Tc in the pertechnetate form is extracted with TBP
by the following equations [2]:
adducts salted-out by the preferential extraction of
uranium nitrate and uranium pertechnetate species are
possibly formed. This is illustrated clearly by the
H+ + TcO4 + 3TBP
HTcO4 3TBP,
(1)
1
results for 20 g l Pu.
UO22+ + NO3 + TcO4 + 2TBP
UO2(NO3)(TcO4) 2TBP,
(2)
The Tc distribution ratio is expressed in the absence
of U, Pu and Zr and in the presence of U and Zr by
RADIOCHEMISTRY Vol. 45 No. 3 2003

RESEARCH COMMITTEE ON RUTHENIUM AND TECHNETIUM CHEMISTRY
empirical equations (8) and (9), respectively [3].
221
These equations are obtained by fitting a large body
of experimental data.
aCbNO , aq
3
DTcO = A
,
(8)
1 + cCdNO , aq + eCNf O , aq
4
3
3
b
where C
is the NO concentration in the
3
NO , aq
3
aqueous phase (M); A = 1 (30 vol % TBP), a =
2.324exp(8070 ), b = 0.848exp(230 ), c = 0.157
exp(3240 ), d = 4.69exp(410 ), e = 1.72exp(3150 ),
f = 1.95exp(160 ), = 1/( + 273) 1/298 ( is the
temperature in C);
1.343
DTcO
=
0DTcO + KTc, UCUO2+, org (1 + kC
)
NO , aq
4
4
3
2
0.707
3
+ KTc, Zr(CZr4+
C
TcO , org)CNO , aq
,
(9)
, org
Fig. 2. Concentration profiles of U, Pu, Np, Tc, and nitric
4
acid in the co-decontamination and fission product scrub-
bing steps in the PUREX system: (points) experiment and
(lines) calculation.
where D
presence of UO , Zr ; D
is the TcO distribution ratio in the
TcO
4
4
2+
4+ 0
is the Tc distribu-
TcO
4
2
2+
4+
2+
tion ratio in the absence of UO , Zr ; C
is
2
UO , org
the UO concentration in the organic pha2se (M);
2+
2
C
is the NO concentration in the aqueous
NO , aq
phase; C
3
3
4+
4+
is the Zr
concentration in the
Zr , org
organic phase; C
is the TcO concentration in
TcO , org
4
4
the organic phase; K
= 0.33exp( 1060 ); k =
= 1670exp(2810 ).
Tc, U
4.87exp(980 ); K
Tc, Zr
Equation (9) reflects the effect of U and Zr on the
Tc distribution ratio, illustrated by Figs. 1a and 1b.
Fig. 3. Reduction of Tc with hydrazine at room tem-
perature.
Equations (8) and (9) are included in the program
simulating the extraction step. It can simulate the
extraction behavior of U, Pu and nitric acid based on
the Richardson calculation model.
cludes intermediate species such as Tc(VI) and
Tc(V) [5].
Figure 2 shows the results of calculations using
the ESCCAR program developed in JAERI and incor-
porating the above-given equations [3]. The calcula-
tion results agree well with the experimental data. The
simulation program has been further developed to in-
clude more detailed data on chemical reactions in
the PUREX system.
Tc BEHAVIOR IN INDUSTRIAL PROCESS
Figure 4 shows the distribution of Tc over the
designed flowsheet of the Rokkasho Reprocessing
Plant (RRP). In the first stage, fission products are
separated, and then Pu is separated from U by reduc-
tion with U(IV) and hydrazine. In the second stage,
U and Pu are purified. In the first stage, the organic
phase from the extraction column is effectively
Figure 3 illustrates the reduction of Tc with hy-
drazine at room temperature [4]. The percentage of
Tc(VII) is plotted against time. After 3 h of induction
period, the percentage decreases and reaches a mini-
mum at about 10 h. At this time, almost all Tc exists
as Tc(IV). Then the percentage of Tc(VII) increases
and reaches approximately the initial value. The reac-
tion is generally explained as Tc(VII) reduction to
Tc(IV) with hydrazine and reoxidation of Tc(IV) with
nitrate. The mechanism, however, presumably in-
scrubbed with 2 M HNO in the first scrubbing
3
column and by more concentrated HNO in the sec-
3
ond scrubbing column. The values of 95 and 33% in
Fig. 4 are maximal for the process; therefore, their
sum exceeds 100%.
Figure 5 shows the Tc distribution over the flow-
sheet of the Tokai Reprocessing Plant (TRP). The
TRP flowsheet includes three solvent extraction
RADIOCHEMISTRY Vol. 45 No. 3 2003

222
MATSUMOTO et al.
Fig. 4. Tc balance at Rokkasho Reprocessing Plant.
Fig. 5. Tc balance at Tokai Reprocessing Plant.
cycles. In the first cycle, 70% of Tc is extracted. In
the second, most of Tc is stripped, and a small
amount of extracted Tc goes in almost equal propor-
tions to the U and Pu purification cycles. In both puri-
fication cycles, Tc is quantitatively separated. This Tc
behavior is presumably due to coextraction with Zr,
U, and Pu.
The chemical forms, solubility, and sorption coef-
ficient [K , ratio of the nuclide concentration in
d
1
solid (Bq g ) to the nuclide concentration in liquid
1
(Bq ml )] of Tc are given in Table 1. In shallow
disposal, presumably under oxidizing conditions, the
Table 1. Tc behavior at waste disposal
Parameter
Shallow disposal
Deep disposal
Tc BEHAVIOR IN RADIOACTIVE
WASTE DISPOSAL
Chemical species TcO4
TcO2 or TcO(OH)2
8
Solubility Soluble
4 10
M
1
Radioactive wastes from nuclear power plants and
reprocessing plants contain Tc. Tc is one of the most
important radionuclides in the safety assessment of
waste disposal.
Kd, ml g :
cement
soil, rock
About 0 1
About 0.5
About 100 1000
About 1000
RADIOCHEMISTRY Vol. 45 No. 3 2003

RESEARCH COMMITTEE ON RUTHENIUM AND TECHNETIUM CHEMISTRY
223
Table 2. Main research issues for Ru and Tc chemistry toward advancing PUREX system
Solution, coordination,
extraction chemistry
Radio-, electro-, thermo-,
catalytic chemistry
Operation
Solid chemistry
Analytical chemistry
Dissolution, Undissolved residue; optimal dissolution; Ru accumula- Quantitative analysis Electrochemical dissolu-
filtration
tion; Ru/Tc precipitation; cocrystallization with U(VI); for Tc in multicompo- tion
Ru deposits; colloid formation; nanofiltration; colloid nent radioactive solu-
separation
tions
Solvent
extraction
Distribution ratios; Ru/Tc in Ru/Tc decontamination Ru/Tc chemical species Electrochemical redox
third phase; Ru/Tc complexa- from actinides by crys- and their identification control for more efficient
tion; redox mechanism;
tallization and super-
in PUREX solutions; decontamination of Ru
oxoanions and ion pairs; con- critical fluid extraction polynuclear NMR for and Tc
trol of decontamination fac-
tors; mathematical simulation;
supercritical fluid extraction
structural analysis;
quantitative analysis
for Tc
Evaporation Ru/Tc vaporization; efficient Ru deposition on equip-
Ru/Tc vaporization be-
havior in evaporator;
Ru/Tc vaporization dur-
ing thermal denitration
of uranyl nitrate hydrate
evaporation for LLW volume ment walls
reduction; recycling of distil-
late
Vitrification, Tc chemical forms in repos- Materials for Tc immobi- Microanalysis methods Vaporization of Ru and
disposal itories; environmental diffu- lization, Tc encapsulation, to confirm redox reac- Tc in melter or inciner-
sion; Ru solubility and sorp- Tc vaporization
tion in environment; reaction
thermodynamics and kinetics;
reactivity toward natural or-
ganics (e.g., humus)
tions; pH changes
ator
Decontami- Separation of anions
Properties of precipitated Microanalysis methods Electrochemical decon-
nation,
decomis-
sioning
Ru/Tc, efficient decon-
tamination of precipitates
tamination (e.g., elec-
trocleaning)
Recovery,
Development of novel sep- Ru/Tc recovery from un-
Electrochemical recovery
of Ru/Tc; industrial appli-
cation of fission products;
radiation effects; clear-
ance level; shielding/stor-
age; medical applica-
tions of Tc
purification, aration processes based on dissolved residues
utilization
coordination chemistry and
ion exchange
chemical form of Tc is TcO , and K is low. On the
ficient and economical. In this table, the rows are
the fields of chemistry, and the columns are specific
problems to be studied. These items were formulated
by more than thirty members of the committee.
4
d
other hand, in deep disposal, in which reductive con-
ditions may be realized, the chemical form of Tc is
TcO or TcO(OH) , and K is high. More detailed
2
2
d
studies on this problem are necessary.
Also, a number of scientific and technical problems
were formulated, reflecting the novel strategy on the
PUREX system with consideration of peculiar proper-
ties of Ru and Tc among various fission products. The
problems are related to separation of Ru and Tc in
the ionic or solid/colloid state, decontamination condi-
tions, solubility or mobility in repository, and recover-
ing/purification technology for the future utilization.
MAIN RESEARCH ISSUES FOR Ru AND Tc
CHEMISTRY TOWARD ADVANCING
PUREX SYSTEM
Table 2 summarizes the main research issues on the
Ru and Tc chemistry for either PUREX process or its
surrounding processes, aimed to make them more ef-
RADIOCHEMISTRY Vol. 45 No. 3 2003

224
MATSUMOTO et al.
(1) Basic scientific and technological aspects:
Thus, it is necessary to investigate and summarize
the solid/liquid, coordination, and separation chemis-
try of Tc and Ru, and also the related chemical en-
gineering data. These data will also be usfeul for
ensuring safety operation of the Tokai and Rokkasho
Reprocessing Plants. Furthermore, these data are
required for minimizing generation of radioactive
wastes during future decommissioning of the reproc-
essing plants.
identification of Ru and Tc chemical species in
PUREX solutions and in radioactive waste reposi-
tories; complex formation of Ru and Tc with degraded
extractants and diluents.
(2) Plant engineering and environmental aspects:
Tc in the white inclusions, method for efficient dis-
solution of insoluble residues; distribution behavior in
the multicomponent highly radioactive extraction sys-
tems (e.g., dissolver solution, HLLW); Tc synergistic
extraction with coexisting actinides and fission prod-
uct cations; optimization of the system from the view-
point of decontamination from Tc.
REFERENCES
1. Kleykampf, H., in Proc. 4th Int. Conf. on Nuclear Fuel
Reprocessing and Waste Management RECOD’94,
London, 1994.
(3) Common and future aspects: analytical meth-
ods for identifying chemical species, structure, quanti-
ties; recovering/purification methods as applied to
2. Kolarik, Z. and Dressler, P., Solvent Extr. Ion Exch.,
99
transmutation or utilization of long-lived Tc; ac-
1989, vol. 7, no. 4, p. 625.
cumulation of data on physicochemical and radio-
chemical properties.
3. Handbook on Process and Chemistry, JAERI Review,
Department of Fuel Cycle Safety Research, 2002,
no. 2001-038.
Along with advancing the PUREX process, it is
intended to study more extensively the environmental
behavior and utilization of Tc as a long-lived fission
product. In this context, it is quite necessary to gain
profound knowledge on the separation and purifica-
4. Asakura, T. and Uchiyama, G., Redox Reaction of Tc
in Nitric Acid Solution, JAERI Research Report (in
press).
5. Wilson, P.D. and Garraway, J., in Proc. Int. Meet. on
Fuel Reprocessing and Waste Management, La Grange
Park, Illinois (the United States), 1984, vol. 1, p. 467.
99
tion chemistry, materials science to fabricate Tc
target, and nuclear properties.
RADIOCHEMISTRY Vol. 45 No. 3 2003