Sorption of gaseous RuF5 on granulated fluoride sorbents

Article abstract of DOI:10.1134/S1066362211030106

Published data on sorption decontamination from RuF₅ of UF ₆ formed in the course of gas-fluoride reprocessing of spent nuclear fuel are summarized. Sorption of individual RuF₅ on granulated fluoride sorbents, NaF (at 100

Full text of DOI:10.1134/S1066362211030106

ISSN 1066-3622, Radiochemistry, 2011, Vol. 53, No. 3, pp. 288–291. © Pleiades Publishing, Inc., 2011.
Original Russian Text © M.B. Seregin, A.A. Mikhalichenko, A.Yu. Kuznetsov, G.S. Sokovin, A.M. Chekmarev, 2011, published in Radiokhimiya, 2011,
Vol. 53, No. 3, pp. 246–248.
Sorption of Gaseous RuF5 on Granulated Fluoride Sorbents
M. B. Seregina, A. A. Mikhalichenkoa, A. Yu. Kuznetsov*a, G. S. Sokovinb, and A. M. Chekmarevb
a Leading Research Institute of Chemical Technology, Public Joint-Stock Company,
Kashirskoe sh. 33, Moscow, 115409 Russia; * e-mail: A_Quznetsov@vniiht.ru
b Mendeleev Russian University of Chemical Engineering, Moscow, Russia
Received April 26, 2010
Abstract—Published data on sorption decontamination from RuF5 of UF6 formed in the course of gas-fluoride
reprocessing of spent nuclear fuel are summarized. Sorption of individual RuF5 on granulated fluoride sorbents,
NaF (at 100 and 400°С) and MgF2 (at 125°С), is studied. The capacities of the granulated fluoride sorbents for
RuF5 at these temperatures are determined. Sorption decontamination of UF6 from RuF5 on granulated fluoride
sorbents is studied. The presence of UF6 in the gas does not affect significantly the sorption capacity of sodium
and magnesium fluorides for RuF5.
Keywords: ruthenium pentafluoride, uranium hexafluoride, purification, fluoride sorbents
DOI: 10.1134/S1066362211030106
UF6 decontamination from volatile fluorides of other
fission products (FPs), including RuF5.
In the development of gas-fluoride technology for
spent nuclear fuel (SNF) regeneration, it is important
to find efficient methods for UF6 decontamination
from volatile fluorides of fission products. This paper
deals with sorption decontamination of UF6 from
RuF5.
Trials of fluoride reprocessing of uranium–zir-
conium fuel elements were performed in ORNL (USA)
[4, 5]. The fuel elements were dissolved, using anhy-
drous HF, in a NaF–ZrF4 melt at 600°С. After the dis-
solution, the melt was treated with fluorine at the same
temperature. The released UF6 and excess fluorine
were delivered to twofold purification of UF6 by its
sorption on NaF at 100°С and desorption at 400°С.
The attained decontamination factor in sorption treat-
ment of UF6 to remove RuF5 was 1 × 103.
RuF5 is one of the most radioactive fission prod-
ucts. High degree of its removal from UF6 is an impor-
tant condition for attaining the required purity of UF6.
RuF5 belongs to moderately volatile fission product
fluorides. It has the following properties: mp 86.5°С,
bp 272°С, vapor pressure logP [atm] = 6.66 – 3329/Т
(Т = 363–433 K). Numerous studies on UF6 decon-
tamination from RuF5 have been performed in the
United States and Russia.
Later the implementation of the sorption purifica-
tion was improved by using an apparatus with a mov-
ing sorbent bend [6]. The parameters of UF6 decon-
tamination from RuF5 were considerably improved:
The decontamination factor reached 1 × 103 in the dis-
solution step, 1 × 105 in the sorption–desorption step,
and ~1 × 108 in total.
As shown by Hepworth et al. [1], RuF5 forms with
alkali and alkaline-earth metal fluorides the following
complex salts: LiRuF6, NaRuF6, KRuF6, CsRuF6,
Ca(RuF6)2, Sr(RuF6)2, and Ba(RuF6)2. Detailed data on
synthesis of these complex salts are given in [2].
To remove RuF5 from UF6, a two-step treatment
scheme with different fluoride sorbents was used: first
on NaF at 400°С and then on MgF2 at 120°С. The
RuF5 content was decreased from 250 to 2 ppm [7]. In
tests with barium, calcium, and magnesium, the decon-
tamination factor of UF6 from RuF5 was 374, 55, and
9, respectively.
According to [3], in interaction of RuF5 with alkali
and alkaline-earth metal fluorides at 300°С for 1 h, the
capacity of the fluorides was as follows, mg g–1: NaF
49.6, LiF 10.6, BaF2 9.3, and MgF2 7.7. The best re-
sults were obtained with NaF, but they are insufficient
for the efficient absorption of RuF5.
A number of trials were performed for processes of
Trials on reprocessing of fuel elements of uranium–
288

SORPTION OF GASEOUS RuF5
289
zirconium and uranium–aluminum alloys stored for a
period from 7 months to 5 years were performed at the
Argonne National Laboratory (ANL) in the United
States and in Fontenay-aux-Roses in France [8–10].
The UF6 purification was performed using NaF gran-
ules (400°C), with the subsequent sorption of UF6 on
NaF at 150°С, its desorption at 400°С, and desublima-
tion of the purified UF6. The total degree of decon-
tamination of UF6 from RuF5 was 5 × 103–2 × 104.
After arrangement of an additional sorption column
with MgF2 (T = 125°C), in operation with fuel ele-
ments stored for 3 months, the total degree of decon-
tamination of UF6 from RuF5 was 3 × 105–5 × 105,
which is also insufficient for preparing high-purity
UF6.
200°С). Single and double sorption (100°С)–desorption
(400°С) of UF6 on NaF in a flow of a mixture of nitrogen
(80 vol %) and fluorine (20 vol %) was performed.
The column packed with NaF (d = 60 mm, h =
340 mm) ensured at 400°С selective sorption of 2–7%
of the initial amount of RuF5, and the column with
BaF2 granules, from 3.4 × 10–2 to 22.4% (11.2% on the
average) of the initial amount of RuF5.
The UF6 decontamination appeared to be the most
efficient on NaF granules in the sorption (100°С)–de-
sorption (400°С) cycle. Single sorption–desorption
cycle ensured decontamination from volatile FP fluo-
rides with a factor of 4.2 × 102–6.1 × 102, and double
sorption (100°С)–desorption (400°С) of UF6 ensured
decontamination from volatile FP fluorides, including
RuF5, with a factor of 2 × 104. The total degree of de-
contamination of UF6 from the sum of FPs under opti-
mal conditions was 5 × 107, and from RuF5, 1 × 108
[12]. In separate operations, the following parameters
of decontamination from RuF5 were obtained [12]:
82.7% of Ru remained in the fluorination residue,
14.6% of Ru was trapped on the filter; 0.4%, in the
precondenser; 0.3%, in the recondensation unit; 1.4 ×
10–2%, in the first step of sorption (100°С)–desorption
(400°С) of UF6 on NaF; and 5.5 × 10–4%, in the sec-
ond step.
This problem was solved at the Leading Research
Institute of Chemical Technology by using other refin-
ing methods for UF6 purification. Trials were per-
formed with samples of fuel rods of a BOR-60 reactor
15–30 mm long and 6 mm in diameter, containing ura-
nium 90% enriched in 235U. The burn-up was 4.9 and
9%. The specific activity of the fuel was 0.4–0.6 Ci g–1
[11, 12].
After removing the stainless steel cladding by dis-
solution in molten zinc, the major fraction of fuel was
in the form of pieces 1–3 mm in diameter, which were
charged into a vertical fluorination reactor.
RuF5 in an amount of 5.3 × 10–6% was detected in
the column with NaF, intended for the collection of the
purified UF6. The RuF5 breakthrough was accounted
for [12] by the fact that RuF5 occurred in the form of
aerosol particles migrating through voids in the gran-
ule bed at a small sorbent bed height.
Fluorination was performed first at 400–450°С for
75–110 min with feeding a mixture of fluorine and nitro-
gen (1 : 1 by volume) into the reactor and then
at 500°С for 15–20 min with feeding pure fluorine. The
flow rate of the fluorine-containing mixture was 1 l h–1.
Four alternative flowsheets were tested [11, 12].
The major fraction of moderately volatile FP fluorides
(RuF5, NbF5, ZrF4) was removed from UF6 in the steps
of fluorination, precondensation of FP fluorides, and
filtration of gases. The use of fluoride sorbents ensures
fine decontamination of UF6 from RuF5 and other
volatile FP fluorides.
Thus, in reprocessing of irradiated oxide fuel, to
ensure high degree of decontamination of UF6 from the
sum of FPs (108 and above), it is very important to use
the refining potentialities of all the process operations.
Despite high level of development of the gas-
fluoride technology with SNF at the Leading Research
Institute of Chemical Technology [11–13] and Re-
search Institute of Nuclear Reactors [14], the effi-
ciency of using base fluoride sorbents in the steps of
UF6 decontamination from volatile FP fluorides, in-
cluding RuF5, is still poorly understood, which affects
the choice of an optimal scheme for sorption purifica-
tion of UF6. Data on the capacity of fluoride sorbents
for main volatile fission product fluorides and on
the effect exerted by prevalent amounts of UF6 on the
efficiency of the sorption purification are lacking.
On the whole, the undissolved residue contained
from 36 to 82.7% of the initial amount of Ru (59% on
the average), from 14.6 to 47.9% (31.6% on the aver-
age) was separated with the dust on filters, and up to
0.4% of the initial amount of Ru was recovered in the
precondenser.
The following sorbents were tested in the step of
sorption decontamination of UF6 from volatile FP fluo-
rides: NaF granules (400°C) and BaF2 granules (60–
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290
SEREGIN et al.
Experiments on RuF5 sorption at 400°С were per-
formed as follows: The horizontal fluorinator was
charged with the required amount of the Ru metal
powder, and the fluorination with feeding the required
volume of fluorine was performed at 400°С. After the
fluorination completion, the gas mixture in the reactor
was diluted with argon to attain the total gas pressure
of 760 mm Hg. The gas composition was as follows
(mm Hg): RuF5 12.5, F2 343.7, and Ar 403.8.
Fig. 1. Sorption of RuF5 on NaF granules at 400°С.
A mixture of this composition was fed to a pre-
evacuated horizontal reactor charged with the calcu-
lated amount of NaF granules. The amount of the
sorbed RuF5 was determined by chemical analysis.
The mixture was fed in portions. After feeding each
portion, the gas was kept for 15 min at 400°С, with
monitoring the decrease in the gas pressure. A total of
10 gas portions were fed, each with 15-min keeping,
until the RuF5 pressure fully ceased to decrease. The
results are shown in Fig. 1.
Fig. 2. Sorption of RuF5 in the presence of UF6 on NaF
granules at 400°С.
With each portion fed, the capacity of NaF granules
for RuF5 increased, reaching at 400°С the limiting
value of 0.174 g RuF5/g NaF.
This study deals with selective sorption of RuF5
from a mixture with UF6 using various fluoride sor-
bents. As fluoride sorbents we chose granulated NaF in
the mode of sorption of volatile FP fluorides at 400°C
and in the cycle of UF6 sorption (100°C) and desorp-
tion (400°C), and also granulated MgF2 (125°C).
These sorbents are the most extensively tested and of-
ten recommended in papers published in Russia and
other countries.
The sorption of RuF5 on NaF at 100°С was studied
by successively feeding portions of the gas mixture
obtained in the fluorinator at 400°С. Before feeding
into the sorption reactor, the gas was preliminarily
cooled to 100°С. As a result of repeated feeding of gas
mixture portions over a period of 2.5 h, the limiting
capacity of the NaF granules for RuF5 at 100°С
reached 0.0052 g RuF5/g NaF.
Sorption of RuF5 on MgF2 granules was studied
similarly to sorption on NaF at 100°С. The capacity of
MgF2 for RuF5, determined by chemical analysis, was
0.0023 g RuF5/g MgF2.
The sorption of RuF5 in the presence of UF6 on NaF
and MgF2 was performed from a gas mixture of the
following composition (mm Hg): RuF5 12.5, UF6
250.0, F2 343.7, Ar 153.8. The sorption conditions
were similar to those described above. The sorption
time after feeding each portion to the sorbents being
tested was 15 min, and the total sorption time was
2.5 h. The amount of the sorbed RuF5 was determined
by chemical analysis.
The granulated sorbents synthesized at the Leading
Research Institute of Chemical Technology by the col-
loid-chemical procedure had the following characteris-
tics. NaF: cylindrical granules, diameter 7, height
5 mm; relative porosity 60%; crushing strength
25 kg cm–2; specific surface area 2.4 m2 g–1. MgF2:
cylindrical granules, diameter 3.5, height 3.5 mm; rela-
tive porosity 41%; crushing strength 200 kg cm–2; spe-
cific surface area 13 m2 g–1.
The laboratory installation on which the experi-
ments on RuF5 sorption were performed consisted of
the following main units: cylinders with fluorine and
argon, a horizontal fluorinator for synthesis of RuF5,
and a reactor for studying the RuF5 sorption.
One of our goals was determination of the individ-
ual capacities of NaF and MgF2 for RuF5. Experiments
were performed at a RuF5 pressure of 12.5 mm Hg,
which corresponded to its theoretical content in gases
at complete fluorination of Ru present in SNF.
The results of studies on the RuF5 sorption on NaF
granules at 400°С from a mixture with UF6 are shown
in Fig. 2.
The capacity of NaF granules for RuF5 in 2.5 h
reached 0.158 g RuF5/g NaF, which was somewhat
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SORPTION OF GASEOUS RuF5
291
2. Zvyagintsev, O.E. et al., Khimiya ruteniya (Ruthenium
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In the sorption of RuF5 in the presence of UF6 on
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the value obtained in individual sorption of RuF5 on
MgF2 (0.0023 g RuF5/g MgF2). On MgF2 (125°C), the
UF6 sorption was also observed in an amount of 0.015 g
UF6/g MgF2, in agreement with published data [15].
Thus, the presence of UF6 in the gases does not af-
fect significantly the sorption capacity of NaF and
MgF2 for RuF5, compared to individual sorption of
RuF5.
Our results show that the most efficient way to de-
contaminate UF6 from RuF5 is the sorption of RuF5 on
NaF granules at 400°С. In so doing, the sorbent capac-
ity reaches 0.174 g RuF5/g NaF. The other operations
of sorption purification can be used as additional op-
erations for more exhaustive removal of RuF5 from
UF6.
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ACKNOWLEDGMENTS
13. Shatalov, V.V., Seregin, M.B., Kharin, V.F., and Po-
nomarev, L.A., At. Energ., 2001, vol. 90, no. 3,
pp. 212–222.
The study was supported by the Federal Agency for
Education, State Contract no. P-1581 of September 10,
2009.
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