Identification of a chemical fingerprint linking the undeclared 2017 release of 106Ru to advanced nuclear fuel reprocessing

Article abstract of DOI:10.1073/pnas.2001914117

The undeclared release and subsequent detection of ruthenium-106 (¹⁰⁶Ru) across Europe from late September to early October of 2017 prompted an international effort to ascertain the circumstances of the event. While dispersion modeling, corrobo

Full text of DOI:10.1073/pnas.2001914117

Identification of a chemical fingerprint linking the
106
undeclared 2017 release of Ru to advanced
nuclear fuel reprocessing
Michael W. Cookea,1, Adrian Bottia, Dorian Zokb, Georg Steinhauserb, and Kurt R. Ungara
aRadiation Protection Bureau, Health Canada, Ottawa, ON K1A 1C1, Canada; and bInstitute of Radioecology and Radiation Protection, Leibniz Universität
Hannover, 30419 Hannover, Germany
Edited by Kristin Bowman-James, University of Kansas, Lawrence, KS, and accepted by Editorial Board Member Marcetta Y. Darensbourg May 1, 2020
(received for review February 7, 2020)
The undeclared release and subsequent detection of ruthenium-
106 (106Ru) across Europe from late September to early October of
2017 prompted an international effort to ascertain the circum-
stances of the event. While dispersion modeling, corroborated
by ground deposition measurements, has narrowed possible loca-
tions of origin, there has been a lack of direct empirical evidence to
address the nature of the release. This is due to the absence of
radiological and chemical signatures in the sample matrices, con-
sidering that such signatures encode the history and circumstances
of the radioactive contaminant. In limiting cases such as this, we
herein introduce the use of selected chemical transformations to
elucidate the chemical nature of a radioactive contaminant as part
of a nuclear forensic investigation. Using established ruthenium
polypyridyl chemistry, we have shown that a small percentage
(1.2 0.4%) of the radioactive 106Ru contaminant exists in a poly-
chlorinated Ru(III) form, partly or entirely as β-106RuCl3, while 20%
is both insoluble and chemically inert, consistent with the occur-
rence of RuO2, the thermodynamic endpoint of the volatile RuO4.
Together, these findings present a clear signature for nuclear fuel
reprocessing activity, specifically the reductive trapping of the vol-
atile and highly reactive RuO4, as the origin of the release. Con-
sidering that the previously established 103Ru:106Ru ratio indicates
that the spent fuel was unusually young with respect to typical
reprocessing protocol, it is likely that this exothermic trapping
process proved to be a tipping point for an already turbulent mix-
ture, leading to an abrupt and uncontrolled release.
constitutes the identification of unique signatures. From a radiologi-
cal perspective, there is none. Samples have been shown to be radi-
opure and to carry the stable ruthenium isotopic signature of civilian
spent nuclear fuel (10), while stable elemental analysis by scanning
electron microscopy and neutron activation has revealed no detect-
able anomalies compared to aerosol filter media sampled prior to the
advent of the 106Ru contaminant (1, 11). We are, then, left with the
definition of a limiting case for a nuclear forensic investigation.
Fortunately, we are concerned with an element that has sig-
nificant covalent character to its bonding interactions (12). This
affords the opportunity to perform chemical transformations, with
and without the presence of a stable form of the same element. By
selecting reactions that are well understood and/or by varying
reaction conditions, we can compare the distribution of the ra-
dioactive element to the stable form in the reaction product(s)
(Scheme 1). In this way, it is possible to deduce both general and
highly specific information about the atomic connectivity, that is,
the chemical context, of the radioactive contaminant, given that
the rules for reactivity are well understood.
Results and Discussion
To help refine synthetic targets amenable to the strategy pro-
posed in Scheme 1, we subscribe to the hypothesis that the
Significance
In the fall of 2017, a massive, undeclared release of 106Ru oc-
curred that was detected across Eurasia. To conclusively ad-
dress the nature of the release, we have used carefully selected
and established chemical transformations to reveal a chemical
fingerprint for the 106Ru contaminant that is uniquely consis-
tent with specific methodology employed in the reprocessing
of spent nuclear fuel. In view of international attention and
investigation to date, this chemical fingerprint is the first direct
evidence to this effect. This work serves, by example, as a
potentially valuable addition to the field of nuclear forensics,
considering that it is the only means to extract historical in-
formation from a radiopure contaminant in the absence of
stable elemental anomalies.
ruthenium polypyridyl complex radiochemistry nuclear forensics
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n the fall of 2017, the man-made, high-yield fission product
I
106Ru (half-life, t1/2 = 373.6 d) was detected by monitoring
networks across Europe (1–4), along with sporadic detections of
minute amounts of the relatively short-lived 103Ru (t1/2 = 39.2 d) in
select locations. Although unprecedented in scale (250 TBq) (5),
airborne and surface measurements substantiated a timely as-
sessment that there was no detrimental impact to human health
(6). Nevertheless, the undeclared intrusion of such radioactivity
into the air space and soil of sovereign nations demands in-
vestigation, in support of national and coordinated global security.
To address the location of the 106Ru source, recent reports have
used dispersion modeling and field measurements of 106Ru con-
centration (airborne and ground deposition) to demonstrate an
origin in the Southern Urals of Russia, in the area of the Mayak
industrial complex (1, 5–9). A long history of nuclear-related ac-
tivities in this area, combined with the radiopurity of the field
observations and the detection of the short-lived 103Ru, lends
credence to the scenario of an accidental release during nuclear
fuel reprocessing and serves to dispel some persistent theories on
the origin of the release (e.g., nuclear reactor accident, downed
radioisotope thermoelectric generator satellite, volatilized medical
sources, etc.) (1). However, to make a direct link to fuel reproc-
essing activity on this basis alone is circumstantial and as such
provides room for plausible deniability. Direct evidence to this end
Author contributions: M.W.C. designed research; M.W.C., A.B., and D.Z. performed re-
search; G.S. and K.R.U. contributed new reagents/analytic tools; M.W.C. analyzed data;
M.W.C. wrote the paper; and G.S. and K.R.U. provided samples, personnel, and
technical support.
The authors declare no competing interest.
This article is a PNAS Direct Submission. K.B.-J. is a guest editor invited by the
Editorial Board.
This open access article is distributed under Creative Commons Attribution-NonCommercial-
NoDerivatives License 4.0 (CC BY-NC-ND).
1To whom correspondence may be addressed. Email: michaelw.cooke@canada.ca.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/
doi:10.1073/pnas.2001914117/-/DCSupplemental.
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(30, 31). Among inorganic ruthenium compositions, it is note-
worthy that β-RuCl3 possesses the rare quality of being highly
soluble in dative solvents. Thus, we have chosen to explore the
hypothesis that the passivation of RuO4 in HCl was implicated in
the environmental release.
This hypothesis informs an appropriate selection of chemical
reactions and synthetic targets toward revealing a unique signature,
and other meaningful chemical information, about the 106Ru
contaminant. To this end, polypyridyl chemistry is appealing as a
reactive vehicle to incorporate 106Ru, since such ruthenium com-
plexes are exceptionally robust and formed in high yield under
ambient conditions. Also, characteristic charge-transfer electronic
transitions that lie in the visible region of the spectrum make these
complexes intensely colored, facilitating chromatographic separa-
tion (33, 34). Of the polypyridyl ligands available, those derived
from the tridentate 2,2′:6′,2″-terpyridine ligand are particularly
well-suited as they offer achiral products and superior kinetic sta-
bility (33). Considering the ease of synthesis afforded by one-pot
procedures to form 4′-substituted analogs of 2,2′:6′,2″-terpyridine,
we have elected to synthesize 4′-p-tolyl-2,2′:6′,2″-terpyridine (ttpy;
Fig. 1) according to reported procedures (35, 36), albeit with a
notable exception regarding its purification (Materials and Meth-
ods). With this ligand in hand, we are supported by a plethora of
established ruthenium coordination chemistry, the vast majority of
which is derived from reaction with β-RuCl3. In particular, we have
identified the subsequent monoligated complex, ttpyRuCl3 (37),
and the reduced, bis-ligated complex, Ru(ttpy)22+ (38), as targeted
reaction products for isolation and radiometric measurement, in
adherence to the general strategy presented in Scheme 1. Although
the former has invariably served as a reaction intermediate to the
formation of heteroleptic complexes (39, 40), it takes center stage
in this work. This is due to the fact that the formation of ttpyRuCl3
(or related complexes based upon 2,2′:6′,2″-terpyridine) is expec-
ted to be highly selective to β-RuCl3 and related compounds,
considering that there is no alteration in oxidation state or local
coordination geometry about the ruthenium atom. In fact, to the
best of our knowledge, the formation of ttpyRuCl3 has only ever
been performed from β-RuCl3. Therefore, by investigating the
formation of ttpy106RuCl3 from the relatively small proportion of
the 106Ru contaminant that is extractable in ethanol and by
demonstrating the highly selective nature of the reaction, we can
speak to the existence of β-106RuCl3. In this way, we may de-
termine whether or not passivation of RuO4 in hydrochloric acid
was invoked.
Scheme 1. High-level concept depicting the reactive incorporation of both
stable (green) and radioactive (red) chemical species of an element into
isostructural end products.
release of 106Ru into the environment is associated with fuel
reprocessing activity. Owing to its complex chemistry and high
specific activity (typically 6 to 9% of fission products) in spent
nuclear fuel (12–16), the removal of radio-ruthenium from waste
streams poses a significant challenge. While a myriad of ap-
proaches has been proposed, the vast majority center about the
PUREX (plutonium–uranium extraction) process where the re-
covery of uranium and plutonium is accomplished by liquid ex-
traction with an organophosphate from fuel that is digested in
nitric acid (16–18). To amplify the oxidative power of the mix-
ture, oxidants such as ceric ammonium nitrate can be added to
encourage the evolution of the volatile and highly reactive RuO4,
which can then be driven off with carrier gas and subsequently
trapped or passivated. At this early dissolution stage, and later
during the vitrification of raffinates, the condensation and de-
composition of RuO4 to relatively inert oxides inside the con-
tainment vessels can lead to problems of accumulated dose and
accelerated corrosion (14, 15, 19, 20). To address this, consid-
erable investigative effort has been made to remove ruthenium
at the fuel dissolution phase (17, 21, 22). Nevertheless, the for-
mation of RuO4 is still a very appealing option with respect to
simplicity, cost, and scalability, as evidenced by recent investi-
gations into ruthenium volatilization through RuO4 generation
(23–26). Subsequent chemical reduction of RuO4 to more pas-
sive forms is also appealing with respect to such considerations,
despite introducing additional waste material for management.
Some of the more notable options in this regard are the efficient
formation of RuO4−2 and RuO4− salts under alkaline conditions
(21, 27), the formation of RuO2 on contact with reducing media
(26, 28), and the generation of Ru(III) and Ru(IV) poly-
chlorinated complexes in hydrochloric acid (29–31).
Treatment of filter pieces from both German and Swedish ra-
dionuclide surveillance networks with solvent gave highly re-
producible results for partitioning of the 106Ru contaminant. Good
partitioning reproducibility, in conjunction with autoradiographic
imaging (SI Appendix, Fig. S21), supports a homogenously dis-
Radiochemistry 1: Synthesis and Purification of ttpyRuCl3 in the
Presence of 106Ru. With the tridentate ligand (ttpy) in hand, re-
action of 1.1 equivalents with β-RuCl3 proceeds in high yield
(>80%) from ethanolic solution, provided that the reactant
concentration is >0.01 M. The overall radiochemical reaction is
presented in Scheme 2 (see also SI Appendix, Fig. S16), and
experimental details are provided in Materials and Methods.
Ethanol containing leached 106Ru (74.5 5.3 mBq 106Ru) was
used to synthesize ttpyRuCl3 from stable β-RuCl3 in 88% yield
(0.12 g theoretical yield). The insoluble precipitate formed was
isolated and counted using a high-purity germanium well de-
tector, using the gamma photon emission at 622 keV (9.93%
intensity) associated with the short-lived 106Rh progeny as the
analytical signal. Relative to the ethanol extract, 28.8 3.5% of
the 106Ru activity was found to be localized in the initial pre-
cipitated material. The remaining ethanolic filtrate was chroma-
tographed on silica gel using an eluent mixture of acetonitrile and
saturated, aqueous potassium nitrate solution (7:1, respectively)
to isolate the sole by-product, Ru(ttpy)22+, after work-up with
NH4PF6. This material was similarly gamma-counted and was
tributed contaminant. We have found that 51.3
2.4% of the
106Ru contaminant was associated with the aqueous extract. This
result is in very good agreement with that determined by other
laboratories (1) and clearly indicates that the 106Ru is composed of
more than one chemical form. Partitioning experiments similarly
performed with carbon tetrachloride showed no discernable as-
sociation with the 106Ru contaminant. This indicates an absence of
adsorbed RuO4, which is typically soluble in carbon tetrachloride,
one of the few solvents that does not succumb to attack by this
powerful oxidizing agent (13). This is not surprising, since RuO4
decomposes to RuO2 (hydrated) in water under ambient condi-
tions and further upon exposure to light (15, 32), while reoxidation
of RuO2 back to RuO4 is very slight and reserved for atmospheric
oxidants such as ozone (32). Interestingly, we found that a small
but highly reproducible fraction (7.35
0.70%) of the 106Ru
contaminant was partitioned into ethanol. This result further
supports a multicomponent composition for the 106Ru contami-
nant. In light of the aforementioned discussion regarding the
pragmatic treatment of RuO4 in nuclear fuel reprocessing, found to contain 11.2
2.0% (8.37
1.41 mBq) of the 106Ru
β-RuCl3 is expected to be formed from the passivation of RuO4 in activity from the ethanol extract. In contrast to the fate of the
HCl to an extent that depends directly upon time and temperature stable ruthenium, this is a sizeable proportion of 106Ru. However,
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Fig. 1. Synthetic targets used to evaluate the reactivity and distribution of 106Ru.
this is quite reasonable considering that the 0.1 equivalent excess this range of count time, the concomitant span of detection ca-
of ttpy used in the reaction (i.e., 7.5 mg) constitutes an enormous molar
excess over 106Ru (4.03 × 1012-fold), and that inorganic ruthenium
species other than β-RuCl3 will react to form Ru(ttpy)22+ under these
conditions. Again, we see evidence of a complex, multicomponent
106Ru contaminant.
pabilities, as defined by the critical limit (Lc) and detection limit
(Ld) at 95% confidence, corresponded to Lc = 3.7 mBq and Ld =
7.5 mBq for 1 Ms and Lc = 2.9 mBq and Ld = 5.9 mBq for 2 Ms.
Nevertheless, a very good account of the initial 106Ru in the
crude ttpyRuCl3 was provided by the components of the HPLC
purification process. The elution peak at retention time (Rt) =
2.96 min, corresponding to ttpyRuCl3, was found to contain
10.2 2.6 mBq of 106Ru (Fig. 3), while the fraction collected from
0 to 2.0 min, representing unretained material from the column
hold-up volume, was found to contain 12.7 3.1 mBq of 106Ru
(SI Appendix, Figs. S19 and S20). These quantities represent
48.7 13.2% (ttpyRuCl3) and 60.9 14.8% (unretained) of the
While 28.8 3.5% of the ethanol-extracted 106Ru was found to be
isolated with the stable product, ttpyRuCl3, consideration of poten-
tial flocculation effects precluded the conclusion that ttpy106RuCl3
was formed to any extent. To address this, a high-performance liquid
chromatography (HPLC) method was developed to separate
ttpyRuCl3 from Ru(ttpy)22+, and from other environmental con-
taminants associated with the filter material. This HPLC method was
first developed on an analytical scale and later adapted to a semi-
preparative one, using a C18 stationary phase and a mobile phase
consisting of an isocratic mixture of N,N′-dimethylformamide
(DMF) and methanol (90:10, respectively), optimized at 1.30 mM of
tetrabutylammonium chloride (SI Appendix, Figs. S14 and S15).
Note that method development was confined by the limited solu-
bility of ttpyRuCl3 (appreciably soluble in N,N′-DMF, tolerating no
more than 30% methanol) and that effective separation of ttpyR-
2+
uCl3 and Ru(ttpy)2 was afforded mainly by the addition of the
tetrabutylammonium chloride. The characteristic ligand-to-metal
and metal-to-ligand charge-transfer bands that characterize ttpyR-
uCl3 and Ru(ttpy)22+, respectively, were used to arrive at a detection
wavelength of 450 nm in the electronic spectrum.
The entirety of the crude ttpyRuCl3 isolated in Scheme 2 (108
mg, 20.9
2.5 mBq 106Ru) was purified by HPLC (Fig. 2).
Fractions collected at set times in the HPLC purification process
were combined and concentrated for gamma counting. The very
small amount of 106Ru measured in these reaction components
required long detection count times ranging from 1 to 2 Ms. For
Scheme 2. Reaction targeting the formation of ttpyRuCl3 in the presence of
106Ru. i106Ru obtained from ethanol extraction. iiTwo hours at reflux temper-
ature. iiiPurified by reverse-phase HPLC. ivAdjusted for chemical recovery.
Fig. 2. HPLC purification of ttpyRuCl3 (Rt = 2.96 min) that precipitated from
the radiochemical reaction described in Scheme 2. Monitored at λ = 450 nm.
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106Ru measured in the crude ttpyRuCl3 before purification. The
unretained fraction of 106Ru likely consists of unreactive species
removed by flocculation as ttpyRuCl3 precipitated from the
reaction solution.
Scheme 3. Reaction to form [106Ru(ttpy)2](PF6)2. i106Ru obtained from eth-
anol extraction. iiStable complex added postreaction as a tracer for chro-
matographic isolation. iiiEthanol, 90 °C, 16 h.
Radiochemistry 2: Reaction of 106Ru without Stable Ru Precursor. To
provide additional insight and validation, particularly in light of
the relatively large counting uncertainty associated with the
gamma measurements of the preceding radiochemical experi-
ment (Scheme 2), an analogous experiment was performed
without the use of stable ruthenium precursor. In this instance,
the formation of the reduced, bis-ligated complex [106Ru(ttpy)2]2+
was targeted (Scheme 3 and SI Appendix, Fig. S18). The reac-
tion to form [Ru(ttpy)2]2+ from 2.1 equivalents of ttpy in ethanol
alone was found to be more broadly applicable than previously
believed, proceeding smoothly from both Ru(III) and Ru(IV)
precursors containing suitable counter anions, as demonstrated by
chemical recoveries obtained with β-RuCl3 (89%), (NH4)2RuCl6
(94%), and Ru(NO)(NO3)3 (90%). Thus, we can use an excess of
ttpy to compare the reactive fractions of the 106Ru contaminant
(extractable in ethanol) obtained from both carrier and carrier-
free approaches (Table 1).
Here, a mere 0.20 mg of ttpy amounted to an extraordinary
molar excess (1.7 × 1010-fold) relative to 106Ru. Following ter-
mination of the reaction, stable [Ru(ttpy)2](PF6)2 was added as a
tracer to enable chromatographic purification. Gamma counting
of the isolated fraction revealed that 28.6 2.6% (0.137 0.013
Bq) of the 106Ru from the ethanol extract was colocated with the
added [Ru(ttpy)2](PF6)2. A spectral overlay of the gamma-
counted reaction components is presented in Fig. 3. From Ta-
ble 1, summing contributions of the isolated components from
the β-RuCl3 radiochemical reaction (Scheme 2, ttpyRuCl3 and
Ru(ttpy)22+), we find a total reactive fraction of 27.2 9.0%.
This result is in excellent agreement with the reactive fraction
found for the corresponding carrier-free reaction (Scheme 3).
Such an intersection of results strengthens further the identifi-
cation and contribution of ttpy106RuCl3 and additionally serves
to provide a more accurate uncertainty estimate considering the
higher activity (i.e., lower counting uncertainty) afforded by the
carrier-free reaction (Fig. 3).
Formation of ttpyRuCl3: Reaction Selectivity. While the formation of
ttpyRuCl3 has only ever been performed from β-RuCl3 (37, 40),
we undoubtedly expect the reaction to proceed to an appreciable
extent from higher-order polychlorinated Ru(III) compounds,
given the favorable binding interaction afforded by the tridentate
ligand, ttpy, and considering that their solution equilibria in
dative solvent will include the formation of RuCl3 (41, 42).
Nevertheless, by defining the applicable scope of this reaction,
we refine and gain confidence in the types of contaminant ru-
thenium species that could lead to the observed formation of
ttpy106RuCl3. Instead of testing an exhaustive selection, several
compounds were chosen that reflect a systematic variation from
soluble polychlorinated Ru(III) species, namely, variation with
respect to ruthenium valency, exchangeable supporting ligands,
and physical format. To this end, we had selected the poly-
chlorinated Ru(IV) salt, (NH4)2RuCl6, the Ru(III) mixture,
Ru(NO)(NO3)3 (in nitric acid), and the highly insoluble allo-
trope to β-RuCl3, α-RuCl3. In nitric acid solution, what is
denoted commercially as Ru(NO)(NO3)3 is actually a mixture of
ruthenium (III/IV) complexes varying in proportion of co-
ordinated and exchangeable nitrate, nitrite, and water molecules,
depending on solution conditions (16, 43); moreover, it is di-
rectly representative of the nitric acid-based oxidative mixtures
Fig. 3. Overlay of gamma spectra, in the analytical region of interest (600 to 640 keV) for 106Ru measured in the reaction components leading to the
formation and isolation of ttpyRuCl3 (A) and [106Ru(ttpy)2](PF6)2 (B). Depicted are gamma emission peaks corresponding to 214Bi (609 keV, naturally occurring)
and 106Ru (616 and 622 keV). (A) Ethanol extract (red); crude ttpyRuCl3 (blue); purified ttpyRuCl3 (green). (B) Filter piece (red); ethanol extract (blue); isolated
fraction colocated with added [Ru(ttpy)2](PF6)2 (green).
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Table 1. Allocation of 106Ru in the radiochemical reactions relative to the initial quantity measured in the
ethanol extract
Reaction
ttpyRuCl3 (% 106Ru, k = 1) [Ru(ttpy)2](PF6)2 (% 106Ru, k = 1) Reactive fraction of 106Ru, %
ttpy + β-RuCl3
+
106Ru
14.0 3.9* (16.0 4.4)†
—
11.2 2.0
28.6 2.6
27.2 9.0
28.6 2.6
ttpy + 106Ru
*Isolated ttpyRuCl3 fraction from HPLC (Rt = 2.96 min).
†Adjusted for chemical recovery.
used to dissolve spent fuel early in the PUREX process (16, 17).
Subjecting these model compounds to the reaction conditions used
to synthesize ttpyRuCl3 (Materials and Methods), with the notable
exception of using saturated ethanolic and aqueous ethanolic so-
lutions of potassium chloride in the case of Ru(NO)(NO3)3, pro-
duced no discernable trace of ttpyRuCl3 (Table 2). These tests
demonstrate that the reactive tolerance is limited to polychlorinated
ruthenium (III) compositions. Therefore, we can conclude that the
isolated ttpy106RuCl3 in these experiments originated from 106Ru
filter contaminant that exists as β-106RuCl3 or, more generally,
By investigating the reactive nature of the bulk of the 106Ru
contaminant, we may reveal additional information to help dis-
cern between these scenarios. To do this, we have selected
commercially available ruthenium compounds representative of
the types encountered in such reprocessing activities (Table 2).
These compounds comprise a diverse array, differentiated by
ruthenium valency, supporting ligand, and physical format, and
we can use them to characterize an appropriate reaction (i.e.,
establish the rules of reactivity). Then, by applying the reactive
conditions to the 106Ru contaminant and tracking its subsequent
fate and distribution, we may make reasonable deductions about
the bulk chemical composition of the 106Ru contaminant and
examine how these align with the two plausible scenarios.
Reaction to form the homoleptic complex [Ru(ttpy)2](PF6)2
(Fig. 1) was used as the reactive vehicle. Forceful reaction con-
ditions by way of elevated temperature (154 °C), extended du-
ration (16 h), and provision of dechlorinating agent (AgNO3)
were selected to encourage reaction completion (Materials and
Methods). Interestingly, to date, such reactions have only been
performed using Ru (III) (typically β-RuCl3) or specially pre-
pared Ru (II) halogenated compounds (47, 48). Only one liter-
ature reference was found pertaining to solvent reduction of a
Ru (IV) compound, and this was carried out under microwave
irradiation, unrelated to polypyridyl complexation (49). It is
therefore quite interesting that this reaction system was found to
be very effective for both Ru(III) and Ru(IV) precursors, to the
extent that a potent oxidizing agent used in organic synthesis,
KRuO4 (50, 51), reacted appreciably while the highly refractory
allotrope of RuCl3, α-RuCl3, was found to react smoothly and in
essentially quantitative yield (Table 2). Such marked general
reactivity using these conditions underscores the complete ab-
sence of reactivity observed in the case of RuO2.
[
106RuCln(H2O)6-n 3-n. Considering the sequential fractionation of
]
106Ru in the described experiments, from filter material to ethanolic
extract (7.35 0.70%) to precipitated ttpyRuCl3 (28.8 3.5% with
88% chemical recovery) to HPLC isolated ttpyRuCl3 (48.7
13.2%), we determine that 1.17 0.36% of the filter contaminant
exists in this chemical form.
Assessment of the Bulk 106Ru Contaminant. That a small proportion
of the 106Ru contaminant is composed of polychlorinated
106Ru(III) species is direct evidence that fuel reprocessing was
the origin of the 2017 environmental release. Plausible reproc-
essing activities that could lead to such compounds are limited to
either the reductive trapping of oxidatively generated RuO4 in
hydrochloric acid (29–31) or the electrochemical reduction and
metallization of uranium in spent fuel from molten alkali chlo-
ride mixtures (44–46). Fortunately, the compositions formed
from both approaches differ substantially. An interrupted pro-
cess involving the reductive trapping of RuO4 in hydrochloric
acid would reasonably be expected to contain some measure of
the inert RuO2, owing to the decomposition of RuO4, along with
a mixture of Ru(IV) and Ru(III) chloro complexes, the pro-
portion of which will depend upon temperature, duration, and
HCl concentration but should favor heavily the reduction to
Ru(IV) (30, 31). Mixed nitrosyl–nitrate–nitrite Ru(III/IV) com-
plexes derived from an oxidative nitric acid slurry used to generate
the RuO4 may also become entrained upon release, depending on
the system engineered. In the case of the pyroprocessing of spent
fuel from molten alkali chloride mixtures, chloro complexes are
exclusively formed and, although dependent on melt temperature
and duration, center upon the formation of Ru(III) chloro com-
plexes with some disproportionation/decomposition to metallic
ruthenium occurring very slowly at temperatures >550 °C (44).
Radiochemical details are provided in Materials and Methods.
For all reactions, we have coupled chromatography and radiog-
raphy to demonstrate that 106Ru has been incorporated into the
isolated complex (i.e., [Ru(ttpy)2](PF6)2 + [106Ru(ttpy)2](PF6)2).
Here, we have used thin-layer chromatography (TLC) (normal
phase, preparative scale) to demonstrate the purity of the iso-
lated [Ru(ttpy)2](PF6)2 by elution with an aqueous acetonitrile
solution of high ionic strength. The decay progeny, 106Rh [t1/2
=
30 s, β(mean) = 1,410 keV] is a hard beta-particle emitter, making
it well-suited to autoradiographic imaging. Prolonged exposure
of the phosphor imaging plate (6 wk) yields darkened areas that
Table 2. Chemical yields of [Ru(ttpy)2](PF6)2 and ttpyRuCl3 from representative inorganic
ruthenium compounds
Reactant
Ru ox. state
Solid-state form
[Ru(ttpy)2](PF6)2, % yield
ttpyRuCl3, % yield
Ru(NO)(NO3)3*
(NH4)2RuCl6
β-RuCl3
α-RuCl3
KRuO4
3+
4+
3+
3+
7+
4+
Molecular
Molecular
Polymeric
Polymeric
Molecular
Polymeric
92
96
94
95
19
No reaction†
No reaction
88
No reaction
No reaction
No reaction
RuO2
No reaction
*Mixture in nitric acid. ox., oxidation.
†Reacted in presence of large excess of potassium chloride.
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correlate to the [Ru(ttpy)2](PF6)2 isolated from reactions in-
volving 106Ru-contaminated filter pieces and their aqueous ex-
tracts (SI Appendix, Fig. S22). As with all radiochemical
experiments described herein, every effort to provide a full account
with other empirical evidence pertaining to radiopurity and age
estimation (i.e., time since removal from irradiation) for spent
nuclear fuel obtained from measurement of the 103Ru:106Ru ratio
(1). An age estimate of ∼2 y is considerably less than the usual
of the initially measured 106Ru activity has been made by measuring time (≥3 y) allotted before reprocessing under typical reactor
all subsequently isolated reaction products, by-products, and other
relevant materials. In this way, the reactive and unreactive portions
of the 106Ru contaminant can be independently determined, where
106Ru localized with [Ru(ttpy)2](PF6)2 constitutes the reactive por-
tion and 106Ru measured in other materials and by-products (AgCl)
constitutes the unreactive portion. For all reactions, a reasonably
full account has been provided in consideration of the respective
measurement uncertainty carried by each experimental component
(General) and of the potential loss of material given the consider-
able number of mass transfer steps involved (Radiochemistry).
operating conditions (52). Therefore, a RuO4 trapping process in
HCl would undoubtedly be exothermic, exacerbating an already
energetic mixture that, in light of the high volatility and potential
explosive decomposition of RuO4 (15), gives credence to the oc-
currence of an abrupt, uncontrolled release (i.e., explosion). Such
conditions would certainly aid in the volatilization and dispersion
of otherwise nonvolatile ruthenium species. For a facility un-
dertaking the purification of fission products from spent nuclear
fuel for commercial gain, it is hard to ignore the potential profit
afforded by obtaining 106Ru in high-specific activity, considering
its long-standing medical use in the development of brachytherapy
plaques for the treatment of eye cancer (53, 54). For this appli-
cation, it is commonly electrodeposited from solution onto silver,
along with the addition of carrier RuCl3 (54, 55). Since this re-
quires a highly soluble form of 106Ru, it makes sense to invoke a
reductive technique that generates polychlorinated Ru(III/IV)
species with a high degree of radiopurity and aqueous solubility,
rather than the vast majority of passivation techniques that ulti-
mately produce the highly insoluble RuO2.
Direct Reaction with a Contaminated Filter Piece. Direct reaction of
a portion (0.144 g, 7.99 0.53 Bq) of the 106Ru-contaminated
filter piece in the presence of β-RuCl3 resulted in 79.6 4.9%
(6.36 0.39 Bq) of the 106Ru being localized with the isolated
[Ru(ttpy)2](PF6)2 after gamma counting. The balance of the
activity (unreactive portion) was reasonably well accounted for,
with 8.62 0.68% (0.689 0.054 Bq) and 7.77 0.60% (0.460
0.035 Bq) measured in the remaining filter piece and the in-
soluble AgCl by-product, respectively.
In closing, the detection of 106Ru in aerosol filters across
European surveillance networks in 2017 represents a limiting
case for forensic investigation, considering the high radiopurity
of the contaminant and the absence of detectable signatures or
anomalies from colocated stable elements. However, as this work
has demonstrated, new opportunities arise when the radioiso-
tope pertains to an element capable of covalent bond formation.
By subjecting the radioactive contaminant to iterative, well-
characterized chemical transformations and determining its
subsequent fate and distribution, we create an inferential process
from which we can gain both generalized and highly specific in-
formation about the chemical form(s) of the radioactive contam-
inant. This work constitutes direct evidence for specific nuclear
fuel reprocessing activity and, coupled with other measurements
and atmospheric dispersion modeling, provides irrefutable proof
as to the origin of the 2017 environmental release of 106Ru.
Moreover, this work serves, by example, as a potentially valuable
addition to the established suite of nuclear forensic capabilities.
Reactions Subsequent to Aqueous Extraction of a Filter Piece. To gain
a bit more resolution, a similar reaction was performed on the aqueous
extract of another filter piece. The 106Ru that partitioned into the
water extract (51.3 2.4%, 6.14 0.46 Bq) was found to be almost
completely localized in the isolated [Ru(ttpy)2](PF6)2 (95.3 6.0%,
5.85 0.37 Bq) with the balance of activity reasonably accounted
for in the AgCl precipitate (6.06 0.41%, 0.372 0.025 Bq).
The filter piece was found to retain 27.8 1.9% (3.33 0.22
Bq) of the 106Ru contaminant after aqueous extraction. The
balance of 106Ru activity (∼20.9%) was most likely lost in
transfer to the submicron filtration material which could not be
accommodated in the well-detector space for measurement
(Gamma Spectrometry). Direct reaction of the water-washed fil-
ter piece resulted in 75.2 4.9% (2.50 0.16 Bq) of the activity
being localized in the isolated [Ru(ttpy)2](PF6)2, with the bal-
ance reasonably accounted for in the AgCl by-product (16.2
1.1%, 0.540 0.038 Bq) and the remaining filter piece (9.06
0.71%, 0.302 0.024 Bq). Assuming that the activity remaining
on the filter piece is representative of the entirety of 106Ru that
did not partition into the aqueous phase (i.e., 48.7 2.4%) and
summing reactive and unreactive contributions, we find that
85.6 8.1% and 15.2 1.4% of the initial 106Ru contaminant to be
reactive and unreactive, respectively. These results are in reasonably
good agreement with those obtained by direct reaction on a 106Ru-
contaminated filter piece. Clearly, one or more of the water-
insoluble components is chemically inert to the reaction conditions
employed. According to Table 2, chemical inertness and a high
degree of water insolubility are certainly consistent with the oc-
currence of RuO2. Against the previously mentioned compositional
characteristics for both trapping and pyroprocessing scenarios, the
delineation of a small Ru(III) polychlorinated component (1.17
0.36%) and a substantial water-insoluble, reactively inert compo-
nent (∼20%) consistent with RuO2 support an origin from the re-
ductive trapping of RuO4 in HCl. Such activity further explains the
high radiopurity of radio-ruthenium observed in environmental
samples (1), since volatilization of RuO4 from waste streams stands
alone as the means to provide optimal separation efficiency (14).
Materials and Methods
General. Particulate filters containing 106Ru from the radiological monitoring
networks of Germany (German Meterological Services Deutscher Wetter-
dienst) and Sweden (Swedish Defense Research Agency Totalförsvarets for-
skninginstitut) were graciously donated. Chemical experimentation was
conducted on portions of a particulate filter sample obtained from Vienna,
Austria, and was composed of polypropylene (air collection from 2017-09-28
to 2017-10-04 at a rate of 675 m3/h).
Chromatographic supports consisting of alumina (neutral, type WN-6,
super grade, flash chromatography), silica (high-purity grade, 220 to 440
mesh, flash chromatography), and preparative TLC plates (glass-backed, 2.0-
mm SiO2 layer) were purchased from Sigma-Aldrich, Ltd. Columns (C18
,
XBridge: 3.5 μm, 250 × 4.6 mm and 5 μm, 250 × 10 mm) were purchased from
Waters, Ltd. for analytical and preparative-scale HPLC separations, re-
spectively. All aqueous and ethanolic extracts of contaminated filter pieces
were filtered through conditioned syringe filters (0.1-μm pore size; Fisher
Scientific) prior to gamma counting and subsequent reaction.
Solvents and reagents were used as received. These include N,N′-dime-
thylformamide (Fisher Chemical, ACS grade), acetonitrile (Fisher Chemical,
ACS grade), methanol (Fisher Chemical, Optima grade), dichloromethane
(Fisher Chemical, ACS grade), diethyl ether (Fisher Chemical, ACS grade),
potassium nitrate (Sigma-Aldrich, ≥ 99%), silver nitrate (Sigma-Aldrich, ≥
99%), ammonium hexafluorophosphate (Acros Organics, 99%), 2-acetyl
pyridine (Sigma-Aldrich, 99%), p-tolualdehyde (Sigma-Aldrich, 97%),
β-ruthenium chloride hydrate (Sigma-Aldrich, reagent plus), ammonium
hexachlororuthenate (Strem Chemicals, 99%), ruthenium (III) nitrosyl nitrate
solution (Sigma-Aldrich, 1.5% Ru in 6.8 wt % nitric acid), ruthenium (IV)
Concluding Remarks
We have revealed compositional markers for the 106Ru contami-
nant that are uniquely consistent with nuclear waste reprocessing,
namely the reductive trapping of RuO4 in HCl. This finding aligns
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Cooke et al.

oxide (Sigma-Aldrich, 99.9%), potassium perruthenate (Sigma-Aldrich), α-ruthenium
chloride (Merck), and tetrabutylammonium chloride (Sigma-Aldrich, 98%).
Experimental uncertainty was derived by combining contributions from
counting statistics, counting efficiency, and mass transfer (if applicable) in
quadrature.
Yield (α-RuCl3) = 0.162 g (95%); yield ((NH4)2RuCl6) = 0.163 g (96%); yield
(Ru(NO3)3(NO)) = 0.156 g (92%); yield (KRuO4) = 0.032 g (19%); yield (RuO2) =
no reaction.
Gamma Spectrometry. The gamma detection system used was a small anode
germanium (SAGe) well detector (GSW275L; Mirion Technologies) outfitted
with a cosmic veto (plastic scintillator) detector to reduce the background (CV
System-LM; Mirion Technologies). The SAGe well detector has a diameter of
28.00 mm and a depth of 40.00 mm with an active volume for the germa-
nium crystal of 65.50 mm (thickness) by 86.6 mm (diameter). The resolution
of the detector (full width at half maximum) is 1.835 keV at 1,332.5 keV. The
gamma acquisition software used was Genie 2000 v3.4.1 (Mirion Technolo-
gies), with the counting efficiencies of the samples in the well of the SAGe
simulated using LabSOCS v4.4.1 (Mirion Technologies). The analysis software
used in quantifying the activity of 106Ru was a peak-fitting and peak iden-
tification program called UniSAMPO (v 2.67)-Shaman (v 1.2), developed by
Baryon Oy of Finland. The HPGe well detector was calibrated for energy and
shape resolution using a National Institute of Standards and Technology–
traceable standard (SRS 112559; Eckert & Ziegler) prior to measurement.
Data were collected from 0 to 2,800 keV for 16,384 channels. Samples were
counted in 20-mL glass scintillation vials (61 × 28 mm, outer dimensions).
Detector count times varied greatly depending on the amount of 106Ru in
the sample and ranged anywhere from several thousand to several million
seconds, ideally until an acceptable counting uncertainty was attained
(<10%). Analysis was based upon the 622-keV gamma emission (9.93%
abundance) associated with the 106Ru progeny, 106Rh, while decay correc-
tions were performed using t1/2 = 371.8 d for 106Ru. For the experiment
incorporating 106Ru into ttpyRuCl3, the purified reaction components were
gamma counted until the critical limit (Lc) was exceeded and the peak was
automatically identified by the peak search algorithm of the analysis soft-
ware (UniSAMPO-Shaman, Baryon Oy, Finland). For instance, detection
count times ranging from 1 to 2 Ms corresponded to detection capabilities,
as defined by the Lc and detection limit (Ld) at 95% confidence, of Lc = 3.7
mBq, Ld = 7.5 mBq and Lc = 2.9 mBq, Ld = 5.9 mBq, respectively.
Synthesis and Characterization.
Synthesis of 4′-p-tolyl-2,2′; 6′,2″-terpyridine (ttpy). Synthesis of this ligand re-
sembled previously reported procedures (35, 36); however, these procedures
were found to yield unsatisfactory purity considering the application. In our
hands, 2-acetylpyridine (7.83 g, 0.065 mol and KOH aqueous solution (5 mL,
15 wt %) were stirred briefly in methanol (60 mL) at room temperature (∼5
min). p-tolualdehyde (3.56 g, 0.03 mol) and concentrated ammonium hy-
droxide (25 mL) were then added and the mixture heated to reflux with
vigorous stirring for 48 h. After cooling, the reaction mixture was decanted
into a large separatory funnel. To this was added 600 mL of water and
600 mL of dichloromethane. After agitation, the dichloromethane layer was
removed, washed once more with 600 mL water, then separated and dried
over sodium sulfate. The dichloromethane was removed by distillation and
the remaining residue was recrystallized from 95:5 ethanol/water to yield
2.45 g of slightly impure material. High purity was achieved by flash chro-
matography using 688 g of alumina, previously deactivated by thorough
mixing with water (5% by mass) and using toluene as the mobile phase.
Yield = 1.91 g (20%); 1H NMR (600 MHz, d6-DMSO) δ ppm: 2.40 (s, 3H, Htolyl),
7.40 (d, J = 8.0 Hz, 2H, Htolyl 3,5), 7.53 (dd, J = 7.5 Hz, J = 5.0 Hz, 2H, H5,5′′),
7.83 (d, J =8.0 Hz, 2H, Htolyl 2,6), 8.04 (t, J = 7.5 Hz, 2H, H4,4″), 8.67 (d, J = 8.0
Hz, 2H, H3,3′′), 8.70 (s, 2H, H3′,5′), 8.76 (d, J = 5.0 Hz, 2H, H6,6″). ESI(+)MS found
(calcd) for C22H17N3H: 324.1511 (324.1501).
Synthesis of ttpyRuCl3. This compound was prepared according to literature
procedure. Note that reactant concentration should be maintained ≥0.01 M
to attain high yield. Note also that the ethanol extract from a filter piece
contaminated with 106Ru, after filtration through a submicron filter, would
be used in place of ethanol alone, according to the following typical prep-
aration. The ligand ttpy (0.0825 g, 2.55 × 10−4 mol) and RuCl3 hydrate
(0.0607 g, 2.32 × 10−4 mol) were combined in 20 mL of ethanol (95%) and
heated to reflux with agitation for 2 h. After cooling, the insoluble solid was
isolated by filtration over a glass filter frit then agitated 5 to 10 min in
ethanol (40 mL) and filtered. The residue was then rinsed with an additional
portion of ethanol (40 mL) and finally with diethyl ether (40 mL). The solid
was dried under vacuum. Yield = 0.123 g (88%). ESI(+)MS found (cald) from
DMF solution for C44H34N6RuCl3Na: 554.9445 (554.9416).
NMR and Mass Spectrometry. The 1H NMR spectra were acquired on a Bruker
AVANCE III 600 MHz spectrometer by the University of Ottawa NMR Facility.
Chemical shifts are reported relative to Me4Si as an internal reference. Mass
spectra were obtained using a Micromass Q-TOF II Electrospray Ionization
Mass Spectrometer by the John L. Holmes mass spectrometry facility at the
University of Ottawa.
Synthesis of [Ru(ttpy)2](PF6)2 from β-RuCl3.
Procedure (A). This compound was prepared similarly to literature proce-
dures. Typically, the ligand ttpy (0.111 g, 3.42 × 10−4 mol, 2.1 equivalents),
RuCl3 hydrate (0.0426 g, 1.63 × 10−4 mol), and AgNO3 (4.89 × 10−4 mol) were
combined in N,N′-DMF (reagent grade, 50 mL) and heated to reflux for 16 h.
The reaction mixture was then cooled to room temperature, and the AgCl
by-product was removed by vacuum filtration over a glass filter frit. The
filtrate was then distilled to dryness and the residue chromatographed
on silica using an acetonitrile/saturated, aqueous potassium nitrate (7:1)
mixture as mobile phase. The isolated compound was transferred to a
separatory funnel, followed by addition of water, NH4PF6, and enough
dichloromethane to render a phase separation. After washing, the aqueous
layer was removed and discarded. This process was repeated twice more,
after which the organic phase was collected and distilled to dryness. The
residue was then redissolved in acetonitrile and precipitated from water.
This precipitate was collected by vacuum filtration, redissolved in acetoni-
trile, and precipitated from diethyl ether. This final precipitate was collected
by filtration and dried under vacuum. Yield = 0.160 g (94%); 1H NMR (600
MHz, CD3CN) δ ppm: 2.54 (s, 3H, Htolyl), 7.18 (dd, J = 8.0 Hz, J = 6.0 Hz, 2H,
HPLC. HPLC was performed using a Dionex ICS-6000 instrument equipped
with photodiode array detector (PDA-1; Thermo Scientific), autosampler
(AS-AP; Thermo Scientific), and fraction collector (ASX-280-FC; Thermo Sci-
entific) and employing a C18 column (3.5 μm, 250 × 4.6 mm and 5 μm, 250 ×
10 mm) from Waters, Ltd. Chromeleon 7 (Thermo Scientific) was the soft-
ware package used for instrument control and analysis. All injections and
subsequent runs were monitored at both 254- and 450-nm wavelength.
Radiochemistry. Transformation of β-RuCl3 to form either ttpyRuCl3 or
[Ru(ttpy)2](PF6)2 in the presence of 106Ru adhered to the respective protocols
outlined above in Synthesis and Characterization. Typically, a filter piece was
shaved with a razor into many thin pieces, and these were placed inside a
scintillation vial for gamma counting prior to reaction. Once transferred to
the reaction vessel, the empty scintillation vial was gamma-counted once
more to ensure the efficacy of transfer.
Reactions with β-RuCl3. For reaction to form ttpyRuCl3 (SI Appendix, Fig. S16),
the shaved filter pieces (1.03
0.08 Bq 106Ru) were immersed in 5 mL of
ethanol and agitated in an ultrasound bath for 10 to 20 min, after which the
ethanol was removed via syringe and filtered. The process was repeated
twice more, and the ethanol fractions were combined and gamma-counted
(0.074 0.005 Bq 106Ru). The ethanol washings were then transferred to a
reaction vessel, along with an additional 5 mL of ethanol (rinse). The re-
action protocol described herein for ttpyRuCl3 (Synthesis and Characteriza-
tion) was carried out, giving the target complex in 88% yield. This material
was then gamma-counted, as were the subsequent components from its
HPLC purification.
H5,5′′), 7.42 (d, J = 6.0 Hz, 2H, H6,6″), 7.58 (d, J = 8.0 Hz, 2H, Htolyl 3,5), 7.94
(t, J = 8.0 Hz, 2H, H4,4″), 8.11 (d, J = 8.0 Hz, 2H, Htolyl 2,6), 8.64 (d, J = 8.0 Hz,
2H, H3,3′′), 8.99 (s, 2H, H3′,5′). ESI(+)MS found (calcd) for C44H34N6Ru (M2+):
373.9200 (374.0962).
Procedure (B). Alternatively, the ligand ttpy (0.111 g, 3.42 × 10−4 mol, 2.1
equivalents) and RuCl3 hydrate (0.0426 g, 1.63 × 10−4 mol) were combined in
ethanol (95%, 50 mL) and heated to reflux for 16 h. Distillation of the
ethanol, followed by chromatographic isolation and work-up as outlined
above in Procedure (A) gave the target complex in 89% yield (0.151 g).
Notably, the same molar quantities and conditions applied to (NH4)2RuCl6
and Ru(NO)(NO3)3 gave comparable yields (94 and 90%, respectively) for the
formation of [Ru(ttpy)2](PF6)2.
For direct reaction of a filter piece to form [Ru(ttpy)2](PF6)2 (SI Appendix,
Fig. S17), the gamma-counted filter shavings (7.99
0.53 Bq 106Ru) were
transferred to a reaction vessel followed by the addition of 10 mL of water.
After brief agitation in an ultrasound bath, the reaction protocol outlined
herein for [Ru(ttpy)2](PF6)2 was carried out (Synthesis and Characterization),
giving a comparable yield for the final product (93%). All components
Synthesis of [Ru(ttpy)2](PF6)2 from α-RuCl3, (NH4)2RuCl6, Ru(NO3)3(NO), KRuO4, and
RuO2 in N,N′-DMF. These compounds were reacted, and subsequently isolated,
according to the scale and conditions outlined in Procedure (A) for β-RuCl3.
Cooke et al.
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(i.e., [Ru(ttpy)2](PF6)2, AgCl by-product, and the remaining filter pieces) were
subsequently gamma-counted.
screw cap was secured. The reaction solution was agitated while immersed
in an oil bath set to 90 °C for 16 h (SI Appendix, Fig. S18). After cooling to
room temperature, the stir bar was removed and the reaction solution
transferred to a distillation flask, followed by 20 mg of [Ru(ttpy)2](PF6)2
(stable Ru). The solvent was removed by distillation, and the mixture was
chromatographed on silica gel and worked up as previously described above
in Synthesis and Characterization. The isolated complex was quantitatively
recovered and gamma-counted.
For reaction to form [Ru(ttpy)2](PF6)2 from the aqueous extract (SI Ap-
pendix, Fig. S17), the shaved filter pieces (11.97 0.89 Bq 106Ru) were im-
mersed in 5 mL of water and agitated for 10 to 20 min in an ultrasound bath,
after which the water was separated via syringe and filtered. The process
was repeated twice more, and the aqueous fractions were combined and
gamma-counted. The aqueous extract was then transferred to a reaction
vessel, followed by an additional 5 mL of water (rinse). Complexation was
carried out according to the procedure described herein for [Ru(ttpy)2](PF6)2
(Synthesis and Characterization), with the exception that a higher-proportion
of DMF was used (60 mL). The target complex, [Ru(ttpy)2](PF6)2, was isolated in
95% yield. All components (i.e., [Ru(ttpy)2](PF6)2, AgCl by-product, and the
remaining filter pieces) were subsequently gamma-counted. The remaining,
washed filter pieces were reacted separately in DMF to give the isolated
[Ru(ttpy)2](PF6)2 in 94% yield, which was then gamma-counted along with the
other reaction components.
Autoradiography. Autoradiography of chromatographed reaction products
(SI Appendix, Figs. S21 and S22), isolated in the presence of 106Ru, was
performed on a BAS-5000 Image Analysis System (Fujifilm). Imaging plates
(BAS-SR2025; GE Healthcare) were developed in closed imaging cassettes
(EXPSR CASS. 20 × 25 cm; GE Healthcare) and erased (zeroed) using an IP
Eraser 3 (Fujufilm). Images were acquired using BASReader software (Fuji-
film) and processed using Multi Gauge v.3.1 (Fujifilm).
Carrier-free reaction. Shaved filter pieces (0.48 0.03 Bq 106Ru) were immersed
in 5 mL of ethanol and agitated in an ultrasound bath for 10 to 20 min, after
which the ethanol was removed via syringe and filtered. The process was
repeated twice more, and the ethanol fractions were combined and gamma-
counted. The ethanol washings were concentrated by vacuum distillation
then transferred with rinse solutions to a 3-mL conical reaction vessel. The
ligand (0.20 mg, 6.18 × 10−7 mol) was added from a stock solution in ethanol
to give a final reaction volume of 0.5 mL. A stir bar was added and a Teflon
Data Availability. All relevant data and protocols (synthetic and radiochem-
ical) are provided in the paper and SI Appendix.
ACKNOWLEDGMENTS. M.W.C. and K.R.U. thank Dr. Andreas Bollhöfer (Bun-
desamt für Strahlenschutz) and Dr. Johan Kastlander (Totalförsvarets for-
skninginstitut) for provision of filter samples. D.Z. and G.S. acknowledge
financial support from VolkswagenStiftung.
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