We obtained six samples from a single fuel rod from the Belgian BR3 reactor, a PWR reactor which operated between 1962 and 1987. The samples are from fuel rod 316, which contained low enriched uranium oxide fuel and operated in core 4A/4B of the BR3 reactor between 1976 and 1980 with an average burnup of 39 GWd/tU. The entire fuel rod was transferred to Idaho National Laboratory, where it was sampled for different experimental campaigns as previously described11. Samples in this study include six different locations that were sub-sampled along the fuel rod axis, as noted in Table 2. Dilutions of the six fuel samples were shipped to Los Alamos National Laboratory for analysis. The distance from the spade end reflects the distance between the end of the rod and the location where the sample was obtained, where the rod is approximately 1 meter long. Previous characterization of BR3 fuel rods indicates that the fuel burnup increases near the center of the rod11,12,13.
Purification of Ru
To purify Ru, we used a combination of cation exchange chromatography, anion exchange chromatography, and extraction chromatography. First, the sample was passed through a cation exchange resin (Bio-Rad AG 50W × 8) in dilute HCl (0.5 M). Samples were evaporated to dryness and then dissolved in 4 mL of 0.5 M HCl prior to column chromatography. A Kontes Flex Column (0.7 cm inner diameter, 20 cm long) was filled to 19 cm with AG 50W × 8 resin (100–200 mesh) and washed with 8 M HNO3 (20 ml), 6 M HCl (2 ml), and 0.5 M HCl (50 ml). The sample was loaded onto the column and immediately collected. Under these conditions, the actinides and many other major matrix elements adhere to the resin while Ru has low affinity. The column was washed three times with 0.5 M HCl (2 ml) and the eluent collected (Table S1).
This initial cation exchange column was followed by an anion exchange (Bio-Rad AG1\(\times\)8) column14. A Bio-Rad Polyprep Column (10 ml capacity) was filled with 0.5 ml of the anion exchange resin AG 1\(\times\)8 (100–200). The resin was washed with 8 M HNO3 (2\(\times\)10 ml) and H2O (10 ml) and conditioned with 4 ml of 1 M HCl with 10% bromine water added (freshly prepared as 1 ml bromine-saturated water per 10 ml 1 M HCl). To the eluents from the cation exchange column, concentrated HCl (0.45 ml) and saturated bromine water (1.2 ml) were added to increase the molarity to 1 M HCl and 10% bromine water. This solution was loaded immediately onto the anion exchange column. The column was washed with 0.5 M HCl (5 ml) followed by 0.0012 M HCl (10 ml) to remove a number of potentially interfering elements. Ruthenium is subsequently eluted with concentrated HNO3 (12 ml). The solution was evaporated gently to reduce the volume to 1-2 drops but not allowed to completely evaporate. In the case that the solution goes to dryness, samples were refluxed with concentrated HNO3 before once again gently reducing the volume (Table S2).
Finally, the sample is dissolved in 2% (v/v) HNO3 and passed through an extraction chromatography resin (Eichrom Ln resin) to remove any remaining Mo from the sample prior to analysis by inductively coupled plasma mass spectrometry (ICP-MS). The Eichrom Ln resin utilizes extraction with di(2-ethylhexyl)orthophosphoric acid (HDEHP), which has previously been shown to bind Mo and have a low affinity for Ru in dilute nitric acid15,16. A Bio-Rad polyprep column (10 ml capacity) was filled with 1 ml of Eichrom Ln resin (100–150 \(\upmu\)m). The resin was washed with 2 M HF (2\(\times\)8 ml), then conditioned with 2% (v/v) HNO3 (10 ml). The sample from the anion column was diluted to a volume of 2 ml with 2% (v/v) HNO3 and loaded onto the column (Table S3). The column was washed with 2% (v/v) HNO3 (2\(\times\)0.5 ml). All eluents were collected directly in a Falcon 15 ml conical centrifuge tube for MC-ICP-MS analysis. Recoveries of Ru are typically ~ 50%, but variable (Table S4). The primary matrix of the fuel rod samples purified for Ru in this study was relatively simple compared with that of an environmental sample. Therefore, the use of an additional DEAE column (Biorad weak anion exchange resin) to separate remaining Pd from Ru was not required as there is no natural Pd present in the sample17.
Isotope dilution Ru measurements
To determine the concentration of Ru by isotope dilution, an aliquot of each fuel sample (ca. 20 ng Ru) was combined with an isotopically enriched \(^{101}\)Ru tracer (ca. 5 ng) and slowly evaporated to dryness by heating on a hotplate at 100 \(^{\circ }\)C. The samples were purified as described above. Following purification, the Ru isotopic ratios were measured by MC-ICP-MS. The concentration of the Ru in the samples was calculated from the measured \(^{102}\)Ru/\(^{101}\)Ru ratios in the traced samples, the measured \(^{102}\)Ru/\(^{101}\)Ru ratios in the untraced fuel samples, and the \(^{102}\)Ru/\(^{101}\)Ru ratio measured in the \(^{101}\)Ru tracer solution.
To confirm the accuracy of the method, test samples were prepared and analyzed that contained a mixture of natural U (National Bureau of Standards (NBS) standard reference material, SRM U960) and known concentrations of Ru (obtained from a certified Spex CertiPrep ICP-MS Ru standard and a certified Alfa Aesar ICP-MS Ru standard). The ratio of U to Ru in each of the test samples was chosen to approximate the values in the BR3 fuel samples, with ~ 20 ng of natural Ru and 10,000 ng of natural U per sample. Each test sample was spiked with the \(^{101}\)Ru tracer solution, purified for Ru, and the total amount of Ru in the sample determined by isotope dilution ID-ICP-MS.
Measurement of Ru by MC-ICP-MS
Analysis of Ru isotopes was performed using a Thermo Scientific Neptune Plus MC-ICP-MS equipped with nine Faraday cups. Sample introduction into the MC-ICP-MS was done in dry plasma mode with a Cetac Aridus 3 desolvating nebulizer with a 100 \(\mu\)L/min flow rate nebulizer. The instrument was outfitted with a jet-style skimmer and X-style sample cone. A static Faraday routine was used to measure the samples as follows \(^{97}\)Mo+ (L3), \(^{98}\)Ru+ (L2), \(^{99}\)Ru+ (L1) \(^{100}\)Ru+ (C), \(^{101}\)Ru+ (H1), \(^{102}\)Ru+ (H2), \(^{104}\)Ru+ (H3), \(^{105}\)Pd+ (H4) with 1\(\times\)10\(^{11}\) ohm amplifiers assigned to each Faraday cup (see Table 1). Each measurement is the average of 20 cycles with an 8 second integration. Mass dependent fractionation was corrected using standard-sample bracketing with a natural Ru ICP-MS standard, Alfa Aesar ruthenium. Previously reported values for the natural Ru isotopic composition were used for the isotope ratios of the standard18.
There are direct isobaric interferences from Mo and Pd on \(^{100}\)Ru, \(^{102}\)Ru, and \(^{104}\)Ru, respectively. Fission product decay does not result in the formation of \(^{104}\)Pd as this decay is blocked by \(^{104}\)Ru. Therefore, Pd is not an isobaric interference on \(^{104}\)Ru in this suite of samples. Environmental samples, however, could contain natural Pd and \(^{105}\)Pd is monitored to ensure that there is no significant interference present. Also monitored is \(^{97}\)Mo, however this interference cannot be corrected as we are unable to assume a natural abundance of Mo to perform a correction. If \(^{97}\)Mo voltage is significant, the Ln column is repeated (for decontamination factors for Mo, Zn and Pd, see Tables S5 and S6). BR3 samples in this study recorded voltages on \(^{97}\)Mo of < 0.1 mV and \(^{97}\)Mo/\(^{101}\)Ru of < 0.0002. Not reported in this study is \(^{99}\)Ru as there is an isobaric interference on \(^{99}\)Ru from \(^{99}\)Tc that cannot be accounted for. The average voltages recorded during measurement of the six BR3 fuel samples are included in Table 1.
Plutonium purification
Plutonium concentrations were measured by isotope dilution mass spectrometry using a certified \(^{242}\)Pu tracer. The \(^{242}\)Pu tracer was purchased from NIST as SRM 4334G Plutonium-242 Radioactivity Standard. An aliquot of each sample containing ca. 1 ng \(^{239}\)Pu was combined with \(^{242}\)Pu tracer (ca. 600 pg). The sample was then purified according to a previously published procedure involving pre-concentration by LaF3 co-precipitation, followed by purification via anion exchange chromatography19. After the purification steps, the sample was evaporated, dissolved in 2% (v/v) HNO3 and the plutonium was analyzed by ICP-MS. A second untraced aliquot was purified in parallel and used to measure the isotopic composition of the plutonium (\(^{239,240,241,242}\)Pu isotopes) by ICP-MS. Both the traced and untraced samples were measured using a Thermo Fisher Scientific X-series II ICP-QMS with an Apex Omega introduction system. Instrument performance and hydride formation were monitored and corrected using SRM 948. Mass bias was surveyed and corrected using CRM 128. To measure \(^{238}\)Pu, another aliquot of the sample was purified in a similar fashion. The purified plutonium fraction was electroplated onto a stainless steel planchette and \(^{238}\)Pu and \(^{239+240}\)Pu were determined by alpha spectrometry. Alpha spectra were recorded using an Ortec Ensemble 8-channel alpha spectrometer equipped with a 19 keV FWHM 300 \(\hbox {mm}^{2}\) ULTRA-AS Si-detector.
Uranium purification
The \(^{233}\)U tracer was an internal solution standardized by reverse isotope dilution against a gravimetric preparation of the U metal standard NIST SRM U960. An aliquot of the sample containing ~ 80 ng total U was combined with \(^{233}\)U tracer (ca. 30 ng). The sample was evaporated, redissolved in 5 ml 3 M HNO3, and then loaded onto a column containing 2 ml of Eichrom UTEVA resin, conditioned with 3 M HNO3 (5 ml). The column was washed with 3 M HNO3 (5 ml), 9 M HCl (5 ml), and 5 M HCl-0.05 M oxalic acid (3\(\times\)5 ml). The U fraction was eluted with 1 M HCl (3\(\times\)5 mL) into a 60 ml savillex Teflon jar. The sample was evaporated, the final purified fraction was redissolved in 2% (v/v) HNO3, and the U was analyzed by ICP-MS. The samples were measured using a Thermo Fisher Scientific X-series II ICP-QMS introduced with the standard spray chamber. Instrument performance and hydride formation were monitored and corrected using SRM U-050. Mass bias was surveyed and corrected using IRMM 74/1.