Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2024 Nov 14;35(12):3233–3241. doi: 10.1021/jasms.4c00371

Actinide Elemental Ratios of Spent Nuclear Fuel Samples by Resonance Ionization Mass Spectrometry

Manuel Raiwa 1,*, Michael R Savina 1, Autumn G Roberts 1, Danielle Z Shulaker 1, Brett H Isselhardt 1
PMCID: PMC11622380  PMID: 39539108

Abstract

graphic file with name js4c00371_0009.jpg

While resonance ionization mass spectrometry (RIMS) has demonstrated utility in measuring isotopic compositions of elements in complex matrices without the need for chemical separation to remove isobaric interferences, it has had limited application in measuring elemental compositions. The ability to determine elemental compositions via an in situ method like RIMS would be an exceptional asset in spent nuclear fuel analysis, where they are important in assessing reactor histories and whose chemical separation presents a radiological hazard. However, quantitative elemental analysis by RIMS requires special considerations because each element is ionized by its own set of lasers tuned to element specific resonant ionization wavelengths. We present the first comprehensive study of measuring elemental ratios by RIMS in spent nuclear fuel. All actinides produced by neutron capture are enhanced significantly radially from the center to the edge of a fuel pellet. This edge effect is not readily accessible by conventional bulk measurements.

Introduction

Mass spectrometry is commonly used to measure the isotopic and elemental composition of a sample of interest. The specific analytical method that is used depends on the sample’s properties, such as its state of matter, sample size, concentration of the analyte in question within the sample, and possible interferences in the mass spectrum. In the case of hazardous radiological materials such as spent nuclear fuel, additional safety aspects regarding material handling and analysis need to be addressed. Therefore, it is desirable to analyze the smallest sample required to obtain meaningful isotopic and elemental information to minimize radiological hazard risk.

Bulk analysis of spent nuclear fuel is typically carried out by dissolving a sample of the solid material in a hot cell and subsequent inductively coupled plasma mass spectrometry (ICP-MS) and alpha spectrometry.1,2 This method is well established; with additional chemical pretreatment to separate elements it produces isobar-free results for most isotopes. However, it usually requires milligrams of sample material and is inherently destructive, thereby losing much spatial information or variability contained in the original solid.3 For example, due to the “edge effect” (sometimes also called rim effect or skin effect in the literature), the Pu concentration is several-fold higher within a few hundred micrometers of the edge of a spent fuel pellet compared to the center, which tends to be representative of the bulk concentration.4,5 In bulk analysis, the edge effect cannot be observed as it provides an average over the entire sample.

To probe heterogeneity on the micrometer scale, direct spatially resolved measurements are performed by secondary ion mass spectrometry (SIMS) or laser ablation (LA) ICP-MS. The former is capable of measuring samples with a spatial resolution better than 1 μm but is limited to isotopes free of elemental isobars.6 LA-ICP-MS can be equipped with a collision gas cell which can resolve some isobaric interferences but usually produces laser pits of at least 10 μm size.7 In the case of spent nuclear fuel, the atmospheric ablation of the sample requires extensive shielding of the source region and like SIMS is currently restricted to measurements of isotopes without isobaric interferences.8

Isobar-free analysis of small solid samples is best performed by resonance ionization mass spectrometry (RIMS, or sometimes called resonant laser secondary neutral mass spectrometry: rL-SNMS). While this method is not yet commercially available, it has been established in multiple fields including cosmochemistry, environmental studies, and nuclear forensics.9−11 The unique laser ionization method of RIMS increases the sensitivity and enables the detection of isotopes down to the femtogram or ppt level.12,13 While these studies show great sensitivity and selectivity, they have been limited to measuring isotopic ratios within one element.

Analysis of isotopes between elements presents many challenges in all mass spectrometry methods because the different chemical properties of each element need to be considered. In SIMS, matrix effects and differences in ionization potentials can lead to ionization efficiencies differing by several orders of magnitude between elements.5,14 To correct for this, matrix-matched standards are used to determine relative sensitivity factors (RSFs). These are generally not available for spent nuclear fuel, however the data can be corrected by model calculations.5 In RIMS the ionization process is independent from the desorption process and is thereby less dependent on matrix effects, although a matrix-matched standard is still required. Nevertheless, elemental ratio measurements by RIMS have not been widely reported. Anderson et al. use laser ablation coupled with RIMS to measure the Sr/Rb elemental ratio in terrestrial rocks and meteorites for geochronology applications.15,16 However, their laser ablation method is destructive, creating analytical spots ∼80 μm in diameter, which is much larger than SIMS and in the case of analyzing spent nuclear fuel the instrument can become contaminated after only several analyses.

Here we present the measurement of multiple elemental ratios in spent nuclear fuel by RIMS without isobaric interferences and with spatial resolution on the order of a micrometer, comparable to SIMS. Several micrometer-sized samples of spent nuclear fuel were analyzed for U/Np, U/Pu, U/Am, and Pu/Am ratios. The observed spatial distribution of elements within the fuel pellet are discussed regarding neutron capture differences. The ability to directly measure the interelement ratio of the isobaric decay pair 241Am/241Pu allows for age calculation of spent fuel without chemical separation.

Methods

RIMS Instrument

Measurements were performed on the Laser Ionization of Neutrals (LION) instrument at Lawrence Livermore National Laboratory, which has been described in detail previously.17 In short the time-of-flight mass spectrometer is equipped with a Ga+ ion gun that is used to sputter material from solid samples with a repetition rate of 1500 Hz. All measurements in this work were performed with a 15 keV Ga+ beam focused to a 2–3 μm spot. The dc current was ∼1.3 nA for sputter cleaning from which 100 to 250 ns long primary ion packages are cut for analysis depending on overall signal intensity. Secondary ions are rejected with a bias of +6000 V, and the remaining neutral atoms are irradiated by pulsed lasers that are tuned to atomic transition frequencies of the elements of interest (e.g., U, Np, Pu, Am). Photons are resonantly absorbed, resulting in stepwise excitation and ultimately ionization (Figure 1). Atoms from other elements do not possess energy levels resonant with the lasers and thus remain largely unaffected.

Figure 1.

Figure 1

Open in a new tab

Laser wavelengths used in this work to stepwise excite atoms close to or over the ionization potential (green line). The resonant ionization schemes are described in detail in the literature.18−21

Figure 1 shows the resonance ionization schemes used for this work. These schemes were used in pairs to determine elemental ratios (e.g., U + Pu, Pu + Am). The LION lasers are home-built grating-tuned Ti:sapphire lasers pumped by commercial Photonics Industries DM50–527 Nd:YLF lasers pulsed at 1500 Hz. Frequency doubling was performed by lithium triborate (LBO) crystals where needed. All beams were focused to 1.0–1.5 mm (FWHM) in the laser-atom interaction region above the sample. Spatial overlap of the lasers was confirmed by diverting the beams to a beam profiler while laser pulse timing and wavelength were continuously monitored and stabilized. Laser powers of each excitation step were chosen depending on the saturation power of the atomic transition. A more detailed description including saturation behavior is given elsewhere.18−21

After neutral atoms were resonantly ionized, the ions were accelerated into a time-of-flight mass analyzer. To enable simultaneous measurements of isotopes with the same mass (e.g., 241Am and 241Pu), one of the ionization schemes was delayed by ∼300 ns. Thus, even though the isobars have the same time-of-flight to the detector, the delayed time of birth shifts the arrival time. This results in a peak offset equal to 1.5 m/z and the ability to resolve both isobaric peaks.22 A detailed timing scheme of the analysis cycle is given in the Supporting Information (SI-Figure 1).

For isotopes with strong isobaric interferences, like 238Pu overlapping with the overabundant 238U in nuclear fuel, a background measurement is needed. Because even though the selectivity of stepwise excitation makes nonresonant ionization of atoms from another element unlikely, backgrounds can arise from photofragmentation of molecules into ions; for example, oxide molecules into metal and oxygen ions.23 To quantify the background signal on trace components like 238Pu the laser wavelength of the first excitation step is shifted periodically by 0.1 nm from the atomic resonance during the measurement. This results in the loss of resonant ions while the background signal remains unaffected.12 The off-resonant background spectrum is then subtracted from the on-resonant and only the resonant excess is evaluated further.

Samples

In total 14 spent UO2 nuclear fuel samples were analyzed. Seven were from the Belgium pressurized water reactor No. 3 (BR3), which were the subject of several past RIMS isotopic studies.22−24 These 10 μm-sized cubes were cut with a focused ion beam (FIB) from spent fuel pellets with an initial enrichment of 8.25% 235U and Pt-welded to Cu combs.25 Four cubes originate from a pellet with an average burnup of 40.7 GWd/tU (designated sample set A) and three from a pellet with 48.3 GWd/tU (designated sample set B).

Seven additional samples were taken from the Advanced Test Material 109 (ATM-109, designated sample set C), Quad Cities reactor Illinois, USA (QC-1). This Zr-clad fuel had a lower initial enrichment of 3% 235U and was doped with 2 wt % Gd2O3. Initial analysis performed by Wolf et al. determined a high burnup of 62 GWd/tU.1,26 Seven samples with length scales of 1 to 5 μm were cut using a FIB and welded to a Mo comb, similar to the BR3 samples. Figure 2 shows an ATM-109 cube attached to the tip of a Mo tine.

Figure 2.

Figure 2

Open in a new tab

Secondary electron image of one of the spent nuclear fuel samples from the ATM-109 sample set. The cube (circled) is welded with Pt to the tip of a tine of a Mo comb.

The cube positions relative to the fuel pellets’ edge were recorded at the time of the sample preparation via FIB for both the ATM-109 and BR3 samples. Table 1 lists the samples analyzed in this study, in which the letter designates the fuel pellet, and the number represents the position of the sample relative to the fuel pellets’ edge in micrometers. For example, sample B-100 is a cube cut 100 μm from the edge of the BR3 fuel pellet with average burnup of 48.3 GWd/tU.

Table 1. Samples Analyzed in This Worka.

Fuel origin Sample designation
BR3 A-5 A-40 A-100 A-4000 B-30 B-100 B-4000
ATM-109 C-20 C-60 C-120 C-400 C-800 C-1600 C-3200
Open in a new tab
a

Sample numbers indicate the sampling distance in μm from the pellet edge.

Bulk analysis by ICP-MS was previously performed on substantial portions of the fuel pellets from which the samples were prepared for this study.26−28 The bulk analysis data provides isotope concentrations for several elements, which we take as representative of the average of the entire pellet.

Relative Sensitivity Factor

Each resonant ionization scheme has its own useful yield (UY), defined as the ratio of atoms consumed to atoms detected. Because RIMS operates on ground state neutral atoms, the useful yield depends on the efficiency of photoionization and also on the relative sputtering yields of ground state neutral atoms, excited state atoms, ions, and molecules.29 Because no two elements are likely to have the same useful yield, a sample with known elemental concentrations is measured first and used to define the relative sensitivity factor (RSF):

graphic file with name js4c00371_m001.jpg 1

Despite recent advances in the production of homogeneous reference materials for RIMS, there is a lack of solid certified material with known concentrations of multiple actinides.30 Furthermore, it is known from SIMS studies, that the sputter yield of secondary ions is dependent on a variety of parameters including the sample matrix.14,31 In contrast to secondary ions, the ionization process in RIMS occurs outside the sample and can therefore be considered almost independent from matrix effects.31 Nevertheless, it remains important to choose a reference material with a matrix comparable to the sample’s matrix as the elements of interest can also be sputtered as molecules. This is especially important if the sample matrix is an oxide as in the case of UO2 spent fuel. The sputtering yield for ground state U neutral atoms is far lower from a UO2 matrix than from U metal, as most of the U is sputtered as oxide molecules.17

Given the lack of a suitable certified reference material, we used the BR3 cube from the center of the lower burnup pellet (Sample A-4000) as our reference material and assumed its composition to be equal to the bulk ICP-MS analysis result. This is justifiable as most of the disturbances to the elemental composition are confined to within several hundred micrometers of the edge of the spent fuel pellet. This edge effect is well studied for isobar-free isotopes by SIMS and LA-ICP-MS.4,5,32 Because the volume of this outermost region represents less than 10% of the centimeter-sized spent fuel pellet, we expect a cube from the pellet center to have an elemental composition comparable to the bulk value.

Relative Ionization Probability

In addition to the unequal ionization rates between elements, it is important to assess the ionization efficiency of each isotope within one element. This laser bias depends on two different effects. First, the isotopic shift of the energy levels involved in the laser excitation and their overlap with the laser wavelength. For equal excitation of all isotopes the laser linewidth must be much larger than the isotopic shift. This is generally not the case for actinide studies at LION as their isotopic shifts and the frequency-doubled lasers linewidths are in the range of several GHz. Second, odd isotopes with nuclear spin undergo hyperfine splitting of their energy levels. This creates differences in the excitation efficiencies based on the electronic transition selection rules compared to even isotopes without hyperfine splitting. This odd–even effect in resonance ionization has been well documented and studied.33,34

Both aforementioned effects are dependent on laser parameters like precise irradiance in the extraction volume and laser linewidth, which can differ slightly between analytical sessions. To account for laser-induced isotopic fractionation, an additional reference sample containing several actinides of known isotopic composition was measured at the beginning and end of each day. The deviations from the known compositions were calculated and sample measurements were scaled accordingly.

Measurement procedure: After the laser bias is corrected the RSF measurement is performed on the sample A-4000. Here multiple isotopic pairs between the two elements with bulk data available are considered (e.g., 239,240,242Pu/235,238U) and individual RSFs are calculated according to eq 1. An average RSF from the individual RSFs is then calculated as the weighted average with weighted standard deviation to account for the different counting statistics. The obtained average RSF is used to scale all counts of one element of the measured samples. This includes the A-4000 sample itself as with the use of the average RSF the measurement of isotopic pairs with no reference data becomes possible for this sample (e.g., 238Pu/238U). More details and data on the RSF measurement is given in the Supporting Information (SI-Figure 2).

For all elemental ratio measurements, including the RSF measurement, it is of paramount importance to presputter the sample surface. Previous analysis of UO2 showed that the unaltered surface yields mostly U oxides as the dominant sputtered species.18 As atomic layers are removed by the Ga+ ion beam, the atom-to-oxide ratio in the flux improves as oxygen is preferentially sputtered until the process comes to equilibrium and the sputtering yields of each species stabilize.17 It is therefore important to reach these stable sputter yields for each element of interest in every sample before data acquisition. We ensured stable sputter yields by turning off the pulsing of the Ga+ gun and rastering the beam over the entire sample for several seconds. Next, we set the gun back to pulsed mode for analysis and centered it on the sample without rastering. The process was repeated until the count rates of both elements stabilized. It should be noted that this step only affects the topmost few atomic layers and does not significantly increase sample consumption.

After presputtering, several 5 min measurements were performed to monitor variations in the measured elemental ratios which would be indicative of inadequate presputtering or changes in the laser parameters. Finally, at the end of the analysis session, both the RSF and the laser bias reference standards were reanalyzed to check for instrument drift. Calculated uncertainties include counting statistics and error propagation from both reference sample measurements.

Results

Neptunium

Neptunium is produced in reactors by neutron capture on U. Figure 3 shows the uncorrected elemental U/Np ratios for the sample sets A and B (BR3 reactor) as a function of the measured U/UO ratio after presputtering. All 5 min measurements connected with lines were taken in direct succession without delay. Nonresonant ionization of UO is prominent in RIMS spectra of UO2, and the U/UO can be used as a proxy for the oxidation state of the surface.18 Surface oxidation state has been shown to have a prominent effect on the sputtering yield of ground state atomic U, so it is important to quantify its effect on the relative yields of actinides from the oxide matrix.35

Figure 3.

Figure 3

Open in a new tab

Raw elemental ratio stability after presputtering of the BR3 samples. Each data point represents a 5 min measurement. The two runs from the A-4000 sample were taken at the start and the end of the measurement day and used for RSF calculations. Uncertainties are 1σ.

The U/UO ratio shifts significantly during the 20 min measurement intervals (Figure 3). However, the uncorrected elemental ratios all agree within one standard deviation for each sample. The RSFs also agreed within uncertainty after the presputtering was repeated several hours later at the end of the day.

Figure 4 shows the corrected 237Np/238U ratio for all sample sets. The 237Np/238U ratios from the center cubes from sample sets B and C are in 2σ agreement with their bulk analysis values. This validates the use of the center cube from sample set A as an RSF standard. Even though all sample matrices are UO2, when in the reactor, irradiation causes significant and spatially inhomogeneous changes to the U oxidation state, as well as changes to the elemental composition as fission and activation products accumulate near the edge.36−38 However, the center cube results in Figure 4 show that these changes do not significantly affect RIMS elemental ratios even though the three samples sets had different burnups and came from two different reactors.

Figure 4.

Figure 4

Open in a new tab

Elemental ratios of 237Np/238U in spent nuclear fuel as a function of distance from the fuel pellets edge. Data are corrected for elemental and isotopic fractionation. The circled center cubes reproduce the measured pellet bulk values within 2σ. The 237Np/238U ratios increase by ∼50% from the center to the edge of the pellet. Uncertainties are 1σ.

All three sample sets show the same relative increase of ∼50% in 237Np/238U toward the edge of the fuel pellet. As 237Np does not have an elemental isobar, this observation is in agreement with previous studies by SIMS and LA-ICP-MS.5,32 The 237Np/238U enhancement is due to moderated neutrons arriving from outside the pellet including epithermal neutrons whose energies match neutron capture resonances in U.39 These neutrons are quickly absorbed within the first few 100 μm of the fuel and drive the formation of the edge effect observed in this study and in other spatially resolved studies of spent nuclear fuel.5

Although the average burnup of sample set C (62 GWd/tU) is significantly higher than the burnup of sample set A and B, its lower initial enrichment leads to comparable Np concentrations between the fuels. A detailed explanation of the differences in the Np production pathways is provided in the Supporting Information (SI-Figure 3).

Plutonium

Plutonium-238 is produced by neutron capture on 237Np followed by rapid β-decay. It is notoriously difficult to measure due to strong isobaric interference from 238U, which is several orders of magnitude more abundant even in high burnup samples.40

Figure 5 shows an increase in 238Pu/238U ratio from the center to the edge of spent fuel. In contrast to Np, the data reveals a greater dependence on the fuel type and burnup. There is no 238Pu bulk data for the BR3 samples, however RIMS analysis of the center samples of the ATM-109 fuel agrees well with its bulk analysis. Pu-238 concentration correlates strongly with burnup between the sample sets, and the relative edge effect is more pronounced than that observed for 237Np. In sample set C there is ∼300% increase in 238Pu from the center to the edge of the fuel pellet, as compared to a 50% increase in Np. The 238Pu concentration surpasses that of the parent 237Np for samples closer than 800 μm to the edge (values given in SI-Table 1 and SI-Table 2). This is likely due to the large neutron capture resonances in the neutron energy region above 1 eV in 237Np.41 Arriving moderated neutrons with matching energies are quickly absorbed at the pellets edge and depleted from the spectrum thereby creating a higher 238Pu concentration at the edge compared to the center.

Figure 5.

Figure 5

Open in a new tab

Elemental ratios of 238Pu/238U in spent nuclear fuel as a function of distance from the fuel pellets edge. The 238Pu/238U ratio toward the edge shows a increase in 238Pu depending on the fuel and reactor. The BR3 samples A and B show a moderate relative increase of 50% and 130% with respect to their center cube value. The lower enrichment, higher burnup samples C show an overall higher 238Pu concentration with an ∼300% relative increase toward the edge. Uncertainties are 1σ.

Sample sets A and B both have lower 238Pu concentrations which is explained by the lower overall burnup but comparable 237Np content. Notably the lower burnup of sample set A shows the same 50% increase at the edge as in 237Np. The slightly higher burnup sample set B on the other hand already shows an edge effect of ∼230%.

While the 238Pu concentration heavily depends on fuel type and burnup, the 239Pu concentration plotted in Figure 6 shows comparable concentrations and edge effects between the different sample sets. While 237Np and 238Pu are seeded on 235U, 239Pu and its neutron capture products are seeded on 238U. The 235U concentration is limited by the initial fuel enrichment and changes significantly with burnup, but the 238U concentration remains abundant and essentially constant throughout the fuel’s life cycle. Therefore, the 239Pu concentration and that of the heavier actinides produced by neutron capture on it are mainly dependent on burnup, neutron flux and local neutron spectrum, but not initial enrichment.

Figure 6.

Figure 6

Open in a new tab

Pu-239 as a function of radial position within the fuel pellet. Despite different initial enrichment and burnup, the 239Pu/238U ratio is comparable between the sample sets and shows the same increase in 239Pu concentration toward the edge. Uncertainties are 1σ.

The 239Pu content of the center sample from set B agrees with its bulk value, but there is a discrepancy for the center most sample from set C. While the samples from 800 and 1600 μm agree with the bulk data, the sample from 3200 μm has a lower content than the bulk value. Overall sample set C has a lower 239Pu concentration in the center compared to sample sets A and B. Both have a higher 239Pu content despite having lower burnup. This is confirmed by the bulk measurements and a result of the different reactor types. With rising burnup, a distinct equilibrium between the production of 239Pu and depletion via fission and neutron capture toward 240Pu is reached. This dependency between Pu isotopic concentrations and reactor design is often used in nuclear forensics to assess the reactor type of an unknown sample.42

Analogous to 237Np, several studies on the edge effect of 239Pu are available in the literature.4,5 They agree both on the magnitude of the edge effect and its confinement to within several 100 μm of the edge as seen in this study.

While the differences in 239Pu content between the sample sets are small, heavier Pu isotopes show a strong burnup dependency. With every additional neutron needed for production of the isotope the average burnup difference between the fuels becomes more apparent. SI-Figure 4 shows the trend in the heavier actinide ratio 242Pu/238U which displays a clear separation between the higher and lower burnup fuels. Even though the bulk 242Pu/238U concentration of the high-burnup sample set C is six and 10 times higher than the lower burnup samples sets A and B, the edge effect within each sample set remains comparable: 242Pu is a factor of 5 to six times more abundant at the edge than the center.

Figure 7 summarizes the edge effect for all actinide concentrations analyzed in this work. It shows the ratio between outermost cube of a sample set divided by its center cube concentration. As neutron capture in fresh fuel starts from 235U and 238U independently, this leads to two separate edge-effect chains. For simplicity, we assume that the first chain starts at 235U and ends at 238Pu. Further neutron capture produces 239Pu, however 239Pu production is dominated by neutron capture on 238U which starts the second chain.

Figure 7.

Figure 7

Open in a new tab

Ratios between edge and center cube concentrations of each sample set. All actinide concentrations are calculated in respect to 238U. The two edge-effect chains are separated by a vertical gray line (see text for explanation). Uncertainties are 1σ.

Overall, the first chain shows a weaker edge effect compared to the second chain. Fission and neutron capture resonances lead to a depletion of 235U on the edge compared to the center. Pu-238 is the first isotope with significant enhancement at the edge. It also has a strong burnup dependency as three neutron capture reactions are needed for its production. The second chain immediately shows a strong edge effect with the first neutron capture reaction to 239Pu due to the strong epithermal capture resonances of 238U. The 239Pu/238U ratio increases by a factor of 3 in the BR3 samples and by a factor of 5 in the ATM-109 fuel. With additional neutron capture reactions toward heavier actinides the edge effect generally increases in magnitude as every step has its own cross section resonances which add to the effect. The activation products are then subsequently higher enriched at the edge. The chain leads to an 8-fold increase for the heavy actinide 243Am in the ATM-109 samples.

Americium

The heaviest isotope in this study is 243Am which follows the same trend as 242Pu but with a relative increase of 570% to 810% more 243Am at the fuel edge compared to the center. All Am concentrations are given in SI-Table 1. A particular Am isotope of interest in spent nuclear fuel is 241Am, which is produced by β-decay of 241Pu with a half-life of 14.3 years. The accumulating amount in the fuel during the irradiation time is low as it has a high thermal neutron capture cross section (720 ± 20 barn) which depletes the amount produced by Pu decay during reactor operation (typically only a few years).2 Therefore, 241Am predominantly accumulates after the fuel has been removed from the reactor. Assuming there is no 241Am at the time of fuel removal the 241Am/241Pu elemental ratio provides an estimate of the cooling time t, or time since fuel removal, according to eq 2

graphic file with name js4c00371_m002.jpg 2

where λ is the decay constant.

Adjusting the Pu+ and Am+ ionization times by adjusting the laser pulse timings as described in the Methods section allows RIMS to resolve the isobars and enables the direct measurement of the 241Am/241Pu ratio. Figure 8 shows the results of the age determinations as well as the elemental ratios. Sample set A and B from the BR3 reactor share the same irradiation periods and are therefore combined for simplicity. The known cooling times are 43.5 years for the BR3 and 31.5 years for the ATM-109 and are given as solid lines in the figure.

Figure 8.

Figure 8

Open in a new tab

Cooling times estimated from measured 241Am/241Pu ratios. Sample sets A and B from the BR3 reactor share the same irradiation period and are combined for simplicity. The shaded areas represent the irradiation periods of each fuel with the thicker line indicating the last irradiation day. The dotted lines represent the calculated cooling times the fuel was removed from the reactor calculated by the weighted average of all values from the fuel and shown as the period set by the weighted standard deviation of the calculated age. No systematic correlation to the edge-effect is visible. Uncertainties are 1σ.

The data from the two fuels show distinguishable cooling times with calculated values of 47.8+1.3-1.3 and 33.3+3.3-3.9 years. The high uncertainties are due to higher scatter of the ratios within sample sets, which are outside 1σ counting statistics. This scatter does not correlate with the edge effect as the 241Am is dominantly produced outside the reactor when there is no neutron field driving nuclear reactions.

While the last irradiation day lies within the calculated age window for the ATM fuel this is not the case for the BR3 fuel. Here the samples seem to contain too much 241Am for their age. A likely explanation is that the simplification of no 241Am present at the time of reactor removal is not valid. However, assuming a fixed amount of 241Am at t = 0 would not explain the scatter within the sample sets. For an accurate estimation of the initial 241Am content it would be necessary to better understand the extend of the edge effect on 241Pu and additionally consider the energy dependent depletion of 241Am by neutron capture.43,44

Besides a direct measurement of the 241Am/241Pu ratio by simultaneous Pu and Am RIMS analysis, it is also possible to measure the 241Am/238U and 241Pu/238U ratios separately and take the slope of the resulting plot to calculate a cooling time (SI-Figure 5). However, the scatter of individual measurements larger than the statistical uncertainty around the correct age persists.

Age determinations on similar sized cubes of the BR3 fuel has been done before. Hanson et al. determined the 241Am/241Pu ratio by dissolution, chemical purification, and ICP-MS measurements of several individual samples.45 Their individual results also showed significant scatter outside measurement uncertainties. We therefore believe that this scatter is not rooted in variations in sputter yields between elements and samples but a real variation of the Pu/Am ratio on the micrometer scale. This is a strong indication that the age determination by 241Pu/241Am dating is only precise in larger samples or on sample sets with many smaller particles.

Conclusion

Multiple actinide elemental ratios were measured in spent nuclear fuel by RIMS. This enabled a thorough study of the two edge-effect chains present in nuclear fuel including a previously unobserved strong fuel and burnup dependency of 238Pu content. Although the uncertainties are generally larger in the 238Pu measurements due to the large 238U interference, this is a first indication of enhanced 238Pu production not seen in its predecessor nuclides at elevated burnup.

The second edge-effect chain starting at 239Pu was analyzed up to 243Am, including isobar-free 241Pu. Each neutron capture step adds to the already strong edge effect of 239Pu. This ultimately results in an 8-fold increase of 243Am at the edge compared to the center of spent fuel.

The age calculation of sample sets comprised of micrometer-sized spent nuclear fuel samples have elevated uncertainties as the 241Am/241Pu ratios scatter around the correct date more than counting statistics would explain. This measured scatter confirms a previous result in the literature and appears to be a real effect that is observable at micrometer length scales.45 This result is crucial for the interpretation of the 241Am/241Pu based age estimation in isolated unknown samples.

Acknowledgments

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-JRNL-868670. This work was supported by the National Nuclear Security Agency Office of Defense Nuclear Nonproliferation Research and Development.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jasms.4c00371.

  • Data tables, analysis cycle timing scheme, detailed RSF explanation, detailed discussion of 237Np pathway, additional figures for 242Pu concentration and two-way age determination by 241Pu/238U and 241Am/238U (PDF)

Author Contributions

Manuel Raiwa: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing–original draft, Writing–review and editing. Michael Savina: Conceptualization, Funding acquisition, Investigation, Project administration, Resources, Supervision, Writing–review and editing. Autumn Roberts: Data curation, Investigation. Danielle Ziva Shulaker: Resources, Writing–review and editing. Brett Isselhardt: Conceptualization, Project administration, Supervision, Writing–review and editing.

The authors declare no competing financial interest.

Supplementary Material

js4c00371_si_001.pdf (436.1KB, pdf)

References

  1. Wolf S. F.; Bowers D. L.; Cunnane J. C. Analysis of high burnup spent nuclear fuel by ICP-MS. Journal of Radioanalytical and Nuclear Chemistry 2005, 263 (3), 581–586. 10.1007/s10967-005-0627-7. [DOI] [Google Scholar]
  2. Makarova T. P.; Bibichev B. A.; Domkin V. D. Destructive analysis of the nuclide composition of spent fuel of WWER-440, WWER-1000, and RBMK-1000 reactors. Radiochemistry 2008, 50 (4), 414–426. 10.1134/S1066362208040152. [DOI] [Google Scholar]
  3. Gunther-Leopold I.; Kivel N.; Kobler Waldis J.; Wernli B. Characterization of nuclear fuels by ICP mass-spectrometric techniques. Anal Bioanal Chem. 2008, 390 (2), 503–510. 10.1007/s00216-007-1644-x. [DOI] [PubMed] [Google Scholar]
  4. Walker C. T.; Bremier S.; Portier S.; Hasnaoui R.; Goll W. SIMS analysis of an UO2 fuel irradiated at low temperature to 65MWd/kgHM. J. Nucl. Mater. 2009, 393 (2), 212–223. 10.1016/j.jnucmat.2009.06.017. [DOI] [Google Scholar]
  5. Desgranges L.; Pasquet B.; Valot C.; Roure I. SIMS characterisation of actinide isotopes in irradiated nuclear fuel. J. Nucl. Mater. 2009, 385 (1), 99–102. 10.1016/j.jnucmat.2008.09.032. [DOI] [Google Scholar]
  6. Willingham D.; Groopman E. E.; Grabowski K. S.; Sangely L. Direct, uncorrected, molecule-free analysis of (236)U from uranium-bearing particles with NAUTILUS: a new kind of mass spectrometer. Analyst 2018, 143 (22), 5364–5371. 10.1039/C8AN01451E. [DOI] [PubMed] [Google Scholar]
  7. Burger M.; Hendriks L.; Kaeslin J.; Gundlach-Graham A.; Hattendorf B.; Günther D. Characterization of inductively coupled plasma time-of-flight mass spectrometry in combination with collision/reaction cell technology – insights from highly time-resolved measurements. Journal of Analytical Atomic Spectrometry 2019, 34 (1), 135–146. 10.1039/C8JA00275D. [DOI] [Google Scholar]
  8. Ha Y. K.; Han S. H.; Kim H. G.; Kim W. H.; Jee K. Y. Shielded laser ablation ICP-MS system for the characterization of high burnup fuel. Nuclear Engineering and Technology 2008, 40 (4), 311–318. 10.5516/NET.2008.40.4.311. [DOI] [Google Scholar]
  9. Stephan T.; Trappitsch R.; Davis A. M.; Pellin M. J.; Rost D.; Savina M. R.; Yokochi R.; Liu N. CHILI - the Chicago Instrument for Laser Ionization - a new tool for isotope measurements in cosmochemistry. Int. J. Mass Spectrom. 2016, 407, 1–15. 10.1016/j.ijms.2016.06.001. [DOI] [Google Scholar]
  10. Mandel M.; Holtmann L.; Raiwa M.; Wunnenberg-Gust A.; Riebe B.; Walther C. Imaging of I, Re and Tc plant uptake on the single-cell scale using SIMS and rL-SNMS. J. Hazard Mater. 2022, 423, 127143 10.1016/j.jhazmat.2021.127143. [DOI] [PubMed] [Google Scholar]
  11. Morita M.; Tomita H.; Ohtake R.; Kanenari K.; Nakamura A.; Saito K.; Kawai T.; Okumura T.; Sonnenschein V.; Sakamoto T. Development of a micro imaging system for element-selective analysis by coupling of focused ion beam and resonance ionization mass spectrometry. Progress in Nuclear Science and Technology 2018, 5 (0), 179–182. 10.15669/pnst.5.179. [DOI] [Google Scholar]
  12. Bosco H.; Hamann L.; Kneip N.; Raiwa M.; Weiss M.; Wendt K.; Walther C. New horizons in microparticle forensics: Actinide imaging and detection of Pu-238 and Am-242m in hot particles. Sci. Adv. 2021, 7 (44), eabj1175 10.1126/sciadv.abj1175. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Savina M. R.; Shulaker D. Z.; Isselhardt B. H.; Brennecka G. A. Rapid isotopic analysis of uranium, plutonium, and americium in post-detonation debris simulants by RIMS. Journal of Analytical Atomic Spectrometry 2023, 38 (6), 1205–1212. 10.1039/D3JA00096F. [DOI] [Google Scholar]
  14. Smentkowski V. Trends in sputtering. Prog. Surf. Sci. 2000, 64, 1–58. 10.1016/S0079-6816(99)00021-0. [DOI] [Google Scholar]
  15. Anderson F. S.; Levine J.; Whitaker T. Dating a Martian meteorite with 20 Myr precision using a prototype in-situ dating instrument. Planetary and Space Science 2020, 191, 105007 10.1016/j.pss.2020.105007. [DOI] [Google Scholar]
  16. Scott Anderson F.; Levine J.; Whitaker T. J. Dating the Martian meteorite Zagami by the (8)(7)Rb-(8)(7)Sr isochron method with a prototype in situ resonance ionization mass spectrometer. Rapid Commun. Mass Spectrom. 2015, 29 (2), 191–204. 10.1002/rcm.7095. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Savina M. R.; Isselhardt B. H.; Kucher A.; Trappitsch R.; King B. V.; Ruddle D.; Gopal R.; Hutcheon I. High Useful Yield and Isotopic Analysis of Uranium by Resonance Ionization Mass Spectrometry. Anal. Chem. 2017, 89 (11), 6224–6231. 10.1021/acs.analchem.7b01204. [DOI] [PubMed] [Google Scholar]
  18. Savina M. R.; Trappitsch R.; Kucher A.; Isselhardt B. H. New Resonance Ionization Mass Spectrometry Scheme for Improved Uranium Analysis. Anal. Chem. 2018, 90 (17), 10551–10558. 10.1021/acs.analchem.8b02656. [DOI] [PubMed] [Google Scholar]
  19. Kaja M.; Studer D.; Berg F.; Berndt S.; Düllmann C. E.; Kneip N.; Reich T.; Urquiza-González M.; Wendt K. Resonant laser ionization of neptunium: investigation on excitation schemes and the first ionization potential. European Physical Journal D 2024, 78, 50. 10.1140/epjd/s10053-024-00833-7. [DOI] [Google Scholar]
  20. Grüning C.; Huber G.; Klopp P.; Kratz J. V.; Kunz P.; Passler G.; Trautmann N.; Waldek A.; Wendt K. Resonance ionization mass spectrometry for ultratrace analysis of plutonium with a new solid state laser system. Int. J. Mass Spectrom. 2004, 235 (2), 171–178. 10.1016/j.ijms.2004.04.013. [DOI] [Google Scholar]
  21. Kneip N.; Dullmann C. E.; Gadelshin V.; Heinke R.; Mokry C.; Raeder S.; Runke J.; Studer D.; Trautmann N.; Weber F.; et al. Highly selective two-step laser ionization schemes for the analysis of actinide mixtures. Hyperfine Interact. 2020, 241 (1), 45. 10.1007/s10751-020-01712-4. [DOI] [Google Scholar]
  22. Savina M. R.; Isselhardt B. H.; Trappitsch R. Simultaneous Isotopic Analysis of U, Pu, and Am in Spent Nuclear Fuel by Resonance Ionization Mass Spectrometry. Anal. Chem. 2021, 93 (27), 9505–9512. 10.1021/acs.analchem.1c01360. [DOI] [PubMed] [Google Scholar]
  23. Raiwa M.; Savina M.; Shulaker D. Z.; Roberts A.; Isselhardt B. Zirconium analysis in microscopic spent nuclear fuel samples by resonance ionization mass spectrometry. Journal of Analytical Atomic Spectrometry 2024, 39, 1803. 10.1039/D4JA00065J. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Savina M. R.; Isselhardt B. H.; Shulaker D. Z.; Robel M.; Conant A. J.; Ade B. J. Simultaneous isotopic analysis of fission product Sr, Mo, and Ru in spent nuclear fuel particles by resonance ionization mass spectrometry. Sci. Rep-Uk 2023, 13 (1), 5193. 10.1038/s41598-023-32203-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Reilly D.; Beck C.; Buck E.; Cliff J.; Duffin A.; Lach T.; Liezers M.; Springer K.; Tedrow S.; Zimmer M. Focused ion beam for improved spatially-resolved mass spectrometry and analysis of radioactive materials for uranium isotopic analysis. Talanta 2020, 211, 120720 10.1016/j.talanta.2020.120720. [DOI] [PubMed] [Google Scholar]
  26. Wolf S. F.; Bowers D. L.; Cunnane J. C.. Analysis of spent nuclear fuel samples from Three Mile Island and Quad cities Reactors: Final Report; Argonne National Laboratory, 2000. [Google Scholar]
  27. Rodriguez D. C.; Anderson E.; Anderson K. K.; Campbell L. W.; Fast J. E.; Jarman K.; Kulisek J.; Orton C. R.; Runkle R. C.; Stave S. Measurement and analysis of gamma-rays emitted from spent nuclear fuel above 3MeV. Applied Radiation and Isotopes 2013, 82, 181–187. 10.1016/j.apradiso.2013.08.006. [DOI] [PubMed] [Google Scholar]
  28. Carney K. P.; Hague R. K.. Irradiated BR-3 Fuel Pin I-334 Bulk Analysis Results; Idaho National Laboratory, 2017. [Google Scholar]
  29. Savina M.; Trappitsch R.. Resonance ionization mass spectrometry (RIMS): Fundamentals and applications including secondary neutral mass spectrometry. In Photoionisation and Photo-induced Processes in Mass Spectrometry, Hanley L., Zimmermann R., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA, 2020; p 440. [Google Scholar]
  30. Hanemann P.; Bister S.; Raiwa M.; Reinhard S.; van Eerten D.; Walther C. Production and characterization of standard particles for rL-SNMS. Journal of Radioanalytical and Nuclear Chemistry 2022, 331, 5039. 10.1007/s10967-022-08600-x. [DOI] [Google Scholar]
  31. Vickerman J. C.; Briggs D.. ToF-SIMS: Materials Analysis by Mass Spectrometry; SurfaceSpectra, 2013. [Google Scholar]
  32. Ha Y. K.; Kim J.; Jeon Y. S.; Han S. H.; Seo H. S.; Song K. Local Burnup Characteristics of Pwr Spent Nuclear Fuels Discharged from Yeonggwang-2 Nuclear Power Plant. Nuclear Engineering and Technology 2010, 42 (1), 79–88. 10.5516/NET.2010.42.1.079. [DOI] [Google Scholar]
  33. Savina M.; Trappitsch R. Resonance Ionization Mass Spectrometry (RIMS): Fundamentals and Applications Including Secondary Neutral Mass Spectrometry. Photoionization and Photo-Induced Processes in Mass Spectrometry 2021, 215–244. 10.1002/9783527682201.ch6. [DOI] [Google Scholar]
  34. Bosco H.; Weiss M.; Raiwa M.; Walther C.; Kneip N.; Wendt K. Influence of the hyperfine structure on plutonium in resonant laser-SNMS. Hyperfine Interact. 2020, 241, 34. 10.1007/s10751-020-1696-2. [DOI] [Google Scholar]
  35. Savina M. R.; Trappitsch R.; Isselhardt B. H. Electronic excitation of uranium atoms sputtered from uranium metal and oxides. Spectrochim. Acta, Part B 2018, 149, 214–221. 10.1016/j.sab.2018.08.003. [DOI] [Google Scholar]
  36. Kleykamp H. The chemical state of the fission products in oxide fuels. J. Nucl. Mater. 1985, 131 (2), 221–246. 10.1016/0022-3115(85)90460-X. [DOI] [Google Scholar]
  37. Spino J.; Vennix K.; Coquerelle M. Detailed characterisation of the rim microstructure in PWR fuels in the burn-up range 40–67 GWd/tM. J. Nucl. Mater. 1996, 231 (3), 179–190. 10.1016/0022-3115(96)00374-1. [DOI] [Google Scholar]
  38. Portier S.; Brémier S.; Walker C. T. Secondary ion mass spectrometry of irradiated nuclear fuel and cladding: An overview. Int. J. Mass Spectrom. 2007, 263 (2–3), 113–126. 10.1016/j.ijms.2007.01.016. [DOI] [Google Scholar]
  39. NEA. The JEFF-3.1.1 Nuclear Data Library; OECD Publishing, 2009. [Google Scholar]
  40. Kim J. S.; Jeon Y. S.; Park S. D.; Ha Y.-K.; Song K. Analysis of high burnup pressurized water reactor fuel using uranium, plutonium, neodymium, and cesium isotope correlations with burnup. Nuclear Engineering and Technology 2015, 47 (7), 924–933. 10.1016/j.net.2015.08.002. [DOI] [Google Scholar]
  41. NEA. The JEF-2.2 Nuclear Data Library; OECD Publishing, Paris, 2000. [Google Scholar]
  42. Wallenius M.; Peerani P.; Koch L. Origin determination of plutonium material in nuclear forensics. Journal of Radioanalytical and Nuclear Chemistry 2000, 246 (2), 317–321. 10.1023/A:1006774524272. [DOI] [Google Scholar]
  43. Jandel M.; Bredeweg T. A.; Bond E. M.; Chadwick M. B.; Clement R. R.; Couture A.; O’Donnell J. M.; Haight R. C.; Kawano T.; Reifarth R. Neutron capture cross section ofAm241. Physical Review C 2008, 78 (3), 034609. 10.1103/PhysRevC.78.034609. [DOI] [Google Scholar]
  44. Mendoza E.; Cano-Ott D.; Altstadt S.; Andriamonje S.; Andrzejewski J.; Audouin L.; Balibrea J.; Bécares V.; Barbagallo M.; Bečvář F. Measurement and analysis of theAm241neutron capture cross section at the n_TOF facility at CERN. Physical Review C 2018, 97 (5), 054616. 10.1103/PhysRevC.97.054616. [DOI] [Google Scholar]
  45. Hanson S. K.; Pollington A. D. Measuring signatures of fuel irradiation in large particle samples. Journal of Analytical Atomic Spectrometry 2021, 36 (5), 1018–1027. 10.1039/D1JA00025J. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

js4c00371_si_001.pdf (436.1KB, pdf)

Articles from Journal of the American Society for Mass Spectrometry are provided here courtesy of American Chemical Society

RESOURCES