Critical comparison of INAA and ICP-MS applied in the characterization of purity of TRISO fuel and substrates to its production

High-temperature gas-cooled reactor (HTGR) is an advanced type of nuclear reactor. Nuclear reactors of this type are gaining more and more interest mainly due to their high thermal efficiency, higher safety level, and cost effectiveness [1, 2, 3, 4]. Nuclear fuel used in modern HTGRs consists of spherical uranium dioxide particles covered with a very durable coating consisting of porous pyrolytic carbon (PyC), dense PyC, silicon carbide, and the outer layer of dense PyC. This structure of the fuel with the TRi-structural ISOtropic (TRISO) coating guarantees that the fission products are retained inside the spherical particle, thus preventing the release of activity. TRISO coated nuclear fuel must adhere to very strict quality standards when it comes to the chemical composition of the individual layers. It should be emphasized that nuclear fuel, independently of the type of reactor, must fulfill several criteria. One of them is its purity. To ensure and control the high purity of nuclear fuel, it is necessary to have analytical procedures that can determine the trace and ultratrace amounts of impurities in a wide range of materials. Under certain conditions, even a small amount of impurities can alter the chemical and nuclear properties of a material; such alterations may cause corrosion, or could result in higher radiological hazards for the operation of reactors. From the analytical point of view, it would be most advantageous to have at least two different analytical techniques, based on different physicochemical bases, which would allow: (1) routine analytical activities based on application of first analytical technique; and (2) verification of the developed procedures and the obtained results with the use of the second technique. Both instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS) are widely used in nuclear materials analysis [5, 6, 7, 8, 9].

The accurate determination of trace and ultratrace elements is recognized as a combination of sample digestion and measurement of given analyte. Sample dissolution/digestion is usually first step of the analytical procedures and has a significant impact on the final result of determinations. Depending on the sample material and its origin, different digestion methods are considered to be effective. In the case of materials that are difficult to digest, it is believed that the recovery of 90% is sufficient for analytical purposes. In the case of materials used in widely understood “nuclear reactor industry”, it is necessary to obtain total digestion of analysed materials. For these materials, obtaining a total, quantitative digestion is crucial for having reliable analytical information, which is essential for nuclear safety.

Neutron activation analysis is a leading multielemental analytical technique used in the determination of elements at trace and ultratrace levels in various materials. Approximately 70% of the elements have nuclear properties suitable for NAA. In INAA, the elemental determination is based on the conversion of stable nuclei to other, mostly radioactive nuclei via nuclear reactions ( XZA+n→(radioactive nucleus)*→YZ'A'+b {}_{\rm{Z}}^{\rm{A}}{\rm{X}} + {\rm{n}} \to ({\rm{radioactive}}\,{\rm{nucleus}})* \to {}_{{\rm{Z}}'}^{{\rm{A}}'}{\rm{Y}} + b ), where: X – chemical symbol of the target element; A and Z – mass number and atomic number of the target element, respectively; Y – chemical symbol of the reaction product; A’ and Z’ – mass number and atomic number of the reaction product, respectively; n – neutron; b – γ photons, neutrons, charged particles, or helium nuclei), and the measurements of the reaction products [10]. Generally, NAA consists of three steps: irradiation with the thermal neutrons, measurements of gamma radiation after appropriate cooling time, and the interpretation of the gamma-ray spectra. The non-destructive phenomena of INAA (sample do not have to be digested prior to the analysis) make the technique attractive for chemical characterization of “difficult to digest” or “unique” materials. In 2011, INAA has been recognized as a primary method of measurements (PMMs) – the highest metrological range [11]. ICP-MS is a multielemental technique to determine low- and ultra-low-concentrations of elements. It is the most popular detection method, due to its precision, high sensitivity, low detection limits for majority of elements (LOD), wide linear dynamic range (up to eight orders of magnitude), and possibilities to determine isotopic ratios. Disadvantages and weaknesses of the ICP-MS detection are the occurrence of spectral and non-spectral interferences (e.g., accurate determination of ⁹⁹Tc may be difficult or even impossible, when stable ⁹⁹Ru and ⁹⁸Mo (via molecular ion ⁹⁸Mo¹H) are present in sample solution) [12].

The Institute of Nuclear Chemistry and Technology (INCT) has been realizing the project financed by the National Centre for Research and Development (NCBR), entitled “Research and analysis of selected chemical aspects of the production and use of TRISO fuel in the HTGR nuclear reactor”. As part of this project, analytical procedures have been developed that can be used to achieve control over the purity of substances used for fuel production and control over the fuel purity at each stage of its production [13]. Both ICP-MS (in solution mode and with laser evaporation of solid samples) and INAA were used in that project. This work aims to compare solution ICP-MS and INAA for analysis of selected substrates for TRISO fuel production. Also, the application of LA-ICP, MS, and INAA for the elemental analysis of chemically resistant protective layers was compared.

Table 1 presents a comparison of selected properties important for INAA and ICP-MS, and which are important in inorganic trace analysis [10, 12]. It can be seen that each technique can give reliable results and can be complementary applied in the characterization of nuclear samples.

Both of these analytical techniques, which are used for the determination of trace amounts of elements in a wide range of materials, are available in the Institute of Nuclear Chemistry and Technology.

Comparison of results obtained with LA-ICP-MS and NAA

In the case of materials with a great chemical resistance such as pyrolytic graphite or SiC, it is difficult to find a suitable procedure to digest such materials for obtaining a complete transfer of analysed material to the solution. One of the decomposition methods for these materials is microwave-assisted digestion with a mixture of concentrated acids: HNO₃, H₂SO₄, HF, and other aggressive reagents. The possible limitations to microwave acid digestion of SiC or pyrolytic graphite are incomplete dissolution of sample, losses of volatile analytes, and the possibility of contamination. A fusion with alkaline mixtures can be an alternative to the acid digestion. Owing to the digestion of high salt content during the final fusion, it is necessary to dilute the sample (in ICP-MS measurement, the maximum salt content should not exceed 0.1%) so that higher limits of detection are obtained. Moreover, the contamination of analysed material by the reagents can occur during the fusion procedure.

In the case of materials that are recognized as difficult to digest, application of LA-ICP-MS could be used as a complementary method for analysis. To ensure high nuclear safety of HTRs, highly chemical and thermal resistant protective layers (pyrolytic graphite and silicon carbide) are used. To avoid difficulties with total decomposition of these materials and analyse the composition of the obtained gaseous phase by the ICP-MS, the use of laser evaporation (LA) of the sample has been proposed. The coupling of LSX-500 laser from CETAC (USA) with ICP-MS ELAN DRC II spectrometer (PerkinElmer) was applied for the analysis of single spherical particle. Preliminary tests were carried out. The samples of 1-mm diameter with zirconium nucleus, covered with a layer of PyC and SiC, were used for testing. A stand for analysis of single particles was made, i.e., a table for placing the grains (Fig. 1). To optimize the analysis conditions (maximum evaporation of individual components of the sample), the beam power and the duration of a single laser beam pulse were changed.

Fig. 1

A laser beam pulse with a beam diameter of 150 μm was directed onto the grains placed on the table. Pulse ablation was selected so that the depth of evaporated layer was 20 μm. Figure 2 shows a picture of the microsphere after laser evaporation of individual layers, and in Fig. 3 the obtained spectra (number of counts during sampling during profiling) are made available. The layers were evaporated in several steps, and evaporation was performed each time the hole depth was 20 μm.

Fig. 2

Fig. 3

Based on the spectra obtained, it can be concluded that the use of laser beam allows evaporation (decomposition) of protective layers of TRISO fuel microsphere and metal core. A laser of sufficient power penetrates the protective layers step by step, reaching the kernel. Initially, only the carbon signal is visible, then the silicon signal appears, and finally, in the mass spectrum, the signal from the fuel material (which is zirconium in this case) is visible. The following evaporation conditions are applied, namely laser beam diameter – 150 μm, frequency – 20 Hz, sampling interval – 15 s, and measurement time – 10 s.

Unfortunately, the lack of appropriate solid standards and certified reference materials dedicated to the LA-ICP-MS coupling may make it impossible to quantify selected elements in silicon carbide and pyrolytic graphite. In this work, NBS Glass 610 was used to conduct the screening analysis of both materials [16]. Table 3 shows the results obtained from the determination of contaminants in protective layers by NAA and LA-ICP-MS. Due to differences in the matrix (glass vs. SiC or pyrolytic graphite), different laser evaporation yields under the same conditions are obtained, and thus only semi-quantitative results can be given. For Eu, Hf, and Sc, it was not possible to determine these elements in protective layers due to lack of certified or informative values in NBS610. It is therefore important to have a complementary method such as NAA, which will allow for quantitative results with low uncertainty values.

Table 3

Mean concentrations of elements as determined by NAA and LA ICP-MS in the analysed silicon carbide (n = 3) and pyrolytic graphite

Silicon carbide
Element	NAA (ng/g)	LA-ICP-MS (ng/g)
Co	678	(950)
Eu	42	n.d.
Fe	980	(1390)
Hf	648	n.d.
Sc	140	n.d.
Pyrolytic graphite
Element	NAA (ng/g)	LA-ICP-MS (ng/g)
Ag	320	(560)

n.d., not determined; (...) – semi quantitative, information results.

Both ICP-MS and INAA are reliable for elemental characterization of nuclear fuel and substances used for its production. Both may be suitable complementarily, which allows increase in the amount of the determined elements and controlling of the quality of the results obtained from the analysis. From the obtained results, it can be seen that silicon oil used in sol-gel fabrication could be a source of disturbing contaminants. Therefore, the influence of oil contaminants on fuel production should be investigated.

LA-ICP-MS was found to be useful for the semi-quantitative analysis without any sample preparation. Despite its limitations, LA-ICP-MS can be used to study the composition of materials that are difficult to solubilize, among other things because of the short analysis time compared to INAA measurements. In this work, LA-ICP-MS was developed for the analysis of both single protective layers and single spherical zirconia particle. The measuring equipment was adapted to the requirements of the developed procedure: a target for irradiating single grains was made and qualitative analyses of microspheres were performed. In the case of analysis of commercially available SiC and pyrolytic graphite, it has been shown that the purity of silicon carbide must be carefully checked before it is used to produce protective layers. It has also been shown that there the possibility exists for the evaporation of the protective layers of pyrolytic graphite, silicon carbide, and the metal core. This suggests that it would be possible to study both the thickness of the protective layers and their purity when applied to the metallic kernel.

INAA, because of its unique possibilities, adhere to the Consultative Committee for Amount of Substance (CCQM) requirements for a PMM. The use of NAA allows the verification of analytical results obtained by other commonly used techniques, which is extremely important for ensuring and controlling the quality (quality control/quality assurance, QC/QA) of analytical results in nuclear laboratories. Therefore, this method should be included for testing the purity of materials used in fuel production.