Low level measurements of thorium and neptunium in environmental samples using the (γ, f) reaction

A new highly sensitive track method for the determination of thorium (²³²Th) and neptunium (²³⁷Np) has been developed. The technique includes the radiochemical separation procedure of the isotopes followed by the irradiation of the resultant samples on the MT-25 microtron. The detection limit is équivalent up to 3·10^–13 g of²³²Th and 7·10^–14 of²³⁷Np. The method was used to determine²³²Th and²³⁷Np isotopes in water samples.     

Distribution of global fallout237Np,Pu isotopes,and241Am in lake and sea sediments

In order to investigate the sedimentary behaviour of neptunium,²³⁷Np together with Pu isotopes and²⁴¹Am have been measured for the sediment cores collected from Lake Mikata (freshwater) and from Lake Kugushi (brakish water) both belonging to the Mikata Five Lakes, and from Nyu Bay (sea water). In all sediment core samples²³⁷Np was detected, and its concentrations were far below those of^239,240Pu and²⁴¹Am measured for the same samples. Inventories of²³⁷Np in Lake Mikata, Lake Kugushi and Nyu Bay were estimated to be 0.53, 0.29 and 0.34 MBq/km², respectively. The activity ratio of²³⁷Np/^239,240Pu calculted from the inventories in each sediment core was 0.29% for Lake Mikata, 0.15% for Lake Kugushi and 0.10% for Nyu Bay. These values except for the value for Lake Mikata are two or three times lower than the value of 0.3–0.4% observed for surface soils of 0–20 cm depth, suggesting that Np is more soluble compared with Pu.     

Anomalously high237Np in intertidal sediments from the Ribble Estuary in the Irish Sea

Depth profiles and inventories of²³⁷Np in sediment cores from the Ribble Estuary in the Irish Sea have been studied along with those of Pu isotopes,²⁴¹Am and¹³⁷Cs, to allow a more detailed look of anomalously high²³⁷Np content observed in this estuary previously. The comprehensive data obtained showed that the depth profiles of both²³⁷Np contents and²³⁷Np/^239,240Pu activity ratios were clearly different from those of^239,240Pu,²⁴¹Am, and¹³⁷Cs and their activity ratios. As much as 80–90% of²³⁷Np inventories (0.32–1.06 kBq/m²), found in three cores, were estimated to be derived from a source other than Sellafield, on the basis of comparison of the²³⁷Np/^239,240Pu inventory ratio (0.65–1.74%) found in the Ribble Estuary cores with those (0.10–0.16%) in the Ravenglass Estuary cores.     

Distribution and behaviour of99Tc,237Np,239,240Pu,and241Am in the coastal and estuarine sediments of the Irish Sea

Intertidal coastal and estuarine sediments from 24 sites in the Irish Sea have been analyzed for⁹⁹Tc,²³⁷Np,²³⁸Pu,^239,240Pu and²⁴¹Am. The²³⁷Np activity and²³⁹Pu/²⁴⁰Pu ratio were measured simultaneously by ICP-MS, and⁹⁹Tc was determined by HR-ICP-MS which is ten times more sensitive than Q-ICP-MS.The activities of⁹⁹Tc,²³⁷Np,^239,240Pu and²⁴¹Am were distributed over a wide range of 1.5–70.5, 0.01–13.3, 2.3–1589, 2.6–1894 Bq/kg, respectively. Activities of these radionuclides decreased exponentially with distance from the Sellafield source. The²⁴¹Am/^239,240Pu and²³⁷Np/^239,240Pu ratios were almost constant with distance from the Sellafield. This result suggests that the distribution and behavior of Np and Pu are controlled by complicated factors such as the influence of transport, the variation with time of Np/Pu ratio in the Sellafield discharges and sedimentary mixing processes in the Irish Sea.     

Determination of237Np in large volume samples of sea water by a radiochemical procedure

A radiochemical procedure followed by alpha spectrometry has been developed for the determination of²³⁷Np present at low activity concentrations in seawater. The analytical procedure is based on concentration of actinides from 1800 1 sea water samples by hydroxide precipitations. Neptunium is isolated by ion exchange, fluoride precipitation and extraction with TTA (thenoyltrifluoroacetone). As a radiochemical yield determinant²³⁹Np or²³⁵Np is used. Neptunium is electroplated onto stainless steel discs before alpha-spectrometry for about 10 days. The procedure allows for sequential separation of plutonium, americium, technetium and radiocaesium together with neptunium. The radiochemical yield for neptunium is only 20–50%, but the procedure has been applied with success on several samples contaminated with²³⁷Np at fallout or close to fallout levels.     

Mössbauer spectroscopy of237Np in solid solutions of actinide dioxides

Emission Mössbauer (nuclear gamma-resonance) spectra of²³⁷Np resulting from the alpha-decay of²⁴¹Am in solid solutions AmO₂–ThO₂ and AmO₂–UO₂ prepared by precipitation from LiF–NaF eutectic melts under fluorine-oxygen exchange were first investigated in the temperature range from 77 to 296 K. The resonant absorbers are NpO₂ and NpAl₂. The possibility is stated of managing the form of the emission Mössbauer spectra of²³⁷Np in dioxides by the directed change of the matrix stoichiometry or by its irradiation by visible light. At temperatures from 77 to 230 K the authors measured for the first time the NGR absorption spectra of²³⁷Np in solid solution NpO₂–ThO₂ with various concentrations of neptunium dioxide. An increase of the absorption line width (approximately by a factor of 4) with a decrease in the NpO₂ contents (to 10 mol.%) was explained by the influence of fast electron exchange between Np⁴⁺ and Np⁵⁺ states observed in the NpO₂–ThO₂ solid solution at low concentrations of neptunium dioxide.     

Dosage du237Np en trances dans l'uranium et le plutonium

The²³⁷Np content of²³⁸Pu or²³⁹Pu samples were determined by the gammaspectrometry of²³⁸Np formed by thermal neutron activation. The measurements were carried out on irradiated²³⁸Pu samples directly, and after the chemical separation of²³⁹Pu samples. The²³⁷Np content of natural uranium was determined from the ratio of the alpha-activities of²³⁸Pu and²³⁹Pu isotopes formed from the decay of neptunium isotopes produced by the activation of²³⁷Np and²³⁸U isotopes, respectively.      

3He induced nuclear reactions on237Np targets

The cross sections for the³He induced nuclear reactions on²³⁷Np leading to the formation of^236sNp,²³⁸Np,²³⁶Pu,²³⁷Pu,²³⁸Pu,²³⁷Am and²³⁹Am were studied by the irradiation of thin and thick targets of²³⁷Np. The cross sections were determined in the energy range of 20–26 MeV using the stacked-foil technique with thin targets. The cross sections were used for the calculation of the thick-target yields for the production of²³⁶Pu,²³⁷Pu and²³⁸Pu from²³⁷Np irradiated with³He²⁺ ions in the energy range of 20–26 MeV.     

Determination of237Np in spent fuel by use of inherent239Np as a chemical yield monitor

A method for analyzing the content of²³⁷Np in spent fuel has been developed using inherent²³⁹Np as a chemical yield monitor. After ion-exchange separations for the dissolved fuel solution, the²³⁷Np content in the neptunium fraction was determined from the activity of²³⁷Np or of²³³Pa, which is in radioactive equilibrium with²³⁷Np. The chemical yield in the separations was determined both from the content of²⁴³Am which is in radioactive equilibrium with²³⁹Np before the separations and from the²³⁹Np content in the neptunium fraction after the separations by alpha- and gamma-ray spectrometry.     

Diffusion of actinides in glasses containing simulated radioactive wastes

Diffusion coefficients of radionuclides²³⁷Np,²³⁹Pu and²⁴¹Am in simulated alumina phosphate and alumina borosilicate glasses at temperatures lower than their transformation temperature were determined. Actinides are known to be the least mobile elements. In particular, the diffusion coefficients of actinides in alumina phosphate glasses at 673 K are about 10^–18 m²·s^–1, and in alumina borosilicate at 773 K about 10^–19 m²·s^–1. It is shown that crystallization of glasses leads to increasing²³⁷Np diffusion mobility. It is also shown that a rather small quantity of water absorbed by a crystallized alumina phosphate glass in tensifies low-temperature migration of²³⁷Np.     

Das Ionenaustauschverhalten von Neptunium in salpetersauren Lösungen

Zusammenfassung Das Verteilungsverhalten von²³⁷Np–²³⁹Np zwischen salpetersauren Lösungen verschiedener Konzentration und einem Anionenaustauscher (Dowex 1X8) wurde untersucht. Durch Reduktion mit Fe²⁺+Hydrazin konnte quantitativ das Np(IV) hergestellt und dessen Verteilungskoeffizient (D) in 1n-10n-HNO₃ bestimmt werden. Mit Hydrazin allein und bei Lösungen ohne Reduktionsmittel wurden stark schwankendeD-Werte gefunden, die auf unterschiedliche Prozentsätze an Np(IV), Np(V) und Np(VI) zurückzuführen waren. Durch die Bestimmung der Anteile der einzelnen Oxidationsstufen konnten jedoch die jeweiligenD-Werte [D(IV),D(V),D(VI)] berechnet werden. Die mit dieser Methode erhaltenen Werte stimmten gut mit den Daten überein, die an Systemen gewonnen wurden, in denen jeweils nur eine Neptuniumoxidations-stufe vorlag.

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The ion exchange behaviour of neptunium in nitric acid solutions

The distribution of²³⁷Np–²³⁹Np between nitric acid solutions of different concentration and an anion exchanger (Dowex 1X8) was investigated. By reduction with Fe²⁺+hydrazine, the Np(IV) was obtained quantitatively and its distribution coefficients (D) in 1n to 10n-HNO₃ were determined. With hydrazine alone and without any reduction media, strongly varyingD-values were found. This was due to different amounts of Np(IV), Np(V), and Np(VI) in the solutions. By determining the fractions of the individual oxidation states the correspondingD-values [D(IV),D(V), andD(VI)] could be calculated. The data obtained by this method agreed well with theD-values resulting from determinations of systems, in which the individual Np-oxidation state was the only component of the corresponding solution.

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Mit 4 Abbildungen

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Herrn Prof. Dr.Hans Nowotny gewidmet.     

Determination of129I and127I in environmental samples by neutron activation analysis (NAA) and inductively coupled plasma mass spectrometry (ICP-MS)

In order to assess the levels and behavior of¹²⁹I (half-life: 1.6×10⁷ y) and¹²⁷I (stable) in the environment, we have developed analytical procedures involving neutron activation analysis (NAA). Environmental samples collected around Tokaimura, Ibaraki Prefecture, Japan, have been analyzed using this method. Ranges of¹²⁹I and¹²⁷I concentrations in surface soil were 0.9–180 mBq kg^–1 and 1–60 mg kg^–1, respectively. Higher¹²⁹I concentrations were found in soil samples collected from coniferous forests, suggesting a contribution from tree canopies in the deposition of this nuclide. Most of the¹²⁹I in soil, was found to be retained in the first 10 cm. The¹²⁹I/¹²⁷I ratios in wheat fields were lower than those in rice paddy fields.A soil sample collected by IAEA from an area contaminated by the Chemobyl accident was also determined. The¹²⁹I concentration and the¹²⁹I/¹²⁷I ratio were 1.6 mBq kg^–1 and 1.7×10^–7, respectively. The¹²⁹I level in this sample was higher than the values obtained in areas far from nuclear facilities in Japan. It was suggested that the analysis of¹²⁹I in soils in the Chernobyl area may be useful in evaluating the¹³¹I levels at the time of the accident.Analyses of¹²⁹I and¹²⁷I by ICP-MS in water samples were also made. The analytical speed of this method was very high, i.e., 3 minutes for a sample. However, there is a sensitivity limitation for¹²⁹I detection due to interference from¹²⁹Xe with the¹²⁹I peak. The detection limits for¹²⁹I and¹²⁷I in water samples were about 0.5 mBq ml^–1 and 0.1 ng ml^–1, respectively.     

Neptunium distribution in a high acid first cycle Purex Process

This work deals with the extraction behavior of neptunium in a high acid Purex Process. The composition of PWR fuel type with 3.2% enrichment, 500 MWd/t burn-up and 100 d cooling time was considered. Two consecutive cold runs were performed in a mock-up facility at IPEN-CNEN/SP with simulated feed solutions containing: 3M HNO₃; 1M U; 455 g²³⁷Np labeled with²³⁹Np; 15 mg Zr l^–1, 12 mg Ce l^–1, 7 mg Ru l^–1 and 13 mg Mo l^–1 traced with active isotopes⁹⁵Zr,¹⁴¹Ce,¹⁰³Ru and⁹⁹Mo as FP. A 30 vol% TBP/n-dodecane was used as solvent. Countercurrent experiments were carried out using two 16 stages plexiglass mixer-settlers, at 25°C, during 21 h continuous operation, with O/A ratio of 2 in the extraction section and 9 and 13 in the 1st and 2nd scrubbing sections, respectively. For a 65% organic loading, ca. 77% of neptunium remains in the waste stream, without any Np valence adjustment.     

Determination of131I,134Cs,137Cs in grass and cheese after Chernobyl accident in Austria

Various samples from Styria /grass/ and Salzburg /cheese/ were analyzed for¹³¹I,¹³⁴Cs and¹³⁷Cs concentration during April–July 1986 by -ray spectrometry. The concentrations are reported in nCi kg^–1 wet weight. The concentration values found for¹³¹I 0.2–17.2 nCi kg^–1 /grass/, 0.1–0.5 nCi kg^–1 /cheese/, for¹³⁴Cs 1.1–6.2 nCi kg^–1 /grass/, 0.2–1.3 nCi kg^–1 /cheese/, for¹³⁷Cs 1.6–15.7 nCi kg^–1 /grass/, 0.3–2.2 nCi kg^–1 /cheese/. While radioactivity of¹³¹I,¹³⁴Cs and¹³⁷Cs in cheese samples increases from May to June, it decreases in grass samples from May to July 1986.     

Plutonium concentration distribution in bed load sediment samples along the Romanian sector of the Danube river and the Black Sea coast

The concentrations of²³⁸Pu and^{239, 240}Pu were determined in 12 sediment samples collected from the bed of the Romanian Danube river and Black Sea coast during June–September 1994. After the sample material has been properly prepared and²⁴²Pu tracer added, plutonium was separated from americium and curium by anion exchange. After electrodeposition on stainless steel discs the elements were counted with an -spectrometry system with silicon surface-barrier detectors. The^{239, 240}Pu concentrations range between 150 and 800 mBq kg^–1 dry, while the²³⁸Pu concentrations rise up to max 150 mBq kg^–1 dry. Although the chemical yields are rather low (51%) we appreciate the results as valuable since they report for the first time the distribution of the plutonium contamination along the Danube river and the Black Sea coast-Romanian sector.     

Rapid determination of 237Np in soil samples by multi-collector inductively-coupled plasma mass spectrometry and gamma spectrometry

A radiochemical procedure is developed for the determination of ²³⁷Np in soil with multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) and gamma-spectrometry. ²³⁹Np (milked from ²⁴³Am) was used as an isotopic tracer for chemical yield determination. The neptunium in the soil is separated by thenoyl-trifluoracetone extraction from 1 M HNO₃ solution after reducing Np to Np(IV) with ferrous sulfamate, and then purified with Dowex 1 × 2 anion exchange resin. ²³⁹Np in the resulting solution is measured with gamma-spectrometry for chemical yield determination while the ²³⁷Np is measured with MC-ICP-MS. Measurement results for soil samples are presented together with those for two reference samples. By comparing the determined value with the reference value of the ²³⁷Np activity concentration, the feasibility of the procedure was validated.     

Determination of neptunium in plutonium and mixed uranium-plutonium samples by isotope dilution gamma-spectrometry with243Am as a spike

A method was developed for isolating neptunium from Pu, U or mixed oxide (MOX) samples and its determination by isotope dilution -spectrometry (IDGS) using²³⁹Np (²⁴³Am) as a spike. Extraction chromatography with trilaurylamine fixed on a SGX-C18 support was used for the isolation of Np. The decontamination factors for U, Pu, Am and Pa vary between 1000–2000 and 100, respectively. The average separation yield of Np is (95±3)%. The amount of²⁴³Am required for spiking is about 0.2–0.3 g. It is recommended to use the pair of -rays 86.53 keV (²³⁷Np)-106.13 keV (²³⁹Np) for the assay of neptunium. A relative uncertainty of 4% or better is achievable in the analysis of plutonium samples, containing 0.4–80 g neptunium. The detection limit, under the proposed experimental conditions, is about 0.05 g Np. The results were compared with the results obtained by using high resolution -spectrometry (HRGS).     

Determination of uranium and thorium isotopes in mineral spring waters

Concentrations levels of uranium and thorium isotopes have been analyzed in the m mineral spring waters of a high background region of Brazil: Poços de Caldas and Águas da Prata. The procedure was based on the determination of²³⁸U,²³⁴U,²³²Th,²³⁰Th and²²⁸Th by -spectrometry after separation and purification of the isotopes of interest by using anion-exchange chromatography and preparation of the samples for -measurements by electrodeposition. The concentration varied from <1.1 to 28.4 mBq.l^–1 and from <1.6 to 141 mBq.l^–1 for²³⁸U and²³⁴U, respectively. Thorium isotope measurements varied from <0.2 to 1.8 mBq.l^–1 from <0.3 to 4.9 mBq.l^–1 and from <0.8 to 19.9 mBq.l^–1 for²³²Th,²³⁰Th and²²⁸Th, respectively. Calculations of thorium and uranium isotopic activity ratios were carried out giving values ranging from 1.9 to 7.2, from 1.2 to 3.0 and from 7.7 to 15.3 for²³⁴U/²³⁸U,²³⁰Th/²³²Th and²²⁸Th/²³²Th, respectively. The effective doses due to the intake of²³⁸U and²³⁴U present in these waters are expected to reach values up to 1.4×10^–3 mSv y^–1 and 8.0×10^–3 mSv y^–1, respectively.     

Estimation of daily uranium ingestion by urban residents in Japan

125 types of food were collected mainly from Yokohama City which is one of the typical urban cities in Japan. The samples, were divided into the 18 food categories in the report of the National Nutrition Survey (NNS),¹ were analysed for uranium by -spectrometry after chemical separation. Concentrations of²³⁸U in individual types of food ranged from 9.85·10^–5 Bq·kg^–1 in grain vinegar to 5.90 Bq·kg^–1 in boiled and dried hijiki. The median value of²³⁸U was found to be 4.83·10^–3 Bq·kg^–1 on a raw weight basis. The daily intake of²³⁸U per person by ingestion was estimated to be approximately 14 mBq with more than 50% of it coming from marine products.     

Determination of I-129 and I-127 in soil and tracer experiments on the adsorption of iodine on soil

Neutron activation analysis of¹²⁹I and¹²⁷I in soil has been studied. The limit of detection for¹²⁹I in soil was about 0.05 mBq/kg or 1×10^–9 as¹²⁹I/¹²⁷I atom ratio. The range of¹²⁹I concentration in surface soils collected around Tokaimura (Ibaraki Prefecture) was 0.9–41 mBq/kg.Tracer experiments on the adsorption of iodine were also carried out, in order to obtain information on the behaviour of iodine in soil-water systems. Different adsorption patterns of iodide and iodate on soil were found. It was supposed that iodide was adsorbed by the soil fraction which became unstable at about 200° C and iodate by the fraction which was relatively stable to heating.