An automated delayed neutron counting system for mass determinations of special nuclear materials

An automated delayed neutron counting (DNC) system has been developed at the Royal Military College of Canada (RMC) to enhance nuclear forensics capabilities pertaining to special nuclear material analysis. The system utilises the SLOWPOKE-2 Facility at RMC as a neutron source and ³He detectors. System control and data acquisition occur through a LabVIEW platform. The time dependent count rate of the delayed neutron production has been examined for ²³⁵U, using certified reference materials. Experimental validation according to ISO 17025 protocols suggests typical errors and precision of −3.6 and 3.1%, respectively, and a detection limit of 0.26 μg ²³⁵U.     

Determination of natural uranium in biological samples using the solid state nuclear track detector method

For the solution of most of the problems which are connected to the biological and physiological role of natural uranium in plants and animal organisms about 10⁻¹⁴ g uranium should be determined. However most of the physico-chemical methods for the determination of natural uranium in biomaterials are time-consuming and possess considerable error. On the basis of addition and inner standard methods a version of Solid State Nuclear Track Detectors (SSNTD) method has been developed in order to determine the natural uranium in biospecimens. According to the experimental data simple relations have been obtained for the calculation of uranium concentration in biomaterial and minium uranium concentration in biosolution which can be measured by the detector used. Under irradiation of SSNTD at a thermal neutron flux of (3–5)·10¹⁵n·cm⁻² the detector sensitivity is 2.30·10⁻⁹ g U/ml for glass detectors; 9.60·10⁻¹⁰g U/ml for the detectors made from artificial mica.     

A brief introduction to NAA facilities of China Advance Research Reactor at CIAE

China Advance Research Reactor (CARR) at China Institute of Atomic Energy (CIAE), with a non-perturbed maximum thermal neutron flux of 1 × 10¹⁵ cm⁻² s⁻¹ at the center of active area, is one of the most powerful research reactors in the world. Three neutron channels have been allocated for conventional neutron activation analysis (NAA), thermal neutron prompt gamma activation analysis (PGAA) and cold neutron PGAA, respectively. Two irradiation tube systems are installed in the conventional NAA channel. One of them is for short irradiation with the rabbit size of diameter (Φ)19 × 40 mm, the other one is for long irradiation with the rabbit size of Φ39 × 70 mm. The medium temperature is about 45 °C and the thermal neutron flux is about 3 × 10¹⁴ cm⁻² s⁻¹ at sample positions. The flux gradient is expected to be very small according to the designed neutron flux distribution. Pneumatic systems are used for samples transfer. The speed of rabbits is designed to be about 20 m/s, and it takes 3 s to travel from irradiation position to detector. Three sets of gamma counting systems and one delayed neutron counting system are being equipped for routine analysis. They are designed for running continuously and automatically. And all the functions can be operated at laboratory or office through remote controlled computer. Software has been made domestically for spectrum peak search, concentration calculation with relative method and k₀ method with interference corrections and some other functions for the convenience of users.     

Concentrations of trace and major elements in Mission–Progresso (Texas) soils

Neutron Activation Analysis (NAA) was applied to determine trace and major elements in Mission–Progresso (Texas) soils. The Rio Grande river runs along the USA—Mexico border. The soil samples were collected at Mission and Progresso areas of the Rio Grande riverbank in the USA side. Soils were analyzed for the presence of toxic effluents due to human activities that might affect agricultural products and health because one of the possible paths of intoxication is the agricultural product consumption. Dried, sieved, and blended soil samples (~1.5 g) were irradiated at the UT Austin TRIGA reactor at a thermal neutron flux of 1 × 10¹² n cm⁻²s⁻¹ and epithermal neutron flux of 1 × 10¹¹ n cm⁻²s⁻¹. Different irradiations, decay, and counting times were combined to determine concentration and detection limits of 21 elements which represent four areas in Mission–Progresso (Texas) with the aim to achieve a consistent characterization. NIST certified reference materials were used in relative analysis and also to determine the accuracy and reproducibility values. The neutron flux was monitored using sulfur flux monitor wires. Normal and Compton suppression gamma ray spectrometers were used to detect different gamma ray energy peaks and this Compton system greatly reduces the background. Concentrations are evaluated in per cent and parts per million and errors are within acceptable levels and these values are compared with values reported in literatures from other countries. The results do not show significant contaminations neither from the Rio Grande river nor from nearby industries.     

Rapid and interference free determination of ultra trace level of uranium in potable water originating from different geochemical environments by ICP-OES

Direct determination of uranium in the concentration range of 8 μg L⁻¹ to mg L⁻¹ in water samples originating from different geochemical environments has been done using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). Uranium detection with 2–3% RSD (relative standard deviation) has been achieved in water samples by optimizing the plasma power, argon and sheath gas flow. These parameters were optimized for three different emission lines of uranium at 385.958, 409.014 and 424.167 nm. Interference arising due to the variation in concentration of bicarbonate, sodium chloride, calcium chloride, Fe and dissolved organic carbon (DOC) on the determination of uranium in water samples was also cheeked as these are the elements which vary as per the prevailing geochemical environment in groundwater samples. The concentration of NaHCO₃, CaCl₂ and NaCl in water was varied in the range 0.5–2.0%; whereas Fe ranged between 1 and 10 μg mL⁻¹ and DOC between 0.1–1%. No marked interference in quantitative determination of uranium was observed due to elevated level of NaHCO₃, CaCl₂ and NaCl and Fe and DOC in groundwater samples. Concentration of uranium was also determined by other techniques like adsorptive striping voltametry (AdSv); laser fluorimetry and alpha spectrometry. Results indicate distinct advantage for uranium determination by ICP-OES compare to other techniques.     

Neutron activation analysis of rare earth impurities in metallic uranium

A method with a sensitivity of 2·10⁻⁷ to 1·10⁻¹⁰% has been developed for determining Yb, Ho, Dy, Gd, Eu, Sm and La impurities in metallic uranium by means of neutron activation. The method is based on a preliminary chromatographic separation of the total amount of rare earth elements from uranium by passing the solution in sulphuric acid through KU-2 cation exchange resin and eluting the traces of uranium retained by the resin with a solution of ascorbic acid. The rare earth impurities are then eluted from the resin with 4–5N HCl, evaporated, and irradiated for 20 hours with a neutron flux of 1.2·10¹³ n·cm⁻²·sec⁻¹. Subsequently the traces of the rare earth elements are co-precipitated with Fe(OH)₃, dissolved in concentrated HCl and separated from the iron and other impurities by passing the solution through Dowex 1X8 anion exchange resin in the chloride form. The individual rare earth elements are then separated from each other using KU-2 cation exchange resin and a solution of ammonium α-hydroxyisobutyrate as the eluant.     

Assessment of radon concentration and external gamma radiation level in the environs of Narwapahar uranium mine,India and its radiological significance

In the environs of uranium mining, milling and processing facilities and in the uranium mineralized terrain, a little higher ambient radon concentration and gamma radiation level may be expected in comparison with natural background. The present study gives a brief account of atmospheric radon concentration, gamma absorbed dose rate and radiation dose received by the members of public in the vicinity of Narwapahar uranium mine. The ambient radon concentration in the air in the study area was found to vary from 5 to 107 Bq m⁻³ with geometric mean of 24 Bq m⁻³ and geometric standard deviation of 1.74 Bq m⁻³. The measured gamma absorbed dose rate in air at 1 m above the ground ranged from 87 to 220 nGy h⁻¹ with an overall arithmetic mean of 128 ± 18.5 nGy h⁻¹. The mean annual effective dose received by the members of public from inhalation of radon and its progeny and external gamma exposure was estimated to be 0.32 mSv year⁻¹, which is comparable to other reported values elsewhere.     

Neutron activation analysis of pure uranium: Preconcentration of impurity elements

We have developed a radiochemical neutron activation analysis technique (RNAA) of pure uranium with using extraction chromatographic separation of ²³⁹Np from impurity elements in TBP-6M HNO₃ media. The estimation of influence of fission products of ²³⁵U on the results by radiochemical neutron activation analysis has been carried out. For it we have performed NAA with preconcentration of impurity elements. Experiments show that in this case the apparent concentration of Y, Zr, Mo, Cs, La, Ce, Pr, Nd exceeds the true concentration by 2500–3000 times. Therefore, determination of these elements is not possible by RNAA. This technique allowed to use the determination of 26 impurity elements with detection limit 10⁻⁵–10⁻⁹% by mass. This developed technique may be used for the determination of impurities in uranium and its compounds.     

Cyclic neutron activation analysis for determination of selenium in food samples using <Superscript>77m</Superscript>Se

Cyclic neutron activation analysis method was conducted for determination of Se in food samples. High accuracy and good precision were proved by analyzing certified reference materials (CRMs) of chicken (GBW10018), rice (GBW10010) and cabbage (GBW10014). The detection limits for the three CRMs reached 0.16, 0.66 and 1.2 ng after 6 cycles at the 161.9 keV γ-peak from ^77mSe, under a neutron flux of 9.0 × 10¹¹ n cm⁻² s⁻¹ and the conditions of 30 s irradiation, 2 s decay, 30 s counting and 2 s waiting, significantly lower than those of conventional neutron activation analysis without any cycles, which were 0.94, 3.6 and 4.3 ng, respectively.     

Study on physicochemical properties of U<Superscript>3+</Superscript> in LiCl–KCl eutectic media at 773 K

In order to enhance the efficiency of pyrochemical technology, especially electrorefining process, physicochemical data of trivalent uranium in LiCl–KCl eutectic at 773 K were measured, including molar absorptivity, formal potential and diffusion coefficient of U³⁺ ions. The molar absorptivities of U³⁺ were determined to be 765 ± 48 and 686 ± 39 M⁻¹ cm⁻¹ at 465 and 550 nm, respectively. The formal potential of U³⁺/U⁴⁺ redox couple and diffusion coefficient of U³⁺ ions were measured to be −0.308 V vs. Ag/Ag⁺ and 8.7 × 10⁻⁶ M⁻¹ cm⁻¹, respectively. To elucidate the chemical behavior of U³⁺ ions under the existence of oxide ions, U³⁺ ions were reacted with oxides ions in situ produced at the LiCl–KCl media. Surprisingly, it was revealed from XRD patterns that UO₂ was formed from the reaction between U³⁺ ions and O²⁻ ions with the molar ratio of 1:1.     

Search for uranium in high purity silicon

Detection of small quantities of uranium in silicon wafers has been carried out by neutron activation followed by observation of fission product¹⁴⁰La. Irradiations of about one week were made at a flux of 6·10¹⁴n cm⁻² s⁻¹ and the activity of the 1596 keV line was determined. Counting rates of as low as 1 count per minute have been observed. This indicates uranium concentrations of about 5·10¹⁰ atoms per cubic centimeter of silicon or about 0.01 mg/g, assuming activity from other fissionable nuclides to be negligible.     

Neutron activation analysis of uranium in human bone,drinking water and daily diet

Uranium in human bone, drinking water and daily diet has been determined by neutron activation analysis using the²³⁸U(n, γ)²³⁹U reaction. An improved scheme for the separation of the²³⁹U is proposed; with this scheme, after neutron irradiation in a 100 kW TRIGA reactor, a uranium content as low as 5·10⁻¹¹ g can be determined reliably, rapidly and easily. A wide range of uranium concentrations, from about 0.1 ppb up to about 10 ppb has been found in the bones of normal Japanese. Water from several Japanese city water services, and the daily diet taken in two Japanese cities, have been found to contain an average 9·10⁻⁹ g/l and 1.5 μg per person-day uranium, respectively.     

Characterization of an epithermal irradiation facility

An epithermal energy neutron irradiation facility has been used to perform instrumental activation analysis for iodine, silicon, nickel, zirconium, uranium and thorium. The facility, which is adjacent to the fuel of the University of Virginia 2.0 MW pool reactor, consists of a dry sample region surrounded by a fixed cadmium shield. A boron nitride capsule can be used to hold the sample in the cadmium facility to further enhance the reduction of thermal neutron activation. The neutron fluence rate is 2.2×10¹⁶ n·m^–2·s^–1 for fission spectrum energy neutrons (measured with Ni(n,p)Co) and 8.2·10¹⁵ n·m^–2·s^–1 for resonance energy neutrons (measured with gold).Iodine has been measured at concentrations as low as 0.1 mg/kg with 3% counting statistics in powdered infant formula and 0.15 mg/kg with 5% statistics in liquid infant formula. Silicon has been measured at concentrations of 0.2% in biological samples with counting statistics between 5 and 10% and in coal and soil at concentrations greater than 4% with better than 1% statistics. Nickel has been measured in coal and soil at the 20 mg/kg level and higher with 6% statistics. Zirconium has been determined at 600 mg/kg and greater in ceramics with counting errors less than 3%. Uranium and thorium have been measured at the 10 mg/kg level with 3% counting statistics.     

Analytical use of neutron-capture gamma-rays

Certain elements which are not possible to detect with conventional neutron activation analysis can be measured using thermal neutron-capture gamma-ray analysis. The use of a curved neutron guide at the High Flux Reactor, Grenoble, with a thermal neutron flux of 1.5·10¹⁰n·cm⁻²·sec⁻¹ and the advantage of a low-background counting system (Ge(Li) detector) far from the reactor core are described. Experimental detection limits of a number of elements are given for the low-energy and the high-energy regions. Some applications of the capture gamma-ray method in the whole energy range are studied and are briefly discussed.     

A combined method of neutron activation analysis and radiometric measurements for <Superscript>234</Superscript>U and <Superscript>238</Superscript>U determination in soil samples of low uranium concentration

A method that combines the use of non-destructive neutron activation analysis and high-resolution α spectrometry has been developed for determination of the activities of ²³⁴U and ²³⁸U in geological samples of low uranium content. The ²³⁸U content is determined by k₀-based neutron activation analysis, whereas the ²³⁴U/²³⁸U relationship is measured by α spectrometry after isolation and electrodeposition of the uranium extracted from a lixiviation with 6 M HCl. The main advantage of the method is the simplicity of the chemical operations, including the fact that the steps destined to assure similar chemical state for the tracer and the uranium species present in the sample are not necessary. The method was applied to soil samples from sites of the North Peru Coast. Uranium concentration range 3–40 mg/kg and the isotopic composition correspond to natural uranium, with about 10% uncertainty.     

Liquid–liquid extraction of uranium(VI) using cyanex 272 in toluene from sodium salicylate medium

Liquid–liquid extraction and separation studies of uranium have been carried out from sodium salicylate media using cyanex 272 in toluene. Uranium was quantitatively extracted by 1 × 10⁻³ M sodium salicylate with 5 × 10⁻⁴ M cyanex 272 in toluene. The extracted uranium(VI) was stripped out quantitatively from the organic phase with 1.0 M hydrochloric acid and determined spectrophotometrically with arsenazo(III) at 660 nm. The effect of concentration of sodium salicylate, extractant, diluents, metal ion and strippants has been studied. Separation of uranium(VI) from other elements was achieved from binary as well as from multicomponent mixtures. The method was extended for the separation and determination of uranium(VI) in geological samples. The method is simple, rapid and selective with good reproducibility (approximately ± 2%).     

The determination of uranium in biological materials by neutron activation analysis using the fission product134I

A method for the determination of uranium based on²³⁵U thermal neutron fission, has been developed and employed on samples of ashed fish tissue and seaweed. The method involves a post-irradiation ion exchange separation of iodine isotopes. The 884 keV photopeak of¹³⁴I is used for measurement. Uranium detection limits in the samples concerned have been estimated to be 1·10⁻⁸g in terms of natural uranium. The precision achieved in analysing several series of 3–5 samples was 4–10 per cent. The accuracy of the method was tested by employing an independent neutron activation procedure based on²³⁹U measurement. The accuracy of both methods was checked by analysing NBS SRM 1571 ‘Orchard Leaves’.     

Uranium in natural waters sampled within former uranium mining sites in Kazakhstan and Kyrgyzstan

New data are presented on ²³⁸U concentrations in surface and ground waters sampled at selected uranium mining sites in Kazakhstan and Kyrgyzstan and in water supplies of settlements located in the vicinity of these sites. Radiochemical neutron activation analysis (RNAA) was used for ²³⁸U determination in all cases. In addition, for data accuracy assessments purposes, a sub-set of these samples was analysed by high-resolution alpha spectrometry, following standard radiochemical separation and purification. Our data show that drinking waters sampled at various settlements located close to the uranium mining sites are characterised by relatively low uranium concentrations (1.9–35.9 μg L⁻¹) compared to surface waters sampled within the same sites. The latter show high concentrations of total uranium, reflecting the influence from the radioactive waste generated as a result of uranium ore production.     

Transfer of uranium and thorium from soil to different parts of medicinal plants using SSNTD

The uptake of ²³⁸U and ²³²Th in different parts of some selected plants used in traditional treatment of hypertension and diabetes in south-eastern Morocco (Errachidia area) has been studied using two different types of solid state nuclear track detectors (SSNTDs) LR-115 type II and CR-39. Plant uptake of radionuclides is one of many vectors for introduction of contaminants into the human food chain. Thus, it is critical to understand soil–plant relationships that control nuclide bioavailability. Soil concentrations of uranium ranged from 6.10 to 11.62 ppm, with a mean of 7.90 ppm. Soil concentrations of thorium ranged from 2.70 to 4.80 ppm, with a mean of 3.41 ppm. Mean uranium specific activities were 8.38 Bq kg⁻¹ in root tissue, 5 Bq kg⁻¹ in stem tissue and 6.02 Bq kg⁻¹ in leaf tissue. Mean thorium specific activities were 2.53 Bq kg⁻¹ in root tissue, 1.64 Bq kg⁻¹ in stem tissue and 1.96 Bq kg⁻¹ in leaf tissue. The transfer factors of ²³⁸U and ²³²Th from soil to different parts (root, stem, leaf, seed and fruit) of studied plant samples have been investigated. The transfer factors obtained for root plants were markedly higher than those for leaf, stem, fruit and seed plants. Soil-to-plant transfer factor (TF) is one of the most important parameters to be used in transfer models for predicting the concentration of radionuclides in agricultural crops and for estimating dose impacts to man. This study of uranium and thorium uptake in plants used in traditional medicine is also significant as far as the health hazard effects of uranium and thorium in human being are concerned.     

Zirconium Dioxide Solubility in High Temperature Aqueous Solutions

The heat transport purification system of CANDU nuclear reactors is used to remove particulates and dissolved impurities from the heat transport coolant. Zirconium dioxide shows some potential as a high-temperature ion-exchange medium for cationic and anionic impurities found in the CANDU heat transport system (HTS). Zirconium in the reactor core can be neutron activated, and potentially can be dissolved and transported to out-of-core locations in the HTS. However, the solubility of zirconium dioxide in high-temperature aqueous solutions has rarely been reported. This paper reports the solubility of zirconium dioxide in 10⁻⁴ mol⋅kg⁻¹ LiOH solution, determined between 298 and 573 K, using a static autoclave. Over this temperature range, the measured solubility of zirconium dioxide is between 0.9 and 12×10⁻⁸ mol⋅kg⁻¹, with a minimum solubility around 523 K. This low solubility suggests that its use as a high-temperature ion-exchanger would not introduce significant concentrations of contaminants into the system. A thermodynamic analysis of the solubility data suggests that Zr(OH)₄⁰ likely is the dominant species over a wide pH region at elevated temperatures. The calculated Gibbs energies of formation of Zr(OH)₄⁰(aq) and Zr(OH)₄(am) at 298.15 K are −1472.6 kJ⋅mol⁻¹ and −1514.2 kJ⋅mol⁻¹, respectively. The enthalpy of formation of Zr(OH)₄⁰ has a value of −1695±11 kJ⋅mol⁻¹ at 298.15 K.