Determination of radioisotopes of Ce,Eu, Pu,Am and Cm in low-level-wastes from power reactors

Long-lived isotopes of lanthanides and actinides are very important for the disposal of low-level radioactive wastes. These nuclides serve for risk calculations of accidents. Their determination requires radiochemical separation from high activity main nuclides. Supervision of waste vessels is done by direct non-destructive -spectrometry of the key nuclides:⁶⁰Co for corrosion products and¹³⁷Cs as for fission products as for transuranic elements. The activity ratios of the long-lived nuclides to the key nuclides are called scaling factors. They have to be determined radiochemically in the laboratory in representative samples of each waste type. They are used for activity calculations of long-lived nuclides in the waste vessels. The scaling factors determined are of the order of magnitude of 10^–6 and due to the fact that we have used low-level measurement techniques, we could performe the necessary chemical separations in a laboratory not exceeding the 10-fold free-level limit.     

Laboratory standard for radionuclides in nuclear waste

A laboratory standard for the determination of long-lived radionuclides in nuclear waste has been prepared using concrete as matrix. It will be used to check sample preparation procedures, chemical separations and methods for the determination of nuclides, e.g., nuclear counting techniques or mass spectrometry. The material chosen was a fine-grained ready-to-use cement mixture, to which the following radionuclides were added:⁶⁰Co and¹³⁷Cs as -ray-emitting nuclides,⁹⁹Tc,⁹⁰Sr and⁵⁵Fe as radionuclides emitting pure -radiation or low-energy electrons, and²³³U,²³⁷Np,²³⁸Pu and²⁴¹Am as -emitters. Care was taken to attain a homogeneous distribution of the nuclides in the standard material. Pieces of about 1 g were formed as suitable and representative samples. Repeated analyses were carried out with the standard to check its homogeneity. The analytical procedures are described in brief. The pieces of the standard material have the same content of nuclides within ±5% at a 95% level of confidence.     

Chemical separation procedure for the nuclide analysis of a tantalum target irradiated for 500 days with 800 MeV protons

A rapid radiochemical separation procedure has been developed for the determination of long-lived and stable nuclides produced by spallation and activation in a tantalum target irradiated for 500 days with 800 MeV protons. In this procedure the matrix element tantalum and simultaneously the¹⁸²Ta activity, built-up by activation of the matrix with themalized spallation neutrons is removed from many elements. About 50 mg of the sample is dissolved in a mixture of concentrated nitric and hydrofluoric acid. After dilution tantalum is extracted with a solution of 0.2M tetrahexylammonium bromide in methyl isobutyl ketone (MIBK). The residual amount of tantalum and the remaining¹⁸²Ta activity are 0.0003% and the recoveries of 27 investigated elements are in the range of 96.0–99.9%. A further 22 elements are quantitatively separated according to their chemical behavior. In the final aqueous fraction the separated long-lived and stable nuclides of 49 elements can be measured with high sensitivity by -ray spectrometry and mass spectrometry (ICP-MS).     

An effective scaling frequency factor method for scaling of harmonic vibrational frequencies: Application to 1,2,4-triazole derivatives

Scaling of harmonic frequencies of a molecule is one of the methods of improving the agreement between the calculated from a quadratic force field and experimental vibrational spectrum. An application of the recently proposed effective scaling frequency factor (ESFF) method to the complicated 1,2,4-triazole derivatives is presented. The calculations are based on the DFT/B3LYP/6-311G** quadratic force fields. It is shown that the ESFF method is capable of providing the high-quality spectra with regard to the scaled frequencies, comparable to these obtained with the well-established scaled quantum mechanical (SQM) force field method. Using the recommended scaling factors for the 11-parameter calculations, the RMS value obtained for a set of 293 vibrational modes of four compounds is only 8.7 and 8.5 cm^?1, for SQM and ESFF, respectively, provided the hydrogen bonded CO bond was excluded from the general non-hydrogen XX stretch group, and the scaling factor attributed to this bond was optimized. The new, 9-parameter set of scaling factors provides SQM- and ESFF-scaled frequencies that are of comparable quality to those of the 11-parameter calculations. In addition, it provides (on average) more reliable band splittings in the middle region of the spectrum, and the order of the scaled frequencies corresponds to that of the experimental bands. The straightforward application of the ESFF method to estimate the value of the scaled frequency is also presented.     

Rapid determination of low concentrations of long-lived alpha-emitters in the atmosphere by alpha-spectrometry

A sensitivity of 0.04 to 0.01 pCi/m³ for long-lived α-emitters in air is obtained by α-spectrometry of electrostatically precipitated aerosols in a large Frisch grid ionization chamber after decay of the natural activity for 2 or 4 hrs and 100 min counting. This corresponds to about 0.1 to 0.01 of the maximum permissible concentrations (168 hrs/week) for²³⁹Pu and other toxic long-lived α-emitters. The dependence of the sensitivity of the procedure on variable background contributions by Rn and Tn daughter products was investigated. After²¹²Bi (ThC) has significantly decayed, the sensitivity is 0.0002 to 0.001 pCi/m³ in the 5 MeV region with 1 000 min counting. Direct determination of the natural²¹⁰Po (RaF) content in the atmosphere or the detemination of other α-emitters at that activity level is thus possible without sample preparation or chemical treatment. An example for the low-level detection of transuranium nuclides in the air of a hot laboratory is presented.     

Neutron activation analysis for determination of induced radioactivity in concrete of nuclear reactor shielding

The content of binding agent components and aggregates of shielding concrete (portland cement, gabbro, silicate, granite, hematite, magnetite, limestone) has been studied to identify nuclides contributing to the long-lived radioactivity of concretes used in nuclear reactor shielding. Elemental content was determined by neutron activation analysis at the IBR-2 reactor of the Laboratory of Neutron Physics (JINR). It is shown that the nuclides mainly responsible for the long-lived activity are¹⁵²Eu,¹⁵⁴Eu,⁶⁰Co and¹³⁴Cs. Based upon the experimental data obtained, calculations of the specific activity of concrete ingredients being irradiated for 30 years by a unit neutron flux density have been performed, and diagrams of the dependence of the specific activity versus time are presented. It is recommended that the application of such media as magnetite, serpentinite, chromite to shielding structures should be abandoned. Estimations of the dose rate from concrete structures containing various aggregates for the limiting case of semi-infinite space are given. The results obtained can form the basis for the optimal choice of the materials when constructing the concrete shielding of neutron sources.     

Impact of operational changes on the scaling factors of radioactive wastes from the IEA-R1 research reactor

Nowadays, the scaling factor methodology is widely used in order to estimate the activity concentration of difficult to measure nuclides in low- and intermediate-level waste from nuclear reactors. However, very few experimental studies evaluate how operational changes in the reactors affect scaling factors. The present work examines the impact of operational changes on the scaling factors that were determined for spent ion-exchange resins and spent activated charcoal permanently withdrawn as radioactive wastes from the water cleanup system of the IEA-R1 nuclear research reactor.     

Geometry Optimization and Quantum-Chemical Calculations of Normal Vibrations for Methyl-N-methylurethane and Methyl-N-(α)Naphthylurethane

The equilibrium geometries and IR frequencies of methyl-N-methylurethane (MU) and methyl-N-()-naphthylurethane (MNU) were calculated by various quantum-mechanical methods: ab initio using the HF/6-31G* basis set, AM1 and PM3 semiempirical methods, and density functional theory (DFT) at the PBE/TZ2P level using MOPAC 6.0, GAMESS, and PRIRODA software. The experimental and calculated frequencies in the range 4000-60 cm^-1 are compared and analyzed. The comparison was performed based on well-studied characteristic urethane bands: Amide A, Amide I, Amide II, Amide III, Amide IV, and Amide V. The calculated frequencies of MU and MNU were compared with the experimental frequencies; the best agreement was obtained by using the DFT method. Other methods require scaling factors. For ab initio calculations, only one linear multiplier, a = 0.889, may be introduced. To obtain agreement with experiment in AM1 and PM3 methods, one must take different scaling factors for different spectral regions. All calculations confirm the previous conclusions that urethane fragment vibrations are characteristic and weakly interacting with methyl group and naphthyl ring vibrations.     

Liquid scintillation radioassay for low-activity beta-emitter mixtures by the method of least squares

From the practicable viewpoint for environmental radioactivity measurement and monitoring, the liquid scintillation radioassay has been applied to low-level pure-beta and -emitter mixtures by using the most probable value theory. In low activity measurement, it is required to select a suitable emulsion scintillator and to deal with the calculated data statistically. Detection limit for a solution sample has been roughly estimated to be 10^–2 Bq ml^–1.     

Rapid determination of90Sr by Compton suppression gamma-ray spectrometry

This work presents a low-level background gamma spectrometry system capable of achieving rapid determination of⁹⁰Sr without chemical separation, in a soil sample when the⁹⁰Sr to -ray emitting nuclide concentration ratio exceeds 10. The system consists of a highly efficient HPGe central detector operated in coincidence or anticoincidence with a NaI(Tl) guard detector and a NaI(Tl) backscatter detector. Experimental results indicate that the bremstrahlung radiation from a pure -emitter can be distinguished with respect to the differences in normal and coincidence spectra. The⁹⁰Sr activity can be directly detemined in the sample in less than 1 hour with a minimum detectable concentration (MDC) estimated at 0.002 Bq·g^–1 for a sample containing no appreciable -ray nuclides or other high energy, -emitting nuclides. In actual measurement, a 0.06 Bq·g^–1 MDC for contaminated soils is achieved.     

Successive determinations of210PB,210PO,226RA,228RA and selected actinides in seawater and sea sediment

Plutonium 239,240 was measured using large-volume water samples from the North Pacific and its adjacent seas. The vertical profiles of^239,240Pu show features that are similar to those reported in earlier papers with a subsurface maxima of around 660 m and a significant concentration increase in the North Pacific bottom waters. It became evident that the^239,240Pu concentration in deep bottom water is proportional to the inventory of the nuclides in the overlying water column and that the appearent distribution ratios of the nuclides between deep-sea sediment and bottom water lie in a relatively narrow range of about 2×10⁴ to 10⁵, independent of sea area and warter depth. The latter implies that^239,240Pu may follow a reversible-type partition process at a deep water/sediment interface. In order to substantiate this assumption, it is desirable to study the phenomenon for Pu as well as for other long-lived radionuclides, both artificial and natural. From this point of view, an analytical procedure for the successive determination of selected long-lived radionuclides was studied.     

Evaluation of methods for the assay of neptunium and other long-lived actinides in environmental matrices

Inductively coupled plasma mass spectrometry (ICP-MS) and neutron activation analysis (NAA) have been investigated as alternatives to alpha-spectrometry for the low-level determination of²³⁷Np and other actinides in environmental matrices. ICP-MS in particular, has been shown here to offer suitable sensitivity, precision and accuracy compared to the other techniques, with considerably faster sample throughput relative to radiometric and activation approaches. Added advantages of ICP-MS are found to include the abilities to determine other long-lived actinides simultaneously and to quantify²³⁹Pu:²⁴⁰Pu ratios. The neutron activation analysis approach was found to be particularly prone to interference especially from uranium nuclides.     

Sorption of the long-lived radionuclides cesium-134, strontium-85 and cobalt-60 on bentonite

The sorption of long-lived radionuclides of cesium, strontium and cobalt (¹³⁴Cs, ⁸⁵Sr and ⁶⁰Co) on bentonite under various experimental conditions, such as contact time, pH, sorbent and sorbate concentrations have been studied. The uptake of Cs and Sr was rapid and equilibrium was reached almost instantaneously in both the cases, while Co sorption was time dependent. The sorption of these nuclides increased by increasing pH. The uptake of Cs, Sr and Co increased with increasing the amount of the bentonite clay. The percentage sorption for Cs, Sr and Co decreased with increasing metal concentrations. The desorption studies with 0.01M CaCl₂ and ground water at low-metal loadings on bentonite showed that about 95% of Cs, 85-90% of Sr and 97% of Co were irreversibly sorbed. These results could be helpful for nuclear waste management, for waste water effluents containing low concentrations of cesium, strontium and cobalt.     

Evaluation of some X-rays and low energy gamma-rays in instrumental neutron activation analysis

The dependence of self-absorption of X-rays and low energy -rays on energy and sample matrix has been studied for^94mNb,¹²⁸I,^104mRh,²³⁹U and²³³Th. Experimentally determined attenuation factors have been compared to calculated values. The use of these nuclides in the instrumental neutron activation of practical samples has been evaluated.     

Determination of radioactive nuclides of uranium and thorium series and estimation of α-flux in LSI constituent materials

Gamma-rays of nuclides of uranium and thorium series in silicone resin and ceramics were measured with a Ge(Li) detector. The state of radioactive equilibrium was determined by comparing the concentrations of uranium and thorium with the radioactive concentrations of²¹⁴Pb and²¹²Pb. The radioactive equilibrium was attained in silicone resin but not in ceramics. The -flux was calculated from the concentrations of uranium and thorium and their daughter nuclides. The calculated values of -flux agreed with that of -flux measured.     

Radiochenical determination of technetium-99 in LLW by chelation with sodium diethyl dithiocarbamate (NaDDC) and extraction with chloroform

A radiochemical method has been developed for the determination of⁹⁹Tc in low-level radioactive, waste from nuclear facilities, using^99mTc as an internal tracer. Radioactive contaminants were removed by carrier hydroxide precipitation and chelating extraction with NaDDC/CHCl₃ system at pH 4. The final technetium was chelated with NaDDC in 3N HCl solution and extracted selectively into chloroform. The average of radiochemical recovery for various types of LLW sample is about 90%. The decontamination factors for most radioactive nuclides are higher than 10⁵. The detection limit for⁹⁹Tc in a sample of about 10 g is 0.17 pCi/g (6.5 Bq·kg^–1) for a 100-minute count.     

Investigation on the corrosion of brazed joints by means of scanning auger microprobe

As a model experiment for the corrosion behaviour of brazed joints in water pipes made of stainless steel, some tens ofan broad joints made of three different brazes (Ag/Cu/In/Ni, Ag/Cu/Sn, Ag/Cu) were exposed to streaming water at different Cl^– concentrations. The samples were polarized. The three-dimensional distribution of the corrosion products, obtained by Auger mapping and depth profiling, supplied some information about the corrosion behaviour of the joints.     

Comparative study of cesium adsorption on dioctahedral and trioctahedral smectites

Bentonites which are characterized by good rheological, mineralogical and chemical stability is considered used as sealing barriers in multibarrier Slovak system of deep geological repository for high-level radioactive waste and spent nuclear fuel. In Slovak Republic there are several significant deposits of bentonite, which are characterized by appropriate adsorption properties and meet the geotechnical requirements for this type of barriers. Study of adsorption properties of bentonites and other smectites is an essential step for developing the migration model long-lived corrosion and activation products, and fission products of uranium. Nuclear wastes contain the most important nuclear fission products, radioisotopes ¹³⁴Cs and ¹³⁷Cs. The present paper investigates and compares the cesium adsorption properties of Slovak and North America bentonites composed mainly of dioctahedral smectite montmorillonite (J, L, SAz-1 and STx-1) and trioctahedral smectites saponite (SapCa-2) and hectorite (SHCa-1).     

Interference by neutron induced second order nuclear reaction in activation analysis of platinum group elements after a nickel sulphide fire assay preconcentration

In determining the trace platinum group elements and gold in rocks and ores by the neutron activation analysis after a nickel sulphide fire assay preconcentration, there are interferences due to nuclides produced from second order nuclear reactions. This paper presents the degree of interference calculated over the ranges of long irradiation times and of reactor neutron flux from 1·10¹³ to 1·10¹⁵ n·cm^–2·s^–1. According to the results of these calculations, every one of the second order interfering reactions on the PGE+Au, except the¹⁹⁷Au(n, )¹⁹⁸Au(n, )¹⁹⁹Au reaction, can be neglected under the long irradiation time or high reactor neutron flux. Special attention is given to the interference from gold in the determining platinum.