Patent Description:
Commonly, methods for the quantification of <NUM>U in a system at very low to vanishing concentrations of <NUM>Ra apply gamma-ray spectroscopy to measure the intensity of gamma-rays within the <NUM>U-specific <NUM> keV peak range (Kaspar et al. Since <NUM>U does not emit gamma rays, its amount in a sample is quantified by measuring the peak intensity of <NUM> keV gamma rays emitted by the decay chain product <NUM>Pa under the condition that the first three members of the <NUM>U decay chain <NUM>U, <NUM>Th and <NUM>Pa have reached secular equilibrium. Under the condition that samples contain natural uranium only, i.e. at (<NUM>±<NUM>) % abundance for <NUM>U, the <NUM> keV gamma peak intensity is used to determine the amount of total uranium in a sample. Disadvantageously, <NUM>Ra as a radioactive isotope of the <NUM>U decay chain emits gamma rays with the main gamma line at <NUM> keV. This gamma line superposes the <NUM> keV gamma line from <NUM>U in natural systems, for example in uranium ore, significantly. Thus, the quantification of the <NUM>U concentration is no longer reliable.

However, systems with very low or vanishing concentration of <NUM>Ra are obtained by highly selective leaching of uranium from natural uranium ore. Thus, the quantification of the <NUM> keV peak intensity can be used to determine the concentration of <NUM>U and total U (Khorfan et al. <NUM>, Singh et al. <NUM>, Sundar et al.

Singh et al. describe a method for online measurement of uranium in solution by gamma-ray spectroscopy in a container with inlet and outlet using standard solutions of known concentrations (Singh et al.

<CIT> discloses a device for the online measurement of uranium concentration in a leach slurry of uranium ore based on the Nal(TI) measurement of the <NUM> keV gamma peak area of <NUM>U. Disadvantages of the disclosed method is the interference of <NUM>Ra and volume or matrix effects, respectively, based on calibrating the system by applying a<NUM>U calibration source, a point-source that is not exactly representative for the voluminous slurry sample.

<CIT> and <CIT> describe a device for the online measurement of uranium concentration in solution based on a Nal(TI) scintillator detecting the gamma rays from a rather small solution compartment. Advantageously, matrix effects are decreased. Disadvantages of the disclosed method is the limited efficiency.

<CIT> discloses a method for the online measurement of uranium concentration in solution based on the Nal(TI) measurement of the <NUM> keV gamma peak area of <NUM>U. <CIT> describes the calibration of the system by using <NUM> reference samples at given uranium concentrations in <NUM> nitric acid solution.

Alternatively, measuring principles to quantify the uranium concentrations in metallurgical processing media are based on the spectroscopy of characteristic electromagnetic radiation ranging from photometric, hyperspectral or Raman spectroscopic applications or X-ray fluorescence. <CIT> describes a method and apparatus for measuring low concentrations of radionuclides, in particular uranium (<NUM>U), in liquid media "in-line" within a nuclear fuel fabrication or reprocessing facility by measuring gamma radiation with a gamma-ray detector and comparing it to an external gamma radiation source.

Disadvantageously, these methods are not applicable or at least not reliable in industrial applications, mainly caused by significant radiation attenuation in the sample as well as in the measuring window.

The object of the present invention is to provide a method for the quantification of radionuclides in liquid media which overcomes disadvantages of the state of the art.

The object has been solved by providing a method for the quantification of radionuclides in liquid media according to claim <NUM>.

As used herein, the term "gamma ray spectra template" refers to a response distribution of gamma-ray detector signals for one gamma-ray source as a function of gamma-ray detector signal pulse-height given in gamma-ray energy equivalent, wherein the template is obtained by computer simulation.

As used herein, the term "calibration factor" refers to an individual factor for each radionuclide, which correlates a concentration of the radionuclide in the liquid media with the intensity parameter from the spectral fitting.

As used herein, the term "simulation" refers to a modelling of radiation transport from a gamma-ray source, preferably the voluminous liquid media comprising at least one radionuclide in the measuring cell, to the gamma-ray detector considering transport processes in the medium itself, in the measuring cell construction materials, in the detector and its housing.

Advantageously, the method according to the invention quantifies radionuclides in liquid media in real-time. As used herein, the term "real-time" refers to a quantification of radionuclide concentrations within a short time, preferably in the range of <NUM> to <NUM>, more preferably <NUM> to <NUM>, most preferably <NUM> to <NUM>. The time for quantification of the radionuclide concentration with the method according to the invention depends on the time of measuring a gamma ray spectrum of the liquid media with the gamma ray detector according to step b), in particular the concentration of the radionuclide, its specific gamma-ray emission intensity and the gamma-ray detector response.

Further advantageously, the method according to the invention is self-compensated, in particular compensating for matrix effects, temperature effects and background effects. The term "self-compensated" refers to the compensation or correction, respectively, of the matrix effects, the temperature effects and the background effects within the method itself, in particular by providing simulated gamma ray spectra templates for radionuclides and the calibration factors, wherein the gamma ray spectra templates are corrected for matrix effects. Temperature effects that lead to a scaling of the measured gamma ray pulse-height spectrum are considered by a pulse-height scaling factor in the spectral fitting procedure. The method further includes the computer-implemented identification and quantification of the radionuclide using the simulated gamma ray spectra templates for radionuclides and calibration factors, wherein the measured gamma ray spectrum is corrected for background effects.

Further advantageously, using the method according to the invention for real-time quantification, an automated process control can be applied.

In embodiments, the at least one radionuclide is selected from the group of most common natural gamma-ray emitting radionuclides, preferably from the group comprising <NUM>U, <NUM>Pa, <NUM>Th, <NUM>Ra, <NUM>Pb, <NUM>Bi, <NUM>Ac, <NUM>Pb, <NUM>Tl, <NUM>K and artificial radionuclides. Advantageously, all gamma-ray emitting radionuclides listed above are considered in the data processing according to step c) and step d) usually. The data interpretation incorporates the half-lives of all radionuclides within the radioactive decay chains.

Preferably, at least one radionuclide is <NUM>U, at least one radionuclide from the <NUM>U decay chain, in particular <NUM>Pa, <NUM>Th, <NUM>Ra, <NUM>Pb, <NUM>Bi; at least one radionuclide from the <NUM>Th decay chain, in particular<NUM>Ac, <NUM>Pb, <NUM>Tl; <NUM>K, or any artificial radionuclide.

As used herein, the term "radioactive decay product" refers to a radionuclide arising from a radioactive decay chain, preferably from <NUM>U, <NUM>U, or <NUM>Th.

Advantageously, the method according to the invention enables the quantification of artificial radionuclides. In embodiments, artificial radionuclides are fission products as part of nuclear reactor waste or from nuclear weapon tests, preferably <NUM>Cs and <NUM>Ba; or isotopes used for medical or technical applications, preferably <NUM>Co.

Advantageously, the method according to the invention is applicable for liquid media comprising more than one radionuclide, in particular complex mixtures of radionuclides, in particular for conditions of disequilibrium of radionuclides and their decay products.

In further embodiments, the liquid media comprising at least one radionuclide according to step a) is selected from hydrometallurgical process media, preferably leach slurries, leach solutions, concentrated processing solutions and waste solutions. Preferably, the liquid media comprising at least one radionuclide is selected from uranium mining solutions or rare-earth element processing solutions.

Advantageously, leaching of uranium ores, e. in leach slurries or leach solutions, results in selective dissolution of uranium while decay products of uranium, in particular <NUM>Ra, are not dissolved or are immobilized by secondary precipitation. Thus, the <NUM>U spectrum with the specific <NUM> keV gamma line is not superposed by the <NUM>Ra spectrum with the specific <NUM> keV gamma line and the <NUM>U and consequently, the uranium concentration can be quantified precisely. In some embodiments, the spectral fitting corrects for the remaining interference by gamma rays from residual <NUM>Ra. Advantageously, significant <NUM>Ra interference can be corrected with the method according to the invention.

In embodiments, the liquid media comprising at least one radionuclide according to step a) is provided with a continuous flow. Advantageously, a continuous flow enables the measurement in real-time, in particular the measurement of any changes of the radionuclide composition and concentration.

In embodiments, the liquid media is provided with a plug-like flow. As used herein, the term "plug-like flow" refers to a velocity profile of a liquid media, wherein the velocity of the fluid is nearly constant across any cross-section of the measuring cell perpendicular to the axis of the measuring cell. Advantageously, the plug-like flow minimizes the time of the exchange of the liquid media in the measuring cell. A very low exchange time is required to minimize the off time of the measuring device during the transition from processing media and pure water for background measurement or during the transition between different inflow lines (multiplexer).

In embodiments, the liquid media is provided with a flow rate in the range of <NUM>/min to <NUM>/min. Advantageously, this flow rate enables an exchange of the liquid media in the measuring cell in a short time, preferably in the range of <NUM> to <NUM>.

In further embodiments, the measuring cell is a flow-through measuring cell. Alternatively, the measuring cell is a processing unit, preferably a pipe or preferably agitated tank.

Preferably, the gamma-ray detector is positioned in the center of the measuring cell. As used herein, the term "center" is the point equally distant from the outer limits.

In embodiments, the gamma-ray detector is surrounded by a watertight containment, preferably a double walled cylindrical container.

In further embodiments, the distance from the outer limits of the measuring cell to the gamma-ray detector wall is in the range of <NUM> to <NUM>, preferably in the range of <NUM> to <NUM>. Advantageously, the gamma-ray detector in the center of a measuring cell with a distance from the outer limits of the measuring cell to the gamma-ray detector wall in the given range enables the measurement of gamma rays from a large sample volume, preferably an effectively infinite sample volume, which means that an increase of the sample volume would not result in an increase of the gamma-ray detector response. Advantageously, a large sample volume results in an increase of the count rate and a decrease of the statistical uncertainty of the measurement. Thus, the measurement time is decreased and a real-time measurement is possible. Advantageously, the geometry of the measuring cell can be optimized with regard to the main target radionuclides emitting specific gamma rays with corresponding ranges in the liquid media itself.

In embodiments, a liquid media comprising at least one radionuclide according to step a) is provided by leaching and/or processing of a material or a solution comprising at least one radionuclide.

In a further embodiment, the leaching and/or processing of a material or a solution comprising at least one radionuclide according to step a) and measuring the gamma-ray spectrum of the liquid media according to step b) are done in-line or in-situ.

As used herein, the term "in-line" refers to measurement of radionuclide-containing media in a processing line, whereas the measuring cell is part of or connected to the processing line or processing unit itself.

As used herein, the term "in-situ" refers to the measurement of radionuclide concentrations in the processing media directly, i.e. without taking samples to be measured off-site in an analytical laboratory.

Preferably the method according to the invention is carried out in the order of the steps a), b), c) and d) or c), a), b) and d) or c).

In embodiments, the measurement time of the gamma spectrum according to step b) is in the range of <NUM> to <NUM>, preferably in the range of <NUM> to <NUM>.

According to the invention, simulated gamma-ray spectra templates for radionuclides and calibration factors are provided, wherein the gamma-ray spectra templates are corrected for matrix effects by the simulation set up for the measuring geometry, i. the position of the gamma ray detector in the measuring cell and the geometry of the measuring cell itself.

As used herein, the term "matrix effects" refers to changes in the gamma-ray spectrum due to adsorption and scattering of gamma rays in the liquid media, e. the <NUM> keV gamma ray of <NUM>U is absorbed to <NUM>% in the thickness of <NUM> of water. Advantageously, the method according to the invention corrects matrix effects, in particular strong matrix effects in a large sample volume.

As used herein, the term "temperature effects" refers to changes in energy scale and resolution shifts and thus, the pulse height for an electron in the pulse-height spectrum, resulting from the interaction of a gamma ray within the scintillator due to changes of the temperature. Advantageously, using spectral fitting in step d) also corrects temperature effects, in particular energy scale and resolution shifts.

Preferably, the temperature effects are corrected by a pulse-height scaling factor applied to the spectral fitting procedure according to step d).

In further embodiments, the simulated gamma-ray spectra templates for radionuclides and calibration factors in step c) are obtained by simulation with a statistical radiation transport software for complex systems, preferably with a Monte Carlo N-Particle (MCNP) code as provided by Los Alamos National Laboratory (LANL) or GEANT4. Preferably, the software MCNP6. <NUM> is used (https://mcnp. Advantageously, the statistical radiation transport software for complex systems takes account of the measuring geometry and the material.

In embodiments, the simulated gamma-ray spectra templates for radionuclides and calibration factors in step c) are provided for the most common natural gamma-ray emitting radionuclides, preferably for the radionuclides <NUM>U, <NUM>Pa, <NUM>Th, <NUM>Ra, <NUM>Pb, <NUM>Bi, <NUM>Ac, <NUM>Pb, <NUM>Tl and/or <NUM>K. Advantageously, all natural gamma-ray emitting radionuclides listed above are considered in the data processing according to step c) and step d) usually.

Preferably, the computer simulated gamma-ray spectra templates for radionuclides and calibration factors are validated against chemical analysis of radionuclide samples, preferably with inductively coupled plasma mass spectrometry (ICP-MS) or precision titration methods.

According to the invention, the at least one radionuclide in the liquid media is identified and its concentration in the liquid media is quantified using a spectral fitting algorithm of the measured gamma ray pulse-height spectrum by a weighted combination of the simulated gamma ray pulse-height spectra templates for radionuclides and the calibration factors, wherein the spectral fitting corrects temperature effects.

As used herein, the term "spectral fitting" refers to a procedure using a model-function which is a weighted sum of radionuclide-specific pulse-height templates. Each radionuclide-specific pulse-height template is obtained by transformation of gamma-ray energy-spectra (i.e. gamma ray spectra templates) to pulse-height spectra. The fitting procedure varies the weights of the summands of the model-function (also called weight-factors) and it also varies parameters for energy scale and energy resolution. The spectral fitting procedure further comprises the transformation of weight-factors to concentrations with the calibration factors. The spectral fitting procedure corrects for the gamma-ray background.

In embodiments, the spectral fitting is a least chi square fit, a likelihood fit or similar, preferably a least chi square fit.

In embodiments, the correction of background effects is done by measuring a background spectrum with clean water, preferably in the range of every hour to every week, more preferably once a day. Advantageously, measuring the background spectrum enables the identification of an increase scaling or fouling effects, i.e. the accumulation of radionuclide-bearing substances at the surface of the gamma-ray detector or the inner surface of the measuring cell. If the scaling or fouling effects exceed a tolerable limit, the measuring cell has to be cleaned. The tolerable limit is defined by the user in dependence on the specific application.

In embodiments, a control system monitors the background effects and/or signals exceeding the tolerable limit.

In further embodiments, the method according to the invention, wherein the at least one radionuclide is uranium, comprises a further step, wherein the interference of <NUM>Ra on the measurement of <NUM>U and/or U concentrations is corrected. The correction of the interference of <NUM>Ra is achieved by quantification the dominant gamma-ray emitting radionuclides of the <NUM>U decay-chain, in particular <NUM>Bi, <NUM>Pb, and <NUM>Ra, and by constraining their weighting factors in the gamma-ray fitting to a fixed ratio, i.e. the ratio corresponding to secular equilibrium between <NUM>Bi, <NUM>Pb, and <NUM>Ra.

In further embodiments, the method according to the invention, wherein the at least one radionuclide is uranium, comprises a further step, wherein the total U concentration is calculated from the <NUM>U concentration with the fixed <NUM>U isotope abundance of (<NUM>±<NUM>) %.

In embodiments, the gamma-ray spectra are stored, preferably every <NUM> to every <NUM>. Advantageously, the gamma-ray spectra to be analysed by the spectral fitting are deduced from the stored gamma-ray spectra as sliding average in a way to realize appropriate statistical uncertainties.

Another aspect of the invention is a device for the quantification of radionuclides in liquid media according to the method according to the invention.

Preferably, the device according to the invention comprises a data processing unit adapted to execute step d) of the method according to the invention.

Another object of the invention is a device for the quantification of radionuclides in liquid media according to claim <NUM>.

In embodiments, the at least one measuring cell is flow-through measuring cell. Alternatively, the measuring cell is a processing unit, preferably a pipe or a tank, more preferably an agitated tank with at least one inlet and outlet.

In further embodiments, the measuring cell has a volume in the range of <NUM> I to <NUM> I, preferably in the range of <NUM> I to <NUM> I.

In embodiments, the at least one measuring cell is shielded against external background gamma-ray sources. Preferably the at least one measuring cell is lead (Pb) shielded.

In further embodiments, the Pb shielding has a thickness of at least <NUM>, preferably in the range of <NUM> to <NUM>. In further embodiments, the Pb shielding is selected from lead mats, lead rings and lead plates. Advantageously, the Pb shielding shields the measurements, in particular the gamma-ray detector, against external background gamma-ray sources. Thus, the detection limit of the quantification is decreased.

In embodiments, the gamma-ray detector is a gamma scintillator, preferably a cerium or lanthanum halide or a Nal(TI) scintillator. Advantageously the gamma-ray detectors are high-performance and/or large-volume gamma-ray detectors with a high detection efficiency and a good resolution. Further advantageously, the high detection efficiency combined with a large sample volume minimizes the statistical uncertainty and the measurement time.

Preferably, the gamma scintillator is selected from the group comprising CeBr<NUM>, LaBr<NUM>:Ce, LaCl<NUM>:Ce and Nal(TI) scintillator, more preferably the gamma scintillator is a CeBr<NUM> scintillator.

In further embodiments, the gamma-ray detector is a large size detector. Advantageously, a large size detector increases the effective spectral count rate and thus, decreases the measurement time.

In embodiments, the watertight containment and/or an inner lining of the measuring cell is made of plastics. Advantageously, plastics adsorb a minimum of gamma rays. Further advantageously, plastics withstand liquid media with high salt concentrations and/or low pH values, preferably down to pH <NUM>. Preferably, the watertight containment and/or an inner lining of the measuring cell is not made of polyvinyl chloride (PVC). Disadvantageously, thorium is adsorbed by PVC and thus, would contribute to the background and influence the detection limit.

In embodiments, the device according to the invention further comprises at least one processing unit, preferably a tank or pipe. In a further embodiment the processing unit is connected to the measuring cell directly by an inlet or by a by-pass or branch-off flow-through measuring cell.

In embodiments, the device according to the invention further comprises at least one further element selected from the group comprising a rate meter, a peak stabilizer, a cooler, in particular an electric cooling or nitrogen (N<NUM>) cooling, and a multiplexer, preferably a hydraulic multiplexer.

Advantageously, a multiplexer enables the measurement of radionuclide concentrations in at least two processing units, preferably <NUM> to <NUM> processing units, more preferably <NUM> to <NUM> processing units, in a sequential manner. In an embodiment, the multiplexer is setup by a control unit.

In preferred embodiments, the device according to the invention comprising a multiplexer includes one inlet from a pure-water reservoir. Advantageously, the pure-water reservoir is used to measure the background of the gamma-ray spectrum. Further advantageously, the device with a multiplexer comprising one inlet from a pure-water reservoir enables flushing of the device and measurement of background spectrum in an optimum time.

Advantageously, measuring the background spectrum enables the identification of increased scaling or fouling effects, i.e. the accumulation of radionuclide-bearing substances at the surface of the gamma-ray detector or the inner surface of the measuring cell.

Another aspect of the invention is the use of the method according to the invention and/or the device according to the invention for the quantification of radionuclide concentrations in hydrometallurgical processing media, preferably the quantification of uranium and/or radioactive uranium decay product concentrations in uranium mining solutions or the quantification of thorium and/or radioactive <NUM>Th decay products in rare-earth element processing solutions.

In some embodiments, the method according to the invention and/or the device according to the invention are used for assessing the efficiency of the removal of thorium and/or radioactive <NUM>Th decay products from the rare-earth elements by:.

In further embodiments, the recently described embodiments can be combined, in particular the embodiments of the method according to the invention can be applied to the device and the use according to the invention and the embodiments of the device according to the invention can be applied to the method and the use according to the invention.

The present invention will now be further explained by the following non limiting figures and examples.

Examples of the device according to the invention are shown in <FIG> A, B and <FIG>. In the first example the measuring cell comprises a gamma-ray detector, wherein the gamma-ray detector is installed at the wall of the measuring cell (<FIG>). In the second example the flow-through measuring cell is an elbow pipe, wherein the gamma-ray detector is installed (<FIG>). In both examples, the gamma-ray detector comprises a scintillator, a photomultiplier tube and an integrated base of the γ-spectrometer comprising a high-voltage power supply, a pre-amplifier and a multi-channel analyzer. Furthermore, the device according to the invention comprises a computer and programmable logic controller for the full scale spectroscopic data analysisand on-line access to the data measured by the device according to the invention.

A further example includes an inlet multiplexer (<FIG>). The flow-through measuring cell is a large-volume compartment (<NUM> inner diameter, <NUM> inner height) for the liquid media with one inlet and one outlet and with a Pb shielding of <NUM> thickness and comprises a gamma-ray detector, wherein the gamma-ray detector is installed at the wall of the measuring cell. The gamma-ray detector comprises a scintillator, a photomultiplier tube and an integrated base of the γ-spectrometer comprising a high-voltage power supply, a pre-amplifier and a multi-channel analyzer. Furthermore, the device according to the invention comprises a computer and programmable logic controller for the full scale spectroscopic data analysis and on-line access to the data measured by the device according to the invention. The computer is connected to a hydraulic control for the multiple setup of the inlet multiplexer in order to unambiguously allocate the measured data to the origin (e.g. pipe) of the liquid media.

A uranium-bearing liquid media (sulfuric-acid leach solution) containing <NUM>/l uranium with strongly suppressed concentrations of uranium decay products (achieved by selective leaching of uranium) was measured with the device according to <FIG> and compared with a fitted spectrum deduced by a mathematical chi-square fit procedure for decomposition based on template spectra for the radionuclides <NUM>K, <NUM>Bi, <NUM>Pb, <NUM>Pa, <NUM>Th, <NUM>Ra and <NUM>U (computer-simulated with Monte Carlo N-Particle code as provided by Los Alamos National Laboratory) and background (measured with water). <FIG> and <FIG> represent the measured spectrum (in logarithmic and linear scale, respectively) in comparison with the fitted spectrum as well as all template spectra considered.

The effect of the background spectrum and measurement time on the relative statistical uncertainty of radionuclide concentrations and the lower level of detection is shown in <FIG> as function of uranium concentration in a leachate solution as an example, wherein the <NUM>Ra concentration is vanishing. By increasing the thickness of the lead shielding from <NUM> to higher values the idealized case at minimum statistical uncertainty and lower level of detection shown in <FIG> can be approached, wherein actual conditions dependent on local background level and thickness of lead shielding. <FIG> shows the uncertainty conditions for a moderate <NUM> lead shielding.

Prepared for the U. Department of Energy under Contract DE-AC05-76RL01830.

Claim 1:
Method for the quantification of radionuclides in liquid media comprising the following steps
a) Providing a liquid media comprising at least one radionuclide in a measuring cell, wherein a gamma-ray detector is positioned in the measuring cell,
b) Measuring a gamma-ray pulse-height spectrum of the liquid media with the gamma-ray detector,
characterized in that the method further comprises
c) Providing simulated gamma-ray pulse-height spectra templates for radionuclides and calibration factors,
wherein the gamma-ray pulse-height spectra templates are corrected for matrix effects,
d) Computer-implemented identification and quantification of the at least one radionuclide in the liquid media using a spectral fitting of the measured gamma-ray pulse-height spectrum by a weighted combination of the simulated gamma-ray pulse-height spectra templates for radionuclides and the calibration factors, wherein the spectral fitting corrects temperature effects and background effects,
wherein the spectral fitting comprises the transformation of weight-factors from the weighted combination of the simulated gamma-ray pulse-height spectra templates for radionuclides to concentrations of the radionuclides in the liquid media with the calibration factors.