Uranyum örneklerinde safsızlık verimlerinin plazma emisyon spektrometresiyle tayini

Abstract

ÖZET Nükleer güç, ekonomik enerji temininde en önemli enerji kaynaklarından biri olarak kullanılmaktadır. Nükleer güç reaktörleri yapımında kullanılan, özellikle reaktör kalbi ve yakınındaki malzemelerin doğru ve güvenilir olarak belirlenmesi gerekmektedir. Nükleer saflıkta uranyum dioksit, diğer özel liklerinin yeterliliği ile birlikte, içerdiği saf3izlık ele mentleri konsantrasyonlarının toplamının 1500 ppı'i geçmemesi sağlanarak nükleer güç reaktörlerinde yakıt olarak kullanıl maktadır. Nükleer yakıt içindeki safsızl ıkların reaktör verimini düşürmesi yanında düzensiz kullanım zamanına da neden olduğu, ve ayrıca atık malzemelerinde radyoaktivite miktarını istenmeyen düzeyde artırdığı bilinmektedir. Bu çal ışmada, uranyum içindeki birçok safsızl ık elementinin belirlenmesi için uran i I nitrat % 100 Tri-n-butil fosfat (TBP) ile ekstrakte edilmiş olup uranyum organik faza alınarak beiirienen 15 safsızl ık elementinin sulu fazda kaima verimleri Endüktif Bağlantılı plazma Atomik Emisyon Spektrometresi ile incelenmiştir. Uranyumun tamamen uzaklaştırılması yanında safsızl ık ele mentlerinin maksimum verimle kalmaları için, farklı uranyum konsantrasyon ara Ilgındaki uran i I nitrat çözeltileri üç değişik metodla hazırlanmışlardır. Konsantrasyonları 70-120 gU/it ara sındaki uran i i nitirat çözeltilerinin, 2:1 Organ i k/fisi 1 1 i su(0/R) ekstraksiyon, 4:1 (0/fi) yıkama ve 1:1 (O/fi) ekstraksiyon metodu iie yapılan çalışmada, örnekde 2.5-5 ppm uranyum kalmış, ve yüksek oranda (% 85' in üzerinde) safsızl ık elementleri verimleri sağlanabilmiştir. Konsantrasyon arası 6-70 gü/lt uranı I nitrat çözeltilerinin, /:/ (0/fl) ekstraksiyon, 2:1 (Û/fi) yıkama (1:1 (0/fi) ekstraksiyon metodu ile, örnekde 0-2 ppm uranyum)ve % 95-100 arası verimle safsızl ık elementleri kalmıştır. Bu çalışmada uranyumun-TBP ekstraksiyon süresi ve sıcaklık değişikliklerinin etkisi ile ICP-flES de seçilen dalga boylarında % 95 güvenilirlik içinde tayin sınırları belirlenmiştir. ~y-

sunnRRY Nuclear power has been used as an economical and major source of energy supply. Hater i al s used in power plant construction that are inclose proximity to the reactor core must be monitored carefully for composition. Even more important is the accurate evaluation of nuclear fuel itself. Nuclear-grade uranium dioxide is utilized in nuclear fuel production and it must satisfy stringent requirements as to purity. The specifications for maximum concentration limits of impurities in nuclear-grade uranium dioxide state that the total impurity content, based on the summation of the contribution from each impurity, shall not exceed 1500 ppm, and that the total equivalent boron content shall also not exceed 4 ppm as determined from summation of the contribution from each impurity element. This implies that all the impurity elements encountered in nuclear-grade uranium, require quantitative determination. The purity requirements for nuclear-grade uranium are very stringent. Impurities not only affect the metallurgical properties of the uranium, but some impurities also affect the chain reaction efficiency even at concentrations as low as the fractional ppm range. Trace contaminants in fuel have been found to act as `neutron absorbing` poisons, and there by decrease the amount of radioactivity in reactor waste materials. In this work, procedures based on dissolution of the sample prior to the separation of the impurities from the uranium matrix via I i quid- i i qui d extraction followed by Inductively Coupled Plasma-fitoroic Emission Spectrometry (ICP-fiES) analysis have been described. ICP-fiES with its wide acceptance as a versalite technique for the determination of trace elements, lends itself as a possible alternative. The plasma, is a highly ionized gas. In ICP-fiES technique the more common gas used is the argon. The excitation energy is supplied by an induction coil belonging to an oscillant circuit through which passes a high frequency current. Following several ways the argon gives the energy su I tab I e to excite the elements introduced in the plasma. -ya*Recording to their excitation and ionization potentials the different elements present in the plasma mill generate the emission of atom and ion lines, Uhen an element is introduced in the plasma by collisions or by absorbing a radiation, it will reach an excitation level ionized or not, depending on the amonut of energy transmitted, fin excited level is not stable. Then the tendency for the excited element is to come back to the base leuel. For a given element the energy levels ape very well defined, and a transition from one level to another can be done only if this transition is spectroscopical ly allowed. To each transition corresponds an energy given in terms of wavelengths emitted, and these wavelengths are characteristic to each element. The determination of the element concentration, by measuring its intensity of emission line is the working principles of the emission spectrometry. Depending on the type of samples to be measured with the ICP-flES technique, several factors like viscosity, matrix composition, concentration levels, interelement effects, overlaps of emisson lines have an influence on the analytical response. Performing an analysis, the main topic is to be able to correlate the sample analytes responses with responses produced by the standard solutions. If the viscosity, the matrix composition and the temperature are the same for the samples and the standard solutions, there is, in general no use to apply a correction, except if there is a spectral overlap, In fact, it often happens that one parameter changes from one solution to another, then a correction is requested. The main corrections used in ICP-RES are: (i) Spectral interferences correction, (ii) Background changes correction, On the instrumental point of view, some parameters can modify the solution transport, therefore the emission in the plasma. The way of calibrating the spectrometer can correct for these modifications or for the drift with time. Uranium emission lines interfere with the analytical lines of many elements, resulting In poor ICP-flES detection limits and -VİİTbiased deter» i nations. The direct excitation of these laatrices for trace impurity determination is not acceptable owing to severe spectral interferences found in the complex atomic emission spectra of these materials. Consequently, separation procedures are commonly used to enhance the impurity signals in spectrometry analysis. Solvent extraction separation of uranium from the impurities prior to analysis is most effective «ay to eliminate uranium spectral interference. The choice of solvent for a solvent extraction separations procedures is generally governed by the following requirements: (i) Low extract abi I ity of impurities and relatively high extractabi I ity for uranium, (ii) Low mutual solubility with an aqueous phase in contact, (Hi) Adequate radiation and chemical stability, (iv) Low vapor pressure and high flash point, (v) Suitable density, viscosity and inter facial tension in the systems, (vi) Reasonable cost A detailed study was conducted to establish the optimum conditions for uranium extraction with Tri-n-butyl phosphate (TBP), namely; (I) The effect of uranyl nitrate concentration, (ii) Percent recoveries of impurity elements such as fll, Ca, Cd, Cr, Cu, Fe, K, Li, fig, Un, Ho, Na, Ti, 2n and Zr. (iii) The effect of temperature, (Iv) The effect of shaking time (v) The effect of the remained uranium in the sample to the analysis of the impurities.in this work 100 X TBP and uranyl nitrate in 6 N HNO3 mas used and taken as constant in all extractions. The partition coefficient of uranium »as found to increase with increasing TBP concentration to a maximum for pure TBP. This increase may be explained on the basis of free solvent concentration, since more free solvent is obviously available as a result of increasing the solvent concentration. The partition coefficient of uranium depends upon the second power of the free solvent concentration. The effect of uranium concentration on partition coefficient is also a function of nitric acid concentration. For 100 % TBP, the partition coefficient of uranium »as also found to increase with increasing nitric acid concentration. To increase the nitric acid concentration more than 6N, not much increases the partition coeffecient of uranium, but decreases the percent revocery of impurities in the aqueous phase. The extraction of uranium by TBP decreases with increased temperature. -Phosphate, sulfate and flour ide ions reduce the extraction of uranium by TBP from nitrate media. Uranium is extracted from chloride solution but less efficiently than from nitrate solution. Silica couses poor phase separation and the formation of emulsion. The partition of uranium and other metal nitrates between tributyl posphate by the presences of hydrolysis products (mono-and-di-butyl phosphate) in the organic phase. To optimize the extraction process, high purity acids and deionized, distilled water had been used. The aim was to remove uranium and to remain all impurities in aqueous phase, Prior to the determination of impurities, a separation of the major constituent from the trace elements is performed, By means of a reference solution the contamination of the sample by the regents and by the equipment used in demonstrated in the case of the uranium separation, fl multielement standard solution was produced for calibration of the spectrometer. Detection limits in 95 % confidence limit for elements in an uranium extracted solution calculated from the concentration required to give a signal equal to twice the standard deviation of the consecutive exposures. The analytical wavelengths of the analyte lines were evaluated and the lines chosen were free from uranium spectral interferences with in the concentration range of 1 to 10 microgram uranium present per mi of the solution analyzed, It is readily apparent that increase in uranium concentrations will cause a significant possltive bias due to a weak on-line interference that is not explicitly compensated by the simple background correction method employed. -ix-To effect adequate phase separation, the maximum uranium concentration in uranyl nitrate solution prepared from U3OQ, is adjusted to 120 gU/lt. The uranium concentration varied between 6-120 gU/lt in uranyl nitrate solutions are prepared from this stock. The aim is to prepare synthetic standards that simulate some of the extreme compositions and demonstrate that a three different methods will provide reliable analyses. Each method has different organic/aqueous (Q/fl) phase ratios and stages. To compare the relative selectivity and efficiency of each method, synthetic standards containing 6-120 gU/lt and different concentraitons of 15 added impurity elements are extracted and then analyzed by ICP-RES. For the uranyl nitrate solutions contain more than 120 gU/lt, 2:1 (0/fl) extraction, 4:1 (0/fl) scrubbing, 1:1 (0/fl) extraction and 1:1 (0/fl) extraction »as found as suitable procedure to prepare samples. This method Is not recommended for the uranium with high levels of impurities. The better »ay is to dilute the uranyl nitrate solution, The percent recoveries of the impurities In uranium may change to related to their concentrations level, with same method. It means that the same method not always gives the same recoveries of impurities at constant uranium concentration. Suitable procedure to prepare samples was found that uranyl nitrate solutions contain between 70-120 gU/lt, 2:1 (0/fl) extraction, 4:1 (0/fl) scrubbing and 1:1 (0/fl) extraction. These solutions may contain 2.5-5 ppm uranium after extractions, fit these level of uranium concentration, the analyse of impurities is not effectted by ICP-flES. The recoveries of impurities are more than 85 % with this procedure. For the uranium with high level of Impurities, to prepare 70 gU/lt uranyl nitrate solution and the uranium with low level of impurities to prepare 120 gU/lt uranyl nitrate solution is recommended to save the impurities in moderate I eve I s. For the uranyl nitrate solutions contain between 6-70 gU/lt, 1:1 (0/fl) extraction, 2:1 (0/fl) scrubbing and 1:1(0/fl) extraction was found as suitable procedure to prepare samples. These solutions may contain 0-2 ppm uranium after extractions. The recoveries of impurities are between 95-100 % with this procedure. The recoveries of all elements may differ after extraction and these must bu known for an accurate analysis. Zirconium has poor recoveries due to the extraction with TBP. The recoveries of zirconium after extractions are between 1-2 X. The recoveries --X-decreases. The recovery of molybdenum prepared by these procedures is between 50-70 %. The object iye of this work is to establish the conditions for an ICP-fiES analysis procedures that can be replaced several of the currently used techniques, thus achieving a faster analysis, time and a reduction in analysis cost. It has been shown that the uranyl nitrate solutions contain 6-70 gli/lt the, recoveries of impurities ore between 95-100 % with proposed method. For low levels of impurities in uranium, it is better to prepare uranyl nitrate solution that contains 70 gU/lt. find for high levels of impurities, to prepare uranyl nitrate solution that contain between 6-12 gU/lt give better results with this method. Extraction processes allow for the improvement of detection limits by using large sample sizes and smaller final volumes. In summary, the solvent-extraction method discussed here is quite compatible with the multielement ICP-fiES technique, The ICP-fiES1 s precision, accuracy, sensitivity, and multielement capability depend upon the use suitable method for sample preparation. -JUt.