ISO/TC 85/SC 5/WG 1 - Analytical methodology in the nuclear fuel cycle

This document specifies a method of determining the apparent density and tap density of free-flowing uranium dioxide (UO2) powder which will be used for pelleting and sintering of UO2 pellets as a nuclear fuel. This method can be used for different UO2 powder types including grains, granules, spheres or other kinds of particles. The method can also be applied to other fuel powders as PuO2, ThO2 and powder mixtures as UO2-PuO2 and UO2-Gd2O3. This document is based on the principle of using a flowmeter funnel (see 4.1). Other measurement apparatus, such as a Scott volumeter, can also be used.

This document specifies an analytical method by spectrophotometry, for determining the plutonium concentration in nitric acid solutions, with spectrophotometer implemented in hot cell and glove box allowing the analysis of high activity solutions. Commonly, the method is applicable, without interference, even in the presence of numerous cations, for a plutonium concentration higher than 0,5 mg·l−1 in the original sample with a standard uncertainty, with coverage factor k = 1, less than 5 %. The method is intended for process controls at the different steps of the process in a nuclear fuel reprocessing plant or in other nuclear facilities.

This document specifies a method for the determination of the isotopic and elemental uranium and plutonium concentrations of nuclear materials in nitric acid solutions by thermal-ionization mass spectrometry. The method applies to uranium and plutonium isotope composition and concentration measurement of irradiated Magnox and light water reactor fuels (boiling water reactor or pressurized water reactor), in final products at spent-fuel reprocessing plants, and in feed and products of MOX and uranium fuel fabrication. The method is applicable to other fuels, but the chemical separation and spike solution are, if necessary, adapted to suit each type of fuel.

This document specifies the dissolution of powder samples of plutonium oxide for subsequent determination of elemental concentration and isotopic composition.

This document specifies the dissolution of samples consisting of MOX pellets or powders to provide suitable aliquots for subsequent analysis of elemental concentration and isotopic composition.

This document provides a method for evaluation of the measurement uncertainty arising when an impurity content of uranium solution is determined by a regression line that has been fitted by the "method of least squares". It is intended to be used by chemical analyzers. Simple linear regression, hereinafter called "basic regression", is defined as a model with a single independent variable that is applied to fit a regression line through n different data points (xi, yi) (i = 1,?, n) in such a way that makes the sum of squared errors, i.e. the squared vertical distances between the data points and the fitted line, as small as possible. For the linear calibration, "classical regression" or "inverse regression" is usually used; however, they are not convenient. Instead, "reversed inverse regression" is used in this document[2]. Reversed inverse regression treats y (the reference solutions) as the response and x (the observed measurements) as the inputs; these values are used to fit a regression line of y on x by the method of least squares. This regression is distinguished from basic regression in that the xi's (i = 1,?, n) vary according to normal distributions but the yi's (i = 1,?, n) are fixed; in basic regression, the yi's vary but the xi's are fixed. The regression line fitting, calculation of combined uncertainty, calculation of effective degrees of freedom, calculation of expanded uncertainty, reflection of reference solutions' uncertainties in the evaluation result, and bias correction are explained in order of mention. Annex A presents a practical example of uncertainty evaluation. Annex B provides a flowchart showing the steps for uncertainty evaluation. In addition, Annex C explains the use of weighting factors for handling non-uniform variances in reversed inverse regression. NOTE 1 In the case of classical regression, the fitted regression line is inverted prior to actual sample measurement[3]. In the case of inverse regression, the roles of x and y are not consistent with the convention that the variable x represents the inputs, whereas the variable y represents the response. For these reasons, the two regressions are excluded from this document. NOTE 2 The term "reversed inverse regression" was suggested taking into account the history of regression analysis theory. Instead, it can be desirable to use some other term, e.g. "pseudo-basic regression".

This document describes the ceramographic preparation of uranium dioxide (UO2) sintered pellets for qualitative and quantitative microstructure examinations. These examinations can be carried out before and after thermal or chemical etching. They enable — observations of fissures, inter- or intra-granular pores and inclusions, and — measurement of pore and grain size and measurement of pore and grain size distributions. The measurement of average grain size can be carried out using a classical counting method as described in ISO 2624 or ASTM E112[3], i.e. intercept procedure, comparison with standard grids or reference photographs. The measurement of pore-size distributions is usually carried out by an automatic image analyser. If the grain-size distributions are also measured with an image analyser, it is recommended that thermal etching be used to reveal the grain structure uniformly throughout the whole sample.

ISO 12800:2017 gives guidelines on the determination of the specific surface area of as-fabricated uranium dioxide powder by volumetric or gravimetric determination of the amount of nitrogen adsorbed on the powder, and can be applied to other similar materials, e.g. U3O8, UO2-PuO2 powders, and other bodies with similar surface areas, e.g. powder granules or green pellets, provided that the conditions described are fulfilled. Modifications using other adsorbing gases are included. The method is relevant as long as the expected value is in the range between 1 m2/g and 10 m2/g.

ISO 15366-2:2014 describes procedures to chemically separate and purify uranium and plutonium in dissolved solutions of irradiated light water reactor fuels and in samples of high active liquid waste of spent fuel reprocessing plants, prior to their isotopic analysis by e.g. mass spectrometric method or alpha spectrometry. ISO 15366-2:2014 describes a slightly different separation technique from ISO 15366-1, based on the same chemistry, using smaller columns, different support material and special purification steps, applicable to samples containing plutonium and uranium amounts in the nanogram range and below. The detection limits were found to be 500 pg plutonium and 500 pg uranium.

ISO 15366-1:2014 describes procedures to chemically separate and purify uranium and plutonium in dissolved solutions of irradiated light water reactor fuels and in samples of high active liquid waste of spent fuel reprocessing plants, prior to their isotopic analysis by e.g. mass spectrometric method or alpha spectrometry. ISO 15366-1:2014 describes a technique for the separation of uranium and plutonium in spent fuel type samples based on chromatographic method. The procedure applies to samples containing 1 μg to 150 μg Pu (IV) and (VI) and 0,1 mg to 2 mg U (IV) and (VI) in up to 2 ml of 3 mol·l-1 nitric acid solution. It is applicable to mixtures of uranium and plutonium in which the U/Pu-ratio can range from 0 up to 200.

ISO 15646:2014 describes a procedure for measuring the densification of UO2, (U,Gd)O2, and (U,Pu)O2 pellets, achieved by heat treatment under defined conditions. The densification of fuel in power operation is an important design feature. Essentially, it is dependent on structural parameters such as pore size, spatial pore distribution, grain size, and in the case of (U,Gd)O2 and (U,Pu)O2, oxide phase structure. A thermal re-sintering test can be used to characterize the dimensional behaviour of the pellets under high temperature. The results of this test are used by the fuel designer to predict dimensional behaviour in the reactor, because thermal densification in the reactor is also dependent on these structural parameters, albeit in a differing manner in terms of quantity.

ISO 8300:2013 specifies a precise and accurate gravimetric method for determining the plutonium content in plutonium dioxide (PuO2) of nuclear grade quality, containing a mass fraction of less than 0,65 % of non-volatile impurities. The method is used to cross-check accountancy analyses of plutonium dioxide.

ISO 8425:2013 specifies a precise and accurate gravimetric method for determining the concentration of plutonium in pure plutonium nitrate solutions and reference solutions, containing between 100 g and 300 g of plutonium per litre, in a nitric acid medium.

ISO 21483:2013 specifies an analytical method for determining the solubility in nitric acid of plutonium in pellets of unirradiated mixed oxide fuel (light-water reactor fuels). The results provide information about the expected dissolution behaviour of irradiated pellets under industrial reprocessing conditions. In this aspect, the specific conditions (e.g. time of the test) may vary depending upon the need to match to a specific reprocessor's requirements. The test is aimed at determining solubility under equilibrium conditions rather than the kinetics of dissolution and hence allows for a second dissolution period.

ISO 13465:2009 specifies an analytical method for determining the neptunium concentration by spectrophotometry, with a standard deviation of about 5 %, in nitric acid solutions of nuclear reactor irradiated fuels, at different steps of the process in a nuclear fuel reprocessing plant. The method is applicable to aliquots containing a concentration of neptunium between 10 mg×l-1 and 200 mg×l-1.

ISO 18213-3:2009 presents statistical procedures that can be applied to tank calibration and volume measurement data for nuclear materials accountancy tanks. In particular, ISO 18213-3:2009 presents several diagnostic plots that can be used to evaluate and compare tank calibration data; a procedure for estimating the uncertainties of tank calibration measurements (i.e., determinations of height and volume); a model for estimating either a tank's calibration equation or its inverse (the measurement equation), together with related uncertainties, from a set of standardized tank calibration data (i.e., from a series of standardized height-volume determinations); and a method for computing uncertainty estimates for determinations of liquid volume. It is intended that the methods in ISO 18213-3:2009 be used within the context of the other parts of ISO 18213. Specifically, the methods presented in ISO 18213-3:2009 are tailored to the general methodology described in ISO 18213-1 and to appropriate related algorithms in ISO 18213-2, ISO 18213-4, ISO 18213-5 or ISO 18213-6. Although the methodology in ISO 18213-3:2009 is intended for application specifically within the context of the other parts of ISO 18213, the methods are more widely applicable. In particular, the statistical model presented in Clause 5 for estimating the tank's measurement equation from a set of standardized calibration data can be applied, regardless of whether or not these data are acquired in accordance with the methods of ISO 18213. A similar statement holds for (propagation) methods of variance estimation: it is intended that the results in ISO 18213-3:2009 be applied to the specific models for which they were derived, but the methods themselves are more widely applicable. An option is presented for a facility to develop equivalent plant- or tank-specific methods of statistical analysis as an alternative to ISO 18213-3:2009. However, if a facility adopts ISO 18213 and chooses not to develop equivalent alternative methods of statistical analysis, it is necessary to use the methods of ISO 18213-3:2009.

ISO 21614:2008 describes a method for determining the carbon content in UO2, (U,Gd)O2 and (U,Pu)O2 powder and sintered pellets by combustion in an induction furnace and infrared absorption spectroscopy measurement. ISO 21614:2008 is applicable for determining 10 µg/g to 500 µg/g of carbon in UO2, (U,Gd)O2 and (U,Pu)O2 powder and pellets.

ISO 9278:2008 specifies a method for determining the bulk density and the amount of open and closed porosity of sintered UO2 pellets. The method can be applied to other materials, for example green pellets, and UO2‑PuO2 or UO2‑Gd2O3 pellets.

ISO 21847-3:2007 describes a method for determining trace amounts of 232U in uranium hexafluoride, uranium oxides or uranyl nitrate.

ISO 21847-2:2007 describes a method for determining trace amounts of 238Pu, 239Pu + 240Pu in uranium hexafluoride, uranium oxides or uranyl nitrate.

ISO 21847-1: 2007 describes a method for determining trace amounts of 237Np in uranium hexafluoride, uranium oxides or uranyl nitrate.

ISO 9005:2007 specifies an analytical method for the determination of the oxygen/uranium ratio in uranium dioxide powder and sintered pellets. The method is applicable to reactor grade samples of hyper-stoichiometric uranium dioxide powder and pellets. The presence of reducing agents or residual organic additives invalidates the procedure.

ISO 15647:2004 specifies a method of isotopic analysis of uranium hexafluoride (UF6) with 235U concentrations less than or equal to 5 % by mass and 234U and 236U concentration between 0,001 % by mass and 1,5 % by mass. The method is in routine use to determine conformance to UF6 specifications. ISO 15647:2004 is applicable if the mass spectrometer uses Faraday cups.

ISO 16796:2004 is applicable to the determination of Gd2O3 in powder blends and sintered pellets of Gd2O3 + UO2 from 1 % to 10 %, by the ICP-AES method.

ISO 16795:2004 covers the determination of Gd2O3 in sintered fuel pellets, by X-ray fluorescence spectrometry using the Gd L-alpha line. The fuel pellets are polished before X-ray examination. This method has been tested for mass fractions of from 2 % to 10 % Gd2O3.

ISO 12795:2004 specifies a method of determining the mass fraction of uranium and the oxygen-to-uranium atomic ratio in hyperstoichiometric uranium dioxide (UO2+X) powders and pellets. ISO 12795:2004 is used for the determination of the U mass fraction and the O/U-ratio of nuclear grade uranium dioxide. An impurity correction in the U mass fraction and the O/U-ratio determination should be performed, if the amount of total impurities in oxide form exceeds 1 500 micrograms per gram of sample. Lower impurity levels influence the O/U-ratio by less than 0,000 5 and can be neglected. The non-volatile impurities shall be determined by an appropriate technique, and the correction applied. If the total content of the non-volatile impurities in oxide form is greater than 1 500 micrograms per gram of sample, the overall precision of the method depends on the accuracy of these impurity measurements.

ISO 7097-1:2004 describes an analytical method for the determination of uranium in pure product material samples such as U metal, UO2, UO3, uranyl nitrate hexahydrate, uranium hexafluoride and U3O8 from the nuclear fuel cycle. This procedure is sufficiently accurate and precise to be used for nuclear materials accountability. This method can be used directly for the analysis of most uranium and uranium oxide nuclear reactor fuels, either irradiated or unirradiated, and of uranium nitrate product solutions. Fission products equivalent to up to 10 % burn-up of heavy atoms do not interfere, and other elements which could cause interference are not normally present in sufficient quantity to affect the result significantly. The method recommends that an aliquot of sample is weighed and that a mass titration is used, in order to obtain improved precision and accuracy. This does not preclude the use of any alternative technique which could give equivalent performance. As the performance of some steps of the method is critical, the use of some automatic device has some advantages, mainly in the case of routine analysis.

ISO 7097:Part 2 describes an analytical method for the determination of uranium in pure product material samples such as U metal, UO2, UO3, uranyl nitrate hexahydrate, uranium hexafluoride and U3O8 from the nuclear fuel cycle. This procedure is sufficiently accurate and precise to be used for nuclear materials accountability. This method does not generate a toxic mixed waste as does the potassium dichromate titration. The method may not be applied to scrap or waste samples until such time as it is qualified by obtaining results statistically equivalent to those obtained by the potassium dichromate method on the same sample types. The method recommends that an aliquot of sample is weighed and that a mass titration is used, in order to obtain improved precision and accuracy. This does not preclude the use of any alternative techniques which could give equivalent performance. As the performance of some steps of the method is critical, the use of some automatic device has some advantages, mainly in the case of routine analysis.

ISO 10981:2004 specifies an analytical method for determining the uranium concentration between 0,1 g/l and 400 g/l in nitric acid solutions of irradiated fuel from light-water reactors, gas-cooled reactors and fast-breeder reactors. It specifies how interference by nitrite and plutonium ions is prevented. The other constituents of fuel solutions do not interfere. This method is suitable for process control, but not for accountancy purposes.

ISO 16794: 2003 specifies a test method for the determination of hydrocarbons, chlorocarbons and partially or completely substituted halocarbons or halohydrocarbons contained as impurities in uranium hexafluoride (UF6) by infrared spectrometry. This method cannot be used for compounds giving IR rays with interference by UF6 (for example CF4). The test method is quantitative and applicable in the mole fraction from 0,000 1 % or 0,001 0 %, depending on the type of impurity, up to 0,100 %. The test method can also be used for the determination of hydrofluoric acid (HF) and several elements existing as fluorides; boron in BF3, silicon in SiF4, phosphorus in PF5, molybdenum in MoF6 and tungsten in WF6.

Describes a method of subsampling suitable for taking aliquots from a representative sample of uranium hexafluoride in the liquid phase. The subsamples are intended for isotopic analysis (1 g bis 3 g), impurity analysis (10 g bis 200 g) and uranium assay (5 g bis 10 g). Carbon halides, hydrocarbons and certain metal halides can be measured directly from the sample.

The methods specified are based on heating a portion of the test sample at a temperature of at least 1100 °C to 1200 °C in an oxygen atmosphere, passing the evolved oxidation products over a purification trap filled with manganese dioxide catalyst and silver permanganate catalyst (that oxidises CO to CO2), trapping the CO2, restoring the initial pH continuously by addition of hydroxyl ions either by potentiostatic titrimetry or by coulometry, or alternatively determining the CO2 by IR absorption and integration of the signal obtained. Is applicable to the determination of 5 µg to 500 µg of carbon.

Specifies a titrimetric/coulometric/infrared absorption method for determining the carbon content in uranium dioxide powder and sintered pellet. Applicable to the determination of 5 µg to 500 µg of carbon in uranium dioxide powder and pellets. Interference from sulfur and halogens is prevented by the use of appropriate traps.

Specifies an analytical method for determining the nitrogen content in uranium metal and uranium dioxide powder and pellets. Applicable to the determination of nitrogen, present as nitride.

Specifies an analytical method which can be used within the concentration range of 1 µg to 0,001 g of fluorine per gram of the sample. Specifies the principle, the reagents, the apparatus, the sampling, the procedure, the expression of results and the test report.

Describes the principle, the apparatus, the procedure, the expression of results and the contents of the test report.

ISO 16793:2005 describes the ceramographic preparation of uranium dioxide (UO2) sintered pellets for qualitative and quantitative microstructure examinations.

ISO 9161:2004 specifies a method of determining the apparent density and tap density of free-flowing uranium dioxide (UO2) powder which will be used for pelleting and sintering of UO2 pellets as a nuclear fuel. The method specified by ISO 9161:2004 can be used for different UO2 powder types including grains, granules, spheres or other kinds of particles. The method can also be applied to other fuel powders as PuO2, ThO2 and powder mixtures as UO2-PuO2 and UO2-Gd2O3.

IS0 12800:2003 covers the determination of the specific surface area of as-fabricated uranium dioxide powder by volumetric or gravimetric determination of the amount of nitrogen adsorbed on the powder, and can be applied to other similar materials, e.g. U308, UO2-PuO2 powders, and other bodies with similar surface areas, e.g. powder granules or green pellets, provided that the conditions described are fulfilled. Modifications using other adsorbing gases are included.

Specifies an analytical method for the determination of the oxygen/uranium ratio in uranium dioxide powder and sintered pellets. The method is applicable to reactor grade samples of hyperstoichiometric uranium dioxide powder and pellets. The limit of detection for deviation from stoichiometric composition is 2,002 for uranium dioxide powder and 2,000 2 for sintered pellets.

Describes for two methods the principle, the apparatus, the procedure of the boiling water method and of the m-Xylene impregnation method, the precision, the expression of results and the contents of the test report.