Measurements: Radiochemical separation methods and radiometric analysis. Equipment and radionuclides.

Transcription

1 Measurements: Radiochemical separation methods and radiometric analysis. Equipment and radionuclides. Peter Ivanov Acoustics & Ionising Radiation National Physical Laboratory, UK IAEA WS RER9106/9009/01, Visaginas, Lithuania, August 2015

2 Contents Introduction MARLAP approach Sample preparation- receiving, inspection, tracking Radiochemistry separations- precipitation, solvent extraction, ion-exchange, extraction chromatography Quantification of Radionuclides- sample pre-treatment, instrument calibration, gamma-detection methods, alphadetection methods, beta detection methods, specialized analytical methods Data acquisition and reporting

3 Introduction ETM vs. DTM nuclides Stages of radionuclide analysis and quantification When sample pretreatment is required Types of radiochemistry procedures Radiometric counting methods Specialised analytical methods

4 MARLAP approach (EPA) Multi-Agency Radiological Laboratory Analytical Protocols (MARLAP) Provides guidance in the relevant areas of radioanalytical laboratory work Highlights common radioanalytical planning issues Provides a framework and information resource for using a performancebased approach for planning and conducting radioanalytical work Guidance on linking project planning, implementation, and assessment Provides guidance for evaluating radioanalytical laboratory data, i.e., data verification, data validation, and data quality assessment

5 MARLAP approach (EPA) MARLAP is a performance-based approach to laboratory measurements, applicable to a wide range of projects and activities that require radioanalytical laboratory measurements, i.e. Site characterization activities; Site cleanup and compliance demonstration activities; License termination activities; Decommissioning of nuclear facilities; Remediation and removal actions; Effluent monitoring of licensed facilities; Emergency response activities; Environmental site monitoring; Background studies; Waste management activities.

6 Key MARLAP concepts Data life cycle, includes planning, implementation, assessment

7 Key MARLAP concepts Analytical process- compilation of activities from the sample collection to the reporting of data Main stages: Sample receipt, tracking and inspection Sample preparation, homogenising, dissolution Radiochemical separation Preparation of counting sources Measurement Data reduction and reporting

8 Sample preparation Receiving Sample logging and tracking Crashing, drying, homogenising Dispensing, weighting Sample dissolution (for destructive analysis) Preparing of counting sources (for NDA)

9 Radiochemical pre-treatment Radiochemical separations: Precipitation Solvent extraction; Ion-exchange chromatography; Extraction chromatography

10 Precipitation Fractional crystallisation- historically the first separation method used by M. and P. Curie to separate 226 RaCl 2 from BaCl 2 based on solubility differences Precipitation- only applicable if the solubility limit is exceeded, hence it works at high concentrations If a small amount of RN has to be precipitated- adding a carrier is needed Isotopic carriers lead to dilution and decreasing the specific activity, so better use non-isotopic carrier, which can be separated at later stage Hydroxides such as Fe(OH) 3 are suitable for co-precipitation and have high sorption capacity Actinides Soluble Nitrates (NO 3- ) Halides (Cl -, Br -, I - ) Sulfates (SO 2-4 ) Perchlorates (ClO 4- ) Insoluble Fluorides (F - ) Oxalates (C 2 O 2-4 ) Hydroxides (OH - ) Phosphates (PO 3-4 )

11 Solubility and the Solubility Constant K sp Solubility equilibrium refers to the equilibrium that describes a solid forming from solution, i.e. formation of insoluble strontium carbonate: Sr 2+ + CO 3 2- SrCO 3 (s) Then, the solubility product constant is expressed in terms of the concentrations of ions in solution: K sp =[M n+ ] m [N m- ] n For strontium carbonate, K sp is defined in terms of the concentrations of Sr 2+ and CO 3 2- : K sp = [Sr 2+ ][CO 3 2- ] = 1.6x10-9 In order for the carbonate to precipitate, the value of the K sp must be exceeded, but the concentration of each common ion doesn t have to be equal. For example, if [Sr 2+ ] is 1x10-6 M, then [CO 3 2- ] must be greater than M for precipitation to occur because (1x10-6 ) x (0.0016) = 1.6x10-9.

12 Co-precipitation In many solutions, especially those of environmental samples, the concentration of the radionuclide of interest is too low to cause precipitation, even in the presence of high concentrations of its counter-ion, because the product of the concentrations does not exceed the solubility product. For example, Ra in most environmental samples is not present in sufficient concentration to cause its very insoluble sulfate (RaSO 4 ) to precipitate. The radionuclide can often be brought down by co-precipitation with an alternate insoluble compound, i.e. Ra is often co-precipitated with another insoluble sulfate, BaSO 4. Radium can be also co-precipitated with lanthanum fluoride, even though radium fluoride is soluble itself. Adding of stable carrier of the RN ( nat Sr salt to 90 Sr solution) is another way of increasing the overall concentration in order to exceed the solubility limit and to cause precipitation.

13 Precipitation CaCO 3 Pre-concentration of Sr from aqueous samples Ca-oxalate Sr from large seawater samples Ca-phosphate Oxalate precipitation is frequently used in the pre-concentration of actinides and Sr 2+ (and Y 3+ ), and to remove interfering elements as K + and Fe 3+ as they are left in the solution. BaSO 4 Pre-concentration and separation of Ra LaF 3 Co-precipitate with actinides in oxidation state III and IV, and do not co-precipitate actinides in oxidation state V and VI. Fe(OH) 3 Co-precipitation of actinides is extensively used to pre-concentrate actinides from large aqueous sample volumes MnO 2 Scavenges actinides in III and IV oxidation state as well as Ra

14 Co-precipitation methods Radionuclide Co-precipitate Carrier Cs(I) Phosphomolibdate PMo 12 O Cs + Co(II) 1-Nitroso-2-naphthol Sulfide (S 2- ) Am(III) Hydroxide (OH - ) Iodate (IO 3- ) Fluoride (F - ) Phosphate (PO 4 3- ) Sulfate (SO 4 2- ) Oxalate (C 2 O 2 2- ) Co 2+ Co 2+ Am 3+, Fe 3+ Ce 4+, Th 4+, Zr 4+ Ln 3+ Ln 3+ Ln 3+ Ca 2+ Pu(III) Fluoride (F - ) Ln 3+ Pu(IV) Hydroxide (OH - ) Sulfate (SO 4 2- ) U(VI) Hydroxide (OH - ) Phosphate (PO 4 3- ) Cupferron (C 6 H 9 N 3 O 2 ) Peroxide (H 2 O 2 ) Zr 4+, Th 4+, Fe 3+ Ln 3+ Fe 3+ (no carbonates) Al 3+ U(VI) Th 4+, Zr 4+

15 Advantages and Disadvantages of precipitation and co-precipitation Advantages The only practical method of separation or concentration in some cases. Highly selective and virtually quantitative. High degree of concentration. Could be scaled up ( from mg to industrial). Convenient, simple and energy efficient process compared to other techniques. Carrier can be removed and procedure continued with tracer amounts of material (e.g., carrier iron separated by solvent extraction). Disadvantages Time consuming to digest, filter, or wash the precipitate. Precipitate can be contaminated by carrying of ions or post-precipitation. Large amounts of carrier might interfere with subsequent separation procedures. Co-precipitating agent might contain isotopic impurities of the analyte radionuclide.

16 Solvent extraction Solvent extraction has been an important separation technique since the early days of the Manhattan Project, when scientists extracted UO 2 (NO 3 ) 2 into diethyl ether to purify the uranium used in the first reactors. In our days the SF reprocessing plants utilise PUREX process or extraction of (IV and VI valent) actinides with tri-butyl phosphate (TBP) in kerosene. Pu 4+ +4NO 3- +2TBP Pu(NO 3 ) 4.2TBP UO NO 3- +2TBP UO 2 (NO 3 ) 2.2TBP HDEHP (bis(2-ethylhexyl) phosphoric acid)- extracts A(III) and Ln(III) quantitatively from HCl or HNO 3

17 Advantages and Disadvantages of Solvent extraction Advantages Rapid, highly efficient and very selective separations. Partition coefficients independent of RN concentration. Can usually be followed by back-extraction into aqueous solutions. Wide scope of applications the composition of the organic phase and the nature of complexing or binding agents can be varied so that the number of practical combinations is virtually unlimited. Can be performed with simple equipment, but can also be automated. Disadvantages Often requires toxic or flammable solvents. Time consuming, especially if the equilibrium is slow. Can require costly amounts of organic solvents and generate large volumes of organic waste. Can be affected by small impurities in the solvent. Multiple extractions might be required, thereby increasing time, consumption of materials, and generation of waste.

18 Ion-exchange Based on the reversible exchange of ions between a solution and the resin. Ion-exchange resin- insoluble, inert polymeric matrix containing fixed charged groups (exchange sites) associated with mobile counter-ions of opposite charge, exchanged for ions from the solution. The exchange sites are acid or base groups (amines, phenols, and carboxylic or sulfonic acids) used over a specific ph range where they are in their ionic form. Typical functional groups for cation exchangers are sulfonate RSO 3- H + or carboxylate RCOO - H +. The quaternary-amine cation, RNH 3+ Cl - is a common exchange group for anion exchange resins.

19 Ion-exchange In a practical description of ion-exchange equilibria, the weight distribution coefficient K d, and the separation factor, α, are significant. The weight distribution coefficient is defined as: [ A1 ] K d [ A ] where А 1 is the activity of the RN adsorbed on 1 g of the dry resin, and A 2 is the remaining RN activity in 1 ml of solution after equilibrium has been reached. The separation factor refers to the ratio of the distribution coefficients for two ions that were determined under identical experimental conditions: [ K [ K where A and B refer to a pair of ions. d d 2, A], B]

20 Ion-exchange Sorption behaviour of radionuclide ions on anionexchange resin from HCL solutions

21 Ion-exchange: Advantages and Disadvantages Advantages Highly selective Highly efficient as a pre-concentration method Works as well with carrier-free tracer quantities as with weighable amounts Produces a high yield (recovery) Can separate radionuclides from interfering counter-ions Simple process requiring simple equipment Wide scope of applications Can handle high volumes of sample May require high volume of eluent Usually a relatively slow process Requires narrow ph control Disadvantages

22 Extraction chromatography Extraction chromatography combines the selectivity of liquidliquid extraction with the rapidity of chromatographic methods. The separation of the radionuclides is based on the distribution of RNS between an organic and an aqueous phase. The extractant is adsorbed on the surface of an inert support and corresponds to the organic phase. A wide variety of extractants is used : Acidic extractants (e.g. HDEDP) which exchange protons Amines and ammonium salts (e.g. Aliquat 336) which exchange simple anions against anionic complexes Organic molecules containing P=O groups (e.g. TBP) which exchange water molecules in the hydration sphere Crown-ethers which retain cations in function of their size E.P. Horwitz et. al. Separation and preconcentration of actinides from acidic media by extraction chromatography. Analytica Chim Acta 281(2) p (1993)

23 Extraction chromatography

24 Actinide Resin The resin exhibits an extraordinarily high affinity for actinides (Kd> ) The resin is useful for the pre-concentration of actinides out of large volume aqueous samples. The retention of actinides on this resin is so high, that it is not efficient to strip the actinides from it, instead it is necessary to dissolve the DIPEX extractant with isopropanol. The resin may be contacted in batch mode and counted directly by LSC making for a very rapid analysis.

25 TEVA Resin Active component- aliphatic quaternary amine. The differences between the uptakes for HNO 3 and HCl can be exploited to separate certain actinides. I.e. all the tetravalent actinides can be loaded from 3M nitric acid and then by switching to 6M HCl, Th(IV) can be selectively eluted. The retention of Tc(VII), pertechnetate, is very high in solutions of lower nitric or hydrochloric acid concentration. The use of TEVA Resin in the analysis of Tc has become an industry standard.

26 UTEVA Resin The extractant in the UTEVA Resin- diamyl, amylphosphonate DAAP UTEVA Resin works for U measurements in environmental samples, determination of U, Pu and Am, measurement of actinides in high level waste, etc. The large difference in Kd for U and Th in the range of 4-6M HCl allows U/Th separation. Am(III) is not retained at any nitric acid concentration, which is important in developing analytical separation schemes. Pu(IV) can be reduced to Pu(III) (NH 4 I) and will behave like Am(III).

27 Sr Resin 4,4'(5')-di-t-butylcyclohexano 18- crown-6 in 1-octanol The uptake of Sr increases at high [HNO 3 ]. At 8M nitric acid, Kd is 10 2 and it falls below 0 at low [HNO 3 ]. Among the alkaline earth metals, calcium has lowest uptake and it is easy to separate Sr from Ca. Ba retention is high, but it falls off at higher concentrations. To ensure adequate decontamination of Ba in Sr analysis, load Sr on the resin from 8M nitric acid, Ba is eluted, leaving a pure Sr fraction.

28 Tracers and carriers: Carriers: Why ne need to add carriers? 1Bq of Ra-226 (T 1/2 =1600y) is M 1Bq of Zr-95 (T 1/2 =64d) is M Carriers are chemically identical materials to the radionuclide of interest that have a significant, non-radioactive mass. Losses of radionuclides during the analytical separation processes is due to irreversible adherence to dust, container walls, ion exchange resins, and filters, etc. If we add a material that is chemically identical to the radionuclide of interest, it will also occupy these sites and reduce the radionuclide losses. The surface becomes saturated mainly with stable strontium atoms because of the vast mass excess above the radioactive atoms.

29 Tracers and carriers: Two different types of cariers: Isotopic Non-Isotopic. Isotopic carrier is a stable isotope of the radionuclide of interest. For 90 Sr, stable strontium is added to the sample at the beginning of the sample analysis. Radioactive and stable strontium are chemically identical. If the stable strontium precipitates as the carbonate, the radioactive strontium will precipitate as well because the total mass of strontium present will exceed the solubility product constant for strontium carbonate. Non-isotopic carrier- different element with similar chemical behaviour, i.e. barium is used in radium analysis. Since radium has no stable isotopes, a chemical homologue is used. Barium and radium are both in Group II in the periodic table and have very similar chemical properties. When barium is added as a non-isotopic carrier and precipitates as the sulfate, radium will co-precipitate with the barium.

30 Tracers and carriers: What do we expect from a tracer? The tracer must exhibit the same chemical behaviour as the analyte: -Implies that the same element should be employed The tracer should not interfere with the measurement of the analyte: -Preferable to measure by the same technique, or if the tracer does not register in the analyte measurement (and vice versa) Chemical equilibrium between the tracer and analyte should be established at the earliest possible point in the analysis: -Add the tracer as soon as possible and (for solids) employ total dissolution

31 Tracers and carriers: Furthermore: The tracer should not be present in the samples being analysed: -Using nuclides present in the samples being analysed complicates analysis The tracer should be pure and not introduce contamination into samples being measured: -Purity requirement: may differ for mass spectrometry and radiometric measurements The tracer activity should be traceable to national or international standards: -Not strictly so: Measurements of γ emitting tracers may be relative

32 Tracers and carriers: U-232 (T 1/2 =68.9y, α) U-236 (T 1/2 = y, α) U-237 (T 1/2 =6.75d, ƴ (B - )) Pu-236 (T 1/2 =2.9y, α) Pu-237 (T 1/2 =68.9y, ƴ (EC)) Pu-242 (T 1/2 = y, α) Np-236 (T 1/2 = y, ƴ (EC)) Np-239 (T 1/2 =2.4y, ƴ (B - )) Am-243 (T 1/2 =7370y, α) Th-229 (T 1/2 =7340y, α) Po-209 (T 1/2 =102y, α) α- Spectrum of U, Pu and Am isotopes, with tracers added (red ones)

33 Redox sensitive nuclides Ce(III) Ce(IV) U(IV) U(VI) Np(IV) Np(V) Pu(III) Pu(V) Hydroxylamine NH 2 OH HN 4 I KBrO 3 NaNO 2 Fe(III) + Ascorbic acid H 2 O 2 Ferrous sulfamate [Fe(NH 2 SO 3 ) 2 ]

34 Examples of separation methods: Gross Alpha Radioactivity in Water (EICHROM) The α-emitting RN are sorbed on Actinide Resin from a water sample in a batch mode. The resin is transferred to a LSC vial and counted directly by LSC. 0.5g Actinide resin is added to 100 ml sample in a beaker at ph=2 The sample is stirred overnight Filtered through a 0.45µm filter to separate the resin The resin is transferred to a LSC vial with 1ml 0.5 M HCl 10 ml LSC cocktail added The total alpha activity in the sample and blank vials is measured on a liquid scintillation counter LSC with α/β-discrimination. Alpha window: kev

35 Examples of separation methods: Ion-exchange separation of actinides Am(III), Pu(IV), Np(V), U(VI) 8M HNO 3 (NaNO 2 ) 8M HNO M NaNO 2 AG1 2M HCl 9M HCl AG1 Np Am 1M HCl 9M HCl + 0.1M NH 4 I U Pu(III)

36 Separation methods: Np/U/Pu/FPs U, Np, Pu, FPs 1M HNO 3 /0.1M NaNO 2 U, FPs 1M HNO 3 /0.1M NaNO 2 TEVA 10M HCl FPs 0.2M HNO 3 9M HCl/0.1M NH 4 I Pu236 FPs 10M HCl 0.2M HNO 3 TRU FPs Np236 1M HCl/0.1M H 2 C 2 O 4

37 Separation methods: Uraniumparation Solvent extraction: Water phase: 8 M HNO3 Organic phase: 30% TBP in OK Wash: 8 M HNO 3 Wash with 1.5 M HCl (Th) (repeated) Elute U with H 2 O (repeated) Extraction chromatography: Column: UTEVA-Resin Load: 3M HNO 3. 1 M Al(NO 3 ) 3 Wash: 3M HCl U elution: 0.01 M HCl

38 Quantification of radionuclides A typical laboratory may be equipped with the following radiometric instrumentation: Gas proportional detectors for alpha and beta-particle counting High resolution germanium HPGe detectors for gamma detection and spectrometry Solid-state detectors for alpha spectrometry Liquid scintillation counters suitable for both alpha- or beta-emitting radionuclides (LSC) Photon Electron Rejecting Alpha Liquid Scintillation (PERALS)

39 Quantification of radionuclides Some labs may also be equipped with atom and ion counting instrumentation: Mass Spectrometric Analyses Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) Thermal Ionization Mass Spectrometry (TIMS) Accelerator Mass Spectrometry (AMS) Neutron Activation

40 Alpha spectroscopy Solid state (PIPS) detectors Low background High counting efficiency Resolution kev Energy calibration with calibration sources Counting efficiency measured using tracers (U232, Pu242, Am243, Po209 etc) Electroplating or micro precipitation for SP

41 Electrodeposition Sample evaporation Re-dissolving in c.hno 3 Treatment with H 2 O 2 Evaporation Re-dissolving in 0.4 M H 2 SO 4 ph indicator (thymol blue) Add ammonia until reach light pink colour (ph=2) Transfer to ED cell Electroplate for 2 h at 0.5A Add excess of ammonia Electroplate for 5 more min (NH 4 ) 2 SO 4 / H 2 SO 4 ph=2 E=0.5A T=2h

42 Micro-precipitation Method used for preparation of sources for alpha-spectroscopy Typically, 1 to 20 µg of a highly insoluble lanthanide (commonly Nd, Ce, or La) is added to the. This is followed by the addition of hydrofluoric acid to the solution, which causes precipitation of the lanthanide and co-precipitation of the actinide. A quantitative, micropore filter (usually 0.45 µm) is used to separate the precipitate from the supernate. The procedure is faster and more reliable than those involving electrodeposition and gives consistently higher yields.

43 Micro-precipitation Vacuum filtration often is used to speed the operation and is required for efficient source preparation.

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45 ALMERA validated methods A combined procedure for determination of Plutonium isotopes, Am-241 and Sr-90 in environmental samples A procedure for rapid simultaneous determination of Sr-89 and Sr-90 in milk using Cerenkov and scintillation counting A procedure for the sequential determination of Polonium-210, Lead-210, Radium-226, Thorium and Uranium isotopes in phosphogypsum by liquid scintillation counting and alpha spectrometry A procedure for the rapid determination of Radium-226 and Radium-228 in drinking water by prompt liquid scintillation counting Determination of Po-210 in water samples by alpha spectrometry Analytical Methodology for the Determination of Radium Isotopes in Environmental Samples

46 Thank you!