Fast methods for determination of antropogenic actinides and U/Th-series isotopes in aqueous samples.

Rapid and simple methods are applied at the PSI radioanalytical laboratory for determining anthropogenic actinides in waste and nuclear reactor waters (U, Pu, Am, Cm) as well as for analysis of naturally occurring alpha-emitters in continental river and ground water. Anion exchange chromatography followed by alpha-spectrometry as well as alpha/beta-LSC is applied for the reactor coolant waters. To avoid alpha-spectrum interference between 238Pu and 241Am at 5.5 MeV, the Pu-fraction is purified using anion exchange resin. Prior to the separation of the Pu-fraction, all actinides (U, Pu, Am, Cm) are adsorbed batch-wise under stirring onto Actinide Resin and subsequent decomposition of the reagent. The residue is then re-dissolved in a sulfate buffer solution for electrolytic deposition. In tabular water samples isotopes of Ra and Po are analyzed additionally via sorption onto manganese coated discs (Ra) and deposition on silver discs (Po). For counting times of 1 day and use of 0.1-1l sample aliquots, detection limits of a few mBql(-1) can be obtained easily.

Determination of 228Ra, 226Ra and 224Ra in natural water via adsorption on MnO2-coated discs.

A fast procedure based on sorption of Ra on MnO2 coated polyamide discs is presented for determination of radium isotopes (i.e. 228Ra, 226Ra, 224Ra) in aqueous samples. The sample discs can be used directly for low-level alpha-spectrometry without the need for further separation and preparation methods to produce planar sample sources. While the activity of alpha-emitting 224Ra and 226Ra can be determined during a first measurement, beta-emitting 228Ra is obtained via ingrowth of the progeny 228Th on the same sample disc after a standing time of about six months. Calculations are presented for optimizing the analytical accuracy as well as for predicting the sorption yield or chemical recovery of radium on the sample disc as a function of exposure time because the sorption uptake proceeds with first-order kinetics. The analyses can be carried out on small samples of 0.5-11 and, for long counting times of one week and use of high-purity silicon surface barrier detectors, a detection limit of 0.15 mBq l-1 is obtained for 226Ra. Since the half-life of 224Ra is only 3.7 d and since 228Th (as a measure for 228Ra) is built up only partially on the sample disc, a slightly higher detection limit of 0.24 mBq l-1 results for the latter isotopes. The procedure is therefore sufficiently sensitive to allow the investigation of Ra isotope relationships in aquifers at typical environmental levels.

228Ra/226Ra/224Ra and 87Sr/86Sr isotope relationships for determining interactions between ground and river water in the upper Rhine valley.

Ground and river waters of the upper Rhine valley (Alsace, France) were investigated for chemical composition of the major elements, Sr isotopes and radionuclides from the U and Th series. In particular, the isotope ratios and concentrations of Ra and Sr were used as geochemical tracers to distinguish between different types of water and their interactions. The bulk chemical analyses suggest that the surface waters in the Rhine valley can be described as mixtures between Ca-Na-HCO3-rich ground water and less mineralized slightly acidic river waters which have migrated through crystalline (mainly granitic) basement rocks of the Vosges mountains. Mixing of these waters yields positive correlation between bulk Sr, U, Ca and HCO3, indicating that carbonate-rich sediments are the main source of U and (non-radiogenic) Sr in the Rhine valley aquifers. The combination of the Ra and Sr isotope systems (228Ra/226Ra, 87Sr/86Sr) shows, however, that probably three sources contribute to the surface river waters in the upper Rhine valley, i.e. (i) a highly radiogenic crystalline component, (ii) a ground water source and, (iii) a third component from infiltrating Rhine water along the flow path of the parallel running river Ill in the northerly direction. The Sr and Ra isotope systems were also used to calculate small-scale mixing fractions of tributaries along the flow path of the Ill. Mixing ratios of non-pure end-member waters were determined using three isotope diagrams (i.e. 224Ra/226Ra vs. 228Ra/226Ra) and the results obtained with the Ra isotope system were found to be consistent with the data using Sr isotope relationships (i.e. 87Sr/86Sr vs. 1/Sr).

Separation and measurement techniques for the determination of 228Th, 230Th and 232Th in various matrices.

Three analytical techniques are presented which are used at PSI for determination of U and Th isotopes (234U, 235U, 238U, 228Th, 230Th, 232Th) in different materials, i.e. environmental samples (soils, minerals) and dental ceramics as well as in urine for in vitro monitoring of potentially exposed workers. Depending on the sample quantity available and/or required detection limits the measurements are performed either directly via gamma spectrometry or via alpha particle counting with preceding separation chemistry. The separation methods applied are based on either extraction chromatography or on sorption of U and Th on actinide selective resin. Following sample digestion, chemical yield spike additions (232U, 225Th or 225Th), chemical purification and electro-depositional source preparation, alpha particle measurement is carried out using low-level alpha spectrometry. This technique allows detection limits of less than 0.2 mBq per counting source if the assay lasts over a few days and is therefore suitable for determination of trace quantities of short-lived 225Th that can be hardly detected by means of mass spectrometric techniques.