M. Skålberg

The stability of some metal EDTA, DTPA and DOTA complexes: Application as tracers in groundwater studies

Studies of the stability of various metal EDTA, DTPA and DOTA complexes in order to evaluate their applicability as non-sorbing tracers have been performed. In laboratory tests, the stability generally increases for the individual metal ions in the EDTA<DTPA<DOTA order. For most metal ions, the same trend can be observed for the thermodynamic stability constants. In the in situ experiment, various metal EDTA tracers were used in very low concentrations; YbEDTA−, for example had a breakthrough and recovery which were very similar to the non-sorbing tracers used. According to the extremely low tracers concentrations used, thermodynamic data indicate that all metal EDTA tracers should have been decomplexed as a result of the competition with the naturally occurring cations in the groundwater. This was not found, which indicates that the decomplexation rate and sorption mechanism are important in estimating the applicability of the metal complexes as tracers. The DOTA complexes of elements in the middle of the lanthanide series have indicated high stability in the laboratory tests and therefore appear to be good candidates as non-sorbing tracers. However, in contrary to the metal EDTA, tracers, the DOTA complexes of La3+ and Lu3+ seemed to be slightly delayed in the in situ experiment.

MOBILITY OF U, NP, PU, AM AND CM FROM SPENT NUCLEAR FUEL INTO BENTONITE CLAY

The mobility of uranium, neptunium, plutonium, americium and curium from spent nuclear fuel (U02) into compacted bentonite was studied. Pieces of spent BWR U02 fuel was embedded in a compacted bentonite clay/low saline synthetic groundwater system. After a contact time of six years the bentonite was sliced into 0.1 mm thick slices and analysed for its content of actinides. Radiometric as well as inductively coupled plasma mass spectrometry (ICP-MS) were used for the analysis. The influence on the mobility by the addition of metallic iron, metallic copper and vivianite (Fe(II)-mineral) to the bentonite clay was investigated. The results show a low mobility of actinides in bentonite clay. Except for uranium the mobility of the other actinides could, after six years of diffusion time, only be detected less than 1 mm from the spent fuel.

Matrix Diffusion of Some Alkali- and Alkaline Earth-Metals in Granitic Rock

Static through-diffusion experiments were performed to study the diffusion of alkali- and alkaline earth-metals in fine-grained granite and medium-grained Aspo-diorite. Tritiated water was used as an inert reference tracer. Radionuclides of the alkali- and alkaline earth-metals (mono- and divalent elements which are not influenced by hydrolysis in the pH-range studied) were used as tracers, i.e . 22 Na + , 45 Ca 2+ and Sr The effective diffusivity and the rock capacity factor were calculated by fitting the breakthrough curve to the one-dimensional solution of the diffusion equation. Sorption coefficients, K d , that were derived from the rock capacity factor (diffusion experiments) were compared with K d determined in batch experiments using crushed material of different size fractions. The results show that the tracers were retarded in the same order as was expected from the measured batch K d . Furthermore, the largest size fraction was the most representative when comparing batch K d with Kd evaluated from the diffusion experiments. The observed effective diffusivities tended to decrease with increasing cell lengths, indicating that the “transport” porosity decreases with increasing sample lengths used in the diffusion experiments.

In Situ Migration Experiments at Äspö Hard Rock Laboratory, Sweden: Results of Radioactive Tracer Migration Studies in a Single Fracture

Three radially converging in situ migration experiments over a distance of 5 m were performed in a single fracture at a depth of 400 m in the Äspö Hard Rock Laboratory. Injection and breakthrough curves were obtained for uranine,H3HO (HTO), 22Na+, 42K+, 47Ca2+, 58Co(II), 82Br−, 85Sr2+, 86Rb+, 99mTc (no breakthrough), 131I−, 131,133Ba2+ and 134,137Cs+. The in situ experiments lasted for nearly 1.5 years, with single experimental times up to 10,000 hours. The tracer concentrations span over seven orders of magnitude between injection and sampling under practically undisturbed chemical conditions. Dynamic ranges in the breakthrough curves of up to four orders of magnitude were obtained. Thus, the use of radioactive (especially γ-emitting) tracers showed to be a most useful tool for in situ tracer experiments. The relative retardation sequence obtained in the field experiment was Na < Ca ≈ Sr < K < Ba ≈ Rb < Co≈ Cs, which was the same as the relative sequence of the sorption coefficients obtained in the laboratory experiments using crushed rock material. Thus, no scale effect was indicated in the relative retardation sequence between laboratory and field experiments. High recoveries, >90%, were obtained for uranine, HTO, Br, I, Na and Sr and lower recoveries for Ba, Rb, Cs and Co. However, there were indications that there would have been higher recoveries of these elements if it had been possible to continue monitoring over longer experimental times. The low recoveries of Cs and Co indicate either slowly reversible or non-reversible sorption behavior. The laboratory diffusion experiments showed lower diffusivities and porosities and somewhat lower sorptivity of all studied tracers in the site-specific rock samples, dominated by mylonite, than in the diorite host rock. Matrix diffusion and associated sorption within the rock matrix is indicated in the in situ experiments, although this can not be verified without modeling that involves such processes.

Transport and leaching of technetium and uranium from spent UO2 fuel in compacted bentonite clay

The transport properties of Tc and U in compacted bentonite clay and the leaching behaviour of these elements from spent nuclear fuel in the same system were investigated. Pieces of spent UO2 fuel were embedded in bentonite clay (ρd=2100 kg/m3). A low saline synthetic groundwater was used as the aqueous phase. After certain experimental times, the bentonite clay was cut into 0.1 mm thick slices, which were analysed for their content of Tc and U. Measurements were made using inductively coupled plasma mass spectrometry. Tc analysis comprised chemical separation. The analysis of U was done by means of detecting 236U, since the natural content of U in bentonite clay made it impossible to distinguish between U originating from the fuel and the clay. The influence of different additives mixed into the clay was studied. The results showed an influence on both transport and leaching behaviour when metallic Fe was mixed into the clay. This indicates that Tc and U are reduced to their lower oxidation states as a result of this additive.

Nuclear Studies with the Fast On-Line Chemical Separation System SISAK

A survey is given on the development of the Z-separator SISAK, which continuously isolates nuclides with half-life down to ~ 1 s according to their atomic number. The separation equipment is described, as well as the connection to gas jet recoil transportation systems and the data acquisition and evaluation systems used. The use of the SISAK technique is exemplified by results obtained for neutron-rich nuclei in the mass regions A ~ 150, A ~ 110 and A ~ 245. Other applications of the technique are also briefly discussed.

A separation procedure for the analysis of90Sr,154Eu, and the actinides237Np,239Pu,241Am and244Cm

A procedure for the analysis of90Sr,154Eu,237Np,239Pu,241Am and242−244Cm was developed. Separation was done with the separating agent, di-2(ethyl hexyl)-phosphoric acid (HDEHP), and results in fractions containing the different elements to be analysed.90Sr analysis was done by analysing its daughter nuclide90Y, detected through the Cherenkov radiation emitted by this high energy β−-emitter.154Eu was detected using γ-spectrometry with a lower Compton background as a result of the removal of other fission products.241Am could also be detected with γ-spectrometry or together with242−244Cm with α-spectrometry. The long-lived radionuclides237Np and239Pu were detected using inductively coupled plasma mass spectrometry (ICP-MS).

Leaching/migration of UO2-fuel in Compacted Bentonite

The release and migration of the fission products cesium, europium and technetium, the actinides plutonium, americium and curium, and the activation product cobalt from spent nuclear fuel pellets in highly compacted bentonite clay has been measured after contact times of 101 and 386 days. Experiments at longer contact times are in progress. In some cases small amounts (0.5–1%) of powdered copper or iron metal, or vivianite (Fe 3 (PO 4 ) 2 ) have been mixed with the bentonite clay. The results indicate as expected a high mobility of cesium. The actinides have a very low mobility. After 386 days, plutonium has diffused less than 0.5 mm away from the fuel, while americium and curium appear to be somewhat more mobile. The behaviour of europium is similar to that of trivalent actinides. Very little technetium has been leached from all samples. Cobalt shows a strong retention in pure bentonite as well as in the presence of vivianite, while the mobility is much larger when iron or copper is added.

A Technique for Reducing Interferences in Epithermal Neutron Activation Analysis

A new epithermal neutron activation technique is described. The technetium is based on the existence of non-overlapping resonance peaks in the neutron absorption cross-section spectra for the nuclides present in the sample to be analyzed. By this method it is possible to reduce some of the interfering activities with respect to the sought activity by using appropriàte filters. An experiment has been carried out to demonstrate the validity of the technique.

Diffusion of Radionuclides from Spent Oxide Fuel into Compacted Bentonite

Abstract An experimental technique for studies of diffusion of radionuclides from spent oxide fuel into compacted bentonite is described. Results obtained for the diffusion of caesium will be briefly discussed.