Volume 103, Issue 3 - Special Issue: Radiochemistry in Northern Europe / Stepan N. Kalmykov, Jukka Lehto

The strategies adopted in Russia for a closed nuclear fuel cycle with fast reactors (FR), selection of fuel type and recycling technologies of spent nuclear fuel (SNF) are discussed. It is shown that one of the possible technological solutions for the closing of a fuel cycle could be the combination of pyroelectrochemical and hydrometallurgical methods of recycling of SNF. This combined scheme allows: recycling of SNF from FR with high burn-up and short cooling time; decreasing the volume of stored SNF and the amount of plutonium in a closed fuel cycle in FR; recycling of any type of SNF from FR; obtaining the high pure end uranium-plutonium-neptunium end-product for fuel refabrication using pellet technology.

Plutonium partitioning and speciation was studied under the conditions of a liquid nuclear waste repository site. The dominating effect of corrosion products (Fe and Cr) oxides on plutonium partitioning was established in batch sorption tests at 150 ℃ and by using sequential extraction. Various techniques including Mössbauer spectrometry, XRD and TEM were used to characterize Fe/Cr oxide phases that form both precipitates and surface coatings which sorb plutonium. The sorption and speciation of plutonium was modeled thermodynamically using Pourbaix diagram that takes into account both surface complexation reaction and precipitation of insoluble PuO 2 .

To assess the long term environmental impact of radioactive contamination of ecosystems, information on source terms including radionuclide speciation, mobility and biological uptake is of high importance. The speciation of radionuclides deposited or occurring naturally depends on source term and release scenario characteristics, transport and dispersion mechanisms and ecosystem properties. If mobile species are present, ecosystem transfer is relatively fast, whereas the ecosystem transfer is delayed if radionuclides are present as particles or incorporated in mineral lattices. This paper discusses cases showing important factors influencing the mobility of different radionuclides and metals. As examples can be given: the difference between 137 Cs and 90 Sr from Chernobyl fallout and stable Cs and Sr in the soil showing that the fallout has not reached a steady state and therefore shows different mobility properties, thus the time since contamination is important. the presence of organic matter affect the mobility of plutonium in soils and sediments even though the source term is the same. physical properties ( i.e. pH, OM, clay content, grain sice etc. ) is important to interpret mobility data between similar sites. the influence of industrial activity on radionuclides and metals shows that any man-made activity might change the mobility.

Technetium sulfide formation kinetics was studied in the pH range 8 − 12 in presence of Na 2 S and phosphate buffer solution. The conditions for separation of Tc sulfide micro and nanoparticles were found with ultramicrocentifugation and the values of Tc sulfide solubility were demonstrated to be dependent on the Na 2 S concentration as C (Tc 3 S 10+ x ) =− 9 E  − 5 ln [Na 2 S] − 2 · 10 – 5  M. The composition of Tc sulfide precipitate was elucidated with EXAFS, RBS and chemical analyses as Tc 3 S 10+ x or [Tc 3 (μ 3  − S)(S 2 ) 3 (S 2 ) 3/3 ] n in agreement with recent Lukens data.

Detonation nanodiamonds (ND) present a significant part of nanocarbons group, which could be produced on commercial scale by detonation of explosives in a closed chamber. Their unique properties of high surface area, low weight and radiation resistance make ND a prospective candidate for applications in sorption processes in radiochemistry. To study the influence of surface chemistry on sorption properties, apristine sample of ND was treated with acids and hydrogen. The surface chemistry of the samples was characterised by infrared spectroscopy, X-ray photoelectron spectroscopy and Boehm titration. The sorption properties of ND were tested fordifferent radionuclides. The sorption capacity of ND was shown to be higher than those of commonly used radionuclide sorbents like activated carbon and compariable to other members of nanocarbon group like graphene oxide and carbon nanotubes. The sorption properties were shown to be influenced by the presence of oxygen-containing groups on the surface of ND. This represents an opportunity to increase the sorption capacity of ND.

Three recent studies on cesium sorption in soil and bedrock are reviewed. 137 Cs, originating from fallouts of nuclear weapons tests and the Chernobyl accident was found to decrease in an exponential manner in forest soil, and the highest fraction in soil profiles was found in the organic layer. Also, the mineral layer below the organic layer contained a large fraction of cesium inventory but at depths below 20 cm only a very small fraction was observed. In the bedrock of Olkiluoto, where the final repository for spent nuclear fuel from the Finnish nuclear power plants will be constructed, mica mineral biotite plays the most important role in cesium sorption. The selectivity of biotite decreases in the order Cs > K > Na > Ca and the overall selectivity coefficient for Cs/Ca exchange was approximately five and seven orders of magnitude higher than those for Cs/Na and Cs/K exchange reactions, respectively. Ion exchange isotherms for Cs/Na and Cs/K exchange were modelled by assuming three different ion exchange sites: frayed edge sites (FES), basal plane sites and intermediate sites. The selectivity coefficients derived for these sites were successfully used to predict cesium sorption in a mica gneiss rock. Sorption of cesium in mineral soil layers from the Olkiluoto overburden were studied using three different approaches: model batch experiments, an in-situ method and calculations. All three approaches gave the same trend but the distribution coefficient values varied in range of one order of magnitude.

Electrophilic radiofluorination was successfully carried out in the early years of PET radiochemistry due to its ease and fast reaction speed. However, at the present, the use of electrophilic methods is limited due to low specific activity (SA). Post-target produced [ 18 F]F 2 has significantly higher SA compared to other electrophilic approaches, and it has been used in the production of clinical PET radiopharmaceuticals at the Turku PET Centre for years. Here, we summarize the synthesis and use of these radiopharmaceuticals, namely [ 18 F]FDOPA, [ 18 F] CFT, [ 18 F]EF5 and [ 18 F]FBPA.

On-column [ 18 F]fluoride trapping and radiofluorination of 2-(naphthalen-1-yl)ethyl-4-methylbenzenesulfonate (C 10 H 7 (CH 2 ) 2 OTs), performed on polystyrene supported phosphazene base PS-P 2 tBu yielded [ 18 F]1-(2-fluoroethyl)naphthalene ([ 18 F]C 10 H 7 (CH 2 ) 2 F) in 50% radiochemical yield but left up to 43% of activity unreacted on the resin. This activity could be eluted with Kryptofix/K 2 CO 3 and then used for conventional radiofluorination of the same substrate, suggesting that the column-retained activity was present in the form of [ 18 F]fluoride entrapped in polymer matrix. An approach to minimize the amount of entrapped [ 18 F]fluoride by use of glass beads functionalized with alkylsilane-derivatized phosphazene residues was attempted but was stymied by fluorolysis/hydrolysis of the alkylsilane spacer. The results suggest that the key to high yield of on-column radiofluorination is to minimize the residual [ 18 F]fluoride absorption in the matrix by the judicious choice of solid support.