Natural zeolite minerals have strong ion exchange selectivity for alkali and alkaline earth radionuclides, such as strontium and cesium, but only limited sorption affinity for actinides such as uranium. In a previous study, surfactant-modification of the zeolite surface was shown to enhance uranium(6+) sorption at solution pHs greater than about 6. In this study, experiments were conducted to evaluate the ability of surfactant-modified zeolite (SMZ) to sorb both strontium(2+) and uranium(6+). A surfactant-modified zeolite was prepared by treating specimens composed mainly of clinoptilolite with the cationic surfactant hexadecyltrimethylammonium-bromide (HDTMA). The surfactant-modified clinoptilolite was reacted with strontium solutions having a range of strontium concentration, and with a mixed solution of strontium and uranium. Experiments were conducted as a function of pH and in equilibrium with atmospheric CO2 (g). Surfactant-modification of clinoptilolite decreases the ion exchange selectivity of clinoptilolite for strontium at acidic to near-neutral pH. Strontium exchange data were compared to previously developed model predictions of strontium ion exchange behavior on clinoptilolite. The presence of both uranium and strontium in solution had little effect on either the ion exchange selectivity of the surfactant-modified clinoptilolite for strontium or on the enhanced sorption of uranium by SMZ at pH greater than 6.

Recent literature recognizes a lack of proper chemical data that may be necessary for the calculation of realistic values of the nickel solubility in sulphidic groundwaters. Consequently, in order for the performance assessment of radioactive waste repositories to be conservative, solubility values that clearly exceed those obtained from modelling and experiments have been used to compensate for uncertainty. The safety analysis of decommissioning waste from the Olkiluoto NPP indicates that 59Ni, together with 94Nb, will dominate the dose rates in the waste after about 65 000 years. However, the estimated solubility limit of nickel is uncertain, and more studies are needed.

This paper presents the results from an experimental study of the nickel solubility in sulphidic groundwater under anoxic conditions. The waters used were natural groundwater from Olkiluoto and synthetic saline water, to which sodium sulphide had been added. Two sulphide concentrations, 0.1 and 3 mg/L, were used.

This paper describes the most recent work in a programme of generic experimental and modelling sorption studies undertaken to increase confidence in the performance assessment for a potential high-level radioactive waste repository in Japan. The sorption of plutonium onto three rock samples was studied as a function of carbonate concentration under reducing conditions. Geochemical modelling was used to assist with experimental design and interpretation of results.

Uranium and its fission product Tc in aerobic environment will be in the forms of UO22+ and TcO4−. Reduced forms of tetravalent U and Tc are sparingly soluble. As determined by transmission electron microscopy, the reduction of uranyl acetate by immobilized cells of Desulfovibrio desuifuricans results in the production of black uraninite nanocrystals precipitated outside the cell. Some nanocrystals are associated with outer membranes of the cell as revealed from cross sections of these metabolic active sulfate-reducing bacteria. The nanocrystals have an average diameter of 5 nm and have anhedral shape. The reduction of Re7+ by cells of Desulfovibrio desulfuricans is fast in the media containing H2 electron donor, and slow in the media containing lactic acid. It is proposed that cytochrome in these cells has an important role in the reduction of uranyl and Re7+ that is a chemical analogue for one uranium fission product Tc7+ through transferring electron from molecular hydrogen or lactic acid to the oxyions of UO22+ and ReO4-.

We have investigated the interaction of U(VI) and Np(V) actinide ions with cementitious materials relevant to nuclear waste repositories using X-Ray Absorption Fine Structure (XAFS) Spectroscopy. The actinide ions were individually loaded onto untreated as well as hydrothermally treated cements. The mixtures were then equilibrated at varying pH's for periods of 1 month and 6 months.

In all cases uranium was observed to remain in the initial UO22+ form in the Near Edge (XANES) spectra. The uranium samples show evidence of inner-sphere interactions on both treated and untreated cements at all pH's, with the uranyl complexing with the mineral surface via sharing of equatorial oxygens. On treated cement near-neighbor U-U interactions are also observed, indicating the formation of oligomeric surface complexes or surface precipitates.

Neptunium was observed to undergo a reduction from the initial NpO2+ to Np4+ Calculated % reduction showed ca. 15% of Np(V) is reduced to Np(IV) after a 1 month equilibration time. After 6 months higher % reduction of between 40% and 65% was observed. No Np-Np interactions were observed in the EXAFS spectra, which suggests that surface precipitation of NpO2 is an unlikely mechanism for sorption.

Leaching tests have been carried out on concrete with fly ash or blast furnace slag as additives. Different models for leaching processes are proposed and analyzed, and physical/chemical interactions that may occur during a leaching process are discussed. Furthermore, simulations with different conditions and with different mathematical solutions are shown. The results of the leaching tests are displayed as a time-dependent leaching whose behavior varies significantly for each metal.

In order to investigate the validity of the local equilibrium assumption, a dimensional analysis of various ionic transport problems currently encountered in civil engineering has been performed. The dimensional analysis allows comparing, on a theoretical basis, the rate of ionic transport to the rate of chemical reaction. This approach has been applied to various practical cases. The analysis clearly shows that the local equilibrium assumption is verified in most practical cases.

In this paper, experimental results and modelling of the coupling between diffusivity and cracks in cement-based systems are presented.

The cracks were generated in samples by means of a compression test. In this test, displacement is controlled, thus controlling the opening of the cracks. Diffusion tests on samples obtained at three different compression levels corresponding to three cracked states were then performed using tritiated water. These tests showed diffusivity was affected by the existence of cracks and the main effect was observed when cracks crossed the whole specimen (e.g. in the post-peak regime of the compression test).

Finally the diffusion test was modelled in a simple way, assuming that the cracks were circumscribed by parallel planes. With this model, an equivalent opening of the cracks, which was of the same order as the opening estimated using strain measurements or image analysis, was obtained.

In the Japanese concept for the disposal of the high-level radioactive wastes (HLW), the potential pathways for radioactive contaminant transport would be sealed by a combination of tunnel plug, backfilling and grouting. The candidate material for these engineered barriers would be bentonite or a bentonite-based mixture taking into consideration long-term stability of the seals. Two tests of bentonite grouting for sealing an excavation damage zone (EDZ), that exists in the rock in the immediate vicinity of the tunnel, were conducted in the granitic rock at Atomic Energy of Canada Limited's (AECL's) Underground Research Laboratory (URL). One test was the trial for the development of grouting procedure and the evaluation of grouting effectiveness, and the second test was a demonstration of the grouting around the clay-block bulkhead of the Tunnel Sealing Experiment (TSX). In the trial, the injection proportion of 4.0% of grout slurry was the most efficient. The result of the seepage test showed that grouting resulted in a reduction of hydraulic conductivity of the EDZ in the floor of the tunnel. In the demonstration, although the hydraulic pulse test didn't indicate that the grouting significantly reduced the rock hydraulic conductivity, the test was a useful site-scale demonstration of bentonite grout injection for the purpose of EDZ sealing around a tunnel bulkhead.

This paper discussed the effects of solution velocity thorough the cementitious materials on formation of secondary minerals. These minerals were produced by the reaction of hydrates in cement and chemicals in groundwater. The chemicals estimated were NaHCO3, Na2SO4 and NaCl. Calcite yielded by the reaction of carbonate ion and Ca2+ was found to cause a change in the porosity volume of cement, and thus, to decrease the flow rate. The existence of sulfate ion did not affect the flow rate. However, in the case of a solution containing both chloride and carbonate ions, the flow rate increased because Ca dissolution from hydrates was induced.

A can-in-canister waste package design has been proposed for disposal of pyrochlore rich ceramics containing excess weapons plutonium. The can-in-canister configuration consists of a high-level waste (HLW) canister fitted with a rack that holds minicanisters containing the ceramic. The HLW canister is then filled with glass. The pressurized unsaturated flow (PUF) technique was used to investigate waste form/waste form interactions that may occur when water penetrates the waste containers and contacts the waste forms. Volumetric water content was observed to increase steadily from accumulation of water mass as waters of hydration associated with alteration phases formed on the glass surface. Periodic excursions in effluent electrical conductivity and pH were monitored and correlated with secondary phases formed during the test. Plutonium exited the PUF system primarily as filterable particulates. However, effluent Pu and Gd concentrations were found to decrease with time and remained at near detection limits after approximately 250 days, except during transient pH excursions. These results indicate that both Pu and Gd will be retained in the can-in-canister waste package to a very high degree.

Aqueous durability has been assessed for fourteen pyrochlore- and zirconolite-rich titanate waste forms designed for the immobilisation of excess Pu. The ceramics used in this study contained about 12 wt% Pu and about 15 wt% of Hf and Gd oxides as neutron absorbers and were fabricated by cold-pressing and sintering or hot isostatic pressing.

Total release rates (i.e. unfiltered solution + vessel wall inventory) have been measured withthe MCC-1 test method at 90 °C in deionised water. For all samples, 7-day release rates of Pu are between 4 × 10−5 and about 10−3 g.m−2.d−1, reducing to between about 8 × 10−6 and 3 × 10−5 g.m−2.d−1after more than 300 days of leaching. Release rates of U from the baseline ceramic were found to decrease to values between 6 × 10−4 and 1 × 10−5 g.m−2.d−1after 200 days. Hf leach rates were generally < 5 × 10−6 g.m−2.d−l after more than 7 days. Further, the addition of several % of chemical impurities to the baseline sintered formulations was found to reduce U and Gd releases by about a factor of 10 after 200 days.

To investigate the long-term alteration behavior of brannerite, we have undertaken a study of twelve natural samples from a range of geological environments. Our results indicate that seven of the samples exhibit only minor alteration, usually within veinlets or around the rim of the sample. The remaining five samples consist of variable amounts of unaltered and altered brannerite. SEMEDX analyses of unaltered areas indicate that the chemical formulae may deviate from the ideal stoichiometry. The U content ranges from 0.45 to 0.88 atoms per formula unit (pfu). Maximum amounts of the other major cations on the U-site are 0.48 Ca, 0.22 Th, 0.14 Y, and 0.07 Ln (lanthanide = Ce, Nd, Gd, Sm) atoms pfu. The Ti content ranges from 1.86 to 2.10 atoms pfu. Maximum values of other cations on the Ti-site are 0.15 Fe, 0.14 Si, 0.09 Al, 0.06 Nb, 0.04 Mn, and 0.04 Ni atoms pfu. Altered regions of brannerite contain significant amounts of Si and other elements incorporated from the fluid phase, and up to 40-90% of the original amount of U has been lost as a result of alteration. SEM-EDX results also provide evidence for TiO2 phases, galena, and a thorite-like phase as alteration products. Electron diffraction patterns of all samples typically consist of two broad, diffuse rings that have equivalent d-spacings of 0.31 nm and 0.19 nmi, indicating complete amorphization of the brannerite. Many of the grains also exhibit weak diffraction spots due to fine-grained inclusions of a uranium oxide phase and galena. Using the available age data, these samples have average accumulated alpha-decay doses of 2-170 × 1016 alphas/mg. Our results indicate that brannerite is subject to amorphization and may lose U under certain P-T-X conditions, but the overall durability of the titanate matrix remains high.

Crystalline phases of pyrochlore (e.g., CaPuTi2O7, CaUTi2O7) have been proposed as a durable ceramic waste form for disposal of high level radioactive wastes including surplus weapons-usable plutonium. In this paper, we use a linear free energy relationship to predict the Gibbs free energies of formation of pyrochlore phases (CaMTi2O7). The Pu-pyrochlore phase is predicted to be stable with respect to PuO2, CaTiO3, and TiO2 at room temperatures. Pu-pyrochlore is expected to be stable in a geologic repository where silica and carbonate components are absent or limited. We suggest that a repository in a salt formation be an ideal environment for disposal of high level, pyrochlore-based ceramic wastes. In such environment, adding CaO as a backfill will make pyrochlore minerals thermodynamically stable and therefore effectively prevents actinide release from these mineral phases

Corrosion resistances of titanium-based ceramics are quantified using single-pass flow-through (SPFT) experiments. The materials tested include simple pyrochlore group (B2Ti2O7, where B=Lu3+ or Gd 3+) and compositionally complex pure phase pyrochlore (PY12) or zirconolite- and brannerite-bearing pyrochlore-dominated (BSL3) ceramics. Experiments are conducted at 90°C over a range of pH-buffered conditions with typical duration of experiments in excess of 120 days. Apparent steady-state dissolution rates at pH=2 determined on the Gd2Ti2O7 and Lu2Ti2O7 samples indicate congruent dissolution, with rates of the former (1.3×10-3to 4.3×10-3) slightly faster than the latter (4.4×10−4 to 7.0×10−4g m−2d−1). Rates for PY12 materials into pH=2 solutions are 5.9×10−5 to 8.6×10−5 g m−2 d−1. In contrast, experiments with BSL3 material do not reach steady-state conditions, and appear to undergo rapid physical and chemical corrosion into solution. Dissolution rates of PY12 display a shallow amphoteric behavior, with a minimum (2.7×10−5 g m−2 d−1) near pH values of 7. Dissolution rates display a measurable increase (∼10X) with increasing flow-through rate indicating the strong influence that chemical affinity exerts on the system. These results step towards an evaluation of the corrosion mechanism and an evaluation of the long-term performance of Pu-bearing titanate engineered materials in the subsurface.

From elastic recoil detection analysis (ERDA) of 2 MeV He ions and secondary ion mass spectroscopy (SIMS), exposure of Synroc-C to D2O at 150°C for ∼ 30 days produced surface deuteration products of a few nm in thickness, with surface roughness after polishing down to 0.25 µm diamond finish not being of critical importance in the thickness determination. Reaction at 250°C produced more extensive deuteration and general surface alteration, over depths of about a micron. SIMS did not show any surface enhancement of rare earths or Zr on Synroc-C surfaces reacted at 90°C for up to 336 days. Pu-doped SynrocC exposed to deionised water at 70°C showed surface depletion of Pu by alpha-spectroscopy. Zirconolite-rich Synroc showed less surface deuteration than Synroc-C after reaction for 3 weeks at 150°C in D2O. Admixtures of 0.001 M of fluoride ions to dilute HCI (pH = 2) produced deposits of anatase, ∼ 20 µm thick, on perovskite after a few weeks at 90°C; these deposits were much thicker than those produced by the dilute HCI without the fluoride ions being present.

The aqueous corrosion behavior of a zirconolite-rich titanate ceramic was investigated with the aim of describing the rate-controlling process or processes. This titanate ceramic is similar to SYNROC and is proposed as immobilization materials for surplus Pu. The corrosion behavior was described with results from MCC-I and PCT-B static dissolution tests. Three important observations were made: a) Ca is released at a constant rate [7×10−5 g/(m2 day)] in PCT-B tests for up to two years; b) the leachates from PCT-B tests are saturated with respect to both rutile and anatase, and c) the release rates for Pu and Gd increase with time (up to two years) in PCT-B tests. The first observation suggests that the ceramics continue to corrode at a low rate for at least 2 years in PCT-B tests. The second observation suggests that the approach to saturation with respect to these TiO2 phases does not limit the corrosion rate in PCT-B tests. The third observation suggests that the release rate of Pu and Gd are controlled by some unique process or processes, i.e., some process or processes that do not affect the release rate of other elements. While these processes cannot be fully described at this point, two possible explanations, alteration phase formation and grain boundary corrosion are forwarded.

Neptunium has an extremely long half-life and will be the main toxic element in the later stage of disposal. Yttria-Stabilized Zirconia (YSZ), with a fluorite structure, samples doped with Np-237 in high concentration (20, 30, 40 mol %) were fabricated (sintered in Air or Ar, at 1773 K, for 80 hours), and their leaching test (in deionized water, at 423 K, for 84 days) was carried out. The results indicated that the obtained leaching rates were much smaller than those of the Synroc and glass waste form, and that the increase in Np content did not cause any drastic changes in the leaching rates of Zr, Y, or Np. They were also compared to the results of YSZ doped with Ce or Nd used as surrogates for actinides. The work showed that YSZ doped with Np in high concentration has an excellent chemical durability

We report extended x-ray absorption fine structure (EXAFS) spectra from the plutonium LIII edge and x-ray absorption near edge structure (XANES) from the cerium LII edge and plutonium LIII edge in prototype titanate ceramic hosts. The titanate ceramics studied are based upon the hafnium-pyrochlore and zirconolite mineral structures that will serve as an immobilization host for surplus fissile materials. Our samples approximate the composition envelope expected for production materials, which will contain as much as 10.5 weight % fissile plutonium and 21 weight % (natural or depleted) uranium. Three ceramic formulations were studied: one employed cerium as a “surrogate” element, replacing both plutonium and uranium in the ceramic matrix, another formulation contained plutonium in a “baseline” ceramic formulation, and a third contained plutonium in a formulation representing a high-impurity plutonium stream. The cerium XANES from the surrogate ceramic clearly indicates a mixed IIIIV oxidation state for the cerium. In contrast, XANES analysis of the two plutonium-bearing ceramics shows that the plutonium is present almost entirely as Pu(IV) and occupies the calcium site in the zirconolite and pyrochlore phases. The plutonium EXAFS real-space structure shows a strong second-shell peak, clearly distinct from that of PuO2, with remarkably little difference in the plutonium crystal chemistry indicated between the baseline and high-impurity formulations

In two-phase matrices based on pyrochlore-oxide, perovskite-oxide, and pyrochlore-zirconolite assemblages as much as 40 wt.% of the product can consist of an incorporated rare earth - actinide fraction of high level wastes (HLW). In zirconolite, with a nominal stoichiometry of AVIIIBVICV-VI2O7 the actmide and rare earth ions occupy VIII- and VII-fold coordinated sites. Charge compensation is achieved by replacement of Ti4+ with lower valence ions such as Al3+, Fe2+/3+. Mg2+, etc, that have similar radii. The highest lanthanide incorporation (45 wt.% σREE2O3) was exhibited by a calcium free zirconolite containing 8 wt.% A12O3 or 0.7 formula units (f.u.). In pyrochlore (AVIII2BVI2O7−x) actinides and rare earths occupy A-sites. Their incorporation does not require simultaneous ionic substitution in the B-sites, for example Al3+ or Fe 2+/3+ for TI4+, to achieve charge compensation; this simplifies the required composition of the system. Pyrochlore ceramics are suitable for immobilization of wastes with relative elevated actinide content. For a high zirconia, waste ZrO2-based or two-phase pyrochlore-dioxide, perovskite-dioxide or pyrochlore-zirconolite ceramics are preferable. All these wasteforms may be produced via melting