Strontium Release Research Articles

A biocompatible hybrid material with simultaneous calcium and strontium release capability for bone tissue repair

The increasing interest in the effect of strontium in bone tissue repair has promoted the development of bioactive materials with strontium release capability. According to literature, hybrid materials based on the system PDMS–SiO2 have been considered a plausible alternative as they present a mechanical behavior similar to the one of the human bone. The main purpose of this study was to obtain a biocompatible hybrid material with simultaneous calcium and strontium release capability. A hybrid material, in the system PDMS–SiO2–CaO–SrO, was prepared with the incorporation of 0.05mol of titanium per mol of SiO2. Calcium and strontium were added using the respective acetates as sources, following a sol–gel technique previously developed by the present authors. The obtained samples were characterized by FT-IR, solid-state NMR, and SAXS, and surface roughness was analyzed by 3D optical profilometry. In vitro studies were performed by immersion of the samples in Kokubo's SBF for different periods of time, in order to determine the bioactive potential of these hybrids. Surfaces of the immersed samples were observed by SEM, EDS and PIXE, showing the formation of calcium phosphate precipitates. Supernatants were analyzed by ICP, revealing the capability of the material to simultaneously fix phosphorus ions and to release calcium and strontium, in a concentration range within the values reported as suitable for the induction of the bone tissue repair. The material demonstrated to be cytocompatible when tested with MG63 osteoblastic cells, exhibiting an inductive effect on cell proliferation and alkaline phosphatase activity.

The effect of lanthanum oxide and gallium oxide on the biocompatibility of high borate glasses

Event Abstract Back to Event The effect of lanthanum oxide and gallium oxide on the biocompatibility of high borate glasses Kathleen O'Connell1, Caitlin Pierlot2, 3, Helen O'Shea1 and Daniel Boyd2, 3 1 Cork Institute of Technology, Department of Biological Science, Ireland 2 Dalhousie University, Department of Oral Sciences, Canada 3 Dalhousie University, Department of Biomedical Engineering, Canada Introduction: Borate (B) glasses exhibit excellent potential as bioactive materials, particularly for hard tissue augmentation [1],[2]. Historically, the low chemical durability of vitreous B was considered a negative attribute. However, it can be ascertained from the literature that compositional modifications may modulate material properties via structural alterations within the glass network. Accordingly, emphasis has been placed on achieving materials that (i) provide for indication specific material characteristics (i.e. appropriate durability) and, (ii) allow the release of biologically active therapeutic inorganic ions to modulate host responses. This study illustrates that substituting La2O3 or Ga2O3 into high B glasses provides significant modulation of composition-structure-property relationships and provides an interesting materials platform for applications in bone tissue engineering. Materials and Methods: 11 glass compositions, across two series, (70B-20Sr-10-x(La/Ga)Na (x = 0, 2, 4, 6, 8, 10, mol%) were melted and processed to a particle size distribution of 45-150µm. Materials were subjected to structural analysis via magic angle spinning nuclear magnetic resonance spectroscopy. Glass transition temperatures (Tg) were determined by differential scanning calorimetry, while density measurements were obtained using helium pycnometery. Ion release kinetics for each element in each composition was determined by ICP-OES (up to 30-days). One glass, which provided linear strontium release (LB102, from the La glass series) was implanted in a femur healing defect model in New Zealand White Rabbits to evaluate its influence on bone repair. Histopathology was conducted after a 6-week implantation period using Goldner’s trichrome staining. Results and Discussion: Ga2O3 appeared to stabilize the glass systems to a lesser degree than La2O3. Increasing substitutions of La2O3 for Na2O enhanced the durability of the glasses as observed from the release of B, Sr and Na ions. La provided significant control over the modulation of Sr release kinetics; an influence, which was not mirrored with the inclusion of Ga. Linear Sr release profiles were observed for one particular composition of glass (LB102). Upon implantation this glass provided both osteoconduction and osseointegration local to the implants, as characterized by moderate bone formation (green stained areas) and multifocal bone-biomaterial apposition at the implant sites. Conclusion: La2O3 additions, up to 10mol%, provided the greatest stabilization with respect to durability in ion release when compared to Ga2O3. The compositional modification of the La series provided for a B glass particulate with appropriate biocompatibility and bioactivity, so as (i) not to induce an adverse host response, (ii) allow for osteoconduction and osseointegration, and (iii) enhance bone formation. With respect to these data, the authors believe that this study provides an initial platform for the utilisation of B glasses for hard tissue augmentation. AccelLabs, Montreal, Canada; CIT RÍSAM PhD Scholarship Programme (RDRP); NSERC (Discovery Award Program); The Canadian Foundation for Innovation

An in vivo study on the metabolism and osteogenic activity of bioabsorbable Mg–1Sr alloy

Previous studies indicated that local delivery of strontium effectively increased bone quality and formation around osseointegrating implants. Therefore, implant materials with long-lasting and controllable strontium release are avidly pursued. The central objective of the present study was to investigate the in vivo biocompatibility, metabolism and osteogenic activity of the bioabsorbable Mg–1Sr (wt.%, nominal composition) alloy for bone regeneration. The general corrosion rate of the alloy implant as a femoral fracture fixation device was 0.55±0.03mm·y−1 (mean value±standard deviation) in New Zealand White rabbits which meet the bone implantation requirements and can be adjusted by material processing methods. All rabbits survived and the histological evaluation showed no abnormal physiology or diseases 16weeks post-implantation. The degradation process of the alloy did not significantly alter 16 primary indexes of hematology, cardiac damage, inflammation, hepatic functions and metabolic process. Significant increases in peri-implant bone volume and direct bone-to-implant contact (48.3%±15.3% and 15.9%±5.6%, respectively) as well as the expressions of four osteogenesis related genes (runt-related transcription factor 2, alkaline phosphatase, osteocalcin, and collagen, type I, alpha 1) were observed after 16weeks implantation for the Mg–1Sr group when compared to the pure Mg group. The sound osteogenic properties of the Mg–1Sr alloy by long-lasting and controllable Sr release suggesting a very attractive clinical potential. Statement of significanceSr (strontium) has exhibited pronounced effects to reduce the bone fracture risk in osteoporotic patients. Nonetheless, long-lasting local Sr release is hardly achieved by traditional methods like surface treatment. Therefore, a more efficient Sr local delivery platform is in high clinical demand.The stable and adjustable degradation process of Mg alloy makes it an ideal Sr delivery platform. We combine the well-known osteogenic properties of strontium with magnesium to manufacture bioabsorbable Mg–1Sr alloy with stable Sr release based on our previous studies.The in vitro and in vivo results both showed the alloy’s suitable degradation rate and biocompatibility, and the sound osteogenic properties and stimulation effect on bone formation suggest its very attractive clinical potential.

Therapeutic Ion-Releasing Bioactive Glass Ionomer Cements with Improved Mechanical Strength and Radiopacity

Bioactive glasses (BG) are used to regenerate bone, as they degrade and release therapeutic ions. Glass ionomer cements (GIC) are used in dentistry, can be delivered by injection and set in situ by a reaction between an acid-degradable glass and a polymeric acid. Our aim was to combine the advantages of BG and GIC, and we investigated the use of alkali-free BG (SiO2-CaO-CaF2-MgO) with 0 to 50% of calcium replaced by strontium, as the beneficial effects of strontium on bone formation are well documented. When mixing BG and poly(vinyl phosphonic-co-acrylic acid), ions were released fast (up to 90% within 15 minutes at pH 1), which resulted in GIC setting, as followed by infrared spectroscopy. GIC mixed well and set to hard cements (compressive strength up to 35 MPa), staying hard when in contact with aqueous solution. This is in contrast to GIC prepared with poly(acrylic acid), which were shown previously to become soft in contact with water. Strontium release from GIC increased linearly with strontium for calcium substitution, allowing for tailoring of strontium release depending on clinical requirements. Furthermore, strontium substitution increased GIC radiopacity. GIC passed ISO10993 cytotoxicity test, making them promising candidates for use as injectable bone cements.

Comparison of silver, cesium, and strontium release predictions using PARFUME with results from the AGR-1 irradiation experiment

The PARFUME (PARticle FUel ModEl) code was used to predict the release of fission products silver, cesium, and strontium from tristructural isotropic coated fuel particles and compacts during the first irradiation experiment (AGR-1) of the Advanced Gas Reactor Fuel Development and Qualification program. The PARFUME model for the AGR-1 experiment used the fuel compact volume average temperature for each of the 620 days of irradiation to calculate the release of silver, cesium, and strontium from a representative particle for a select number of AGR-1 compacts. Post-irradiation examination (PIE) measurements provided data on release of these fission products from fuel compacts and fuel particles, and retention of silver in the compacts outside of the silicon carbide (SiC) layer. PARFUME-predicted fractional release of silver, cesium, and strontium was determined and compared to the PIE measurements. For silver, comparisons show a trend of over-prediction at low burnup and under-prediction at high burnup. PARFUME has limitations in the modeling of the temporal and spatial distributions of the temperature and burnup across the compacts, which affects the accuracy of its predictions. Nevertheless, the comparisons on silver release lie in the same order of magnitude. Results show an overall over-prediction of the fractional release of cesium by PARFUME. For particles with failed SiC layers, the over-prediction is by a factor of up to 3, corresponding to a potential over-estimation of the diffusivity in uranium oxycarbide (UCO) by a factor of up to 250. For intact particles, whose release is much lower, the over-prediction is by a factor of up to 100, which could be attributed to an over-estimated diffusivity in SiC by about 40% on average. The release of strontium from intact particles is also over-predicted by PARFUME, which also points towards an over-estimated diffusivity of strontium in either SiC or UCO, or possibly both. The measured strontium fractional release from intact particles varied considerably from compact to compact, making it difficult to assess the effective over-estimation of the diffusivities. Furthermore, the release of strontium from particles with failed SiC is difficult to observe experimentally due to the release from intact particles, preventing any conclusions to be made on the accuracy or validity of the PARFUME predictions and the modeled diffusivity of strontium in UCO.

Strontium-Containing Apatite/Poly Lactide Composites Favoring Osteogenic Differentiation and in Vivo Bone Formation

Strontium was shown to enhance bone growth; however, its oral administration may lead to severe side effects. The application of strontium in orthopedic biomaterials may therefore be an alternative to achieve targeted and sustained strontium treatment to the surgery site in aid of bone growth locally. In this study, strontium-containing composites were prepared by introducing various levels of strontium into amorphous apatite and extruding the obtained apatite powders into polylactide in a weight ratio of 1:1. Strontium substitution increased apatite dissolution and enhanced the degradation behavior of the resulting composites. Sustained and dose-dependent strontium release from the composites was observed for up to 24 weeks in simulated physiological solution, indicating that strontium release can be tuned by its substitution level in the embedded apatite. Strontium-containing composites enhanced the osteogenic differentiation of murine mesenchymal stromal cells in vitro in a dose-dependent fashion. After heterotopic implantation with the combination of rhBMP-2, bone formation on strontium-containing composites was enhanced. These results suggest that strontium-releasing composites provide a local ion-rich (i.e., Sr2+ as well as Ca2+ and PO43–) environment that favors osteogenic differentiation and in vivo bone formation

PH Effect on the Dissolution Behavior of the Microspheres Containing Strontium Ranelate

Strontium is known for efficient actions on bone formation and resorption. Strontium ranelate (SrR) is a commercial drug which maintains this balance during bone turnover, reducing the risks of vertebral fractures in the patients. Calcium phosphate bioceramics associated with alginate matrices containing strontium (Sr) could improve bone regeneration due to gradual Sr release. In this report, the strontium ranelate was incorporated on microspheres of alginate (ALG)/β-tricalcium phosphate in a single route of the production. Energy dispersive spectroscopy showed that strontium was incorporated on the surface of the microspheres produced. Thedissolution behaviour into a buffer solution at pH 4.0 and at 7.4 was evaluated, measuring Sr content on the microspheres after in vitro assays by atomic absorption spectrometry. Dissolution tests showed a rapid strontium release in both assays, however, it was more pronounced at pH 4.0. Fourier transform infrared spectra indicated the presence of a new precipitated phase at pH 7.4 up after 14 days. Scanning electron microscopy of the microspheres submitted to in vitro revealed that the microspheres at pH 4.0 buffer underwent erosion up to 7 days, while the ones in pH 7.4 buffer, eroded in 48h. This behaviour is due to the high swelling rate of the microspheres in neutral pH. The solubility of the microspheres favors its use as a great material for a local strontium release and remodeling bone.

Strontium Incorporation on Microspheres of Alginate/β-tricalcium Phosphate as Delivery Matrices

Strontium (Sr) is known to positively affect the mechanism of bone remodelling. Consequently, calcium phosphate bioceramics associated with alginate matrices containing strontium could improve bone regeneration due to gradual strontium release. This work aims to incorporate Sr on microspheres of alginate (ALG)/β-tricalcium phosphate (β-TCP) and evaluates the in vitro release of strontium ions into a buffer solution at pH 4.0 and 7.4. In this study, strontium chloride was employed as a cross-linking agent and Sr source. Energy dispersive spectroscopy (EDS) showed that strontium was incorporated mainly at the surface of the microspheres produced. The in vitro experiments revealed that there is a rapid strontium release up to 24 h at pH 7.4 due to Sr location on microspheres' surface. At pH 4.0 both calcium and strontium were released due to the β-TCP dissolution.

Mass spectrometric monitoring of Sr-enriched bone cements—from in vitro to in vivo

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a well-established technique in materials science, but is now increasingly applied also in the life sciences. Here, we demonstrate the potential of this analytical technique for use in the development of new bone implant materials. We tracked strontium-enriched calcium phosphate cements, which were developed for the treatment of osteoporotic bone, from in vitro to in vivo. Essentially, the spatial distribution of strontium in two different types of strontium-modified calcium phosphate cements is analysed by SIMS depth profiling. To gain information about the strontium release kinetics, the cements were immersed for 3, 7, 14 and 21days in α-MEM and tris(hydroxymethyl)-aminomethane solution and analysed afterwards by ToF-SIMS depth profiling. For cements stored in α-MEM solution an inhibited strontium release was observed. By using principal component analysis to evaluate TOF-SIMS surface spectra, we are able to prove the adsorption of proteins on the cement surface, which inhibit the release kinetics. Cell experiments with human osteoblast-like cells cultured on the strontium-modified cements and subsequent mass spectrometric analysis of the mineralised extracellular matrix (mECM) prove clearly that strontium is incorporated into the mECM of the osteoblast-like cells. Finally, in an animal experiment, the strontium-doped cements are implanted into the femur of osteoporotic rats. After 6weeks, only a slight release of strontium was found in the vicinity of the implant material. By using ToF-SIMS, it is proven that strontium is localised in regions of newly formed bone but also within the pre-existing tissue.

Bone formation induced by strontium modified calcium phosphate cement in critical-size metaphyseal fracture defects in ovariectomized rats

The first objective was to investigate new bone formation in a critical-size metaphyseal defect in the femur of ovariectomized rats filled with a strontium modified calcium phosphate cement (SrCPC) compared to calcium phosphate cement (CPC) and empty defects. Second, detection of strontium release from the materials as well as calcium and collagen mass distribution in the fracture defect should be targeted by time of flight secondary ion mass spectrometry (TOF-SIMS). 45 female Sprague–Dawley rats were randomly assigned to three different treatment groups: (1) SrCPC (n = 15), (2) CPC (n = 15), and (3) empty defect (n = 15). Bilateral ovariectomy was performed and three months after multi-deficient diet, the left femur of all animals underwent a 4 mm wedge-shaped metaphyseal osteotomy that was internally fixed with a T-shaped plate. The defect was then either filled with SrCPC or CPC or was left empty. After 6 weeks, histomorphometric analysis showed a statistically significant increase in bone formation of SrCPC compared to CPC (p = 0.005) and the empty defect (p = 0.002) in the former fracture defect zone. Furthermore, there was a statistically significant higher bone formation at the tissue–implant interface in the SrCPC group compared to the CPC group (p < 0.0001). These data were confirmed by immunohistochemistry revealing an increase in bone-morphogenic protein 2, osteocalcin and osteoprotegerin expression and a statistically significant higher gene expression of alkaline phosphatase, collagen10a1 and osteocalcin in the SrCPC group compared to CPC. TOF-SIMS analysis showed a high release of Sr from the SrCPC into the interface region in this area compared to CPC suggesting that improved bone formation is attributable to the released Sr from the SrCPC.

Pulsed electrodeposition for the synthesis of strontium-substituted calcium phosphate coatings with improved dissolution properties

Strontium-substituted calcium phosphate coatings are synthesized by pulsed electrodeposition on titanium alloy (Ti6Al4V) substrates. Experimental conditions of the process are optimized in order to obtain a coating with a 5% atomic substitution of calcium by strontium which corresponds to the best observations on the osteoblast cells activity and on the osteoclast cells proliferation. The physical and chemical characterizations of the obtained coating are carried out by scanning electron microscopy associated to energy dispersive X-ray spectroscopy (EDXS) for X-ray microanalysis and the structural characterization of the coating is carried out by X-ray diffraction. The in vitro dissolution/precipitation properties of the coated substrates are investigated by immersion into Dulbecco's Modified Eagle Medium (DMEM) from 1h to 14days. The calcium, phosphorus and strontium concentrations variations in the biological liquid are assessed by Induced Coupled Plasma - Atomic Emission Spectroscopy for each immersion time. The results show that under specific experimental conditions, the electrodeposition process is suitable to synthesize strontium-substituted calcium phosphate coatings. Moreover, the addition of hydrogen peroxide (H2O2) into the electrolytic solution used in the process allows us to observe a control of the strontium release during the immersion of the prosthetic materials into DMEM.

Bactericidal strontium-releasing injectable bone cements based on bioactive glasses

Strontium-releasing injectable bone cements may have the potential to prevent implant-related infections through the bactericidal action of strontium, while enhancing bone formation in patients suffering from osteoporosis. A melt-derived bioactive glass (BG) series (SiO2–CaO–CaF2–MgO) with 0–50% of calcium substituted with strontium on a molar base were produced. By mixing glass powder, poly(acrylic acid) and water, cements were obtained which can be delivered by injection and set in situ, giving compressive strength of up to 35 MPa. Strontium release was dependent on BG composition with increasing strontium substitution resulting in higher concentrations in the medium. Bactericidal effects were tested on Staphylococcus aureus and Streptococcus faecalis; cell counts were reduced by up to three orders of magnitude over 6 days. Results show that bactericidal action can be increased through BG strontium substitution, allowing for the design of novel antimicrobial and bone enhancing cements for use in vertebroplasty or kyphoplasty for treating osteoporosis-related vertebral compression fractures.

A novel hybrid material with calcium and strontium release capability

The preparation of PDMS–TEOS–CaO hybrid materials by sol–gel techniques has been widely described in previous works. Calcium nitrate is the most common source of calcium used in these preparations. However, to remove possible toxic nitrate by-products a thermal treatment is necessary at temperatures above 500°C, which leads to the degradation of the polymeric components of the hybrids. Strontium has already shown some promising results in the therapeutic area, being used in cases of osteoporosis and low bone density. In this study a new potential bioactive hybrid material was prepared, by sol–gel techniques, using calcium acetate as a novel calcium source. Also, for the first time, incorporation of strontium in a PDMS–TEOS hybrid system was evaluated. Samples were characterized before and after immersion in Kokubo's Simulated Body Fluid (SBF) by SEM, EDS, ICP and FT-IR spectroscopy.

Strontium- and Zinc-Alginate Hydrogels for Bone Tissue Engineering

The development of bone replacement materials is an important healthcare objective due to the drawbacks of treating defects with bone autografts. In this work we propose a bone tissue engineering approach in which arginine-glycine-aspartic acid (RGD)-modified alginate hydrogels are crosslinked with bioactive strontium and zinc ions as well as calcium. Strontium was chosen for its ability to stimulate bone formation, and zinc is essential for alkaline phosphatase (ALP) activity. Calcium and strontium gels had similar stiffnesses but different stabilities over time. Strontium gels made with alginate with a high percentage of guluronic acid residues (high G) were slow to degrade, whereas those made with alginate rich in mannuronic acid (high M) degraded more quickly, and supported proliferation of Saos-2 osteoblast-like cells. After an initial burst, strontium release from alginate gels was steady and sustained, and the magnitude of release from high M gels was biologically relevant. Saos-2 cultured within alginate gels upregulated the osteoblast phenotypic marker genes RUNX2, collagen I (COL1A1) and bone sialoprotein (BSP), and ALP protein activity was highest in alginate gels cast with strontium ions. This strategy has the potential to be combined with other alginate-based systems for bone tissue engineering, or adapted to other tissue engineering applications.

Removal of strontium from an aqueous solution using co-precipitation followed by microfiltration (CPMF)

This study characterized the removal of strontium from an aqueous solution via co-precipitation followed by microfiltration (CPMF) on a lab-scale. Sodium carbonate was used as the precipitating agent. Ferric chloride was added to a settler at a dosage of 20 mg/L to improve strontium removal, and it was added to a membrane separator at a dosage of 10 mg/L to reduce membrane fouling. The concentration of strontium in the raw water used in this study was about 5 mg/L. In two intermittent tests, where dosages of sodium carbonate were 2000 mg/L and 1000 mg/L, the mean decontamination factors (DFs) were 237 and 158 and the mean concentration factors (CFs) were 288 and 462, respectively. Although the mean DF value was lower when the sodium carbonate dosage was 1000 mg/L instead of 2000 mg/L, the rate at which the specific flux (SF) of the membrane declined decreased as the amount of the effluent treated increased. The problem of strontium release at the beginning of the operation was controlled by a continuous test in which the dosage of sodium carbonate still was 1000 mg/L and the other parameters were the same as in the other tests. The results showed that the mean DF and CF were 157 and 480, respectively.

The effect of composition on ion release from Ca–Sr–Na–Zn–Si glass bone grafts

Controlled delivery of active ions from biomaterials has become critical in bone regeneration. Some silica-based materials, in particular bioactive glasses, have received much attention due to the ability of their dissolution products to promote cell proliferation, cell differentiation and activate gene expression. However, many of these materials offer little therapeutic potential for diseased tissue. Incorporating trace elements, such as zinc and strontium, known to have beneficial and therapeutic effects on bone may provide a more viable bone graft option for those suffering from metabolic bone diseases such as osteoporosis. Rational compositional design may also allow for controlled release of these active ions at desirable dose levels in order to enhance therapeutic efficacy. In this study, six differing compositions of calcium-strontium-sodium-zinc-silicate (Ca-Sr-Na-Zn-Si) glass bone grafts were immersed in pH 7.4 and pH 3 solutions to study the effect of glass composition on zinc and strontium release in a normal and extreme physiological environment. The zinc release levels over 30 days for all zinc-containing glasses in the pH 7.4 solution were 3.0-7.65 ppm. In the more acidic pH 3 environment, the zinc levels were higher (89-750 ppm) than those reported to be beneficial and may produce cytotoxic or negative effects on bone tissue. Strontium levels released from all examined glasses in both pH environments similarly fell within apparent beneficial ranges--7.5-3500 ppm. Glass compositions with identical SrO content but lower ZnO:Na(2)O ratios, showed higher levels of Sr(2+) release. Whereas, zinc release from zinc-containing glasses appeared related to ZnO compositional content. Sustainable strontium and zinc release was seen in the pH 7.4 environment up to day 7. These results indicate that the examined Ca-Sr-Na-Zn-Si glass compositions show good potential as therapeutic bone grafts, and that the graft composition can be tailored to allow therapeutic levels of ions to be released.

Estimating groundwater mixing and origin in an overexploited aquifer in Guanajuato, Mexico, using stable isotopes (strontium-87, carbon-13, deuterium and oxygen-18)†

Stable Isotopes (strontium-87, deuterium and oxygen-18, carbon-13) have been used to reveal different sources of groundwater and mixing processes in the aquifer of the Silao-Romita Valley in the state of Guanajuato, Mexico. Calcite dissolution appeared to be the main process of strontium release leading to relatively equal 87Sr/86Sr ratios of 0.7042–0.7062 throughout the study area which could be confirmed by samples of carbonate rocks having similar Sr ratios (0.7041–0.7073). δ13C values (−11.91– −6.87‰ VPDB) of groundwaters confirmed the solution of carbonates but indicated furthermore influences of soil-CO2. Deuterium and 18O contents showed a relatively narrow range of−80.1– −70.0 ‰ VSMOW and−10.2– −8.8 ‰, VSMOW, respectively but are affected by evaporation and mixing processes. The use of δ13C together with 87Sr/86Sr revealed three possible sources: (i) carbonate–controlled waters showing generally higher Sr-concentrations, (ii) fissure waters with low–strontium contents and (iii) infiltrating water which is characterized by low δ13C and 87Sr/86Sr ratios. The third component is affected by evaporation processes taking place before and during infiltration which might be increased by extraction and reinfiltration (irrigation return flow).

Identification of the mechanism limiting the alteration of clad spent fuel segments in aerated carbonated groundwater

Leaching experiments were performed with five spent fuel samples (20 mm segments of clad fuel rods) from French power reactors (four UO 2 fuel samples with burnup ratings of 22, 37, 47 and 60 GW d t HM −1 and a MOX fuel sample irradiated to 47 GW d t HM −1) to determine the release kinetics of the matrix containing most of the radionuclides. The experiments were carried out with carbonated groundwater on previously leached sections of clad fuel rods in static mode, in an aerated medium at room temperature (25 °C) in a hot cell. Until 313 days of leaching and below uranium saturation, the Sr/U congruence ratios for all the UO 2 fuel samples ranged from 1 to 2; allowing for the experimental uncertainty, strontium can thus be considered as a satisfactory matrix alteration tracer. No significant burnup effect was observed on the alteration of the UO 2 fuel matrix. The daily strontium release factor was approximately 2.7 × 10 −8 d −1 for UO 2 fuel after 706 days of leaching, and seven to eight times higher for MOX fuel. Several alteration mechanisms (radiolysis, solubility, precipitation/clogging) are examined to account for the experimental findings. All the available experimental data (characterization of secondary phases and leaching data) indicate that the mechanism limiting the spent fuel alteration kinetics, for the conditions studied, is likely based on the transport and accessibility of oxidizing species and/or water within the segment.

Effect of Spent Fuel Burnup and Composition on Alteration of the U(Pu)O2 Matrix

ABSTRACTFor a potential performance assessment of direct disposal of spent fuel in a nuclear waste repository, the chemical reactions between the fuel and possible intruding water must be understood and the resulting radionuclide release must be quantified.Leaching experiments were performed with five spent fuel samples from French power reactors (four UO2 fuel samples with burnup ratings of 22, 37, 47 and 60 GWd·tHM−1 and a MOX fuel sample irradiated to 47 GWd·tHM−1) to determine the release kinetics of the matrix containing most (over 95%) of the radionuclides. The experiments were carried out with granitic groundwater on previously leached sections of clad fuel rods in static mode, in an aerated medium at room temperature (25°C) in a hot cell.After 1000 or 2000 days of leaching, the Sr/U congruence ratios for all the UO2 fuel samples ranged from 1 to 2; allowing for the experimental uncertainty, strontium can thus be considered as a satisfactory matrix alteration tracer. No significant burnup effect was observed on the alteration of the UO2 fuel matrix. The daily strontium release factor was approximately 1 à 10−7 d−1 for UO2 fuel, and five to six times higher for MOX fuel. Several alteration mechanisms (radiolysis, solubility, precipitation/clogging) are examined to account for the experimental findings.

Cytosolic Calcium Transient and Light Response in Limulus Ventral Nerve Photoreceptors: Supralinearity in Response-Intensity Curves

Abstract Light-evoked intracellular electrical responses were measured simultaneously with the changes in intracellular free Ca2+ or Sr2+ concentration by means of arsenazo III. The response-versus-stimulus intensity characteristics was measured in two states of relative light adaptation in strontium-containing saline (10 mᴍ Sr2+; 0.25 mᴍ Ca2+). Supralinear parts of the curves with slopes &gt; 1 were found for the receptor current amplitude and the current­-time integral as well as for the amplitude of the arsenazo signal within at least 0.3 log units of identical stimulus intensity range. Maximum slopes of response-versus-intensity curves are on average approximately 1.7 for the receptor current and the arsenazo signal in both adaptational states. The rise in time of the arsenazo signal evoked by a light flash shows two phases. In strontium saline we observe a fast increase at the onset of the arsenazo signal which is detectable within the first 20-100 ms of the signal rise and a slow further increase to the maximum, with a duration of 0.7-6 s. In the detailed analysis of both phases we observed that the amplitude of each phase increases supralinear, i.e. disproportionate to the stimulus intensity. The fast rising phase of the arsenazo signal is not visibly voltage-dependent. This fast rising phase might be a monitor of an IP3-sensitive calcium or strontium release. Since the amplitude of the fast rising phase increases supralinear with the stimulus intensity, we propose the interpretation that more than one IP3-molecule must bind to the IP3-receptor to open this channel of the intracellular calcium store. Only the slow rising phase is voltage-dependent. The kinetic of this slow rising phase was accelerated with more negative membrane potentials. This predominant slow phase correlates with the receptor current-time integral. Therefore the slow rising phase of the arsenazo signal might be a monitor for influx of calcium or strontium through light activated ion channels in the plasma membrane. The supralinearity of the amplitude of the slow rising phase might indicate the supralinear increase in calcium or strontium, which is carried by the receptor current. Additionally, we calculated the fraction of strontium and calcium ions of the total receptor current. When calcium was replaced by strontium in the superfusate about 3-30% of the receptor current are carried by strontium and calcium ions. In physiological saline calcium contributes about 1-1.5% to the receptor current.