Chemical states of fallout radioactive Cs in the soils deposited at Fukushima Daiichi Nuclear Power Plant accident

The chemical states of radioactive Cs (caused by Fukushima Daiichi Nuclear Power Plant accident) in the contaminated soils have been characterized by the desorption experiments using appropriate reagent solutions and size fractionation of the contaminated soils. More than 65% of radioactive Cs remained in the residual fraction of the soil samples after treatment of 1 mole L^?1 NH₄Cl solution and 1 mole L^?1 CH₃COOH solution. Approximately 70% of radioactive Cs in the residual fraction were associated with the size fractions larger than the elutriated one, even though mica-like minerals were present in the elutriated one. These results strongly suggest that radioactive Cs was irreversibly associated with soil components other than mica-like minerals in the contaminated soil.     

Sorption Behavior of Selenium(—II) on Rocks under Reducing Conditions

The sorption of radionuclides on rocks is an important factor for performance assessment of geologic disposal of radioactive wastes. Batch sorption experiments under reducing conditions were carried out to investigate the sorption behavior of selenium onto granodiorite, sandy mudstone, tuffaceous sandstone, and their major constituent minerals and accessory minerals. Rock cores and groundwater employed in the experiments were sampled with special care to avoid exposing the cores and groundwater to air to minimize their oxidation. Selenium was spiked as HSe^? and Se₄ ^2- in the experimental solutions, and reducing conditions were maintained throughout the sorption periods. Distribution coefficients, Kd (m³ kg^?1), were obtained in the ranges of 2.2 × 10^?4 to 4.0 × 10^?3 m³ kg^?1 for granodiorite, 3.3 × 10^?2 to 5.6 × 10^?2 m³ kg^?1 for sandy mudstone, and 2.9 × 10^?2 to 8.2 × 10^?2 m³ kg^?1 for tuffaceous sandstone at pHs 8.5 to 11.5. The dominant sorbent minerals for selenium over a neutral to alkaline pH range were determined to be biotite for the granodiorite samples and pyrite for the sandy mudstone and tuffaceous sandstone samples.     

Theoretical study of the adsorption of Cs,Cs+, I,I−, and CsI on C60 fullerene

Theoretical investigation for the adsorption of the cesium atom (Cs), the cesium iodide molecule (CsI), the iodine atom (I), the cesium cation (Cs⁺), and the iodide anion (I^?) onto the surface of a single fullerene molecule (C₆₀) are reported. A hybrid exchange–correlation functional using the Coulomb-attenuating method (CAM-B3LYP) is employed. The adsorption energies, i.e., the opposite of enthalpy change through adsorption, are calculated to be 143, 12, 9, 46, and 49 kJ mol^?1 for Cs, CsI, I, Cs⁺, and I^?, respectively. The equilibrium constant for Cs is calculated to be 7×10³ atm^?1 at the temperature of 1000 K and is seven orders of magnitude higher than that for CsI, indicating that the C₆₀ molecule adsorb the Cs atom highly selectively against the CsI molecule.     

Argon ion impact desorption cross sections of chlorine and carbon from molybdenum

Total desorption cross sections have been measured for Cl (σ_Cl) and C(σ_C) on molybdenum by argon ion bombardment for an incidence angle of 60° from the surface normal. For the bombardment an ion gun with low current density (i₀ ~ 1 × 10 ^?7 A cm^?2) at low system pressure (~10^?9 Torr) was used. The detection was performed by AES and the data were sensitivity factor corrected. The AES analysis of the surface after adsorption showed that Mo, C and Cl contributed to more than 94% of the atomic composition. With known i₀, it is possible to obtain σ from the adsorbate signal vs ion bombardment time curve. For ion energies between 0.2 keV to 1.0 keV the measured value for σ_Cl and σ_C are 0.5?3 × 10^?15 cm² and 0.2?4 × 10^?15 cm², respectively. The possible effects of the surface roughness due to prebombardment are discussed.     

Radioactivity of fission product and heavy nuclides deposited on soil in Fukushima Dai-Ichi Nuclear Power Plant accident

Based on periodically performed radioactivity measurements on soil samples in the site of Fukushima Dai-Ichi Nuclear Power Station, activity ratios to ¹³⁷Cs of fission product and heavy nuclides were obtained for Sr, Nb, Mo, Tc, Ru, Ag, Te, I, Ba, La, Pu, Am, and Cm isotopes. By exponentially fitting or averaging, the activity ratios at the core shutdown were estimated. Using correlations of activity ratios of ¹³⁴Cs to ¹³⁷Cs, and ²³⁸Pu to the sum of ²³⁹Pu and ²⁴⁰Pu against fuel burnup, burnup of the fuel sourcing the deposited activity of the soil was estimated. The activity ratios to ¹³⁷Cs of each nuclide on the deposited activity were divided by those calculated on the fuel at the shutdown to obtain the deposited activity fraction of each nuclide as a relative value to ¹³⁷Cs, which also corresponds to the deposited fraction of each element as a relative value to Cs. The obtained deposited fractions relative to Cs are the orders of 10^?4 to 10^?2 for Sr, 10^?5 to 10^?3 for Nb, 10^?2 to 10^?1 for Mo, 1 to 10 for I, 10^?3 to 10^?2 for Ba, 10^?2 for La, 10^?6 to 10^?3 for Pu, 10^?6 to 10^?4 for Am, and 10^?7 to 10^?5 for Cm. The deposited fractions for Tc, Ag, and Te were not estimated due to the lack of the calculated inventories in the fuel for the relevant measured radioactive nuclides.     

Tectonic and radioactivity impacts of 238U on groundwater-based drinking water at Gosa and Lugbe areas of Abuja,North Central Nigeria

Tectonic contribution of activity level of ²³⁸U in groundwater-based drinking water in Gosa and Lugbe areas of Abuja was measured using inductively coupled plasma mass spectrometry (ICP-MS). The highest activity level of 2736 µBq L^?1 reported in Lugbe borehole, whereas the lowest value of 443 µBq L^?1 reported at Gosa borehole. The inhabitants permanently used water from the boreholes for daily consumption. The group receives 5.55 × 10^?5 mSv of the annual collective effective dose due to ²³⁸U in drinking water. The radiological risks of ²³⁸U in the water samples were found to be low, typically in magnitude of 10^?7 with cancer mortality value of 1.03 × 10^?7 and morbidity value of 1.57 × 10^?7. The chemical toxicity risk of ²³⁸U in drinking water over a lifetime consumption has a mean value of 4.0 × 10^?3 μg kg^?1 day^?1. It could be that the human risk due to ²³⁸U content in groundwater supplies from ingestion may likely be the chemical toxicity of ²³⁸U as a heavy metal rather than radiological risk. Significantly, Lugbe subsurface may have developed some fractions of granitic strata that contributed to the distribution of radioactive of ²³⁸U in tectonically weak zones.     

A novel technique of in situ phase-shift interferometry applied for faint dissolution of bulky montmorillonite in alkaline solution

The effect of alkaline pH on the dissolution rate of bulky aggregated montmorillonite samples at 23 °C was investigated for the first time by using an enhanced phase-shift interferometry technique combined with an internal refraction interferometry method developed for this study. This technique was applied to provide a molecular resolution during the optical observation of the dissolution phenomena in real time and in situ while remaining noninvasive. A theoretical normal resolution limit of this technique was 0.78 nm in water for opaque material, but was limited to 6.6 nm for montmorillonite due to the transparency of the montmorillonite crystal. Normal dissolution velocities as low as 1 × 10^?4 to 1 × 10^?3 nm/s were obtained directly by using the measured temporal change in height of montmorillonite samples set in a reaction cell. The molar dissolution fluxes of montmorillonite obtained in this study gave considerably faster dissolution rates in comparison to those obtained in previous investigations by solution analysis methods. The pH dependence of montmorillonite dissolution rate determined in this study was qualitatively in good agreement with those reported in the previous investigations. The dissolution rates to be used in safety assessments of geological repositories for radioactive wastes should be obtained for bulky samples. This goal has been difficult to achieve using conventional powder experiment technique and solution analysis method, but has been shown to be feasible using the enhanced phase-shift interferometry.     

Improvement of bitumen-waste product in leachability—Leachability of bitumen product containing BWR's evaporator concentrate

The method has been developed to improve the bitumen product which incorporates an evaporator concentrate from a BWR, with respect to the swelling and leaching. The leachability of the product has been measured by the method recommended by the IAEA. The swelling of the product is successfully prevented by the addition of calcium chloride. The specimen containing the waste up to Wa/B (Weight ratio of Na₂SO₄ + CaCl₂ and bitumen) = 60:40 shows no pronounced swelling, when it is immersed in water. The cumulative fractions of ¹³⁷Cs and ⁶⁰Co leached from a specimen which does not contain CaCl₂ are 0.65 and 0.2 at the leaching time of 30 days. On the hand, the corresponding value at 100 days for the specimen with calcium chloride addition is 5 × 10^?4 for ¹³⁷Cs and 1 × 10^?4 for ⁶⁰Co. The coating of the specimen surface with a fresh bitumen (5 mm thickness) reduces the leachability further. These results indicate that this method is effective to improve the bitumen product incorporating BWR's evaporator concentrate.     

Estimation of the radiation dose equivalent for the hypothetical submergence of a sea-transport package of low-level radioactive waste

Japanese Electric Power Utilities plans to transport low-level radioactive waste (LLW) in CSD-C (36 canisters) and CSD-B packages (10 canisters) by sea from France to Japan. In this study, we carried out assessments of the dose to the public from a hypothetical release of radioactive materials from submerged LLW packages into the sea. The estimated dose equivalents from the CSD-C and CSD-B packages were 2.8 × 10^?8 and 7.9 × 10^?9 mSv year^?1, respectively, for the near shore case. For the deep sea case, the estimated dose equivalents were 8.6 × 10^?8 and 5.8 × 10^?8 mSv year^?1, respectively. These estimated results were much smaller than those found in a previous study of Type-B packages (spent fuel, high level waste, and mixed oxide fuel) and the ICRP recommendation (1 mSv year^?1).     

A Study on Hydraulic Properties of Compacted Fe(III)-montmorillonite

Hydraulic conductivities were determined for compacted Fe(III)-montmorillonite sample. The Fe(III)-montmorillonite sample used in this study, which was prepared by the ion-exchange treatment of Namontmorillonite in FeCl₃ solution, contains less than 20% of Na⁺ ions as exchangeable cations and negligibly small amounts of iron precipitates. The measured hydraulic conductivities of compacted Fe(III)-montmorillonite were determined to be the order of 10^?9, 10^?11, and 10^?13 ms^?1 at dry densities of 0.80, 1.01, and 1.20 Mgm^?3, respectively. These values were found to be kept constant during the experimental period, suggesting no significant change in the physicochemical properties of Fe(III)-montmorillonite during the experiment. When compared with Na-montmorillonite, remarkably high values of hydraulic conductivities were found for Fe(III)-montmorillonite at the dry densities of 0.8 and 1.0Mgm^?3. On the other hand, almost the same value of hydraulic conductivity was obtained at the dry density around 1.2Mgm^?3. This different effect of exchangeable cations on the hydraulic conductivity of montmorillonite could be attributed to the different sizes of macropores in compacted montmorillonite and/or the different thicknesses of electrical double layers formed over montmorillonite sheets.     

Assessment of <Superscript>137</Superscript>Cs decontamination of concrete by the reagent method

The particulars of radioactive contamination of concrete wastes by ¹³⁷Cs were studied. x-Ray phase analysis and chemical analysis show that clayey materials, including Al₂O₃, Fe₂O₃, K₂O, and MgO, on which ¹³⁷Cs sorption is possible, were present in the concrete wastes. The content and form in which ¹³⁷Cs was found in radioactive concrete wastes from nuclear power facilities as well as in model samples were determined. When the wastes were treated with nitric acid the binder dissolved and a polydisperse suspension formed. The suspension consisted of three phases: solution, fine suspension, and rapidly settling precipitate of heavy filler particles. x-Ray phase analysis was performed and the ¹³⁷Cs mass ratio and distribution in the phases were determined. The possibility of decontaminating the concrete by a reagent method was evaluated.     

Preparation of potassium iron(III) hexacyanoferrate(II) supported on activated carbon and Cs uptake performance of the adsorbent

Synthesis of potassium iron(III) hexacyanoferrate(II) (K/Fe-Fe(CN)₆) in the pores of activated carbon (AC) was attempted by impregnating AC with K₄[Fe(CN)₆] and FeCl₃, and the Cs uptake performance of the resulting adsorbent was examined. K/Fe-Fe(CN)₆ supported on AC was prepared by varying the reaction conditions such as the supplied amounts and molar ratios of the reagents, and the Cs uptake performance was optimized. The impregnated product was characterized by XRD, EPMA, and porosimetry to elucidate the condition to which Fe₄[Fe(CN)₆]₃ was filled in the AC pores. The K/Fe-Fe(CN)₆-on-AC was immersed in seawater containing 0.075 mmol · dm^?3 Cs and agitated for 1 day to obtain the Cs uptake. The Cs uptake was large at pH < 10 but decreased abruptly at pH > 10.5. The maximum Cs uptake was 10.4 μmol · g^?1 at the equilibrium Cs concentration of 49 μmol · dm^?3 and the distribution coefficient was 45.5 dm³ · g^?1 at the equilibrium concentration of 0.015 μmol · dm^?3, respectively. When K/Fe-Fe(CN)₆-on-AC was immersed in Cs-containing seawater, K⁺ ions in the adsorbent were completely exchanged for Na⁺ ions in seawater, and the added Cs⁺ ions were then substituted for the Na⁺ ions in the adsorbent.     

Cesium solubility,diffusion and permeation in zirconium carbide

To compare the relative effectiveness of ZrC vis-a-vis SiC as a fission product barrier in fuel structures for high temperature gas cooled reactor (HTGR) applications, a series of cesium infusion experiments on various ZrC powders, and ZrC coated graphite structures was performed to study the cesium solubility, diffusivity, and permeability of this coating material. The ZrC powder results yield a solubility of Cs in ZrC, S(ppm wt) = (1.7 × 10^?6) exp[229 kJ/RT], over the temperature range 1485–1896 K. The diffusion coefficient of Cs in ZrC is 10^?18–10^?16 m²/s over a similar temperature interval. The activation energy of diffusion is estimated to be ≈ 50 kJ/mole.The results of experiments in which both SiC and ZrC coated graphite samples were exposed to cesium are more difficult to interpret. The results support the conclusions of the ZrC powder experiments that ZrC is comparable to SiC as a diffusion barrier to cesium.     

Development of methods for recovering uranium from sludge-like uranium generated in decontamination of metal wastes

Sludge-like uranium wastes (SUWs) have been generated with neutralization of acidic aqueous solutions used for decontamination of metal wastes containing a large amount of iron. We have examined the method for recovering uranium from such SUWs using N-cyclohexyl-2-pyrrolidone (NCP) as a precipitate. As a result, it was found that precipitation ratios (PRs) of uranium in the solutions prepared by dissolving SUWs in HNO₃ increase with increasing an amount of added NCP and are 97.7% at [NCP]/[U(VI)] = 20, and that the PRs of iron, aluminum, fluorine, and sulfate species are less than 1% at [NCP]/[U(VI)] = 3–20. This indicates that uranium species are precipitated selectively. The content ratios of U, Fe, Ca, F, and S in the materials after calcining precipitates obtained at [NCP]/[U(VI)] = 20 were 73, 2.2 × 10^?1, <3 × 10^?3, 2.3 × 10^?2, and 2.2 × 10^?2 wt%, respectively. These values are in accordance with the conditions of uranium ore concentrate defined by ConverDyn Corp and ASTM C967. From these results, it is expected that highly purified uranium can be efficiently recovered from SUWs by using NCP as the precipitant.     

Deuteriding of thin titanium films: The effect of carbon monoxide surface contamination

The effect of adsorbed CO on the deuteriding of thin titanium films at room temperature was measured at D₂ pressures between 10 to 25 mtorr on films contaminated with CO from exposures ranging between ～10^?8 torr-sec to 10^?4 torr-sec. In all measurements, for deuterium/ titanium atom ratios > 0.2, the deuteriding appeared to be initially limited by the sticking of D₂ on the clean or contaminated titanium deuteride surface; the effective sticking coefficient on a clean titanium deuteride surface was determined to be ～3 × 10^?3, while on a surface contaminated by an exposure of 10^?4 torr-sec to CO, the coefficient was reduced to ～2 × 10^?4. The pumping speeds of a Ti film was dramatically different when the film was evaporated over TiD₂. These changes were attributed to the presence of deuterium which diffused from the substrate film into the overlayer film.     

Chemical State and Deposition Rate of Nickel Ion Deposit Produced on Heated Surface in High Pressure Boiling Water

Nickel ion deposit was produced on a heated rod surface in high pressure boiling water (150–285°C, 0.4–7.0 MPa). The deposit under the same temperature and pressure conditions as those for BWR reactor water (285°C, 7.0 MPa) was identified as NiO by spectrum profile analysis of the NiLα, NiLβ and 9th-order NiKα₁ lines. Deposition rate was obtained from in situ measurements of deposit thickness, by a photoacoustic method, and from chemical analysis of deposit amount. The deposition rate coefficients obtained in temperature and pressure ranges of 150–250°C and 0.4–4.0 MPa were 2 × 10^?3–5 × 10^?2, which were 0.15–0.45 times as large as those of iron crud. This was attributed to a dissolution effect of Ni ion from NiO. The deposition rate coefficient at 285°C, 7.0 MPa was estimated to be 4.4 × 10^?2–1.3 × 10^?1.     

Distribution and Fixation of Cesium and Strontium in Zeolite A and Chabazite

The selective removal and fixation of Cs and Sr have been studied in zeolite A and chabazite. Cesium ion was preferentially distributed into chabazite with a high distribution coefficient (K _Cs>10³ cm³·g^?1) in the presence of NaCl (10^?1 mol·dm^?3). The K _Sr values for zeolite A attained about 10³ cm³·g^?1 in the pH range of 8～10, and they gradually decreased with an decrease in pH.

The initial rate of Cs adsorption was fairly fast in chabazite, and the adsorption ratio reached almost 100% within a few hours. The adsorption ratio of Sr in binderless A zeolite reached almost 100% after 15 h. The adsorption of Cs and Sr on these zeolites was followed by Langmuir-type isotherm. Cesium forms of these zeolites recrystallized to pollucite (CsAlSi₂O₆) above 900°C for zeolite A and above 1,200°C for chabazite. As for Sr forms, these zeolites changed to SrAl₂Si₂O₆ above 900°C.

These recrystallized phases were suitable hosts for the immobilization of Cs and Sr in the nuclear waste solutions.     

Analysis of Tritium Behavior in Very High Temperature Gas-Cooled Reactor Coupled with Thermochemical Iodine-Sulfur Process for Hydrogen Production

The tritium concentration in the hydrogen product in Japan's future very high temperature gas-cooled reactor (VHTR) system coupled with a thermochemical water-splitting iodine-sulfur (IS) process (VHTRIS system), named GTHTR300C, was estimated by numerical analysis. The tritium concentration in the hydrogen product significantly depended on undetermined parameters, i.e., the permeabilities of a SO₃ decomposer and a H₂SO₄vaporizer made of SiC. Thus, the estimated tritium concentration in the hydrogen product for the conservative analytical condition ranged from 3.4 × 10^?3 Bq/cm³ at STP (38 Bq/g-H₂) to 0.18 Bq/cm³ at STP (2,000 Bq/g-H₂). By considering the tritium retained by core graphite and the reduction in permeation rate by an oxide film on the heat transfer tube of the IHX and the HI decomposer, the tritium concentration in the hydrogen product decreased to the range from 3.3 × 10^?5 Bq/cm³ at STP (0.36 Bq/g-H₂) to 5.6 × 10^?3 Bq/cm³ at STP (63 Bq/g-H₂), which were smaller than those for the conservative analytical condition by factors of about 3.2 × 10^?2 and 9.6 × 10^?3, respectively. The effectof the helium flow rate in the helium purification system on the tritium concentration in the hydrogen product was also evaluated.     

A study of the systematic adsorption of radioactive strontium by montmorillonite and its fixation by roasting

The conditions of adsorbing Sr^89,90 by montmorillonite have been studied in detail. Various cations may be arranged in the following order according to their capacity to lower the adsorption of radioactive strontium: Al⁺³ > Fe⁺³ Ba⁺² Ca⁺² > Mg⁺² H⁺ > NH ₄ ⁺ > K⁺ > Na⁺.It has been shown that the adsorption of radioactive strontium by montmorillonite is an ionic exchange process and obeys the law of mass action. The presence of the anions CO₃ ^–2, SO₄ ^–2, and C₂O₄ ^–2 in a solution, which form relatively insoluble salts with strontium, does not change the mechanism of the adsorption, but decreases the quantity of radioactive strontium adsorbed, apparently by forming radioactive colloids.The process of fixing radioactive strontium on montmorillonitic clays during roasting has been studied. Roasting at temperatures above 850–900 ° and extending the roasting time over 1–2 hours do not affect the degree of fixation of radioactive strontium. In these experiments, the activity lost by washing in stream and sea water amounted to about 2%.It is suggested that, up to the beginning of alteration in the crystal lattice (T = 800 °C), the fixation is controlled by the formation of slightly soluble chemical combinations of strontium and the adsorbent. Above 800 °C the process is determined by changes in the crystal lattice and by a gradual vitrification of the mineral.     

Distribution of radionuclides in surface seawater obtained by an aerial radiological survey

We investigated the distribution in seawater of anthropogenic radionuclides from the Fukushima Daiichi Nuclear Power Plant (FNPP1) as preliminary attempt using a rapid aerial radiological survey performed by the U.S. Department of Energy National Nuclear Security Administration on 18 April 2011. We found strong correlations between in-situ activities of ¹³¹I, ¹³⁴Cs, and ¹³⁷Cs measured in surface seawater samples and gamma-ray peak count rates determined by the aerial survey (correlation coefficients were 0.89 for ¹³¹I, 0.96 for¹³⁴Cs, and 0.92 for¹³⁷Cs). The offshore area of high radionuclide activity extended south and southeast from the FNPP1. The maximum activities of ¹³¹I, ¹³⁴Cs, and ¹³⁷Cs were 329, 650, and 599 Bq L^?1, respectively. The ¹³¹I/¹³⁷Cs ratio in surface water of the high-activity area ranged from 0.6 to 0.7. Considering the radioactive decay of ¹³¹I (half-life 8.02 d), we determined that the radionuclides in this area were directly released from FNPP1 to the ocean. We confirm that aerial radiological surveys can be effective for investigating the surface distribution of anthropogenic radionuclides in seawater. Our model reproduced the distribution pattern of radionuclides derived from the FNPP1, although results simulated by a regional ocean model were underestimated.