Out-of-pile tritium release study on Li4SiO4 pebbles from TRINPC-I experiments

Out-of-pile tritium release examinations of irradiated Li₄SiO₄ pebbles were performed in TRINPC-I experiments for evaluating material performance and verifying the system design. To generate tritium the specimens were irradiated with neutrons. Li₄SiO₄ pebbles were made by a freeze-drying method. In the experiments, concentrations of tritium in the form of tritium gas (HT + T₂) and tritiated water (HTO + T₂O) in the outlet streams of a reactor tube were measured separately with an ionization chamber and a liquid scintillation radiometer. The results show that the percentage of tritium gas (HT + T₂) and tritiated water trapped by the breeder pebbles were about 72% and 19% of totally released tritium, respectively. Thus, more tritium was released in the form of tritium gas in this work. In addition to tritium trapped by the breeder pebbles, the amount of free tritium was also measured by breaking on-line a quartz capsule containing Li₄SiO₄ pebbles, the percentage of which was 9% of totally released tritium. The temperature peaks of tritium gas mainly appeared at about 477 °C and 654 °C, while the temperature peak of tritiated water appeared at about 402 °C, under which most of tritiated water released.     

X-ray diffraction of UC-UC2 and UC-UN alloys at elevated temperatures

Unit-cell dimensions of U-C-N alloys were measured at temperatures of 760 to 2250 °C with an X-ray diffractometer. Lattice dimensions of the face-centered cubic phase in both the UC-UC₂ and UC-UN solutions are found to be linear functions of the UC mole fraction. In carbon saturated body-centered tetragonal α-UC₂ between 1000 and 1400 °C, a small but distinct decrease in rate of volume change (slope) occurs with increasing temperature. The data suggest that the C/U ratio in α-UC₂ at 1000 °C and below is 1.97± 0.03, a value in excess of that prevailing in carbon-saturated α-UC₂ at higher temperatures.     

Thermal decomposition of tetravalent and trivalent plutonium oxalates

Using a Kurnakov registering pyrometer, we investigated the thermal decomposition of tetra- and trivalent plutonium oxalates. The composition of the intermediate products was determined with a Berg gas burette, by potentiometric titration and by Penfild's method. It was established that freshly precipitated Pu (IV) oxalate lost three molecules of water at 110 ° C. After standing for 3–4 days at 110 ° C the oxalate also gave 1.5–2.7% of CO+CO₂ due to decomposition induced by the -radiation of the plutonium. At the same time partial reduction to trivalent plutonium occurred. In the temperature range 170–200 ° C two more molecules of water and 13% CO+CO₂ were liberated and the plutonium was reduced to the trivalent state to form mainly Pu₂(C₂O₄)₃H₂O; at 380 ° the oxalate was converted to plutonium dioxide. At 140 ° Pu (III) oxalate was completely dehydrated and at 270 ° C in air it was converted to plutonium dioxide. In an inert medium at 330 ° C the oxalate decomposed to give oxalate-carbonate. At 460 ° C the oxalate-carbonate decomposed and the trivalent plutonium was oxidized to the tetravalent state with the formation of the dioxide.     

Iodine-steel reactions under severe accident conditions in light-water reactors

Owing to large surface areas, the reaction of volatile molecular iodine (I₂) with steel surfaces in the containment may play an important role in predicting the source term to the environment. Both wall retention of iodine and conversion of volatile into non-volatile iodine compounds at steel surfaces have to be considered. Two types of laboratory experiment were carried out at Siemens (KWU) in order to investigate the reaction of I₂ at steel surfaces representative for German power plants.

• 1. 

  (1) For steel coupons submerged in an I2 solution at T = 50, 90 or 140 °C the reaction rate of the I2−I− conversion was determined. No iodine loading was observed on the steel in the aqueous phase tests. I2 reacts with the steel components (Fe, Cr or Ni) to form metal iodides on the surface which are all immediately dissolved in water under dissociation into the metal and the iodide ions. From these experiments, the I2−I− conversion rate constants over the temperature range 50–140 °C as well as the activation energy were determined. The measured data are suitable to be included in severe accident iodine codes such as IMPAIR.

• 2. 

  (2) Steel tubes were exposed to a steam-I2 flow under dry air at T = 120 °C and steam-condensing conditions at T = 120 and 160 °C. In dry air, I2, was retained on the steel surface and a deposition rate constant was measured. Under steam-condensing conditions there is an effective conversion of volatile I2 to non-volatile I− which is subsequently washed off from the steel surface. The I2−T− conversion rate constants suitable for modelling this process were determined. No temperature dependence was found in the range 120–160 °C.

     

Investigation of systems of fused salts based on thorium fluoride

The system ThF₄-NaCl-KCL, which is significant in the selection of an electrolyte for the production of thorium, by electrolysis, was investigated by thermal and x-ray phase-analytical methods. The salts were melted in an atmosphere of argon. The following structural diagrams were constructed: the system NaCl-ThF₄-an eutectic type with complete insolubility in the solid state (the eutectic lies at 712 °C and 52 mole % ThF₄); the system KCl-ThF₄-the same type (eutectic at 704 °C and 23 mole % ThF₄); and a section of the ternary system NaCl-KCl-ThF₄ along the line (1NaCl: 1KCl)-ThF₄ with the point of intersection with the eutectic crystallization curve at 628 °C and 40% (by weight) ThF₄.     

Fluorooxalates of tetravalent uranium

The new compounds, the mixed fluorooxalates of tetravalent uranium (UF₂)C₂O₄ · 1¹/2H₂O and (UF)₂ · · (C₂O₄)₃ are described. The former is obtained by the action of a saturated oxalic acid solution on UF₄ · 2¹/2H₂O at temperatures from 25 to 100 °C and the latter by fusion of UF₄ · 2¹/2H₂O with H₂C₂O₄ · · 2H₂O with subsequent heating of the melt to 200 °C.     

The effects of Co γ-ray on poly(ethylene-co-vinyl acetate)/carbon black composites

Cables used in a nuclear power plant are irradiation suppressing ones. Until now, researches on the irradiation suppressing cables have mainly been focused on insulation materials. Therefore, in this paper, the non-isothermal crystallization behaviors and degradation characteristics of ethylene vinyl acetate-carbon black (EVA-CB), used as a shielding material, were investigated by means of the Differential scanning calorimetry (DSC) and chemiluminescence analyzer (CL). The specimens were cooled after removing thermal history at 150 °C for 5 min by changing the cooling rates to 5, 7.5, 10, 15 and 20 °C/min with DSC. In addition, after maintaining a thermal equilibrium at each temperature of 25, 50, 75, 100, 125, 150 and 175 °C, their thermoluminescence was measured for 20 min with CL equipment. The ⁶⁰Co γ-ray was used for irradiation. T_c, T₀, T_∞ and t_1/2 in the DSC experiments are found to decrease gradually as radiation dose increases. Secondly, with the CL experiment, the 0.1, 0.25 and 0.5 MGy EVA-CB composites were found to show a much smaller thermoluminescence than the intact EVA-CB composites, while the 0.75 and 1 MGy EVA-CB composites were found to show a much higher thermoluminescence than ones.     

Evolution of the oxide structure of 9CrODS steel exposed to supercritical water

The corrosion behavior and oxide structure of 9CrODS steel in supercritical water has been studied. Samples were exposed to supercritical water at 500 and 600 °C for times of 2, 4 and 6 weeks. The oxide structure was studied using microbeam synchrotron X-ray diffraction and fluorescence analysis. The 600 °C samples exhibited a three-layer structure with Fe₃O₄ in the outer layer, a mixture of FeCr₂O₄ and Fe₃O₄ in the inner layer, and a mixture of metal and oxide grains (FeCr₂O₄ and Cr₂O₃) in the diffusion layer. Between the 2 and 4-week samples exposed to 600 °C supercritical water, a Cr₂O₃ film appeared at the diffusion layer-metal interface which appears to be associated with slower oxidation of the metal. The 500 °C samples also showed a three-layer structure, but both the outer and inner oxide layers contained mainly Fe₃O₄, and the diffusion layer contained much fewer oxide precipitates and was a solid solution of oxygen ahead of the oxide front.     

Study of electron beam curing process using epoxy resin system

Polymeric matrix composite (PMC) has been used in engineering applications instead of metal in the last few years, due to its corrosion resistance and excellent relation between tensile strength/density and elastic modulus/density. However, PMC materials cured by thermal process require high temperature and are time-consuming. The electron beam (EB) curing technology allows its use at room temperature and reduced curing times, and this is one of the main advantages over thermal technology. The aim of this work is to investigate electron beam curable epoxy formulations to use in filament winding processes to produce composite material with similar or better properties than thermal curable composites. The study has been made with commercial epoxy resins and cationic initiators. The epoxy resin samples were irradiated for few minutes with total dose of 150 kGy. The glass transition temperatures (T_g) were determined by dynamic mechanical analyzer (DMA) and the result was 137 °C. The thermal process was carried out in a furnace following three steps: 4 h at 90 °C, increasing temperature from 90 °C to 130 °C during 4 h and 12 h at 130 °C. The total process time was 20 h. The T_g of this sample was 102 °C.     

Phase relations in the U-Mo-Al ternary system

The phase relations in the U-Mo-Al system of quenched samples annealed at 800 °C for 2 weeks and at 400 °C for 2 months have been established using X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analysis performed at room temperature. Two ternary Al-rich phases, UMo_2−xAl_20+x and U₆Mo_4+xAl_43−x are found stable at 800 °C and 400 °C. They show significant homogeneity ranges resulting from Mo/Al substitution mechanism on various mixed crystallographic sites, as evidenced by single-crystal structure refinements. Substitution of up to 25 at.% of Al by Mo atoms is also observed for UAl₂ (cubic MgCu₂-type) giving a quite large extension (UAl_2−xMo_x, 0 < x < 0.5) into the ternary system. Larger substitution (0.6 < x < 0.7 at T = 800 °C) stabilizes another ternary Laves phase, UAl_2−xMo_x with the hexagonal MgZn₂-type. There is no detectable solubility of Mo in UAl₄, and it is of the order of 1 at.% in UAl₃. The interaction layers between the γU-Mo alloys and the Al matrix in nuclear fuel plates can be successively estimated as composed of the two- and three-phase fields equilibrium indicated on the assessment of the phase relations drawn for samples heat-treated at 400 °C.     

Oxidation of Plutonium Metal

The processes leading to the production of plutonium oxide powders as a result of the interaction of plutonium metal with moist (5% H₂O) air and moist (5% H₂O) argon are investigated. The kinetic curves in the range 250–400°C are obtained. The oxidation is shown to occur in stages; the activation energy of each stage is calculated. A mechanism of the oxidation of plutonium is proposed.It is shown that the plutonium oxides obtained in moist air and moist argon at 300–400°C possess a high reactivity and/or are suitable for further chemical processing – dissolution in nitric acid, fluoridation, and chlorination. 6 figures, 2 tables, 7 references.     

Synthesis and characterization of low-temperature precursors of thorium-uranium (IV) phosphate-diphosphate solid solutions

Several compositions of new precursor of thorium-uranium (IV) phosphate-diphosphate solid solutions (Th_4−xU_x(PO₄)₄P₂O₇, called β-TUPD) were synthesized in closed PTFE containers either in autoclave (160 °C) or on sand bath (90-160 °C). All the samples appeared to be single phase. From XRD data and TEM observations, the diffraction lines matched well with that of pure thorium phosphate-hydrogenphosphate hydrate (TPHPH), Th₂(PO₄)₂(HPO₄) · H₂O, which confirmed the preparation of a complete solid solution between pure thorium and uranium (IV) compounds. TGA/DTA experiments showed that samples of thorium-uranium (IV) phosphate-hydrogenphosphate hydrate (TUPHPH) prepared at 150-160 °C were monohydrated leading to the proposed formula Th_2−x/2U_x/2(PO₄)₂(HPO₄) · H₂O. The variation of the XRD diagrams versus the heating temperature showed that TUPHPH remained crystallized and single phase from room temperature to 200 °C. After heating between 200 °C and 800 °C, the presence of diphosphate groups in the solid was evidenced. In this range of temperature, the solid was transformed into the low-temperature monoclinic form of thorium-uranium (IV) phosphate-diphosphate (α-TUPD). This latter compound finally turned into well-crystallized, homogeneous and single-phase β-TUPD (orthorhombic form) above 930-950 °C for x values lower than 2.80. For higher x values, a mixture of β-TUPD, α-Th_1−zU_zP₂O₇ and U_2−wTh_wO(PO₄)₂ was obtained. By this new chemical route of preparation of β-TUPD solid solutions, the homogeneity of the samples is significantly improved, especially considering the distribution of thorium and uranium.     

Alloying and microstructure stability in the high-temperature Mo-Si-B system

Multi-phase alloys in the Mo-Si-B system are identified as high-temperature structural materials due to their high melting points (above 2000 °C) and excellent oxidation resistance attributed to the self-healing characteristics of borosilica layer up to 1400 °C. In the current study, the effect of alloying additions to achieve a reduced weight density has been examined in terms of changes in the microstructure and phase stability. The critical factor underlying the microstructural changes is related to the influence of the alloying additions on the stability of the high melting temperature ternary-based Mo₅SiB₂ (T₂) borosilicide phase.     

Gamma radiolysis of NaCl brine: Effect of dissolved radiolysis gases on the radiolytic yield of long-lived products

Five moles per litre NaCl solution is γ-irradiated in autoclaves at 35 °C and 90 °C. H₂, O₂ and are formed as long-lived products and reach equilibrium concentrations at a gas partial pressure of some 10 bar. Since formation of a gas phase is completely suppressed, H₂ and O₂ are kept dissolved. The equilibrium concentration level depends on dose rate and pH. Experiments at 90 °C and with added H₂ show no gas production at all. From the kinetic simulation of the experiments we deduce that the reaction of OH and radicals with H₂ limits or inhibits sustaining solution decomposition. If species are present which compete with the radicals for H₂, (such as Br⁻) the decomposition is promoted. In such a case the gas partial pressure exceeds 100 bar.     

Structural study of the oxidation process and stability of NbOxNy coatings

In the present work, we study the oxidation behaviour of NbON multilayer films. The films were deposited by DC magnetron sputtering with a reactive gas pulsing process. The nitrogen flow was kept constant and the oxygen flow was pulsed. Pulse durations of 10 s produced multilayered coatings with a period of λ = 10 nm. Three different films with increasing duty cycles have been deposited.Rutherford backscattering spectroscopy (RBS) was used to study the chemical composition variations at different annealing temperatures (as-deposited, 400 °C, 500 °C and 600 °C) combined with X-ray diffraction (XRD) to identify the crystalline phases formed. At 400 °C, for all films a very thin layer starts to form at the surface with enhanced O concentration. The composition of the deeper part of the samples remains unchanged. At 500 °C, the oxide scale grows, encompassing about half the film thickness. At 600 °C, the process is finished and a single layer is formed with reduced Nb and increased O concentration. Fourier-transformation infrared spectroscopy (FTIR) results confirmed the increase of this surface oxidation, while XRD revealed that crystallization of Nb₂O₅ occurs at 600 °C.     

Investigation of high temperature phase stability, thermal properties and evaluation of metallurgical compatibility with 304L stainless steel, of indigenously developed ferroboron alternate shielding material for fast reactor applications

Towards the cause of serving economic power production through fast reactors, it is necessary to bring in functionally more efficient and innovative design options, which also includes exploration of cheaper material alternatives, wherever possible. In this regard, the feasibility of using a commercial grade ferroboron alloy as potential alternate shielding material in the outer subassemblies of future Indian fast reactors has been recently investigated from shielding physics point of view. The present study explores in detail the high temperature thermal stability and the metallurgical compatibility of Fe-15.4B-0.3C-0.89Si-0.17Al-0.006S-0.004P-0.003O (wt.%) alloy with SS 304L material. In addition, the high temperature specific heat and lattice thermal expansion characteristics of this alloy have also been investigated as a part of the present comprehensive characterisation program. The Fe-15 wt.%B alloy is constituted of principally of two boride phases, namely tetragonal Fe₂B and orthorhombic FeB phases, which in addition to boron also contains some amount of C and Si dissolved in solid solution form. This Fe-B alloy undergoes a series of phase transformation as a function of increasing temperature; the major ones among them are the dissolution of Fe₂B-lower boride in the matrix through a eutectic type reaction, which results in the formation of the first traces of liquid at 1500 K/1227 °C. This is then followed by the dissolution of the major FeB boride phase in liquid and the melting process is completed at 1723 K/1450 °C. In a similar manner, the thermal stability studies performed on combined Fe-B + 304L steel reaction couples revealed that a pronounced pre-melting or liquid phase formation occurs at a temperature of 1471 K/1198 °C, which is lower than the melting onset of both Fe-B and SS 304L. It is found that within the limits of experimental uncertainty, this pre-melting phenomenon occurred at the same fixed temperature of 1471 K/1198 °C, irrespective of the mass ratios of Fe-B and 304L steel. Further, it is also found that SS 304L is completely soluble in Fe-B alloy and the fused product upon solidification formed a mixture of complex intermetallic borides, such as (Fe,Cr)(B,C), (Fe,Cr)₂(B,C) and (Fe,Ni)₃B. In the temperature range 823-1073 K (550-800 °C), the SS 304L clad is found to interact strongly with the Fe-B alloy. The diffusion layer thickness or the attack layer depth (x) is found to vary with time (t) up to about 5000 h, according to the empirical rate law, x² = k(T)t. The temperature sensitivity of the rate constant, k(T) is found to obey the Arrhenius law, k(T) = k_o exp(−Q/RT), with Q = 57 kJ mol⁻¹, being the effective activation energy for the overall diffusional interaction of Fe-B and SS 304L. The room temperature specific heat capacity of Fe-B alloy is found to be 538 kJ kg⁻¹ K⁻¹. The C_P values measured over 300-1350 K, is found vary smoothly with temperature according to the expression, C_P/kJ kg⁻¹ K⁻¹ = 0.62094 + 0.00012T + 10685.81T⁻². The lattice thermal expansion of both FeB and Fe₂B phases are found to be anisotropic in that the c-axis expansion is found to be more than that along a and b axes. The room temperature volume thermal expansivity of FeB and Fe₂B phases are found to be of the order of 48 × 10⁻⁶ K⁻¹ and 28 × 10⁻⁶ K⁻¹, respectively. The thermal expansion of FeB is found to be more temperature sensitive than that of Fe₂B.     

Investigation of the PuO2F2 - HF - H2O system (20°C isotherm)

The PuO₂F₂ — HF — H₂O system was investigated by using the method of isomermic solubility at 20 °C. It was established that there are three phases in the sytem: plutonyl fluoride dihydrate PuO₂F₂ · 2H₂O, tetrafluoroxyplutonic acid H₂PuO₂F₄ · 4H₂O, and plutonium oxyfluoride PuO₂F₂, the composition of which was determined by chemical analysis and by using the Schreinmacher method. The chemical individuality of the phases was confirmed by electron absorption spectra. The existence of the H₂PuO₂ F₄ · 4H₂O acid was proved by using the electromigration and the potentiometric titration methods.     

An investigation of solid state relationships in the system u-te-c

Phase equilibria in the ternary system U-Te-C have been investigated in argon and vacuum atmospheres at temperatures between 600 °C and 1400 °C. A ternary diagram showing the solid state compatibility relationships at 1200 °C is presented and X-ray data for the compounds U₂TeC₂, U₇Te₁₂, UTe₃ and UTe_3.38 are given. Compatibility relationships in the quaternary system U-Te-C-O are discussed and a phase diagram for the system is shown. X-ray powder diffraction data for UOTe and the previously unreported compound U₂O₂Te are presented.     

Enthalpy measurements of La2Te3O9 and La2Te4O11

Enthalpy increment measurements on La₂Te₃O₉(s) and La₂Te₄O₁₁(s) were carried out using a Calvet micro-calorimeter. The enthalpy values were analyzed using the non-linear curve fitting method. The dependence of enthalpy increments with temperature was given as: H°(T) − H°(298.15 K) (J mol⁻¹) = 360.70T + 0.00409T² + 133.568 × 10⁵/T − 149 923 (373 ? T (K) ? 936) for La₂Te₃O₉ and H°(T) − H°(298.15 K) (J mol⁻¹) = 331.927T + 0.0549T² + 29.3623 × 10⁵/T − 114 587 (373 ? T (K) ? 936) for La₂Te₄O₁₁.