Vaporization and Surface Ionization of LaC2, CeC2, PrC2, NdC2, ThC2 and UC2

The Langmuir vaporization and the surface ionization of LaC₂, CeC₂, PrC₂, NdC₂, ThC₂ and UC₂ from a heated graphite filament have been studied mass spectrometrically. It was found that there were present small amounts of neutral and ionic metal dicarbide molecules in addition to neutral and ionic metal atoms in the LaC₂, CeC₂, PrC₂, ThC₂ and UC₂-C systems with the exception of NdC₂-C, where neither neutral nor ionic metal dicarbide molecules were observed. The reason for this exceptional behavior of the NdC₂-C system is explained by the very small vaporization coefficients, as estimated from the measurements of neutral MC₂/M ratios and ionic MC₂/M ratios.

From the measurements of the heats of vaporization, it was surmised that the ionization potential of Th measured by the surface ionization comparison technique might be too high.     

Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (I)

In order to evaluate the effects of hydrogen peroxide (H₂O₂) on intergranular stress corrosion cracking, a high temperature high pressure water loop, which can control H₂O₂ concentration with minimal oxygen (O₂) co-existence, is required. This loop is characterized by

1. A once-through type loop to prevent accumulation of decomposed O₂ in the loop

2. Minimized autoclave volume to prevent bulk thermal decomposition of H₂O₂

3. A polytetrafluoroethylene (PTFE) lining to prevent surface decomposition of H₂O₂, and

4. A H₂O₂ monitoring system with an off-line H₂O₂ detector to determine concentration in the sampled water which is combined with an in-line dissolved O₂ detector to determine the decomposed O₂ concentration.

The authors developed such a loop previously. In the present work, performance tests were carried out and measured data were evaluated by comparing with predicted values to verify whether the target characteristics were met. The measured H₂O₂ remaining in the sampled water agreed with the predicted amount within 5%. It was confirmed that the ratio of H₂O₂ remaining in the loop autoclave was more than 90% and the concentration could be monitored continuously with the in-line dissolved O₂ detector installed after the cooler in the loop. Electrochemical corrosion potential (ECP) and frequency dependent complex impedance were measured successfully by changing H₂O₂ concentration.     

Radiation Treatment of Exhaust Gases, (IX)

The computer simulation method has been applied to the analysis of the NO and NO₂ decomposition in the NO-N₂ and NO₂-N₂ mixtures by electron beam irradiation. The calculated results regarding the NO and NO₂ decomposition were in good agreement with the experimental results in both mixtures. The NO and NO₂ decomposition is mainly induced by the radical reactions and is hardly induced by the ion reactions. That is, the NO decomposition in the NO-N₂ mixture is mainly induced by the attack of N formed by the radiolysis of N₂. The NO₂ decomposition in the NO-N₂ mixture is induced not only by the attack of N, but also by the attack of O formed through the reaction of NO₂ with N at low doses. At high doses, the decomposition of NO formed through the reactions of NO₂ with N and with O is mainly induced by the attack of N, in contrast to the NO₂ decomposition by the attacks of N and O at low doses. The G value of the formation of N(G(N)) by the computer simulation was 2.0.     

Solubility of Uranyl Nitrate Precipitates with N-Alkyl-2-pyrrolidone Derivatives (Alkyl = n-Propyl,n-Butyl,iso-Butyl,and Cyclohexyl)

The solubility of UO₂(NO₃)₂(NRP)₂ (NRP = N-alkyl-2-pyrrolidone) in aqueous solutions with HNO₃ (0–5.0 M) and the corresponding NRP (0–0.50M) has been studied. As a result, the solubility of each speciesof UO₂(NO₃)₂(NRP)₂ generally decreases with increasing concentrations of HNO₃ and the corresponding NRP (C _HNO3 and C _NRP, respectively) in the supernatant. The solubility of UO₂(NO₃)₂(NRP)₂ also depends on the type of NRP; a higher hydrophobicity of NRP generally leads to a lower solubility of UO₂(NO₃)₂(NRP)₂. The logarithms of effective solubility products (K _eff) of UO₂(NO₃)₂(NProP)₂, UO₂(NO₃)₂(NBP)₂, UO₂(NO₃)₂(NiBP)₂, and UO₂(NO₃)₂(NCP)₂ at different C_HNO3 values and 293K were evaluated. For instance, at C_HNO3 = 3:0 M, logK ^NProP _eff = ?1:07 ± 0:03, log K ^NBP _eff = ?2:23 ± 0:02, log K ^NiBP _eff = ?2:59 ± 0:03, and log K ^NCP _eff = ?3:80 ± 0:05. The solubility of UO₂(NO₃)₂(NRP)₂ is determined by the balance among the common-ligand effect, ionic strength, and variation of log K _eff with C _HNO3.     

Radiation Treatment of Exhaust Gases, (VIII)

The NO₂ decomposition by electron beam irradiation was studied in the NO₂-N₂ and NO₂-rare gas mixtures. The NO₂ decomposition yields, G (-NO₂), at low doses in the case of the 500 ppm NO₂ initial concentration were 2.9, 10.4 and 5.9 for the NO₂-N₂, NO₂-He and NO₂-Ar mixtures respectively. A large amount of NO was formed by irradiation of these mixtures. The G (NO) was almost equal to the G (-NO₂). The N₂O was also formed in the NO₂-N₂ mixture. The NO₂ decomposition is mainly attributable to the attack of N, formed by the radiolysis of N₂, on NO₂ in the NO₂-N₂ mixture, and to the attacks of R⁺ and R*, formed by the radiolysis of a rare gas (R), on NO₂ in the NO₂-rare gas mixture. The NO₂ decomposition in the NO₂-N₂ mixture was depressed markedly by the addition of a small amount of O₂. This may be mainly attributable to the regeneration of NO₂ by the reaction of NO with O.     

Low cost, proliferation resistant, uranium–thorium dioxide fuels for light water reactors

Our objective is to develop a fuel for the existing light water reactors (LWRs) that, (a) is less expensive to fabricate than the current uranium-dioxide (UO₂) fuel; (b) allows longer refueling cycles and higher sustainable plant capacity factors; (c) is very resistant to nuclear weapon-material proliferation; (d) results in a more stable and insoluble waste form; and (e) generates less high level waste. This paper presents the results of our initial investigation of a LWR fuel consisting of mixed thorium dioxide and uranium dioxide (ThO₂–UO₂). Our calculations using the SCALE 4.4 and MOCUP code systems indicate that the mixed ThO₂–UO₂ fuel, with about 6 wt.% of the total heavy metal U-235, could be burned to 72 MW day kg⁻¹ (megawatt thermal days per kilogram) using 30 wt.% UO₂ and the balance ThO₂. The ThO₂–UO₂ cores can also be burned to about 87 MW day kg⁻¹ using 35 wt.% UO₂ and 65% ThO₂with an initial enrichment of about 7 wt.% of the total heavy metal fissile material. Economic analyses indicate that the ThO₂–UO₂ fuel will require less separative work and less total heavy metal (thorium and uranium) feedstock. At reasonable future costs for raw materials and separative work, the cost of the ThO₂–UO₂ fuel is about 9% less than uranium fuel burned to 72 MW day kg⁻¹. Because ThO₂–UO₂ fuel will operate somewhat cooler, and retain within the fuel more of the fission products, especially the gasses, ThO₂–UO₂ fuel can probably be operated successfully to higher burnups than UO₂ fuel. This will allow for longer refueling cycles and better plant capacity factors. The uranium in our calculations remained below 20 wt.% total fissile fraction throughout the cycle, making it unusable for weapons. Total plutonium production per MW day was a factor of 3.2 less in the ThO₂–UO₂ fuel than in the conventional UO₂ fuel burned to 45 MW day kg⁻¹. Pu-239 production per MW day was a factor of about 4 less in the ThO₂–UO₂ fuel than in the conventional fuel. The plutonium produced was high in Pu-238, leading to a decay heat about three times greater than that from plutonium derived from conventional fuel burned to 45 MW day kg⁻¹ and 20 times greater than weapons grade plutonium. This will make fabrication of a weapon more difficult. Spontaneous neutron production from the plutonium in the ThO₂–UO₂ fuel was about 50% greater than that from conventional fuel and ten times greater than that from weapons grade plutonium. High spontaneous neutron production drastically limits the probable yield of a crude weapon. Because ThO₂ is the highest oxide of thorium while UO₂ can be oxidized further to U₃O₈ or UO₃, ThO₂–UO₂ fuel appears to be a superior waste form if the spent fuel is to be exposed ever to air or oxygenated water. And, finally, use of higher burnup fuel will result in proportionally fewer spent fuel bundles to handle, store, ship, and permanently dispose of.     

Electric-dipole,quadrupole, and magnetic-dipole susceptibilities and shielding factors for closed-shell ions of the He,Ne, Ar,Ni (Cu+), Kr,Pb, and Xe isoelectronic sequences

Theoretical values of electric and magnetic susceptibilities (α₁, α₂, χ₁) and shielding factors (γ₁, γ₂, σ₁) calculated in the relativistic random-phase approximation are presented in tabular form for ions with closed 1s, 2p, 3p, 3d, 4p, 4d, and 5p shells. The table includes all ions in these sequences with nuclear charge up to Z = 56 and a representative selection of ions with 56 < Z ? 92.     

Radiation Treatment of Exhaust Gases, (X)

Effects of NH₃, NO, NO₂ and SO₂ on the N₂O formation were studied in the mixtures of H₂O, O₂ and N₂ irradiated with electron beams of 1.5 MeV energy. In the H₂O-O₂-N₂ mixture, the N₂O formation was enhanced markedly by addition of a small amount of NH₃. This enhancement may be brought about by the reaction of NH₃ with OH radical to form NH₂ radical leading to the formation of N₂O. In the NH₃-H₂O-O₂-N₂ mixture, the formation of N₂O was suppressed effectively by addition of NO, NO₂ and SO₂. The N₂O formation was not affected by the irradiation temperature range of 80–200°C.     

Radiation Treatment of Exhaust Gases, (VII)

The NO decomposition by electron beam irradiation was studied in the NO-N₂ and NO-rare gas mixtures. The NO decomposition yields, G (?NO) at low doses in the case of the 500 ppm NO initial concentration were 4.04.4 and 1.2 for the NO-N₂, NO-He and NO-Ar mixtures respectively. A small amount of NO₂ was formed by irradiation of these mixtures. The NO decomposition is mainly attributable to the attacks of N and N₄ ⁺ (or N₂ ⁺), formed by the radiolysis of N₂on NO in the NO-N₂ mixtureand to the attacks of R⁺ and R^*, formed by the radiolysis of a rare gas (R)on NO in the NO-rare gas mixture. The NO decomposition in the NO-N₂ mixture was depressed markedly by the addition of a small amount of O₂. This may be mainly attributable to scavenging of N and N₄ ⁺ (or N₂ ⁺) by O₂.     

Behavior of cesium molybdate,Cs2MoO4, in severe accident conditions. (1) Partitioning of Cs and Mo among gaseous species

In order to better understand the behavior of cesium in severe accident of Light Water Reactor (LWR), the high-temperature chemistry of Cs₂MoO₄ in H₂O + H₂ gas was studied. The pseudo–binary system, Cs₂MoO₄–MoO₃, was thermochemically modeled with Redlich–Kister formulation to form a basis to analyze the high-temperature behavior of Cs₂MoO₄. The model prediction was compared with the thermogravimetric measurements of Cs₂MoO₄ in dry and humid argon, which revealed that the mass-loss rate was enhanced in humid atmosphere. The thermochemical model was further applied to predict the partitioning of cesium and molybdenum among gaseous species in the boiling water reactor-core degradation condition typical of short-term station blackout. Effects of the total pressure (3.5–75 bar) as well as the H₂/H₂O ratio (1/4000–2) were examined. CsOH(g) is the predominant cesium species, when the damaged fuel temperature is higher than 2000 K at higher steam pressures, but Cs₂MoO₄(g) would become more important as the steam cools toward the steam dome. The condensation of Cs₂MoO₄ occurs below ～1900 and ～1550 K at 75 and 3.5 bar, respectively. Besides, the ideal mixing of complex component model has also been examined for its simplicity. The latter gave satisfactory prediction as far as the condensed phase composition is concerned.     

Thermographic investigation of UO2, UCl,and KCl ternary and binary systems

The systems UCl₄-KC1, UO₂-KCl, UO₂-UCl, and UO₂-UCl₄-KCl were investigated thermographically. The presence of compounds K₂UCl₆ and UOCl₂ in the binary systems UCI₄-KCl and UO₂-UCl₄ allows a breakdown of the ternary system UO₂-UCl₄-KCl into the subsystems UOCl₂-K₂UCl₆-UCl₄, UO₂-UOCl₂-K₂UCl₆, and UO₂-K₂UCl₆-KCl. The first of these has the lowest melting point and is regarded as an independent ternary system.Translated from Atomnaya Énergiya, Vol. 18, No. 6, pp. 616–623, June, 1965     

A revised additivity rule for electron scattering from ethylene, propene, butene, ethane, propane and butane

Considering the difference between the bound atom in a molecule and the free atom, the original additivity rule is revised. Using the revised additivity rule, the total cross sections for electron scattering by ethylene (C₂H₄), propene (C₃H₆), butene (C₄H₈), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀) are calculated over the energy range 10-1000 eV. The results of the revised additivity rule are compared with those obtained by experiments and the revised additivity rule can give better agreement with experimental values than the original additivity rule.     

Chloride Pyrometallurgy of Uranium Ore, (II)

Selective extraction of uranium from a phosphate ore was studied by using the mixed gas of Cl₂ and O₂. On heating the ore and carbon mixture in Cl₂, the volatilized chloride of uranium is accompanied by iron, aluminum, phosphorus and silicon chlorides. The addition of O₂ gas effectively lowered the volatilization ratios of aluminum, phosphorus and silicon. The ratio decreased with increasing oxygen flow rate up to 50ml/min at 1,223 K (Cl₂: 100ml/min, O₂+N₂: 400m//min). The volatilization ratio of uranium was almost unchanged at 90% up to 50 ml/min O₂ (carbon amount: 15wt%). The effect of the other parameters, i.e. Cl₂ flow rate, carbon amount, reaction temperature and time was examined.     

Effect of Gamma Radiation on Silica Gel, (II)

Radiolysis of N₂O adsorbed on silica gel degassed at 200, 450 and 600°C has been studied to investigate the behavior of electrons on the gel surface. The G-value of nitrogen as the major radiolytic product increases with an increase of N₂O concentration adsorbed approaching a plateau value which depends on degassing temperature of the gel prior to irradiation. By the competitive electron scavenging of N₂O with several electron scavengers, such as CCl₄, SF₆ and nitrobenzene, initial yield of electrons G (e ^?), and relative rate constant for reaction of electrons with scavenger to that with N₂O, k _s /k _N2O, have been obtained. The G (e ^?) is 3.8 and 2.0 for the gel degassed at 200 and 600°C respectively. In the former gel, the value of k _s /k _N2O is comparable with that in aqueous system, while in the latter, with that in non-polar hydrocarbon such as neopentane. The observed difference may be attributed to the presence of silanol groups and residual water which facilitate some form of hydration of electrons on the gel degassed at low temperature. Despite of the complexity of the heterogeneous system, Hammett's relationship is observed among the reactions of electrons with monosubstituted derivatives of benzene, providing the reaction constant ρ as 2.8.     

Characterization of (Th,U)O2 pellets made by advanced CAP process

Coated Agglomerate Pelletization (CAP) process is being developed by Bhabha Atomic Research Centre (BARC) for the fabrication of ThO₂-UO₂ mixed oxide fuel pellets. In order to improve the microstructures with better microhomogeneity, a study was made to modify the CAP process. The advanced CAP (A-CAP) process is similar to the CAP process except that the co-precipitated powder of mixed oxide, ThO₂-30%UO₂ or ThO₂-50%UO₂, is used for coating instead of U₃O₈ powder. The choice of ThO₂-UO₂ powders as the coating material is advantageous compared to U₃O₈, since the presence of large quantities of ThO₂ in UO₂ powder gives better self-shielding effect. In this paper, ThO₂ containing 4%UO₂ (% in weight) was prepared by the A-CAP process. Property measurements including microstructure and microhomogeneity were made by optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), etc. It was found that the pellets sintered in air at 1400 °C showed a duplex grain structure and those sintered in Ar-8%H₂ at 1650 °C showed a very uniform grain structure with excellent microhomogeneity.     

Behavior of Ruthenium in Fluoride-Volatility Processes, (IV)

The products of the following three reactions were studied in relation to the reprocessing of oxide fuels: (i) fluorination of Ru by F₂ (ii) fluorination of Ru by a O₂-F₂ mixture, and (iii) secondary process of RuO₂-F₂ reaction. The product of Ru-F₂ reaction was only RuF₅; the mass spectrum of RuF₅ was obtained. Fluorination of Ru by a O₂-F₂ mixture resulted in the production of RuF₅ (85～75%) and RuOF₄ (15～25%); these results are different from those reported by earlier workers. The use of radioactive Ru^*O₂ traced the behavior of RuOF₄ in the apparatus. RuOF₄ decomposes on the wall which was not preliminarily coated with ruthenium; refluorination was effective for removal of the deposit. These results suggest that the fluorination of irradiated oxide fuels volatilizes the ruthenium as a mixture of RuOF₄, RuF₅, and a small amount of RuO₄.     

Theory of He trapping, diffusion, and clustering in UO2

We have performed ab initio total energy calculations to investigate the behavior of helium and its diffusion properties in uranium dioxide (UO₂). Our investigations are based on the density functional theory within the generalized gradient approximation (GGA). The trapping behavior of He in UO₂ has been modeled with a supercell containing 96-atoms as well as uranium and oxygen vacancy trapping sites. The calculated incorporation energies show that for He a uranium vacancy is more stable than an oxygen vacancy or an octahedral interstitial site (OIS). Interstitial site hopping is found to be the rate-determining mechanism of the He diffusion process and the corresponding migration energy is computed as 2.79 eV at 0 K (with the spin-orbit coupling (SOC) included), and as 2.09 eV by using the thermally expanded lattice parameter of UO₂ at 1200 K, which is relatively close to the experimental value of 2.0 eV. The lattice expansion coefficient of He-induced swelling of UO₂ is calculated as 9 × 10⁻². For two He atoms, we have found that they form a dumbbell configuration if they are close enough to each other, and that the lattice expansion induced by a dumbbell is larger than by two distant interstitial He atoms. The clustering tendency of He has been studied for small clusters of up to six He atoms. We find that He strongly tends to cluster in the vicinity of an OIS, and that the collective action of the He atoms is sufficient to spontaneously create additional point defects around the He cluster in the UO₂ lattice.     

Thermal and mechanical properties of (U,Er)O2

Erbium is considered as a slow burnable poison suitable for use in light water reactors (LWRs). Addition of a small amount of Er₂O₃ to all UO₂ pellets will make it possible to develop super high burnup fuels in Japanese nuclear facilities which are now under the restriction of the upper limit of ²³⁵U enrichment. When utilizing the (U,Er)O₂ fuels, it is very important to understand the thermal and mechanical properties. Here we show the characterization results of (U_1−xEr_x)O₂ (0 ? x ? 0.1). We measured their thermal and mechanical properties and investigated the effect of Er addition on these properties of (U,Er)O₂. All Er completely dissolved in UO₂, and the lattice parameter decreased linearly with the Er content. Both the thermal conductivity and Young’s modulus of (U,Er)O₂ decreased with the Er content. These results would be useful for us in evaluating the performance of the (U,Er)O₂ fuels in LWRs.     

Spectroscopic quantitative analysis of the isotopic composition of gas mixtures containing hydrogen, deuterium, and tritium

A detailed experiment is carried through to construct a calibrating curve in the concentration interval from 0.7 to 97% H₂ in D₂. The results obtained are in good agreement with theory. It is shown that it is possible to perform an isotopic analysis of tritium in mixtures with hydrogen and deuterium. An investigation is performed of the relative intensity of the isotope lines as a function of the pressure in a discharge tube.     

Development of fuel-model interfaces: Investigations by XPS, TEM, SEM and AFM

The presented work aims to reproducibly prepare UO₂-Pd thin film model systems for spent nuclear fuel in order to further investigate surface reactions of these films under relevant redox conditions. The sputter co-deposition of U and Pd (fission product) in the presence of O₂ results in the homogenous distribution of Pd in a crystalline UO₂ matrix. Heating the films causes the diffusion of film components. Hereby, the formation of ε-particles has to be clarified. First electrochemical studies show the influence of the nobel metal Pd on the redox behaviour of UO₂. With increasing Pd concentration the matrix dissolution is decreased. However, we could demonstrate that blocked oxidation processes are of temporary nature. The passivation of the Pd reactive sites with increasing number of cycles finally induces the approximation of the mixed system to the redox behaviour of the pure UO₂ system.