Studies of Ceramic Fuels with Use of Fission Gas Release Loop, (IV)

In-pile release of fission gas from sintered UC pellets in the presence of 8–230 ppm of water vapor in the He sweep gas was measured over the temperature range of 160°–1,000°C. A very complex release behavior was observed and the mechanisms of release were deduced from the manner in which the release depended on the decay constant. It was established that the release of short-lived fission gases during irradiation was controlled mainly by pseudo-recoil, while chemical reaction between UC and water vapor, as well as knock-out, appeared to contribute much more significantly in the case of the longer-lived fission gases. The release of fission gas after reactor shutdown was shown to be governed by the UC-H₂O reaction. The ratio of the release due to this reaction in reference to the total release was found to be dependent not only on the concentration of the water vapor but also on the amount present of the accumulated reaction products. Also, a discussion is given on the inordinately high release of ^135mXe observed at 600°C.     

1993 National Symposium on Atomic Energy,Tokyo, Japan

A steady-state simulation model of the gas separation system using a hollow-filament type membrane has been proposed. The mass transfer coefficients in the non-porous thin layer, in the porous support layer of the membrane and in the boundary layer of the membrane surface are estimated in the model. The four types of flow patterns: cross flow, mixing flow, concurrent flow and counter current flow, are also considered in the model. The mass transfer through the non-porous thin layer of the membrane controls the overall mass transfer by ~99%. The experimental observations of TPL (Tritium Process Laboratory in JAERI) for N₂–H₂ and Air—H₂ systems agreed with the calculated results of the cross flow under a set of typical conditions (disposal volume of 2.78×10^?3 Nm³/s, feed-side pressure of 3.44×10⁵Pa, and permeated-side pressure of 1.07×10⁴ Pa). The validity of the simulation method was thus proved. For Air-H₂-H₂O system also, the recovery ratios calculated for H₂ are in good agreement with the experimental observations. However, the calculated recovery ratios of water vapor were slightly smaller than the experimental observations. This discrepancy may result from the difference in separation mechanism between H₂ and water vapor, or the construction change of membrane caused by the existence of water vapor.     

氢-水同位素交换反应热力学理论研究

本文采用量子化学从头计算法对氢同位素分子及相应的氢同位素水分子进行了几何结构优化和振动频率计算,得到了6种氢-水同位素交换体系的热力学函数和气相反应平衡常数。研究结果表明:平衡常数的理论计算值与实验结果吻合,在0.1 MPa和283.2~373.2K的反应条件下,HD-H2O体系平衡常数的计算值与实验值间相对偏差小于6%。     

A new method for detecting pressure tube failures in Indian PHWRs

For the annulus gas system (AGS) of the standardised Indian pressurised heavy water reactor, an elaborate pressure tube (PT) crack monitoring and detection system is envisaged to ensure safety through leak-before-break. The parameters that are monitored relate to the detection of D₂O moisture leaking in from the primary heat transport (PHT) system through a cracked PT. Since a slow build-up of moisture in the AGS may also occur for reasons other than PT failure, it is desirable that a diverse measurement technique should be available. This paper suggests such a technique, based on the observation that a small reference concentration of fission gases is normally present in the annulus gas. This concentration would change sharply upon PT failure, when the heavy water from the leaking PHT system releases the dissolved fission gas content into the annulus. This paper presents a theoretical study of the parameters that influence the build-up of fission product noble gases in the AGS and shows that leakage rates as low as 10 g h⁻¹ from a PT crack can be detected in a few tens of minutes by this method. This is expected to substantially increase the available time between the leak detection and the PT failure, thus serving as an important tool in meeting the leak-before-break criterion of a critical component in PHWRs.     

Preliminary Study of Isotopic Methanes Analysis by Laser Raman Spectroscopy for In-Situ Measurement at Fusion Fuel Gas Processing

Application of laser Raman spectroscopy for fusion fuel gas processing was studied by measuring isotopic methanes exchanged with hydrogen isotopes, which are considered to be a major impurities in the processing. For experimental gases, isotopically equilibrated deuterium and methane were prepared in the presence of solid catalyst. Large Raman scattering peaks of v ₁, bands were observed at 2,917 cm^?1 for CH₄ and at 2,100-2,200 cm_?1 for deuterated derivatives of methane C(H,D)₄. Under a spectral resolution of 5 cm_?1, the v ¹ bands of CH₃D and CH₂D₂ were observed as an overlapped peak, the relative absolute Raman intensity ratio of each isotopic methane was obtained as CH₄: CH₃D+CH₂D₂: CHD₃: CD₄=230: 74: 144: 100. On the other hand, the Raman intensity ratio obtained from pure deuterated standard methane was CH₄: CH₃D: CH₂D₂: CHD₃: CD₄=230: 53: 33: 115: 105. It was confirmed that isotopically equilibrated hydrogen isotopes and methane mixed gas would be applicable for an alternative standard gas for fusion fuel processing gas analyzing system.     

Experimental verification of the fast reactor safety analysis code SIMMER-III for transient bubble behavior with condensation

Experimental verification of a reactor safety analysis code, SIMMER-III, was undertaken for transient behaviors of large-scale bubbles with condensation. The present study aimed to verify the code for numerical simulations of relatively short-time-scale multi-phase, multi-component hydraulic problems. Among these, vaporization and condensation, or simultaneous heat and mass transfer, play important roles. In this study, a series of transient bubble behavior experiments dedicated to condensation phenomena with noncondensable gases was carried out. In the experiments, a pressurized mixture of noncondensable gas and steam was discharged as a large-scale single bubble into a cylindrical pool filled with stagnant subcooled water. The concentration of noncondensable gas was taken as an experimental parameter as was the species of noncondensable gas. The characteristics of transient behavior of large-scale bubbles with condensation observed in the experiments were estimated through experimental analyses using SIMMER-III. In the experiments with steam condensation, dispersion of the gas mixture discharged into the liquid pool was accompanied by vapor condensation at the bubble surface. SIMMER-III simulations suggested that the noncondensable gas had a less inhibiting effect on the condensation of large-scale bubbles. This is a different characteristic to that of the quasi-steady condensation of small-scale bubbles observed in our previous experiments.     

Molten fuel-coolant interaction during a reactivity initiated accident experiment

The results of a reactivity-initiated accident experiment, designated RIA-ST-4, are discussed and analyzed with regard to molten fuel-coolant interaction (MFCI). In this experiment, extensive amounts of molten UO₂ fuel and zircaloy cladding were produced and fragmented upon mixing with the coolant. Coolant pressurization up to 35 MPa and coolant overheating in excess of 940 K occurred after fuel rod failure. The initial coolant conditions were similar to those in boiling water reactors during a hot startup (that is, coolant pressure of 6.45 MPa, coolant temperature of 538 K, and coolant flow rate of 85 cm³/s). It is concluded that the high coolant pressure recorded in the RIA-ST-4 experiment was caused by an MFCI and was not due to gas release from the test rod at failure, Zr/water reaction, of UO₂ fuel vapor pressure. The high coolant temperature indicated the presence of superheated steam, which may have formed during the expansion of the working fluid back to the initial coolant pressure; yet, the thermal-to-mechanical energy conversion ratio is estimated to be only about 0.3%.     

Removal of Tritiated Water Vapor by Adsorption

Abstract

Experiments were conducted on the dynamic adsorption of tritiated water vapor, using a column packed with either silica-gel, activated alumina, or molecular sieve 5A. The column was 4 mm I.D. and 5–30 cm long. The runs were performed at 30°–70°C. Tritiated water vapor (HTO) was loaded into the column with a concentration of 170 μμCi/ml, either in pulses of predetermined duration or continuously (breakthrough). The concentrations of H₂O and HTO at column outlet were measured. Particular interest was attached to observing the effect of differences in the pretreatment applied to the adsorbents (whether dried or saturated with water), and in the H₂O partial pressure of the carrier gas. It was found that the adsorption characteristics shown for HTO were not influenced to any appreciable extent by differences in the pretreatment applied to the adsorbents. On the other hand, adsorbent performance depended sensitively on the H₂O pressure in the carrier gas.

Adsorption models for a two-component mixture composed of H₂O and HTO were sought for describing the adsorption mechanism. For the particular case of the molecular sieve 5A, the Langmuir-type mixed adsorption model was found to give values agreeing fairly well with experiment. A model postulating chemisorption of H₂O into hydroxyl group on the adsorbent surface, to be subsequently replaced by tritium, was found suitable for explaining the adsorbent behavior in the case of silica-gel.     

Oxidation of Zircaloy-4 by oxygen and the production of water

The interaction of isotopic oxygen (¹⁸O₂) with Zircaloy-4 (Zry-4) at 150 and 300 K has been studied using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) methods. AES reveals the oxidation of the Zry-4 surface, reflected in shifts of the Zr(MNV) and Zr(MNN) features by about 5.5 and 3.0 eV, respectively, for both adsorption temperatures. The O(KLL)/Zr(MNN) Auger peak-to-peak height ratios as a function of exposure show the same trends at both temperatures. Following ¹⁸O₂ adsorption at 150 or 300 K, TPD experiments show hydrogen desorption near 400 K that is attributed to the presence of a surface-stabilized form of hydrogen. Additionally, water (H₂¹⁸O and H₂¹⁶O) desorption below 200 K and above 700 K is observed after 150 K oxygen adsorption. However, after oxygen adsorption at 300 K the only significant desorption features are from isotopic water (H₂¹⁸O). These findings indicate that mass transport involving the near-surface region contributes to the observed desorption, and that this behavior is dependent on the original adsorption temperature. Charging experiments using D₂ prior to and after ¹⁸O₂ adsorption were also performed and support our conclusions about the role of surface–subsurface mass transport in this system.     

Chemical absorbent for catching halogen-containing gases discharged from isotope separation plants

The results of industrial tests of a cascade of chemical absorbents for catching spent halogen-containing processes gases during discharge into the atmosphere are discussed. It is noted that the volume of the discharged gases decreases as a result of CO₂ absorption. The HF and Cl₂ content in passing gas can serve as a criterion for the completion of each catching step; Cl₂ detection after the last step is a service life criterion for all chemical absorbents of a facility without exception. The average catching efficiency is 99.7 and 98.5% with respect to fluorine- and chlorine-containing components, respectively. An approximate calculation of the near-ground concentration of HF and Cl₂ is performed. The gases discharged by the isotope separation plant at the Siberian Chemical Combine were purified at the level of the maximum admissible concentration for the working zone and not at the level of the technical norm established individually for each industrial enterprise. Equipping nuclear fuel cycle enterprises with such systems will make it possible to decrease environmental contamination considerably.     

Heat transfer and thermal conductivity of multi-hundred watt 238PuO2 fuel spheres

The thermal conductivity of porous ²³⁸PuO₂ has been calculated from steady-state core and surface temperatures in spheres having diameters of ≈ 3 and ≈ 3.7 cm assuming linear heat transfer at the sphere surface. The spherical fuel forms had densities of 80 to 85% of T.D. for PuO₂ (T.D. = 11.41 g/cm³). Thermal conductivity in the fuel increased nearly linearly with [$\text{T}{}_{\mathrm{<mtext>avg</mtext>}}\text{(}\text{K}\text{)]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ in the range 350–1100°C and thus was dominated by a gas phase present in the open and closed porosity of the fuel. For spheres heated in helium or vacuum, the gas phase had essentially three degrees of freedom. When the fuel was heated in argon saturated with water vapor, approximately 50 degrees of freedom were indicated and this was apparently largely due to the water vapor. A two-phase, series slab heat flow model adequately predicted the thermal conductivity of the ²³⁸PuO₂ phase as a function of temperature.     

Methane from synthesis gas and operation of high-temperature methanation

The methanation process is an important unit in generating substitute natural gas (SNG) from coal and in providing heat in the Long-Distance Nuclear Energy Transport (NFE) system. Procedures for methanizing synthesis gases containing CO, CO₂ and H₂ have been developed and tested at the Kernforschungsanlage Jülich GmbH (KFA - Federal Republic of Germany) since 1976. This is being carried out together with the partner in the NFE Project, Rheinische Braunkohlenwerke AG, Cologne (FRG).It has been demonstrated in several thousand operating hours at the KFA since 1979 that the procedures and components developed, as well as the catalysts employed satisfy the demands made by high-temperature methanation in the three-stage methanation plants ADAM I and ADAM II with a SNG gas production of 200 or 3300 m³ (STP)h⁻¹ and a useful heat capacity of 300 kJ/s or 5.8 MJ/s.In 1981 a single-stage pilot plant was put into operation at the KFA in which one reactor with cooled stepped reaction tubes and catalytic fixed beds was utilized. The test operation of 1100 hours shows that at a high gas load on the reaction tubes, thermodynamic equilibrium with a high methane content in the product gas can be achieved with simultaneous steam production at 100 bar.     

Active control of oxygen in molten lead-bismuth eutectic systems to prevent steel corrosion and coolant contamination

The thermodynamic basis for controlling oxygen level in lead-bismuth to prevent steel corrosion and coolant contamination is examined. The operational conditions, including the thermodynamic activity of oxygen, cover gas oxygen partial pressure, mixtures of H₂ and H₂O (steam) to obtain such low oxygen partial pressure (<10⁻²⁴ atm or around 10⁻⁶ wt% in lead-bismuth), and the voltage signals of one type of oxygen sensors (with a solid electrolyte and molten bismuth reference electrode) are calculated. These results provide the guidance to implement the oxygen control technique.     

Effects of fluid properties on CCFL characteristics at a vertical pipe lower end

The purpose of this study is to derive a counter-current flow limitation (CCFL) correlation and evaluate its uncertainty for steam generator (SG) U-tubes in a pressurized water reactor (PWR). Experiments were conducted to evaluate effects of the liquid viscosity on CCFL characteristics using air–40 wt% or air–60 wt% glycerol water solution and saturated steam–water at atmospheric pressure with vertical pipes simulating the lower part of the SG U-tubes. The steam–water experiments confirmed that CCFL characteristics could be expressed in terms of the Wallis parameters (J_G* and J_L*) for the pipe diameters of D = 14, 20, and 27 mm. A CCFL correlation was derived using the ratio μ_G/μ_L of the viscosities of the gas and liquid phases, μ_G and μ_L, as a correction term representing effects of fluid properties, where J_G^*1/2(μ_G/μ_L)^?0.07 was expressed by a cubic function of J_L^*1/2(μ_G/μ_L)^0.1. In the correlation, the constant C indicating the value of J_G^*1/2(μ_G/μ_L)^?0.07 at J_L* = 0 was (1.04 ± 0.05), and this uncertainty of ±0.05 would cover most of the previous experimental data including the ROSA-IV/LSTF data at 1, 3, and 7 MPa.     

High pressure corium melt quenching tests in FARO

First-of-a-kind experimental data on the quenching of large masses of corium melt of realistic composition when poured into pressurised water at reactor scale depths are presented and discussed. The tests involved 18 and 44 kg of a molten mixture 80 w% UO₂-20 w% ZrO₂, which were delivered by gravity through a nozzle of diameter 0.1 m to 1 m depth nearly saturated water at 5.0 MPa. The objective was to gain early information on the melt/water quench process previous to tests that will involve larger masses of melt (1.50 kg of mixtures UO₂---ZrO₂---Zr). Particularly, pressures and temperatures were measured both in the gas phase and in the water. The results show that significant quenching occurred during the melt fall stage with 30% to 42% of the melt energy transferred to the water. About two-thirds of the melt broke up into particles of mean size of the order of 4.0 mm. The remaining one-third collected still molten in the debris catcher but did not produce any damage to the bottom plate. The maximum downward heat flux was 0.8 MW m². The maximum vessel overpressurisation, i.e. 1.8 MPa, was recorded with 44 kg of melt poured into 255 kg of water and a gas phase volume of 0.875 m³. No steam explosions occurred.     

LiNO3 Effect on Corrosion Prevention of Aluminum with Complex Shapes

The LiNO₃ effect on aluminum corrosion prevention after land disposal of cement-solidified dry active wastes was examined quantitatively, in the event that the LiH (AlO₂)₂·5H₂O (Li-Al) preservation film was not formed on aluminum surfaces during the solidification process. It is especially probable for these bare surfaces to be left when the wastes include components of complex shapes. LiNO₃ dissolves from the waste forms into underground water to form the Li-Al preservation film. So, we thought that the LiNO₃ addition would prevent the corrosion. We measured the volume of hydrogen gas generation in mortar-soaked water during the Li-Al preservation film formation, as functions of LiNO₃ addition amount, the weight ratio of water to mortar when the mortar-soaked water was produced, and the aluminum surface area, to quantify the effect.

We found that aluminum corrosion was inversely proportional to the LiNO₃ addition. For the corrosion to be less than 10^?5m in 10³h, the initially added amount of LiNO₃ must be 1.5wt% of the sum of cement and sand. Regardless of the weight ratio of water to mortar when the mortar-soaked water was produced, hydrogen gas generation with LiNO₃ was 10% as much as that without it, in 5 x 10³h. Because of the Li-Al preservation film formation reaction, hydrogen gas generation was proportional to the cubic root of the aluminum surface area.     

Hydrolysis of Thorium Nitrides and Carbonitrides

Studies were made on the hydrolysis by water and water vapor of thorium nitrides—ThN, Th₃N₄, Th₂N₂O—and carbonitrides. All the thorium nitrides and carbonitrides were found to decompose in water below 100°C, changing thereby into ThO₂. The order among the thorium nitrides in their propensity toward hydrolysis is: ThC_1-xN_x>ThN>Th₃N₄>Th₂N₂O. Upon hydrolysis, ThN produced NH₃ and H₂, but in the case of the higher nitrides no H₂ was found to evolve.

In the reaction between ThN and water vapor, no higher nitride was produced, in contrast to the case of UN. The difference in behavior between ThN and UN was studied from the standpoint of differences in crystallographic conditions for the transformation from mononitride into higher nitrides. Through the hydrolysis of ThC_1-xN_x in water vapor, products containing C-C, C-C-C and C-N bonds, such as ethane and amines, were found in smaller quantities than for the case of UC_1-xN_x. This fact as well as the difficulty of formation of higher nitrides has resulted in a fairly simple hydrolysis behavior of thorium nitrides.     

Effect of LiNO3 on Corrosion Prevention of Aluminum Wastes after Their Land Disposal

After their land disposal, LiNO₃ added to cement solidified miscellaneous wastes inhibits hydrogen gas generation due to alkaline corrosion of aluminum contained in the wastes. We considered the presence of an Li-Al preservation film prevents hydrogen gas generation, and then, we assumed a scenario in which the amount of LiNO₃ included in the waste packages is lowered by underground water penetration, resulting in dissolution of the Li-Al preservation film. This dissolution allows the alkaline underground water to reach and corrode the aluminum materials. The loss of Na₂O and K₂O in cement by underground water penetration lowers the pH, so that the aluminum corrosion in the waste packages with LiNO₃, expected when the Li-Al preservation film dissolves, is less than that without LiNO₃.

To test this scenario, we measured solubility of the Li-Al preservation film, Li⁺ ion concentration, pH variation by underground water penetration, and aluminum corrosion when the Li-Al preservation film had dissolved. The measured solubility of the Li-Al preservation film was 3 × 10^?4 _M at 283 K. At that time, pH was lowered from 12.9—13.0 to 12.2—12.3. As a result, with LiNO₃ addition the aluminum corrosion amount was reduced to 10% of that without LiNO₃ addition, because of the pH decrease.     

New leak detection technique using ceramic humidity sensor for water reactors

Leak-before-break (LBB) approach has been considered for its application to the main steam line (MSL) of Korean Next Generation Reactor (KNGR) — an advanced pressurized water reactor under development. Unlike the primary water leakage, the MSL leak detection must be based on principles other than radioactivity measurements. Among potential options that are being considered as indicators of leakage, it is believed that humidity at the proximity of the piping system is an effective one. A ceramic-based humidity sensor was developed, which can be qualified for LBB applications. The ceramic material, sintered and annealed MgCr₂O₄–TiO₂, is shown to increase its electrical conductivity upon water vapor adsorption without any negative impact due to gamma radiation over the entire temperature range of interest. In the plant applications, the sensor array can be positioned in the annulus between the piping and surrounding insulation. By the analysis of humidity distribution in the annulus, a leak rate of 1 l h⁻¹ can be detected within an hour when the distance between two adjacent sensors does not exceed 1 m. In order to minimize the number of signal wires, the use of AC impedance technique is shown to be advantageous. In this paper, the results of the development and the performance characterization of ceramic humidity sensor for the LBB application to the MSL of KNGR are summarized.     

Evaluation of UF6 Vapor Release in Postulated Accident

The rupture of UF₆ gas line connected to hot UF₆ cylinder, being one of various accidents in UF₆ vapor leak-out, is considered as a postulated accident for uranium enrichment plants. For this type of rupture, we will estimate the amount of UF₆ vapor release based on a simplified calculation model and then make an evaluation of UF₆ vapor release through a ventilation system of feed vaporization facility. Assuming an instantaneous steady state for the change of UF₆ states, an unsteady state thermodynamics process is solved. Numerical examples show that about 52% of the initial UF₆ quantity are vaporized at 80°C (the temperature of the liquid UF₆ in the cylinder). Furthermore, by using the amount of released UF₆ vapor and the collection capacity of HEPA filter for IiF gas, the amount of gaseous UO₂F₂, HF which may be dissipated to the environment are conservatively estimated.