Comparison of oxidation model predictions with gasification data of IG-110, IG-430 and NBG-25 nuclear graphite

A phenomenological oxidation kinetics model of graphite is presented and its results are compared with the reported experimental gasification data for nuclear graphite of IG-110, IG-430 and NBG-25. The model uses four elementary chemical kinetics reactions, employs Gaussian-like distributions of the specific activation energies for adsorption of oxygen and desorption of CO gas, and accounts for the changes in the effective surface areas of free active sites and stable oxide complexes with weight loss. The distributions of the specific activation energies for adsorption and desorption, the values of the pre-exponential rate coefficients for the four elementary chemical reactions and the surface area of free active sites are obtained from the reported measurements using a multi-parameter optimization algorithm. At high temperatures, when gasification is diffusion limited, the model calculates the diffusion velocity of oxygen in the boundary layer using a semi-empirical correlation developed for air flows at Reynolds numbers ranging from 0.001 to 100. The model also accounts for the changes in the external surface area, the oxygen pressure in the bulk gas mixture and the effective diffusion coefficient in the boundary layer with weight loss. The model results of the total gasification rate and weight loss with time in the experiments agree well with the reported measurements for the three types of nuclear graphite investigated, at temperatures from 723 to 1226 K and weight loss fractions up to ～0.86.     

Direct numerical simulation of turbulent mixed convection in a vertical channel in a wall-normal magnetic field

Liquid metal coolants have a significant role in the design of advanced fusion reactors. There is a need for an investigation of the thermal behavior of the liquid metal in working reactor environment, such as when fluid flow at low Prandtl number (Pr) with a buoyancy effect, is subjected to a magnetic field. In the present study, a direct numerical simulation (DNS) for a low Pr number fluid flow resulting in turbulent heat transfer with buoyancy effect under a magnetic field has been carried out between two vertical plates kept at different temperatures. In this simulation, the values of the Hartmann number (Ha) were 0 and 6, Pr number was 0.06 and Grashof numbers were 6.4 × 10⁵, 9.6 × 10⁵, and 1.6 × 10⁶. The turbulent quantities of the parameters such as the mean temperature, turbulent heat flux, and temperature variance were obtained by direct numerical simulation (DNS). The Reynolds number (Re) for channel flow based on friction velocity averaged by both walls, viscosity, and channel half-width was set to be constant as Re_τ* = 150. A uniform magnetic field was applied in a direction perpendicular to the walls of the channel. The profiles of mean velocity and velocity fluctuations became asymmetric, and the tendency was enhanced with the increasing buoyancy effect. However, by the application of a magnetic field the tendency decreased. In other words, thermal transport between the walls became weak due to the magnetic effect.     

CO2 corrosion of IG-110 nuclear graphite studied by gas chromatography

The CO₂ corrosion behavior of IG-110 nuclear graphite has been investigated using the gas chromatography method which allows the continuous analysis of the CO₂/CO gas mixture at the outlet of the corrosion chamber. The effects of temperature and initial CO₂ concentration are studied based on the Arrhenius-type reaction model. From 745 to 995 °C, the Arrhenius curve shows a linear behavior. For higher temperatures, a non-linear behavior is observed. The activation energy is calculated as 210 kJ/mole and is independent of the initial CO₂ inlet concentrations of 10%, 14% and 17%. The corrosion behavior at 1145 °C, in the diffusion-controlled regime, has also been investigated. At this temperature, the interior of IG-110 graphite is severely attacked by CO₂, and the material's surface morphology is changed drastically. A measurement of the corrosion rate against corrosion time shows that the corrosion rate initially increases to a maximum value at a weight loss degree of 30%–35%, after which it begins to decline.     

The modeling of graphite oxidation behavior for HTGR fuel coolant channels under normal operating conditions

The oxidation of graphite in normal operating conditions is a very important factor when evaluating the service time of the graphite structural material in a high temperature gas-cooled reactor (HTGR). This paper deals with the modeling of graphite oxidation by steam in the helium channel of a fuel block. The FEM software COMSOL is used: the turbulent flow of the coolant is simulated by using the k-? model and the chemical reaction is expressed by the Langmuir-Hinshelwood equation. Calculations were carried out for steam pressures around 1 Pa and for different temperature distributions. The influence of burn-off and the diffusion in graphite porosities were both considered in the oxidation. Results show that oxidation mainly occurred on the graphite surface at the bottom of the core because of the higher temperature. The thickness of graphite with a burn-off higher than 8% was about 1 mm at the core base. Less than 15% of steam was consumed in the coolant channel of the fuel assemblies. Calculations also showed that the mean gasification rate in one channel for the second service time was larger than the first service time.     

Experimental studies on single-phase flow and heat transfer in a narrow rectangular channel

The state-of-the-art studies on single-phase flow and hear transfer in narrow rectangular channels shows some difference in terms of the agreement with the conventional theory. To further make clear this issue, the experimental studies on single-phase flow and heat transfer in a narrow rectangular channel with deioned water as test fluid was carried out. The narrow rectangular channel had the following dimensions: depth (e) = 2 mm, aspect ratio (e/b) = 0.05, length to diameter ratio (L/D_h) = 300, and mean wall relative roughness (?/D_h) = 8.3 × 10^?4. The experiments were performed with the channel oriented uprightly. The parameters that were varied during the experiments included the mass flow rate, inlet temperature and heat flux.Based on the measured temperatures, mass flow rates, pressure drops and heat fluxes, the isothermal and non-isothermal friction factors and the local and mean Nusselt numbers have been calculated. The correlations for the isothermal friction factors and the mean Nusselt numbers have been developed, and have a satisfactory agreement with the conventional theory. Based on the property ratio method, the correlations for non-isothermal friction factors have been proposed, but the new exponent (m) for modifying variable-property effect need to be developed.     

Twisted-tape-induced swirl flow heat transfer and pressure drop in a short circular tube under velocities controlled

The twisted-tape-induced swirl flow heat transfer due to exponentially increasing heat inputs with various exponential periods (Q = Q₀ exp(t/τ), τ = 7, 14 and 23 s) and the twisted-tape-induced pressure drop were systematically measured with mass velocities, G, ranging from 4022 to 15,140 kg/m² s by an experimental water loop flow. Measurements were made on a 59.2 mm effective length which was spot-welded two potential taps on the outer surface of a 6 mm inner diameter, a 69.6 mm heated length and a 0.4 mm thickness of platinum circular test tube. The twisted tapes with twist ratios, y [=H/d = (pitch of 180° rotation)/d], of 2.39, 3.39 and 4.45 were used in this work. The relation between the swirl velocity and the pump input frequency and that between the fanning friction factor and Reynolds number (Re_d = 2.04 × 10⁴ to 9.96 × 10⁴) were clarified. The twisted-tape-induced swirl flow heat transfers with y = 2.39, 3.39 and 4.45 were compared with the values calculated by our correlation of the turbulent heat transfer for the empty tube and other worker's one for the circular tube with the twisted-tape insert. The influence of y and Reynolds numbers based on swirl velocity, Re_sw, on the twisted-tape-induced swirl flow heat transfer was investigated into details and the widely and precisely predictable correlation of the twisted-tape-induced swirl flow heat transfer was derived based on the experimental data. The correlation can describe for the twisted-tape-induced swirl flow heat transfer for the wide ranges of twist ratios (y = 2.39-4.45), mass velocities (G = 4022-15140 kg/m² s) and Reynolds numbers based on swirl velocity (Re_sw = 2.88 × 10⁴ to 1.22 × 10⁵) within −10 to +30% difference.     

Pulsed repetition rate nanosecond laser heating and ablation of the tokamak graphite tile deposited layers

Laser heating and ablation of the plasma-facing surface of a graphite tile from TEXTOR tokamak that was covered by a deposited carbon layer has been studied. Laser heating measurements were performed with a pulsed nanosecond Nd-YAG laser (2nd harmonic, 10 kHz repetition rate, 100 ns pulse duration). Surface temperature measurements were recorded with a home-made pyrometer having a response time of 15 μs (t_99%). The experimental results are simulated with an analytical model of laser heating of a surface covered by a deposited layer and heated repeatedly by laser pulses. The comparison between experimental and theoretical data of the observed temperature excursions enables us to assess the deposited carbon layer physical parameters (thermal conductivity, porosity, etc.) if the thermal and optical properties of the graphite substrate are known. Laser ablation measurements were performed with two pulsed nanosecond Nd-YAG lasers (20 Hz and 10 kHz repetition rate with 5 ns and 100 ns pulse duration, respectively). For a plasma-facing graphite surface covered by a thick (～30–50 μm) deposited carbon layer, the ablation threshold is 4.5 ± 1 kJ/m² regardless of the pulse duration. The obtained ablation threshold is significantly lower than the one measured for a virgin tokamak graphite sample. The comparison of the experimental results and theoretical data demonstrated that the laser ablation mechanisms for tokamak graphite and thick carbon layers deposited on plasma-facing surface are different.     

Experimental investigation of a flow-induced oscillating cylinder with two degrees-of-freedom

The phenomenon of flow-induced vibration of bluff bodies has been studied extensively. The vast majority of these studies have concentrated solely on one degree-of-freedom oscillation in the inline or cross-flow directions. Herein, experiments were carried out with a cylinder in a water channel with two degrees-of-freedom. The cylinder was cantilever mounted with a low natural frequency (typically 65 Hz) in the inline and cross-flow directions. The Reynolds number fell in the range 1.17 × 10³ < Re < 2.6 × 10⁴. The oscillating frequency of the cylinder and the surrounding flow were measured simultaneously using high temporal resolution particle image velocimetry (PIV), which is non-intrusive with respect to the flow and has high spatial and temporal resolutions. The vibration of the cylinder was found to be anisotropic. There was a discrepancy between the vibration frequencies in the inline and cross-flow directions, the difference being a function of reduced velocity.     

Direct measurements of small 14C samples after oxidation in quartz tubes

Small ¹⁴C samples gain importance in environmental research and for dating purposes. However, throughput of such samples is limited by the preparation of graphite targets for accelerator mass spectrometry (AMS) measurements. In our approach, oxidation of samples with copper oxide in quartz tubes was applied to form CO₂ which was measured directly with the gas ion source of the small AMS facility MICADAS. The presented method was designed to meet the requirements for fast and easy handling of small samples (<100 μg carbon). As combustion byproducts are likely to interfere with ionisation processes in the gas ion source, we additionally investigated the effects of several gases on C^? currents.     

Structure design on improving injection performance for venturi scrubber working in self-priming mode

The venturi scrubber working in self-priming mode is one of the most efficient gas cleaning devices to remove the radioactive particles and gases from gaseous stream during severe accident in nuclear power plant. This paper focus on improving injection performance in a split type self-priming venturi scrubber, the static pressure distributions at gas and liquid channels in the scrubber are studied emphatically, the experimental results indicate that pressure at the center of nozzle exit is lower than at the wall, and the variation laws in radial are different with increasing gas velocity. When the average gas velocity at throat U_avg = 64.3 m/s, the static pressure difference between center and wall is 2.1 KPa; with the increase of gas velocity, pressure at centre and wall reduce gradually and the pressure difference become significantly, when the average gas velocity U_avg = 225.8 m/s, pressure at the center is lower 23.2 KPa than at the wall, relative deviation is about 45.6%. However, when U_avg ≥ 230 m/s, the static pressure is not continue to decrease but reverse to recover with increasing gas velocity, and recovery rate at the wall is greater than at the center. The condition before the transition point (U_avg < 230 m/s) is defined as velocity dominate area, in this area, pressure will always decrease with increasing gas velocity; the condition after the point (U_avg ≥ 230 m/s) is defined as resistance dominate area, while pressure in this area will reverse to recover, the venturi scrubber design should be ensured in velocity dominate area. The injection performance of self-priming venturi scrubber is closely relate to pressure distribution characteristics at nozzle exit, in condition of no injection or injecting air, pressure at liquid channel is consistent with the pressure at the wall of nozzle exit, which is higher than the average static pressure; when injecting water, an additional pressure increment will generate at liquid channel duo to the momentum exchange between gas and liquid, and lead to the effective pressure difference for injection reduce further. On this occasion, the influence of liquid channel area and resistance coefficient on injection performance become important, increase liquid channel area is effective for improving injection flow rate.     

Compatibility of FBR structural materials with supercritical carbon dioxide

A key problem in the application of a supercritical carbon dioxide (CO₂) turbine cycle to a fast breeder reactor is the corrosion of structural materials brought about by supercritical CO₂ at high temperatures. In this study, long-term (8000 h) compatibility tests on candidate materials, two high-chromium martensitic steels (12Cr- and 9Cr-steels) and an austenitic stainless steel (316FR), were performed at 400-600 °C in supercritical CO₂ pressurized at 20 MPa, and corrosion allowances for the steels were proposed for application to preliminary reactor design.Although high temperature oxidation was measured in all steels, the behavior differed greatly. For martensitic steels, weight gain exhibited parabolic growth as exposure time increased at each temperature. Neither exfoliation of the oxide nor the breakage was observed during the 8000 h of exposure. The corrosion behavior was equivalent to that seen in supercritical CO₂ at 10 MPa, and it was confirmed that no effects of CO₂ pressure were present under the CO₂ turbine cycle operation conditions. Based on the results, corrosion allowances for temperature-dependant parabolic growth were proposed. For 316FR steel, weight gain was significantly lower than that of martensitic steels, with a maximum value of 6.2 g/m² at 600 °C for 8000 h. Since no dependency of temperature and immersion time on weight gain such as the martensitic steels was noted, corrosion allowances proportional to time was proposed. Estimated corrosion allowances for the martensitic and austenitic steels were 380 μm and 220 μm, respectively, for reactors, whose design life is rated at 60 years.     

Combined Forced and Free Convective Heat Transfer Characteristics in a Narrow Vertical Rectangular Channel with 2.5 mm in Gap Heated from Both Sides

Heat transfer characteristics of mainly combined forced and free convective flow in a vertical rectangular flow channel with a gap of 2 5 mm, which was quite narrow compared with those investigated in previous experiments, were studied experimentally for water. As a result, the following heat transfer characteristics were made clear, using a non-dimensional parameter Gr _x /Re ²¹ _x /⁸ Pr ^1/2 similarly to the case for the 18 mm gap which was already reported by the authors.

(1) When the Gr _x /Re ²¹ _x /⁸ Pr ^1/2 is less than 10^?4, both upward flow and downward flow show the nature of forced convective heat transfer.

(2) When the Gr _x /Re ²¹ _x /⁸ Pr ^1/2 is between 10^?4 and 10^?2, heat transfer coefficients for both upward flow and downward flow are higher than any of those predicted by the previous correlations for turbulent forced convection along a flat plate and turbulent free convection along a vertical flat plate. This is, differently from the case of 18 mm gap, due to the effect of the acceleration of main flow induced by the development of the boundary layer along the channel.

(3) When the Gr _x /Re ²¹ _x /⁸ Pr ^1/2 is larger than 10^?2, the upward flow shows the nature of free convective flow even with the gap as narrow as 2.5 mm in the vertical rectangular flow channel. Heat transfer correlations which have been developed for the 18 mm gap channel, are also available for the described-above regions of 2.5 mm gap channel.     

Simulation of uncontrolled loss of flow transients of a material test research reactor fuelled with low and high enriched uranium dispersion fuels

The effects of using low and high enrichment uranium fuel on the uncontrolled loss of flow transients in a material test research reactor were studied. For this purpose, simulations were carried out of an MTR fuelled separately with LEU and HEU fuel, to determine the reactor performance under loss of flow transients with totally failed external control systems. The coolant pump was assumed to loose its performance and the coolant flow rate reduced according to the relation m(t)/m₀ = exp(−t/25) to a new stable level. The new reduced flows m/m₀ = 0.2, 0.4, 0.6 and 0.8 were modeled. The nuclear reactor analysis code PARET was employed to carry out these calculations. It was observed that the reactors stabilized at new power levels which were lower than the original power level, with the power of HEU fuelled reactor slightly lesser than that of the LEU fuelled reactor. However, at the start of transient, the LEU fuelled reactor had a lower power level resulting in lower fuel, clad and coolant temperatures than the HEU fuelled reactor.     

Comparison of experimental and finite element analytical results for the strength and the deformation of pipes with local wall thinning subjected to bending moment

Fracture behaviors of pipes with local wall thinning are very important for the integrity of power plant piping system. In this study, monotonic bending tests without internal pressure are conducted on 48.6 mm diameter Schedule 80 (thickness, 5.1 mm) STS370 full-scale carbon steel pipe specimens. Fracture strengths of locally wall-thinned pipes were calculated by elasto-plastic analysis using finite element method. The elasto-plastic analysis was performed by FE code ANSYS. We simulated various types of local wall thinning that can be occurred at pipe surface due to coolant flow. Locally wall thinned shapes were machined to be different in size along the circumferential or axial direction of straight pipes. We investigated fracture strengths and failure modes of locally wall thinned pipes by four point bending test. And, the allowable limit of pipes with local wall thinning was investigated. In addition, we compared the simulated results by finite element analysis with experimental data. The failure mode, fracture strength and fracture behavior obtained from FE analyses showed well agreement with experimental results. From the test results, we identified three types of failure modes into ovalization, local buckling and crack initiation. These failure modes could be classified according to thinned depth, thinned length and thinned angle of a pipe. For locally wall-thinned specimens, maximum moments (M_max) were estimated by using the net-section stress criterion. Pipes with local wall thinning can be estimated using σ_u instead of σ_f because of 1.19σ_f  σ_u. Also, the axial strain affects failure modes occurred on local wall thinning. the allowable limit of local wall thinning for carbon steel pipe used can be given as follows; in the case of M_max ≥ M_y, if 10 ≤ l < 25 mm, d/t can be allowed to about 55%, and if 25 ≤ l < 100 mm, d/t can be allowed to about 50%. Also, if 100 ≤ l ≤ 120 mm, d/t can be allowed to about 29%.     

Gamma-rays irradiation effects on polysulfone at high temperature

The temperature dependence of the irradiation effects on polysulfone was studies by measuring the molecular weight, glass transition temperature, gel fraction and evolved gas. Polysulfone was irradiated with gamma-rays at room temperature, 100, 150, 180 and 210 °C. The change of molecular weight distribution and glass transition temperature showed occurrences of a main chain scission at room temperature and cross-linking at high temperature. The decrease of gel dose, the increases of gel fraction and total gas evolution with increasing temperature was observed. The evolution of CO, CO₂ and SO₂ gases increased at high temperature, while yield of evolved H₂ was independent of irradiation temperature. The probability of the cross-linking was clearly increased by irradiation at high temperature above 180 °C, though the chain scission was not changed very much.     

X-ray impact induced desorption of gases from surfaces

Measurements of gases released from 302 stainless steel and gold surfaces before and after discharge cleaning were made in ultrahigh vacuum using X-rays with an energy distribution typical of a tungsten bremsstrahlung spectrum. Similar measurements were also made for Al₂O₃ surfaces which had not been discharge cleaned. For the non-discharge-cleaned surfaces of stainless steel, Al₂O₃, and gold the predominant gas species observed mass spectrometrically was CO₂. For some stainless steel and Al₂O₃ surfaces CO and O₂ were also readily observed. Mean quantum yields for CO, O₂ and CO₂ release from such stainless steel surfaces, for example, ranged from < 6 × 10^?5 to 9 × 10^?4 molecules per photons in the bremsstrahlung spectrum characteristic for 50 keV electron energy. After discharge cleaning a decrease in the mean quantum yields was observed for the stainless steel and gold surfaces.     

棒束通道内定位格架的两相流动局部阻力实验研究

在常温、常压条件下,对竖直3×3棒束通道内定位格架的单相及两相局部阻力特性进行了实验研究。单相流动实验时,水雷诺数的变化范围为290~18 007;两相实验时,气相、液相表观速度变化范围分别为0.013~3.763m/s和0.076~1.792m/s。利用单相实验数据得到的定位格架局部阻力系数计算关系式,用两相实验数据对均相流模型中8种不同的两相等效黏度计算方法进行了评价。Rel9 000时,Dukler模型的预测效果最好;Rel≥9 000时,McAdams计算方法预测效果最好;基于所有数据,Dukler模型的计算值与实验值吻合最好,平均相对误差为29.03%。考虑了质量含气率、两相雷诺数及气液相密度的影响,对Rel9 000时的实验数据进行了拟合,得到的经验关系式的计算值与实验值符合较好。     

Water formation in graphite and boron-doped graphite under simultaneous O and H irradiation

When graphite is exposed to simultaneous irradiation by H⁺ and O⁺, in addition to the H⁺-induced hydrocarbon and O⁺-induced CO and CO₂ formation, water is also formed. The present investigation explores the effect of the presence of boron in graphite on water formation. The results show that B-doped graphite specimens (∼15 at.% B) exposed to simultaneous O⁺ and H⁺ irradiation produce less water when compared to pure graphite under similar irradiation. The concept of the formation of ‘water-precursors’ at the end of the O⁺ range is proposed to explain the observed effect.     

HCLL and HCPB coolant purification system: Design of the copper oxide bed

The coolant purification system (CPS) together with the tritium extraction system (TES) and helium cooling system (HCS) are the principal auxiliary circuits of helium-cooled-lithiium–lead (HCLL) and helium-cooled-pebble-bed (HCPB) test blanket modules (TBMs). To extract heat from TBMs, Helium is used as primary coolant. CPS is used to extract tritium from the helium primary circuit as well as to guarantee removal of impurities which could interact with structural material. The reference process proposed for CPS is composed of 3 main successive steps. Step 1 consists in oxidation of Q₂ and CO to Q₂O and CO₂ using a copper oxide bed (Q represents either: H, D or T). Step 2 is dedicated to the removal of water which is adsorbed together with CO₂ on molecular sieve bed. Step 3 will remove residual impurities using a heated getter.Based on the operating conditions of CPS (pressure, flowrate, temperature) and on an estimation of the impurities foreseen, this paper presents a design of the oxidising bed which fulfils all requirements in terms of efficiency and lifespan. The design is obtained using a phenomenological approach taking into account competition between oxidation of CO and Q₂ on the metal oxide. The model was implemented in matlab software. A column of 0.41 m large and 2 m long containing 480 kg of CuO is proposed to assure complete oxidation of Q₂ for 16 months long.     

Creep-to-rupture of the steel P92 at 650 °C in oxygen-controlled stagnant lead in comparison to air

Creep-to-rupture experiments were performed on 9%-Cr ferritic–martensitic steel P92 in the CRISLA facility. The specimens of P92 were examined at 650 °C and static tensile stress between 75 and 325 MPa in both stagnant lead with 10^?6 mass% dissolved oxygen and air. The steel showed an insignificant difference in time-to-rupture, t_R, and ductile fracture in both environments at >100 MPa, corresponding to t_R < 3,442 h. At 75 MPa in Pb (t_R = 13,090 h), the steel, however, featured purely brittle fracture pointing to liquid metal embrittlement. Structural changes in the steel and surface oxidation in the different environments were studied using metallographic techniques. The Laves phase that forms during thermal aging at 650 °C was found along prior austenite grain boundaries and martensite laths already after relatively short testing time, along with chromium carbides that are already present in the as-received condition of the steel.