Transient gasification in an NBG-18 coolant channel of a VHTR prismatic fuel element

This paper investigates the transient gasification of NBG-18 nuclear graphite with atmospheric air ingress in a 0.8-m long coolant channel of a prismatic Very High Temperature Reactor fuel element. Analysis varied the initial graphite and air inlet temperature, T_o, from 800 to 1100 K at air inlet Reynolds number, Re_in = 5, 10 and 20. The analysis employs a Generic Interface that couples a multi-species diffusion and flow model to readout tables of the CO and CO₂ production fluxes. These fluxes are functions of the graphite local surface temperature, oxygen partial pressure and graphite weight loss fraction and calculated using a chemical-reactions kinetics model for the gasification of nuclear graphite. The analysis accounts for the heats of formation of CO and CO₂ gases, the heat conduction in the graphite sleeve, and the change in the oxygen partial pressure in the bulk gas flow mixture along the channel. Transient calculations performed up to a weight loss fraction of 0.10 at the entrance of the channel, t₁₀. They include the local graphite surface temperature and composition of bulk gas flow, the local and total graphite weight losses and the local and total production rates of CO and CO₂ gases. The heat released in the exothermic production reactions of these gases increases the local graphite surface temperature, accelerating its gasification. At the end of the calculated gasification transient, t = t₁₀, the graphite weight loss is highest at the channel entrance and decreases rapidly with axial distance into the channel, to its lowest value where oxygen in the bulk gas flow is depleted. Increasing T_o decreases t₁₀ and the total graphite loss, while increasing Re_in decreases t₁₀ but increases graphite loss.     

核级石墨微观孔隙有效扩散系数研究

有效扩散系数是描述石墨内部扩散传质的重要参数,传统经验公式中的单一平均孔径假设无法反映石墨内部复杂的孔隙孔径分布规律及其对扩散的影响,现有核级石墨的有效扩散系数计算公式与实验结果相差较大。考虑到受Knudsen扩散影响,并根据核级石墨微观孔隙孔径分布规律及扩散理论将孔径范围分为两类,从而对有效扩散系数计算公式进行了修正。采用压汞仪对IG110核级石墨未氧化及不同温度下等温氧化样品进行了孔径分析,采用本文修正公式计算了有效扩散系数并与实验测量结果进行了比较。同时对失重率、温度和压力对有效扩散系数的影响进行了讨论。结果发现,修正公式计算结果与核级石墨扩散系数的实验结果相符,提高了核级石墨有效扩散系数的计算精度。有效扩散系数随失重率的增加而升高,失重率低于20%时增长明显。有效扩散系数与温度呈0.528次方关系,与压力大致呈-0.33次方关系。     

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.     

The investigation of OH radicals produced in a DC glow discharge by laser-induced fluorescence spectrometry

In this paper the OH radicals produced by a needle-plate negative DC discharge in water vapor,N2 + H2O mixture gas and He + H2O mixture gas are investigated by a laser-induced fluorescence (LIF) system.With a ballast resistor in the circuit,the discharge current is limited and the discharges remain in glow.The OH rotation temperature is obtained from fluorescence rotational branch fitting,and is about 350 K in pure water vapor.The effects of the discharge current and gas pressure on the production and quenching processes of OH radicals are investigated.The results show that in water vapor and He + H2O mixture gas the fluorescence intensity of OH stays nearly constant with increasing discharge current,and in N2 + H2O mixture gas the fluorescence intensity of OH increases with increasing discharge current.In water vapor and N2 + H2O mixture gas the fluorescence intensity of OH decreases with increasing gas pressure in the studied pressure range,and in He + H2O mixture gas the fluorescence intensity of OH shows a maximum value within the studied gas pressure range.The physicochemical reactions between electrons,radicals,ground and metastable molecules are discussed.The results in this work contribute to the optimization of plasma reactivity and the establishment of a molecule reaction dynamics model.     

载荷对HTR-PM高温气冷堆用石墨球摩损性能的影响

采用摩擦磨损试验机研究了载荷对HTR-PM高温气冷堆用石墨球在氦气气氛下的摩损性能影响,并利用光学显微镜分析了磨损表面形貌,利用激光粒度仪和扫描电镜分析了石墨粉尘的粒度和形貌。结果表明:在氦气气氛下,摩擦系数随着载荷的增加呈现减小的趋势;产生的石墨粉尘主要呈片状或絮状体,有较强的粘附性,粒度随着载荷的增加而增加;不同载荷下,随着时间的增加,石墨球的磨损率不断减小,最终趋于一个稳定值。     

定位绕丝结构对棒束通道热工水力特性影响数值分析

定位绕丝设计广泛应用于金属快堆堆芯设计及气冷快堆堆芯设计中,本文基于三维精细化绕丝定位棒束通道网格模型模拟分析了定位绕丝螺距、定位绕丝数量及定位绕丝形状对超临界二氧化碳在棒束通道中流动换热的影响。模拟结果表明定位绕丝螺距比定位绕丝数量及定位绕丝形状对温场流场的影响更大,定位绕丝螺距小于200 mm时,进出口压降大幅增加,表面换热系数增加,温度不均匀度大幅降低;随着定位绕丝数量增加,进出口压降线性增加,表面换热系数变化不大;圆形定位绕丝可以以较小截面积达到与方形定位绕丝相似的效果,梯形定位绕丝对流场影响不如矩形定位绕丝。     

Moisture propagation and resulting stress in heated concrete walls

A mathematical model to simulate the coupled heat and mass transfer in heated porous media, as well as the resulting stress, is described. A finite element analysis, assuming homogeneous elastic material, is used to study the temperature and pore pressure distribution, and the rate of moisture propagation through a concrete containment wall under different time-dependent temperature boundary conditions. Results are also presented for the internal stresses caused by the presence of temperature gradients, pore pressure and the release of chemically bound water at high temperatures. Stress analysis calculations are superimposed over the calculations of the moisture propagation. The temperature, pore pressure and volume change resulting from the loss of bound water, as derived by the thermal mass transport calculation, are used as input for the stress analysis.     

Investigation of oxidation behaviors of nuclear graphite being developed and IG-110 based on gas analysis

The oxidation behaviors of the nuclear graphite being developed were investigated using gas chromatograph at 873–1373 K. The oxidation experiments were carried out with the gas flow rate of 0.2 L/min and the oxygen concentrations of 7, 10 and 20 mol%. The oxidation reaction began at 973 K and was accelerated with the increase of temperature. At 1173–1273 K, the oxidation was limited by oxygen supplied to graphite and the reaction rate held steady. From 1273 to 1373 K, the oxidation rate increased obviously due to the significant reaction between CO₂ and graphite. At the low temperature regime (973–1073 K), the apparent activation energies with the oxygen mole fractions of 7%, 10% and 20% were 298, 324 and 321 kJ/mol, respectively. Scanning electron microscope was applied to reveal the pore development of the graphite oxidized at different temperatures. The effect of CO combustion at temperature below 1173 K was discussed based on the oxidation behaviors of the graphite being developed and IG-110. It was suggested that the ASTM D7542-15 standard should be adjusted to fit some popular graphite, such as graphite IG-110.     

Modelling of the HFR-EU1BIS experiment and thermomechanical evaluation

Numerical modelling has been successfully applied during the design and evaluation of the HFREU1bis HTR fuel pebble irradiation experiment conducted by the Joint Research Centre Institute for Energy (JRC) in the HFR Petten, The Netherlands. HFR-EU1bis contains 5 HTR fuel pebbles that are placed in a graphite shroud. This assembly is placed in a containment tube, which is again placed in another containment, which is in contact with the HFR cooling water.The experimental requirement is to maintain a central temperature of 1250 °C for all pebbles throughout the irradiation time of 250 effective full power days (efpd). This was achieved by tailoring the gas gaps in the sample holder such that the axial heat generation profile is compensated. Determining the dimensions of the components of the experiment has been done using a thermomechanical finite element model. A combination of numerical and analytical techniques has been applied to minimise calculation times. The thermomechanical finite element model includes the thermal influence of changing gas gap dimensions due to expansion. Heat transfer by radiation through the gas gap has been modelled as well.Results of the model have been compared with measured temperatures. After some adjustments of the model parameters, good agreement has been found between calculated and measured temperatures. Additionally, the influence of irradiation on graphite thermal conductivity and thermal expansion has been included in the model. Due to irradiation damage the thermal conductivity of graphite decreases and thermal expansion generally increases, leading to increasing thermal stresses. Due to burn-up however, the heat flux decreases during irradiation, which leads to a decrease in thermal stresses. To evaluate the influence of these competing mechanisms, the thermal stress evolution during irradiation has been calculated and will be discussed.     

堆芯氧化腐蚀产物沉积层内两相沸腾传热传质特性

多孔形貌的氧化腐蚀产物沉积层一方面会增大热阻导致传热恶化,另一方面硼吸附会导致堆芯功率分布发生改变,因此,准确预测沉积层内的传热传质特性对反应堆的经济性和安全性有重要意义。本文将多孔介质层内的沸腾传热、毛细流动、溶质输运、化学反应等多物理现象进行耦合,提出了预测沉积层内的传热传质特性的高保真方法。冷却剂在毛细作用下进入多孔介质,并在烟囱表面蒸发。溶质随毛细流扩散进入多孔介质并被浓缩,各溶质间保持化学平衡。上述方法可以合理预测沉积层内的温度、流速、压力和溶质浓度分布,与Haq等相比,相对误差在±0.83%以内。在此基础上,分析了沉积层孔隙率、厚度、烟囱半径、烟囱直径等形貌参数对传热传质的影响规律。     

Fluorination of Uranium Compounds by Bromine Trifluoride Vapor, (I)

The fluorination of U₃O₈ powder by BrF₃ vapor was attempted. The reaction proceeded even at 67°C and under 10 mmHg BrF₃ partial pressure, producing UF₆. The reaction rate increases with temperature up to about 220°C, and its activation energy is 0.9 kcal/mol. The reaction rate, however, decreases at temperatures slightly above 220°C and rises again after passing through a minimum at 225°–230°C.     

Effect of noncondensable gas in a vertical tube condenser

An experimental study is performed to investigate the effects of noncondensable (NC) gas in the steam condensing system. A vertical condenser tube is submerged in a water pool where the heat from the condenser tube is removed by boiling heat transfer. The design of the test section is based on the passive condenser system in an advanced boiling water nuclear power reactor. Data are obtained for various process parameters, such as inlet steam flow rate, noncondensable gas concentration, and system pressure. Degradation of the condensing performance with increasing noncondensable gas is investigated. The condensation heat transfer coefficient and heat transfer rate decrease with noncondensable gas. The condensation heat transfer rate is enhanced by increasing the inlet steam flow rate and the pressure. The condensation heat transfer coefficient increases with the inlet steam flow rate, however, decreases with the system pressure. For the condenser submerged in a water pool with saturated condition, the strong primary pressure dependency is observed.     

Isotope exchange reaction between tritiated water and hydrogen on SiC

SiC has been considered as a primary candidate material for a first wall component in future fusion reactor because it has been claimed that SiC has excellent high-temperature properties, good chemical stability and low activation. However, the behavior of tritium on SiC has not been discussed yet. In this study, tritium trapping capacity on the surface of SiC was experimentally obtained at the temperature range of 25-800 °C in consideration of tritium trapping to the experimental system. The capacity, which was independent of the water vapor pressure in the gas phase and the temperature, was determined as about 10⁶ Bq/cm². The isotope exchange reaction rate between tritiated water in a gas phase and hydrogen on the surface was quantified at the temperature of 25, 500 and 700 °C in consideration of the behavior of tritium trapping at change of experimental condition by the numerical curve fitting method applying the serial reactor model. The reaction rate was observed to be constant as 3.48 × 10⁻⁵ m/s. Additionally tritium release behavior from the surface of SiC in water vapor atmosphere was predicted and compared with that for graphite and stainless steel.     

气态甲基碘去除特性实验研究

以安全壳过滤排放系统对甲基碘的去除性能为背景，分别以去离子水和碱性硫代硫酸钠溶液为吸收剂，对不同吸收剂温度和浓度条件下的甲基碘去除过程进行实验研究，并在此基础上分析物理传质和化学反应两种机制对甲基碘气体去除过程的影响。结果表明：室温条件下，碱性硫代硫酸钠溶液对甲基碘气体的去除作用主要以物理传质过程为主，化学反应速率较慢是限制甲基碘气体去除效率的主导因素；随着温度的增加，化学反应在甲基碘气体去除过程中的作用不断加强。当化学反应速率增加至某一值附近时，继续强化化学反应过程对甲基碘气体去除效率不再具有明显影响，吸收过程进入化学反应不敏感区域，物理传质速率成为限制甲基碘气体去除过程的主要因素。这时需通过增加气液接触面积等方法强化物理传质过程，方能进一步提升对甲基碘气体的去除效果。     

金属钠蒸发行为研究#br#

钠的蒸发问题在钠冷快堆技术发展中不容忽视,钠蒸气将会引起众多钠冷快堆的运行问题。通过设计钠蒸发釜,搭建钠蒸发装置,对系列温度和压力下的钠蒸发速率进行了研究。经过大量实验,获得了温度200～550 ℃,氩气压力为5 Pa~51 MPa（a）下钠的蒸发速率。同时研究了蒸发面积、冷却面与钠液面距离对钠蒸发速率的影响。经过对实验数据拟合的结果表明,在同一压力下,钠的蒸发速率随温度的升高而呈指数升高,其变化规律符合Langmuir公式;在相同温度下,钠的蒸发速率随着氩气压力的升高呈对数下降,且与氩气压力自然对数的四项式相关;对于温度和压力的综合影响,得到了两个拟合关系式,该蒸发速率关系式能应用于5 Pa～51 MPa(a)、200～550 ℃范围内钠蒸发速率的预估。     

高温气冷堆甲烷蒸汽重整制氢系统重整器性能数值分析

甲烷转化率和产氢量是反映重整器性能的重要指标。本文对匹配高温气冷堆HTR-10的蒸汽重整器性能进行数值分析。设定重整器氦气入口流量不变，研究不同氦气入口温度、压力，不同工艺气入口温度、压力、流量，以及不同水碳比对重整器性能的影响。在所研究的范围内，结果表明：氦气的入口温度对重整器性能有明显的影响；氦气的入口压力、工艺气的入口温度和压力对重整器性能影响较小；提高工艺气流量，甲烷转化率降低，但产氢量增加，而提高水碳比则有相反的变化关系。     

不凝结气体对蒸汽浸没射流压力振荡强度的影响

对含不凝结气体（空气）的蒸汽浸没射流压力振荡特性进行了实验研究。研究发现,含有少量不凝结气体时,蒸汽浸没射流凝结形态发生显著变化。含不凝结气体的压力振荡强度较纯蒸汽时明显变小,但含有少量不凝结气体的压力振荡强度随空气质量分数的增加整体呈缓慢上升趋势。压力振荡强度随径向距离的增加而单调减小,随轴向距离的增加先增大后逐渐减小,存在压力振荡峰值,且随空气质量分数的增加,压力振荡峰值位置沿轴向后移。压力振荡峰值位置在轴向无量纲距离X=3和X=12之间。不同过冷水温度下,压力振荡峰值随空气质量分数变化的趋势不同。压力振荡峰值的位置即压力振荡最强的位置位于核心汽羽的尾部。     

Investigation of swelling of U-Pu mixed carbide

Mixed carbide fuel samples irradiated in various types of capsules were investigated with respect to fuel swelling and fission gas behaviour. The irradiations were carried out in the FR 2 reactor in Karlsruhe at temperatures between 300 and 1750°C up to 5.5% burnup. The swelling was evaluated by immersion density measurements in carbon tetrachloride. The fission gas determinations were carried out by measuring the released gas and by measuring the retained fission gas.The swelling rate of mixed carbide is a strong function of temperature. At temperatures below 1000°C it is between 1 and 1.5% per % burnup. At temperatures above 1000°C the swelling rate increases with temperature. It is about 3% per % burnup at 1300°C and about 12% per % burnup at 1750°C. The swelling rate at high temperatures decreases with increasing burnup due to a saturation of the fission gas bubble porosity.     

非能动余热排出换热器换热能力数值分析

基于多孔介质模型,对AP1000非能动余热排出换热器(PRHR-HX)运行初始阶段进行了数值模拟。一回路的入口温度及流量采用RELAP5的计算结果,并以此作为CFD计算的边界条件。采用多孔介质模型处理C型管束区,添加管束区分布阻力。通过商业CFD软件FLUENT计算得到安全壳内置换料水箱(IRWST)侧冷却剂的三维温度及速度分布,通过用户自定义函数UDF完成一回路侧与IRWST侧的耦合换热计算,获得一回路温度分布及换热量。计算结果表明,随着IRWST内冷却剂温度升高,换热器热负荷降低,并出现明显的热分层现象,同时证明采用多孔介质模型与耦合换热计算是分析PRHR/IRWST系统瞬态热工水力特性的有效方法。     

Effect of water vapor on tritium permeation behavior in the blanket system

It is found that most hydrogen supplied to the purge gas changed to water vapor due to the water formation reaction in the early stage of the blanket operation and that physical or chemical adsorbed water is released in the high concentration into the blanket purge gas when the blanket temperature becomes higher than several hundreds of degrees K if the pre-treatment is not applied to the solid breeder materials. Effect of coexistence of water vapor in the purge gas on permeation behavior of hydrogen through F82H ferritic steel in the breeding part and palladium–silver (Pd–Ag) in the recovery part is discussed because use of them is generally considered for recovery of bred tritium from the solid blanket. Almost no decrease in permeation rate of F82H is observed in this study when water vapor exists in the blanket purge gas. The permeability of hydrogen isotopes through the Pd–Ag pipe gradually decreases when water vapor exists in the blanket purge gas. Properties required in estimation of the hydrogen permeated to the purge gas are experimentally obtained in this study.