HRTEM study of oxide nanoparticles in K3-ODS ferritic steel developed for radiation tolerance

Crystal and interfacial structures of oxide nanoparticles and radiation damage in 16Cr-4.5Al-0.3Ti-2W-0.37 Y₂O₃ ODS ferritic steel have been examined using high-resolution transmission electron microscopy (HRTEM) techniques. Oxide nanoparticles with a complex-oxide core and an amorphous shell were frequently observed. The crystal structure of complex-oxide core is identified to be mainly monoclinic Y₄Al₂O₉ (YAM) oxide compound. Orientation relationships between the oxide and the matrix are found to be dependent on the particle size. Large particles (>20 nm) tend to be incoherent and have a spherical shape, whereas small particles (<10 nm) tend to be coherent or semi-coherent and have a faceted interface. The observations of partially amorphous nanoparticles and multiple crystalline domains formed within a nanoparticle lead us to propose a three-stage mechanism to rationalize the formation of oxide nanoparticles containing core/shell structures in as-fabricated ODS steels. Effects of nanoparticle size and density on cavity formation induced by (Fe⁸⁺ + He⁺) dual-beam irradiation are briefly addressed.     

On the chemical erosion of some low-Z materials by hydrogen plasma and on the possibility of regeneration of the first wall by low pressure plasma CVD

Chemical erosion of various carbon materials and of the carbides of boron and silicon by hydrogen plasma was investigated experimentally. The chemical erosion of the carbides appears negligible at elevated temperatures, but it can be significant for carbon. In addition, the application of low plasma CVD-technique for regeneration of the first wall of a future fusion reactor is proposed and discussed.     

高温气冷堆舱室抗商用飞机撞击的耦合数值分析

本文建立了大型商用飞机撞击典型高温气冷堆核电站反应堆舱室的非线性有限元模型,计算中混凝土舱室直接采用工程用钢筋混凝土的损伤塑性本构模型,飞机结构采用Johnson-Cook本构模型。对飞机高速撞击高温气冷堆核电站反应堆舱室非线性撞击过程进行模拟计算,得出正面和侧面撞击条件下的撞击载荷曲线、撞击位移云图、反应堆舱室混凝土破坏情况等结果。评估表明,反应堆舱室结构在撞击条件下的整体损伤微小,可为保护内部关键设备提供重要的屏障功能。     

运行核电厂堆芯吊篮壳型振动特性研究

反应堆堆芯吊篮的振动反映了吊篮及相关堆内构件的振动情况与设备稳定性，是评估反应堆安全运行的重要依据。本文采用中子噪声技术和信号时频域分析方法，重点研究了堆芯吊篮壳型振动特性，通过分析某核电厂特定机组近几年的监测数据，获得堆芯吊篮壳型振动模态参数的变化趋势。结果表明，在每个燃料周期内，吊篮壳型振动频率有逐渐变小趋势，每经历一次大修后，振动中心频率基本恢复至上一个燃料循环的初始振动频率处。研究结果有助于了解堆芯吊篮在多个燃料循环周期内壳型振动的特性和成因，为堆芯吊篮早期故障诊断奠定基础。      

Core debris behavior and interactions with concrete

The behavior of LMFBR core debris following a hypothetical core-disruptive accident (HCDA) depends upon a wide range of physical and chemical phenomena. Current understanding of the key phenomena are summarized for core-debris behavior within the reactor vessel, within the reactor cavity, and within the concrete base mat below the reactor cavity. In-vessel behavior was the principal consideration of post-accident heat removal for the FFTF reactor. Several concepts of engineered core-retention systems within the reactor cavity have been considered for other reactors, including the cooled crucible concept, the sacrificial barrier concept, the stable barrier concept, and the catch tray concept. Behavior within the concrete base mat is an important part of a general concept of inherent core retention which depends upon an understanding of the complex interactions of core debris with concrete.     

A feasibility assessment of the use of nanofluids to enhance the in-vessel retention capability in light-water reactors

Nanofluids, colloidal dispersions of nanoparticles, exhibit a substantially higher critical heat flux (CHF) compared to water. As such, they could be used to enhance the in-vessel retention (IVR) capability in the severe accident management strategy implemented by certain light-water reactors. It is envisioned that, at normal operating conditions, the nanofluid would be stored in dedicated storage tanks, which, upon actuation, would discharge into the reactor cavity through injection lines. The design of the injection system was explored with risk-informed analyses and computational fluid dynamics. It was determined that the system has a reasonably low failure probability, and that, once injected, the nanofluid would be delivered effectively to the reactor vessel surface within seconds. It was also shown analytically that the increase in decay power removal through the vessel using a nanofluid is about 40%, which could be exploited to provide a higher IVR safety margin or, for a given margin, to enable IVR at higher core power. Finally, the colloidal stability of a candidate alumina-based nanofluid in an IVR environment was experimentally investigated, and it was found that this nanofluid would be stable against dilution, exposure to gamma radiation, and mixing with boric acid and lithium hydroxide, but not tri-sodium phosphate.     

Study of ionic wind based on dielectric barrier discharge of carbon fiber spiral electrode

Based on the idea that a large number of charged particles can be generated by a high-frequency alternating current(AC)dielectric barrier discharge(DBD),and charged particles can be accelerated directionally by a direct current(DC)electric field,a new type of ionic wind formation method is proposed in this paper.To this end,a carbon fiber spiral electrode serves as the generation electrode and a metal rod electrode as the collection electrode,with AC and DC potentials applied respectively to the generation electrode and the collection electrode to form an AC-DC coupled electric field.Under the action of the coupled electric field,a dielectric barrier discharge is formed on the carbon fiber spiral electrode,and the electrons generated by the discharge move from the generation electrode to the collection electrode in the opposite direction of the electric field vectors.During the movement,energy is transferred to the gas molecules by their colliding with neutral gas molecules,thereby forming a directional gas stream movement,i.e.ionic wind.In the research process,it is verified through electric field simulation analysis and discharge experiment that this method can effectively increase the number of charged particles in the discharge process,and the velocity of the ionic wind is nearly doubled.On this basis,the addition of a third electrode forms a distinct discharge region and an electron acceleration region,which further increases its velocity.The experimental result shows that the ionic wind speed reaches up to 2.98 m s^?1.Thanks to the ability of the electrode structure to generate an atmospheric pressure DBD plasma and form an ionic wind,we can create a noise-free air purification device without resorting to a fan,with this device having good application prospects in the field of air purification.     

Thermal response of plasma sprayed tungsten coating to high heat flux

In order to investigate the thermal response of tungsten coating on carbon and copper substrates by vacuum plasma spray (VPS) or inert gas plasma spray (IPS), annealing and cyclic heat load experiments of these coatings were conducted. It is indicated that the multi-layered tungsten and rhenium interface of VPS-W/CFC failed to act as a diffusion barrier at elevated temperature and tungsten carbides were developed after 1 h incubation time when annealing temperature was higher than 1600 °C. IPS-W/Cu and W/C without an intermediate bonding layer were failed by the detachment of the tungsten coating at 900 and 1200 °C annealing for several hours, respectively. Cyclic heat load of electron beam with 35 MW/m² and 3-s pulse duration indicated that IPS-W/Cu samples failed with local detachment of the tungsten coating within 200 cycles and IPS-W/C showed local cracks by 300 cycles, but VPS-W/CFC withstood 1000 cycles without visible damages. However, crack creation and propagation in VPS-W/CFC were also observed under higher heat load.     

Modelling of release of particulate material from transport containers

Abstract

The Nuclear Decommissioning Authority (NDA) is developing a family of Standard Waste Transport Containers (SWTCs) for the transport of unshielded intermediate level radioactive waste packages. The SWTCs are shielded transport containers designed to carry different types of waste packages. The combination of the SWTC and the waste package is required to meet the regulatory requirements for Type B packages. One such requirement relates to the containment of the radioactive contents, with the IAEA Transport Regulations specifying release limits for normal and accident conditions of transport. In the impact tests representing accident conditions of transport, the waste package will experience significant damage and radioactive material will be released into the SWTC cavity. It is therefore necessary to determine how much of this material will be released from the cavity to the external environment past the SWTC seals. Typical assessments use the approach of assuming that the material will be evenly distributed within the cavity volume and then determining the rate at which gas will be released from the cavity, with the volume of radioactive material released with the gas based on the concentration of the material within the cavity gas. This is a pessimistic approach as various deposition processes would reduce the concentration of gas-borne particulate material and hence reduce their release rate from the SWTC. This paper assesses these physical processes that control the release rate and develops a conservative methodology for calculating the particulate releases from the SWTC lid and valve seals under normal and accident conditions of transport, in particular:

a) the flows within the SWTC cavity, especially those near the cavity walls;

b) the aerodynamic forces necessary to detach small particles from the cavity surface and suspend them into the cavity volume;

c) the adhesive forces holding contaminant particles on the surface of a waste package;

d) the breakup of waste material upon impact that will determines the volume fraction and size distribution of fine particulate released into the cavity.

Three mechanisms are specifically modelled, namely Brownian agglomeration, Brownian diffusion and gravitational settling, since they are the dominant processes that lead to deposition within the cavity and the easiest to calculate with much less uncertainty than the other deposition processes. Calculations of releases under normal conditions of transport concentrate on estimating the detachment of any waste package surface contamination by inertial and aerodynamic forces and show that very little of any contamination removed from the waste package surface would be released from the SWTC. Under accident conditions of transport, results are presented for the fraction released from the SWTC to the environment as a function of the volume fraction of the waste package contents released as fine particulate matter into the SWTC cavity. These show that for typical release fractions of 10^-6 to 10^-8 for the release of radioactive material from waste packages into the SWTC cavity, the release fraction of the waste package inventory from the SWTC of typically 10^-9 to 10^-10. Hence, the effective decontamination factor provided by the SWTC is 10² to 10³. Whilst this analysis has been carried out specifically for the SWTC carrying waste packages, it is applicable to other arrangements and its use would reduce the high degree of pessimism used in typical containment assessments, whilst still giving conservative results.     

Atomistic modelling of the Fe-Cr-C system

For the purpose of modelling the impact of carbon on radiation damage phenomena in steels, we have performed an extensive set of first principle calculations on the Fe-Cr-C system. The calculated solution and diffusion enthalpies of carbon in iron and in chromium agree well with experimental data, as do the relative formation energies of mono-carbides, cementite, Hägg and M₂₃C₆ carbides. Our data further indicate that interstitial carbon is attracted to a solute iron atom in bcc chromium, while the reaction between carbon and a solute chromium atom in bcc iron is repulsive. An empirical potential fitted to data for iron carbides is capable of reproducing melting behaviour of cementite, while the predicted interaction with point defects agrees less well with DFT data than a potential recently published by Hepburn and Ackland.     

高温气冷堆反应堆厂房外墙及蒸发器舱室抗飞机撞击数值分析

反应堆抗大飞机撞击是需要考虑的外部事件,对反应堆的安全评价非常重要。本文在耦合冲击动力学有限元模型的基础上,提出了双层平行墙体承受飞机撞击的等效模拟方法,研究了高温气冷堆（HTGR）较薄的方形蒸发器舱室抗商用飞机撞击能力。建立了反应堆厂房外墙受商用飞机撞击穿透评价标准,并进行了商用飞机撞击反应堆厂房外墙仿真计算,得到了飞机剩余动能曲线。飞机撞击蒸发器舱室的计算中,保守假设飞机穿过外墙后无质量损失,形态完好,以剩余速度撞击方形蒸发器舱室。评估表明,蒸发器舱室结构在撞击条件下的整体损伤微小,可为保护内部关键设备提供重要的屏障功能。      

Modeling of a confinement bypass accident with CONSEN,a fast-running code for safety analyses in fusion reactors

The CONSEN (CONServation of ENergy) code is a fast running code to simulate thermal-hydraulic transients, specifically developed for fusion reactors. In order to demonstrate CONSEN capabilities, the paper deals with the accident analysis of the magnet induced confinement bypass for ITER design 1996. During a plasma pulse, a poloidal field magnet experiences an over-voltage condition or an electrical insulation fault that results in two intense electrical arcs. It is assumed that this event produces two one square meters ruptures, resulting in a pathway that connects the interior of the vacuum vessel to the cryostat air space room. The rupture results also in a break of a single cooling channel within the wall of the vacuum vessel and a breach of the magnet cooling line, causing the blow down of a steam/water mixture in the vacuum vessel and in the cryostat and the release of 4 K helium into the cryostat. In the meantime, all the magnet coils are discharged through the magnet protection system actuation. This postulated event creates the simultaneous failure of two radioactive confinement barrier and it envelopes all type of smaller LOCAs into the cryostat. Ice formation on the cryogenic walls is also involved. The accident has been simulated with the CONSEN code up to 32 h. The accident evolution and the phenomena involved are discussed in the paper and the results are compared with available results obtained using the MELCOR code.     

Magnet Safety Assessment for ITER

The ITER magnet system consists of structurally linked sets of toroidal (TF) and poloidal (PF) field coils, central solenoid (CS), and various support structures. The coils are superconducting, force flow Helium cooled with a Kapton-Glass-Epoxy multilayer insulation system. The stored magnetic energy is about 100GJ in the TF system and 20GJ in the PF-CS. Coils and structure are maintained at 4 K by enclosing them in a vacuum cryostat. The cryostat, comprising an outer envelope to the magnets, forms most of the second radioactivity confinement barrier. The inner primary barrier is formed by the vacuum vessel, its ports and their extensions. To keep the machine size within acceptable bounds, it is essential that the magnets are in close proximity to both of the nuclear confinement barriers. The objective of the magnet design is that, although local damage to one of the barriers may occur in very exceptional circumstances, large scale magnet structural or thermal failure leading to simultaneous breaching of both barriers is not credible. Magnet accidents fall into three categories: thermal (which includes arcing arising from insulation failure and local overheating due to discharge failure in the event of a superconductor quench), structural (which includes component mechanical failure arising from material inadequacies, design errors and exceptional force patterns arising from coil shorts or control failures), and fluid (Helium release due to cooling line failure). After a preliminary survey to select initial faults conceivable within the present design, these faults are systematically analyzed to provide an assessment of the damage potential. The results of this damage assessment together with an assessment of the reliability of the monitoring and protective systems, shows that the magnets can operate with the required safety condition.     

中能衍射极限环中电阻壁和高频系统阻抗计算与优化

在衍射极限环中,由于采用了高梯度小孔径磁铁,真空盒尺寸相对于第3代同步辐射光源大幅减小,全环阻抗明显增大,因此在设计阶段需要对真空元件阻抗进行精准的评估与模型优化。本文计算了中能衍射极限环的多层电阻壁阻抗,针对衍射极限环给出了优化的电阻壁的材料选择;利用数值模拟程序,完成了对高频腔与三次谐波腔的设计、优化与阻抗计算;分析了由电阻壁阻抗、高频腔和三次谐波腔中的高次模引起的束流不稳定性,并对不稳定性问题提出了抑制方案。     

Systematic Analysis of Fission Cross Sections of Actinides by means of Double-Humped Barrier Model

Neutron-induced fission cross sections of 24 actinide nuclei were analyzed in terms of the double-humped fission barrier model to deduce the barrier heights. Good fits were obtained by assuming that the first barrier is mass-symmetric and axially asymmetric, while the second barrier is mass-asymmetric and axially symmetric. Systematic trends were observed in the barrier heights of the actinide nuclei; the first barrier height as a function of neutron number tends to be peaked at V?147, whereas the second barrier height increases linearly as a function of (1 -x)³A^2/3, where x is the fissility. By decomposing the barrier heights into liquid-drop and shell correction parts, the surface energy coefficient was deduced to be 17.55 MeV. This value is consistent with existing values obtained from nuclear mass systematics. This fact corroborates the theoretical conjecture that the shell correction is damped at larger deformation corresponding to the second barrier. Near constancy of the fission barrier heights for actinides (fission barrier anomaly) was interpreted in terms of the threecomponent analysis.     

UF6泄漏事故后果评价研究进展

简要介绍了核燃料循环过程中UF6泄漏事故的几类事故情形,以及UF6泄漏后的大气扩散过程。目前,用于UF6泄漏事故后果评价的主要模型是HGSYSTEM／UF6模型和RASCAL模型：一般情况下,两种模型可溶性铀的平均浓度的预测值与实际测量值相比为小于2的数;在D类稳定下RASCAL预测的结果处于高斯模型和HGSYSTEM／UF6之间;而在F类稳定度下,1km内基本上是RASCAL计算结果最低,1km外3个模型预测结果无规律性。     

Plasma Syntheses of Carbon Nanotube-Supported Pt-Pd Nanoparticles

It is reported that the highly dispersed Pt nanoparticles on carbon nanotubes can be synthesized under mild conditions by in situ plasma treatment.The carbon nanotube was pretreated by O₂ plasma to transform into oxide carbon nanotubes（O-CNTs）,and then it was mixed with the precursors（the mixture of H₂ PtCl₆and PdCl₆）.After that,the O-CNTs and the precursors were simultaneously treated by H₂ plasma.The precursors were transformed into Pt-Pd nanoparticles（NPs）and the O-CNTs transformed into CNT.The synthesized CNT-based Pt-Pd nanoparticles were characterized by scanning electron microscopy,transmission electron microscopy,X-ray diffraction and X-ray photoelectron spectroscopy.All the analysis showed that the Pt-Pd nanoparticles were deposited on CNT as a form of face-centered cubical structure.     

II. Steady state thermal measurements on the secondary side of a PWR steam generator (BUGEY-4 nuclear power plant)

Steady state thermal measurements, water and shell temperature, are made in the downcomer of the steam generator of the PWR BUGEY-4. The measurements show that the flow is of a swirling type with the degree of swirl being a function of the load (power). The angular temperature distribution of water and shell exhibits a minimum that rotates from the hot leg to the cold leg with increasing load. The results also admit the conclusion that the feedwater and recirculation water are well mixed in the cylindrical section of the downcomer. Measurements and computations of fluid and shell temperatures are in good agreement for the steam generator in a state of natural circulation. For the state of forced circulation, the agreement is poor when it is assumed that the flow in the downcomer is turbulent and the steam generator outer shell is well insulated. The agreement is excellent for turbulent flow but with air leakage, or infiltration, between the steam generator outer shell and its insulation. If the hypothesis of air infiltration is ruled out, agreement between measurements and computations is much improved when the flow in the downcomer is taken as a turbulent core flow with an attendant laminar boundary layer. The existence of a laminar boundary layer would require the flat plate transition Reynolds number be enhanced by a factor of 100. This enhancement could result from the combined effects of swirl and density gradient in the downcomer.     

Progress in understanding fission-product behaviour in coated uranium-dioxide fuel particles

Supported by results of calculations performed with two analytical tools (MFPR, which takes account of physical and chemical mechanisms in calculating the chemical forms and physical locations of fission products in UO₂, and MEPHISTA, a thermodynamic database), this paper presents an investigation of some important aspects of the fuel microstructure and chemical evolutions of irradiated TRISO particles. The following main conclusions can be identified with respect to irradiated TRISO fuel: first, the relatively low oxygen potential within the fuel particles with respect to PWR fuel leads to chemical speciation that is not typical of PWR fuels, e.g., the relatively volatile behaviour of barium; secondly, the safety-critical fission-product caesium is released from the urania kernel but the buffer and pyrolytic-carbon coatings could form an important chemical barrier to further migration (i.e., formation of carbides). Finally, significant releases of fission gases from the urania kernel are expected even in nominal conditions.     

Effect of cooling temperature of electrodes on Joule-heating flow in cubic cavity

The effect of cooling condition of electrode plates on chaotic Joule-heating flow behavior in a cubic cavity has been experimentally investigated. Two electrode plates are immersed in the fluid body and placed on the opposing side wall. They are connected to an AC power source for internal volumetric ‘Joule-heating’ in the cavity. This chaotic flow which induced by Joule heating is observed within whole the cavity. The chaotic flow is investigated quantitatively by Ultrasonic Velocity Profiling (UVP) method. The chaotic flow behavior is also observed by using Particle Image Velocimetry (PIV) method to understand the effect of temperature distribution in this condition. The chaotic flow behavior generates fluctuations of temperature and velocity. As a result, cooling condition of electrode plates has a strong effect on the Joule-heating flow behavior that the vortex area occurred in the upper part of cavity. On the other hand, in the adiabatic condition, unstable flow appeared in whole cavity. In additional, velocity profiles are analyzed by Fast Fourier Transform (FFT) method about the frequencies of fluctuations. Furthermore, a numerical analysis using Finite Element Method, GSMAC-FEM, is also examined the Joule-heating flow behavior for cubic cavity.