Theoretical and experimental performance analysis for cold trap design

Cold trap is a purification unit used in sodium system of FBR's for maintaining the oxygen/hydrogen level in sodium within acceptable limits. It works on the principle of crystallization and precipitation of oxides/hydrides of sodium in a wire mesh, when the temperature of sodium is reduced below the saturation temperature. The cold traps presently used have lower effectiveness and get plugged prematurely. The plugged cold traps are cleaned and then put back into service. Frequent cleaning of cold trap results in the long down time of the sodium system. New design of cold trap has been conceived to overcome the above problems. The mathematical modeling for the new design was carried out and validated with experimentally tested results for its effectiveness. This paper shares the experience gained on the new design of cold trap for FBR's.     

CEFR冷阱结构设计改进及热工水力分析

冷阱是中国实验快堆（CEFR）运行过程中的重要设备之一，在净化反应堆系统中的冷却剂钠、保证反应堆安全稳定运行等方面起着重要作用。而目前CEFR现有冷阱在使用过程中存在着杂质沉降效率低，易造成冷阱堵塞而使冷阱更换频繁等问题，对CEFR的长期稳定运行非常不利。分析冷阱出现的问题，可能有两方面原因：一是钠在冷阱中的流动路径不合理，易造成区域的堵塞；二是在丝网密度和布置方面，没有采用分区布置也容易使杂质沉积在外层而堵塞流道。针对这两方面的原因，本文提出了现有冷阱的改进方案，从结构设计和水力学两方面来改善冷阱内钠中杂质的沉降模式，达到延长冷阱运行寿命的目的。改进方案主要是将丝网区内筒孔封闭约1/3，并在丝网上部支撑板上开两排导流孔，以改进流场；同时将丝网分成上、中、下3个区域布置，每个区域的丝网采用不同的密度，以满足杂质从冷阱底部开始逐渐沉积的要求。通过对该方案三维建模与数值分析，并与CEFR现有冷阱的分析结果进行比较发现，改进冷阱流场分布更为合理，丝网的布置也更能符合杂质沉积的要求，后续将与CEFR的实际运行工况做进一步的比较研究。     

Behaviors of Impurities in Liquid Sodium, (II)

The behavior of hydrogen in liquid sodium containing relatively large amounts of oxygen were investigated with the aid of the hydrogen sensor of niobium membrane. The partial pressure of the hydrogen in liquid sodium at hot zone of a cold-trapped natural circulation sodium loop was measured as functions of temperatures of the cold trap and the hot zone of the loop. It was observed that at constant cold trap temperature the partial pressure of hydrogen in sodium increases with increasing the hot zone temperature. This study also showed that, keeping the temperature of hot zone constant, a logarithmic plot of hydrogen concentration calculated from the equilibrium hydrogen partial pressure vs. the reciprocal temperature of cold trap yields a straight line whose slope is nearly equal to that on the solubility of sodium hydroxide in sodium. Finally, it was observed that the permeation of hydrogen in sodium through a niobium membrane is a process controlled by diffusion. But the permeability for hydrogen in niobium is a few-hundredths smaller than that in literature, owing to the oxide film on the membrane surface.     

Optimal performance analysis of a sodium cold trap

Cold traps are normally used to purify sodium, and their principle of operation is to cool sodium to temperature near its melting point, where soluble impurities contained in it are easy to precipitate. Two mathematical models are developed to analyze the processes that occur in sodium cold trap. One of them is used to analyze optimal passway shape of air cooler and the other is used to analyze optimal management of Stainless steel mesh. The models for analyzing optimal performance of sodium cold trap are typical optimal control problems. The problem are solved by means of Generalized Pontryagin's Minimum Principle. The numerical results show that air cooler of a cold trap should be two zones, that is, one is cooled by air and the other one is held in isothermal situation. The passway shape of air cooler should be a variable across section when performance of cold trap is optimal. The mesh management of a cold trap should be small surface in top and large surface in bottom when its precipitating performance is optimal and its working period is longer. The obtained conclusions are useful for practical engineering design and operation of a cold trap.     

中国示范快堆高性能冷阱净化能力试验研究

钠冷快堆采用冷阱作为净化钠中杂质的设备,目前中国实验快堆（CEFR）的冷阱净化杂质能力不能满足中国示范快堆（CFR600）各钠回路中杂质的净化需求。本文通过开展高性能冷阱净化能力试验,获得了不同温度下冷阱的溶解速率、净化速率、捕集能力和杂质容量。这些性能指标均达到了预期值,其捕集能力是CEFR一回路冷阱的2.33倍,是二回路冷阱的1.91倍。试验研究结果为CFR600冷阱结构的优化设计及冷阱性能分析软件的开发提供了依据,满足了CFR600一、二回路和乏组件转换桶钠净化系统关键设备的需求,实现了自主研发的高性能冷阱在CFR600工程上的成功应用。     

Design and analysis on tritium system of multi-functional experimental fusion–fission hybrid reactor (FDS-MFX)

As early application of fusion technology, the fusion–fission hybrid systems/reactors could be used to transmute long-lived radioactive waste and produce fissile nuclear fuel. A fusion–fission hybrid reactor named FDS-MFX was designated for checking and validating the DEMO reactor blanket relevant technologies. The reactor design is based on easy-achieved plasma parameters extrapolated from the successful operation of tokamaks and the subcritical blanket is designed based on the well-developed technologies of fission reactors. In this contribution, a concept of the tritium system was designed for the FDS-MFX: the tritium was extracted from LiPb by the helium purge gas which contains a small amount of hydrogen gas, then the impurity gas was removed by cold trap, finally tritium was separated from hydrogen isotope by the cryogenic distillation and supply to reactor core. On the basis of data obtained by present design and experimental research, the system parameters were presented and discussed in detail. The results preliminarily demonstrated the engineering feasibility of the design.     

Experiments on Reactivity Balance as Malfunction Detector for Research Reactor

An amalgamation method is described for the determination of both hydroxide and non-hydroxide hydrogen in sodium. Using a mercury ejector pump and a Toepler pump, hydrogen (non-hydroxide hydrogen) liberated from sodium amalgam at 200°C under vacuum is collected and determined by gas chromatography. Then, the vessel containing the remaining sodium amalgam is filled with argon to a pressure of 600 mmHg and heated at 400°C under argon stream. The hydrogen (hydroxide hydrogen) collected is in the gas phase of a cold trap maintained at liquid nitrogen temperature, which condenses the argon but not hydrogen. The hydrogen thus collected is determined by gas chromatography. The detection limits for non-hydroxide and hydroxide hydrogen possible with this proposed method are 0.05 and 0.1 ppm in sodium, respectively.     

Interaction of hydrogen isotopes with metals: Deuterium trapped at lattice defects in palladium

From an interplay between theory based on the effective-medium scheme and experiments, an extremely simple picture has evolved which is capable of describing a vast number of experimental quantities related to interaction of hydrogen with metals, especially the trapping of hydrogen at defects. It is shown that the trap strengths are determined mainly by the interstitial electron density, and any open structures in the lattice leads to a trap, with the vacancies and voids being the strongest traps. It is also found theoretically and experimentally that up to six hydrogen atoms can be accomodated in a vacancy, and the change in trap strengths with occupancy has been determined. Recent results for the trapping of deuterium to defects in Pd are discussed.     

液态锂铅回路冷阱过滤芯捕集效率数值分析

液态锂铅纯化技术是聚变堆锂铅包层关键技术之一,冷阱是锂铅在线纯化的常用装置。鉴于冷阱过滤芯捕集杂质的效率难以在线测量,本文利用Fluent中离散相模型(DPM模型)对某些影响过滤芯捕集效率因素进行了数值模拟,得出了三种规格单层过滤芯周围的离散相浓度场分布及捕集效率。结果显示:在考虑杂质结晶析出速度的前提下,增大流体速度和降低过滤芯孔隙率可以提高过滤芯对杂质的捕集效率,且速度增大到一定数值之后,过滤芯对杂质捕集的效率增速减缓。相关结果可为冷阱过滤芯的的优化设计及液态金属流速的确定提供理论依据与参考。     

测定钠中杂质的自动阻塞计的研制

采用风机冷却速率自动调节系统，解决低阻塞温度的测量问题，给出不同阻塞温度下自动阻塞计的温度和流量测量曲线。温度和流量的数据采集采用ＩＭＰ分散式数据检测与控制系统以消除干扰，提高测量精度。阻塞计的工作过程基本上由微机控制自动进行。     

阻塞计的校准方法研究

阻塞计是监测钠中杂质浓度水平的最重要的在线测量仪表,为提高阻塞计的测量精度,提出了一种新的校准方法--冷阱控制法。实验通过控制冷阱冷点温度来控制钠中杂质的饱和温度,进而对阻塞计进行校准。实验中采用闭环控制模型准确控制了冷阱冷点的温度,冷阱冷点温度的波动范围为±0.5 ℃。共进行了28组实验,通过实验得到了阻塞温度的经验校准曲线。阻塞温度经校准后,校准值与理论值间偏差的范围为-0.654~0.653 μg/g,平均绝对偏差为0.298 μg/g,平均相对标准偏差为4.40%,校准曲线的残差概率图散点基本上呈直线趋势,残差服从正态分布,校准曲线的可信度较高。冷阱控制法较为简单,易于推广,该方法对阻塞计校准后,可提高其测量准确度。     

钠冷阱丝网布置的最优化分析

本文主要讨论冷阱中丝网布置的优化问题。对于冷阱中的质量传递问题采用了一维模型，对丝网沉积面积随时间而变的因素进行了简化处理，提出了液态钠流经丝网区一次后应尽量满足对杂质浓度的要求，且在整个丝区杂质析出均匀，丝网用量最小情况下的最优化模型。利用诚特里亚金极大值原理对所得模型在等温冷阱情况下进行了求解，并对非等温冷阱的情况进行了性分析。所得结论可为工程设计提供理依据。     

Analyses for Passive Safety of Fusion Reactor during Ex-Vessel Loss of Coolant Accident

Passive safety of nuclear fusion reactors during ex-vessel Loss-of-Coolant Accidents (LOCAs) in the divertor cooling system has been investigated using a hybrid code, which can treat the interaction of the plasma and plasma facing components (PFCs). The code has been modified to include the impurity emission from PFCs with a diffusion model at the edge plasma. We assumed an ex-vessel LOCA of the divertor cooling system during the ignited operation in International Thermonuclear Experimental Reactor (ITER), in which a carbon-copper brazed divertor plate was employed in the Conceptual Design Activity (CDA). When a double-ended break occurs at the cold leg of the divertor cooling system, the impurity density in the main plasma becomes about twice within 2s after the LOCA due to radiation enhanced sublimation of graphite PFCs. The copper cooling tube of the divertor begins to melt at about 3s after the LOCA, even though the plasma is passively shut down a t about 4s due to the impurity accumulation. It is necessary to apply other PFC materials, which can shorten the time Period for passive shutdown, or an active shutdown system to keep the reactor structures intact for such rapid transient accident.     

Cold Trapping of Fission Products in a Stainless Steel Sodium Loop

The behavior of fission products in sodium system was studied using a cold trap installed in the Toshiba Fission Product Loop, which is a stainless steel inpile sodium loop. The fission products were generated within the liquid sodium by irradiating UO² fuel under sodium convection. The cold trapping behavior of ⁹⁵Zr, ⁹⁵Nb, ¹³²Te, ¹³¹I ¹³⁷Cs and ¹⁴⁰La (¹⁴⁰Ba) was examined by measuring the γ-rays in the cold trap and in the sodium dump tank by means of Ge(Li) detectors. Such fission product nuclides as ^{131 132}Te and ¹³⁷Cs were found to be reversively removed by the cold trap at lower temperatures, with negligible hysteresis. The distribution coefficient K(cm) for the cold trap surface was found tobe expressible by log K=(3.08±0.17)-(0.0112±0.0006)×T(°C) for ¹³¹I, and by log K:=-(4.55±0.29) + (1830±1/140) × 1/T(K) for ¹³⁷Cs. The behavior of ¹³¹I and ¹³⁷Cs during cold trapping can be explained clearly by adsorption model using these K values. While ⁹⁵Zr, ⁹⁵Nb, ¹⁰³Ru and ¹⁴⁰La(¹⁴⁰;Ba) also were detected in the cold trap, the efficacy of cold trapping is far lower for these nuclides than for ¹³¹I and ¹³⁷Cs, and changing the cold trap temperature did not appear to have any appreciable effect on the cold trapping. Presence of ¹⁴¹Ce and ¹⁴⁴Ce was not detectable in the cold trap, although small amounts of these nuclides were detected in the sodium dump tank.     

Removal of oxides from sodium and tests for the oxide content

Oxides present in sodium,used as a heat carrier, accelerate the corrosion of the ducts and may cause blockage of the coldest sections of the circuit. The article gives the results of tests on cold oxide traps and work on an apparatus for determining the oxygen content in sodium. The results of the work can be used for planning experimental and industrial apparatuses in which sodium or sodium-potassium alloys are used as heat carriers.     

Modelling of hydrogen isotope retention in the tungsten divertor of EAST during ELMy H-mode

In this work, we study hydrogen isotopes (HI) inventory inside tungsten plasma-facing materials during high confinement mode discharges with repetitive edge localized modes (ELMy H-mode) based on the operating parameters of the EAST device, since tungsten is considered as the primary plasma-facing material and the ELMy H-mode is an important operation regime for EAST and future devices. The upgraded Hydrogen Isotope Inventory Processes Code (HIIPC) is applied with the incident depth profile provided by SRIM-2013 to make the study. The code is first verified by comparison with experimental measurements. The effects of the incident ion energy and ion flux on the retention are then studied. Finally, using the parameters obtained from EAST diagnostics, the HI retention inside the W divertor during ELMy H-mode is studied, which indicates the retained HI can be increased dramatically mainly due to ion-induced trap sites by ELMs.     

Impurity Transport in a Simulated Gas Target Divertor

Future generation fusion reactors and tokamaks will require dissipative divertors to handle the high particle and heat loads leaving the core plasma (100–400 MW/m² in ITER). A radiative divertor is proposed as a possible scenario, utilizing a hydrogen target gas to disperse the plasma momentum and trace impurity radiation to dissipate the plasma heat flux. Introducing an impurity into the target hydrogen gas enhances the radiative power loss but may lead to a significant impurity backflow to the main plasma. Thus, impurity flow control represents a crucial design concern. Such impurity flows are studied experimentally in this thesis. The PISCES-A linear plasma device (n 3 × 10¹⁹ m^–3, kT _e 20 eV) has been used to simulate a gas target divertor. To study the transport of impurities, a trace amount of impurity gas (i.e., neon and argon) is puffed near the target plate along with the hydrogen gas. Varying the hydrogen gas puffing rate permits us to study the effects of various background plasma conditions on the transport of impurities. A 1-1/2-D fluid code has been developed to solve the continuity and momentum equations for a neutral and singly ionized impurity in a hydrogen background plasma. The results indicate an axial reduction in the impurity concentration upstream from the impurity puffing source. Impurity entrainment is more effective for higher hydrogen target pressures (and for higher hydrogen plasma densities). However, if there is a reversal of the background plasma flow, impurity particles can propagate past the plasma flow reversal point and are then no longer entrained.     

An electrochemical hydrogen meter for measuring hydrogen in sodium using a ternary electrolyte mixture

An electrochemical sensor for measuring hydrogen concentration in liquid sodium that is based on a ternary mixture of LiCl, CaCl₂ and CaHCl as the electrolyte has been developed. DSC experiments showed the eutectic temperature of this ternary system to be ∼725 K. Impedance spectroscopic analysis of the electrolyte indicated ionic conduction through a molten phase at ∼725 K. Two electrochemical hydrogen sensors were constructed using the ternary electrolyte of composition 70 mol% LiCl:16 mol% CaHCl:14 mol% CaCl₂ and tested at 723 K in a mini sodium loop and at hydrogen levels of 60-250 ppb in sodium. The sensors show linear response in this concentration range and are capable of detecting a change of 10 ppb hydrogen in sodium over a background level of 60 ppb. Identification of this electrolyte system and its use in a sensor for measuring hydrogen in sodium are described in this paper.     

Development of Safety Assessment Method for Plasma Anomaly Events in Fusion Reactors

The objective of the study is to provide a safety assessment method for plasma transients including thermal response of in-vessel components. We developed a plasma physics model for safety analysis which has been implemented in a safety analysis code (SAFALY). The SAFALY code consists of a 0-D plasma dynamics model and a 1-D thermal behavior model of in-vessel components in the thickness direction. The code can treat hydraulic accidents using the results from a hydraulic code and analyze a passive plasma shutdown due to the impurity release from the wall. The overpower events in International Thermonuclear Experimental Reactor (ITER) were investigated, when the fueling rate and confinement improvement changes. The results show no significant damage to the confinement boundary of ITER is expected, as long as the cooling system works normally.