氟熔盐体系腐蚀杂质及氧化物溶解行为的研究

发布时间：2018-05-28 22:32

本文选题：熔盐堆 + FLINAK盐　；参考：《中国科学院研究生院(上海应用物理研究所)》2017年博士论文

【摘要】：熔盐堆(MSR)作为第四代核反应堆候选堆型之一,具有在线的燃料循环、热功率密度高、固有安全性等诸多优势。氟盐由于具有较小的热中子吸收截面、较高的热容、良好的流动性和导热性能、很宽的液态工作范围、良好的化学稳定性等出色的热化学性能及材料相容性而被选为熔盐堆的冷却剂和燃料盐。然而氟盐具有较强的吸湿性,其吸附的水分在高温下易发生水解反应:H2O+ 2F → 2HF + 02-,水解产物HF在氟盐中具有一定的溶解度,会与结构材料发生氧化还原反应。研究表明,合金材料在氟盐中的腐蚀主要是Cr元素的选择性腐蚀:Cr + xHF →(x/2)H2 + CrFx。另一种水解产物02-在一回路中易与燃料盐UF4发生相互作用而生成UO2沉淀:UF4+2O2-→UO2↓+4F-。核燃料沉淀在反应堆中不断累积将导致局部过热区域的形成,引发超临界问题,给熔盐堆的安全运行带来极大威胁。目前,关于熔盐堆腐蚀产物的存在形态及浓度分析、核燃料铀的沉淀检测都是定期对熔盐堆取样,并采用离线的化学分析法对氟化物熔盐中腐蚀产物Cr、氧、核燃料U进行分析。通过浓度变化进而预估熔盐堆中材料的腐蚀及核燃料的沉淀情况。所涉及到的步骤包括取样、制样、测试等,分析周期较长。发展在线检测手段对氟熔盐体系腐蚀产物的存在形态及浓度、核燃料与氧相互作用及氧化物溶解度进行测定显得尤为重要。基于以上背景,本论文开展了如下研究:采用电化学技术(循环伏安法、方波伏安法)研究了 600 ℃时腐蚀产物Cr(Ⅲ)在FLINAK中的电化学行为,结果表明Cr(Ⅲ)的还原分两步进行:Cr(Ⅲ)还原为Cr(Ⅱ),Cr(Ⅱ)还原为Cr。当FLINAK中仅加入Cr(Ⅱ)时,电化学曲线上出现了 Cr(Ⅲ)还原为Cr(Ⅱ)的信号,说明Cr(Ⅱ)转化为更稳定的Cr(Ⅲ)。通过扫描电子显微镜和X射线衍射技术证明Cr(Ⅱ)转化为Cr(Ⅲ)是通过歧化反应3Cr(Ⅱ)= 2Cr(Ⅲ)+ Cr进行的。高温拉曼光谱技术证明了腐蚀产物Cr的稳定存在形态为[CrF6]3-结构。通过开展静态腐蚀实验研究Cr(Ⅲ)、Cr(Ⅱ)与材料的相容性,结果表明熔盐中Cr(Ⅲ)将加剧材料腐蚀,而Cr(Ⅱ)则可以抑制腐蚀。采用LECO氧分析仪和离子色谱仪研究了氟盐中氧的存在形态主要为O2-和含氧酸根(S042-,NO3-,PO43-)。熔盐总氧含量可以通过LECO氧分析仪测得,而熔盐中的含氧酸根含量则由离子色谱测定,因此氟熔盐中O2-离子浓度应从总氧含量中扣除含氧酸根中的氧含量。采用方波伏安法研究了 600 ℃时FLINAK体系中O2-的电化学行为,结果表明O2-氧化为O2的电极反应是受扩散控制的过程,并建立了 O2-浓度与氧化峰电流的线性定量关系。利用该线性关系可在线测定氟盐体系中O2-离子的浓度。采用方波伏安法测得Zr02在FLINAK熔盐中的溶度积为(5.532-6.327)× 10-7 mol3/kg3。采用循环伏安法和方波伏安法研究了 600 ℃时FLINAK中UF4的电化学行为,结果表明U(Ⅳ)的还原分两步进行:U(Ⅳ)还原为U(Ⅲ),U(Ⅲ)还原为U,且两个反应在0.02-0.3 V s-1范围内均是受扩散控制的可逆过程。在0.01-0.084 mol kg-1范围内,采用循环伏安法建立了 U(Ⅳ)浓度和U(Ⅲ)/U的还原峰电流之间的线性定量关系。当FLINAK-UF4体系中引入O2-后,由于生成UO2沉淀导致U(Ⅲ)/U的还原峰电流逐渐降低。UO2在FLINAK-UF4-Li2O体系中的溶度积为4.75× 10-6mol3/kg3,通过UO2的溶度积数据可以评估燃料盐中所能允许含有的最大氧浓度。采用化学分析法研究了 600 ℃时FLINAK体系中ZrF4添加剂对UO2溶解度的促进作用,电感耦合等离子体发射光谱的结果表明FLINAK中UO2的溶解度为0.247 wt%,而添加ZrF4浓度至2.91 wt%时,UO2溶解度能达到最大值1.422 wt%。采用电化学方法(循环伏安法、方波伏安法、恒电流电解)和X射线衍射技术研究了 FLINAK-ZrF4体系中UO2的溶解行为,结果表明UO2将以U-O-F的络合离子形式溶解。为了进一步探究ZrF4对核燃料沉淀UO2的抑制机理,我们采用LECO氧分析仪和拉曼光谱技术研究了 ZrF4与氧的相互作用机制,结果显示当nZr/nO≤0.5 时,产生 ZrO2 沉淀;当 nZr/nO0.5 时,Zr02 溶解为[Zr2OF10]4-溶于氟盐中,且氧的最大溶解度为0.02 mol kg-1。因此当熔盐中的初始O2-浓度低于或等于0.02 mol kg-1时,添加过量的ZrF4可将自由形式的O2-完全转化为络合氧形式,从而抑制了 UO2沉淀的形成。
[Abstract]:Molten salt reactor (MSR), as one of the fourth generation nuclear reactor candidate reactors, has many advantages, such as on-line fuel cycle, high thermal power density, inherent safety and so on. Fluorine salts are excellent for their small thermal neutron absorption cross section, high heat capacity, good fluidity and thermal conductivity, wide liquid scope, good chemical stability and so on. The thermo chemical properties and material compatibility are selected as the coolant and fuel salt of the molten salt reactor. However, the fluorine salt has a strong hygroscopicity, and its adsorbed moisture is easily hydrolyzed at high temperature: H2O+ 2F to 2HF + 02-. The hydrolysate HF has a certain solubility in the fluoride salt, and it will redox reaction with the structure material. The corrosion of the alloy material in the fluorine salt is mainly the selective corrosion of the Cr element: Cr + xHF, (x/2) H2 + CrFx. another hydrolysate 02- can easily interact with the fuel salt UF4 in the first loop to produce the UO2 precipitation. The UF4+2O2- > UO2 +4F-. nuclear fuel precipitation in the reactor will lead to the formation of the local superheated region, and lead to the formation of the local superheated region. The supercritical problem poses a great threat to the safe operation of the molten salt reactor. At present, the existence form and concentration analysis of the corrosion products of the molten salt pile are analyzed. The precipitation detection of uranium in nuclear fuel is regular sampling of the molten salt reactor, and the off-line chemical analysis method is used to analyze the corrosion products of Cr, oxygen and nuclear fuel in the fluoride molten salt. Through the concentration change, the concentration changes are analyzed. Then the corrosion of the material in the molten salt reactor and the precipitation of the nuclear fuel are estimated. The steps involved include sampling, sampling, testing and so on, and the analysis period is long. It is particularly important to develop the on-line detection method for the corrosion products of the molten salt system and determine the existence of the corrosion products, the interaction of nuclear fuel and oxygen and the determination of the solubility of oxide. In the above background, the following studies have been carried out in this paper: electrochemical techniques (cyclic voltammetry (cyclic voltammetry, Fang Bo voltammetry) were used to study the electrochemical behavior of Cr (III) in FLINAK at 600 C. The results showed that the reduction of Cr (III) was two steps: Cr (III) was reduced to Cr (II), Cr (II) was reduced to Cr. when Cr (II) was only added to FLINAK. The signal of Cr (III) reduction to Cr (II) shows that Cr (II) is converted to a more stable Cr (III). Through scanning electron microscopy and X ray diffraction technique, it is proved that Cr (II) is converted to Cr (III) by the disproportionation reaction 3Cr (II) = 2Cr (III) + Cr. High temperature Raman spectroscopy proves that the stable existence of Cr corrosion products is [Cr. F6]3- structure. By conducting static corrosion experiments to study Cr (III), Cr (II) and the compatibility of the materials, the results show that Cr (III) in the molten salt will aggravate the material corrosion, while Cr (II) can inhibit the corrosion. The existing morphology of the oxygen in the fluorine salt is mainly O2- and oxygen containing roots (S042-, NO3-, PO43-), and the total molten salt of the molten salt. Oxygen content can be measured by LECO oxygen analyzer, and the content of oxyacid in molten salt is determined by ion chromatography, so the concentration of O2- ion in the total oxygen content should be deducted from the total oxygen content in the total oxygen content. The electrochemical behavior of O2- in FLINAK system at 600 C was studied by Fang Bo voltammetry. The results showed that O2- oxidation was O2. The electrode reaction is a diffusion controlled process, and the linear quantitative relationship between the O2- concentration and the peak current of oxidation is established. By using this linear relationship, the concentration of O2- ions in the fluorine salt system can be measured online. The solubility product of Zr02 in the molten salt of FLINAK by square wave voltammetry is (5.532-6.327) x 10-7 mol3/kg3. by cyclic voltammetry and square wave volt. The electrochemical behavior of UF4 in FLINAK at 600 C was studied by ANN method. The results showed that the reduction of U (IV) was carried out in two steps: U (IV) was reduced to U (III), U (III) was reduced to U, and the two reactions were reversible in the range of 0.02-0.3 V s-1. (III) linear quantitative relation between the reduction peak current of /U. When O2- is introduced in the FLINAK-UF4 system, the reduction peak current of U (III) /U decreases gradually and the solubility product of.UO2 in FLINAK-UF4-Li2O system is 4.75 x 10-6mol3/kg3 due to the formation of UO2 precipitation, and the maximum oxygen allowed in the fuel salt can be evaluated by the solubility product data of UO2. Concentration. The effect of ZrF4 additive on UO2 solubility in FLINAK system at 600 C was studied by chemical analysis. The results of inductively coupled plasma emission spectra showed that the solubility of UO2 in FLINAK was 0.247 wt%, and when the concentration of ZrF4 was 2.91 wt%, the solubility of UO2 could reach the maximum value of 1.422 wt%. using electrochemical method (cycle). The dissolution behavior of UO2 in the FLINAK-ZrF4 system was studied by voltammetry, square wave voltammetry, constant current electrolysis and X ray diffraction. The results showed that UO2 would dissolve in the form of U-O-F complex ions. In order to further explore the inhibition mechanism of ZrF4 for nuclear fuel precipitation UO2, we used LECO oxygen analyzer and Raman spectroscopy to study ZrF4 and oxygen. The interaction mechanism shows that ZrO2 precipitation is produced when nZr/nO is less than 0.5, and when nZr/nO0.5, Zr02 dissolves into [Zr2OF10]4- dissolved in fluorine salt, and the maximum solubility of oxygen is 0.02 mol kg-1., so when the initial O2- concentration in the molten salt is lower than or equal to 0.02 mol kg-1, the addition of ZrF4 can completely convert the free form O2-. The formation of UO2 precipitates was inhibited by the complexation of oxygen.
【学位授予单位】：中国科学院研究生院(上海应用物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TL426

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