發(fā)布時間：2018-05-28 22:32

  本文選題：熔鹽堆 + FLINAK鹽��； 參考：《中國科學院研究生院(上海應用物理研究所)》2017年博士論文

【摘要】：熔鹽堆(MSR)作為第四代核反應堆候選堆型之一,具有在線的燃料循環(huán)、熱功率密度高、固有安全性等諸多優(yōu)勢。氟鹽由于具有較小的熱中子吸收截面、較高的熱容、良好的流動性和導熱性能、很寬的液態(tài)工作范圍、良好的化學穩(wěn)定性等出色的熱化學性能及材料相容性而被選為熔鹽堆的冷卻劑和燃料鹽。然而氟鹽具有較強的吸濕性,其吸附的水分在高溫下易發(fā)生水解反應:H2O+ 2F → 2HF + 02-,水解產(chǎn)物HF在氟鹽中具有一定的溶解度,會與結(jié)構(gòu)材料發(fā)生氧化還原反應。研究表明,合金材料在氟鹽中的腐蝕主要是Cr元素的選擇性腐蝕:Cr + xHF →(x/2)H2 + CrFx。另一種水解產(chǎn)物02-在一回路中易與燃料鹽UF4發(fā)生相互作用而生成UO2沉淀:UF4+2O2-→UO2↓+4F-。核燃料沉淀在反應堆中不斷累積將導致局部過熱區(qū)域的形成,引發(fā)超臨界問題,給熔鹽堆的安全運行帶來極大威脅。目前,關(guān)于熔鹽堆腐蝕產(chǎn)物的存在形態(tài)及濃度分析、核燃料鈾的沉淀檢測都是定期對熔鹽堆取樣,并采用離線的化學分析法對氟化物熔鹽中腐蝕產(chǎn)物Cr、氧、核燃料U進行分析。通過濃度變化進而預估熔鹽堆中材料的腐蝕及核燃料的沉淀情況。所涉及到的步驟包括取樣、制樣、測試等,分析周期較長。發(fā)展在線檢測手段對氟熔鹽體系腐蝕產(chǎn)物的存在形態(tài)及濃度、核燃料與氧相互作用及氧化物溶解度進行測定顯得尤為重要�；谝陨媳尘�,本論文開展了如下研究:采用電化學技術(shù)(循環(huán)伏安法、方波伏安法)研究了 600 ℃時腐蝕產(chǎn)物Cr(Ⅲ)在FLINAK中的電化學行為,結(jié)果表明Cr(Ⅲ)的還原分兩步進行:Cr(Ⅲ)還原為Cr(Ⅱ),Cr(Ⅱ)還原為Cr。當FLINAK中僅加入Cr(Ⅱ)時,電化學曲線上出現(xiàn)了 Cr(Ⅲ)還原為Cr(Ⅱ)的信號,說明Cr(Ⅱ)轉(zhuǎn)化為更穩(wěn)定的Cr(Ⅲ)。通過掃描電子顯微鏡和X射線衍射技術(shù)證明Cr(Ⅱ)轉(zhuǎn)化為Cr(Ⅲ)是通過歧化反應3Cr(Ⅱ)= 2Cr(Ⅲ)+ Cr進行的。高溫拉曼光譜技術(shù)證明了腐蝕產(chǎn)物Cr的穩(wěn)定存在形態(tài)為[CrF6]3-結(jié)構(gòu)。通過開展靜態(tài)腐蝕實驗研究Cr(Ⅲ)、Cr(Ⅱ)與材料的相容性,結(jié)果表明熔鹽中Cr(Ⅲ)將加劇材料腐蝕,而Cr(Ⅱ)則可以抑制腐蝕。采用LECO氧分析儀和離子色譜儀研究了氟鹽中氧的存在形態(tài)主要為O2-和含氧酸根(S042-,NO3-,PO43-)。熔鹽總氧含量可以通過LECO氧分析儀測得,而熔鹽中的含氧酸根含量則由離子色譜測定,因此氟熔鹽中O2-離子濃度應從總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 ℃時FLINAK體系中O2-的電化學行為,結(jié)果表明O2-氧化為O2的電極反應是受擴散控制的過程,并建立了 O2-濃度與氧化峰電流的線性定量關(guān)系。利用該線性關(guān)系可在線測定氟鹽體系中O2-離子的濃度。采用方波伏安法測得Zr02在FLINAK熔鹽中的溶度積為(5.532-6.327)× 10-7 mol3/kg3。采用循環(huán)伏安法和方波伏安法研究了 600 ℃時FLINAK中UF4的電化學行為,結(jié)果表明U(Ⅳ)的還原分兩步進行:U(Ⅳ)還原為U(Ⅲ),U(Ⅲ)還原為U,且兩個反應在0.02-0.3 V s-1范圍內(nèi)均是受擴散控制的可逆過程。在0.01-0.084 mol kg-1范圍內(nèi),采用循環(huán)伏安法建立了 U(Ⅳ)濃度和U(Ⅲ)/U的還原峰電流之間的線性定量關(guān)系。當FLINAK-UF4體系中引入O2-后,由于生成UO2沉淀導致U(Ⅲ)/U的還原峰電流逐漸降低。UO2在FLINAK-UF4-Li2O體系中的溶度積為4.75× 10-6mol3/kg3,通過UO2的溶度積數(shù)據(jù)可以評估燃料鹽中所能允許含有的最大氧濃度。采用化學分析法研究了 600 ℃時FLINAK體系中ZrF4添加劑對UO2溶解度的促進作用,電感耦合等離子體發(fā)射光譜的結(jié)果表明FLINAK中UO2的溶解度為0.247 wt%,而添加ZrF4濃度至2.91 wt%時,UO2溶解度能達到最大值1.422 wt%。采用電化學方法(循環(huán)伏安法、方波伏安法、恒電流電解)和X射線衍射技術(shù)研究了 FLINAK-ZrF4體系中UO2的溶解行為,結(jié)果表明UO2將以U-O-F的絡合離子形式溶解。為了進一步探究ZrF4對核燃料沉淀UO2的抑制機理,我們采用LECO氧分析儀和拉曼光譜技術(shù)研究了 ZrF4與氧的相互作用機制,結(jié)果顯示當nZr/nO≤0.5 時,產(chǎn)生 ZrO2 沉淀;當 nZr/nO0.5 時,Zr02 溶解為[Zr2OF10]4-溶于氟鹽中,且氧的最大溶解度為0.02 mol kg-1。因此當熔鹽中的初始O2-濃度低于或等于0.02 mol kg-1時,添加過量的ZrF4可將自由形式的O2-完全轉(zhuǎn)化為絡合氧形式,從而抑制了 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.
【學位授予單位】：中國科學院研究生院(上海應用物理研究所)
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TL426

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