Farklı hidrojen-karbon ortamlarında indirgenen seramik yakıt tabletlerinin karakterizasyonu

Abstract

ÖZET Nükleer yakıt tabletlerinin kalite kontrolü ile ilgili çalışmaların devam ediyor olması bu konunun günümüzde de önemini devam ettirdiğinin bir göstergesidir. Uranyum dioksit taşıdığı özellikler nedeniyle tercih edilen bir nükleer yakıttır. Günümüzde uranyum dioksit tozu daha çok amonyum diuranat (ADU) ve amonyum uranil karbonat (AUC)'den elde edilmektedir. Uranyum dioksit tozlarının sinterlenmesi günümüzde yüksek sıcaklıkta (H2 atmosferinde, 1700-1750 °C de) ve düşük sıcaklıkta (CO2 atmosferinde, 1100 °C de) olmak üzere iki şekilde yapılmaktadır. Yüksek sıcaklık sinterlemesinin yüksek sıcaklık ve uzun süre gerektirmesi gibi dezavantajları nedeniyle düşük sıcaklık sinterlemesi ile ilgili çalışmalar başlatılmış ve günümüzde de halen devam etmektedir. Otoriteler düşük sıcaklık sinterlemesinde AUC'den elde edilen uranyum dioksit tozunu; presleme aşamasında yağlayıcı ve bağlayıcı gerektirmemesi, akıcı olması gibi nedenlerle tercih etmektedirler. Ancak AUC'den uranyum dioksit üretimindeki sıvı atık içindeki uranyum miktarının ADU'ya göre fazla olması büyük dezavantajıdır. Bu çalışmada ADU'dan elde edilen uranyum dioksit tozunun düşük sıcaklıkta sinterlenmesiyle kullanılabilirliği araştırılmıştır. Reaktör performansı açısından önemli olan tane yapısı ve dağılımı, gözenek yapısı ve dağılımı, yoğunluk, tane büyümesi ile ilgili çalışmalar yapılmış ve literatürde yapılan çalışmalarla karşılaştırılarak kullanılabilirliği araştırılmıştır. Uranyum dioksit bileşiği içindeki fazlalık oksijen (stokiyometrik olmayan yapısı veya UO2+X içindeki x) yakıta ve yakıt kınına zarar verdiğinden yapıdan alınması yani indirgenmesi gerekmektedir. Bu amaçla da sinterleme veya indirgeme atmosferlerinde hidrojen gazı kullanılmaktadır. Ancak hidrojenin pahalı ve tehlikeli olması gibi dezavantajları nedeniyle indirgeme atmosferindeki miktarının sınırlandırılması gerekmektedir. Bu çalışmada farklı H2/Ar indirgen atmosferler kullanılarak tabletin mikroyapısı, Oksijen/Uranyum oranı (O/U) ve yoğunluktaki değişmleri incelenerek indirgen atmosfer içindeki hidrojen için bir alt şuur getirilerek ekonomikliği ve güvenirliği sağlanmıştır. Bu çalışmanın sonucunda ADU'dan elde edilen uranyum dioksit tozunun düşük sıcaklıkta sinterlemesi ve düşük hidrojen konsantrasyonunda indirgemesi ile yeni bir ADU-UO2 çevrimi önerilmiştir. xu

SUMMARY THE USABILITY OF EX-ADU URANIUM DIOXIDE POWDER FOR LOW TEMPERATURE SINTERING AND THE EFFECTS OF LOW LEVEL HYDROGEN CONCENTRATION REDUCING ATMOSPHERE IN MICROSTRUCTURE In most of the water-moderated commercial nuclear power reactors, in operation or under construction, uranium dioxide (U02) is used as the fuel material. The selection of UO2 for this application is based primarily on its resistance to radiation damage and its excellent stability in high temperature water, Fuhrman at al. (1963).The good basic properties of U02 (high melting point, sufficient thermal conductivity, low thermal expansion) as well as its good behavior under irradiation (dimensional stability, fission product retention) and in the case of a fuel rod defect (compatibility with water) have made UO2 the most important nuclear fuel, Assmann and Stehle (1979). The standard method of incorporating UO2 in large reactors is in the form of pellets, which are usually produced by compacting a specially prepared oxide powder, derived from the decomposition of ammonium diuranate (ADU) and sintering in hydrogen at temperatures as high as 1700 °C. Such high sintering temperatures are necessary to attain the required high densities of the order of 95% of theoretical. High-density oxide pellets can also be produced by a low temperature sintering process which takes advantages of the benefical effect of nonstoichiometry on sintering behavior, Fuhrman at al.(1963). It is well known that the microstructure of UO2 fuel pellets plays a crucial role in determining their performance in the reactor. The most important microstructure controlled processes are densification, which is dependent on the amount and size distribution of small porosity (approximately less than 2 um in diameter); swelling, which may be ameliorated by the incorporation of large closed porosity into the microstructure, Littlechild and Butler (1985). X1UIn conventional ceramic technology, two characteristics of sintered ceramic products are desired in general for dense and high strength materials: highest bulk density and smallest grain size that can be obtained. This is not the case for sintered ceramic nuclear fuels. Assmann at al. (1984). It is desirable for nuclear fuel pellets to contain a certain amount of well-defined porosity and to have larger grain sizes, Assmann at al. (1984), Turnbull (1974).The performance aspects are classified into the following items : (1) fission-induced dimensional changes, that is, the superposition of densification and swelling; (2) fission gas release under steady-state and transient conditions; (3) pellet/cladding interaction Density and pore structure govern the dimensional behavior of the fuel. The grain structure may effect the mechanical properties of the fuel and the kinetics of fission product release during operation, Assmann and Dörr (1983). The kinetics of densification is governed by the lattice diffusion coefficient of uranium (U) which in UC^+x increases proportionally to x2. Stoichiometry of UOa+x/LLjCVy is controlled by the oxygen activity of the sintering gas without undue delay, since at 600 °C the mobility of the oxygen in these lattices is rather high. Because of the considerable enhancement of U diffusion in hyperstoichiometric uranium oxide, oxidative sintering can be performed at low temperatures (e.g. 1100 °C) and simultaneously during a short period of time. Reduction of the as-sintered uranium oxide is done by changing sintering atmospheres to H2 for a short period without any adverse effect on the integrity of UO2 pellet, Assmann at al. (1986). Microstructural characteristics of sintered U02 pellets-density, ratio of open to closed porosity, pore and grain structure-strongly influence the in-reactor performance of the fuel, e.g. irradiation-induced dimensional changes (densification, swelling), fission product behavior, and interaction with the cladding. The pore structure is characterized by open and closed porosity, shape and arrangement of pores, and pore size distribution. The percentage of open porosity does not constitute any critical quantity, although with closed pores the release of fission products is delayed and the production drying is facilitated. The microstructure of UO2 depends on the powder properties as well as on the pelletizing process, Assmann at al (1986), Assmann and Stehle (1979). Fabrication of uranium dioxide consists of powder production, precompacting, pressing, resintering and loading in reactor steps, Voilath and Wachtendonk (1984). The in-pile behavior of the fuel is influenced by the characteristic properties such as density, microstructure and resintering activity. These characteristics are formed during the sintering of green pellets. In addition sintering is usually the slowest XIVprocess step during pellet fabrication and it is sensitive regarding quality aspects. Therefore an optimization of sintering parameters may well increase the capability of the entire plant, if it allows a reduction in the sintering times, Güldner and Schmidt (1991). The types and composition of the sintering atmosphere influence the grain and pore structure, density and the retain of hydrogen gas of the pellet, Stuart and Adams (1975). The ratio of O/U of pellet is important, because it influences the thermal conductivity, the fission gas behavior and the fuel cladding. Therefore the nonstiochimetric pellets both in low temperature sintering and conventional high temperature sintering requires the reduction step following the sintering. The researches generally focus on this subject. The researches don't focus on the other effect (grain size, pore size and their distributions) of reduction gas. The German scientists developed a low temperature sintering at the beginning of 1980's. In this method the green pellets first sintered in CO2 atmospheres and then reduced in hydrogen atmospheres for a while. The uranium dioxide powder used in these study was prepared via the ADU (ammonium diuranate) at the Department of Nuclear Fuel Technology of Çekmece Nuclear Research and Training Center. The powder was mixed with 0.2% binder and compacted with lubricant under 300 MPa into cylindrical specimens of 10 mm in diameter and approximately 1 1 mm high. The powder compact showed the density in the range of 50%±1% of theoretical density. Compacts were heated at about 300 °C per hour in CO2 atmosphere and sintered at 1 150 °C for an hour and then cooled at sintering atmosphere. The sintered pellets were cut longitudinally and polished. After polishing the reduction of sintered pellets was carried out by different ratio of H2/Ar atmospheres (low level hydrogen concentration) at 1 100 °C for 1, 2, 3 and 6 hours. The O/U ratio of each pellet were determined by oxidation to U3O8 at 900 °C for 2 h in air and by calculation of the original composition from the weight gain. The optimum gas ratio for stoichiometry is determined and compared with literature. Grain and pore size distribution were determined quantitatively with the aid of an image analyzer. More than 1000 grains and pores were counted for the determination of the grain and pore size distribution. The absolute value of density and closed pore ratio of sintered and reduced pellets were done for each one by using Archimedes principle. The relation between mean grain size, mean pore size, O/U ratio, density, closed pore ratio, were searched and concluded. XVThe pore shape influence the thermal conductivity of materials. It is shown that the elliptical pores conduct less heat than spherical ones. The shape also form during the sintering of green pellets. In this work the influence of reduction gases on pore shape is investigated. The effect of reduction atmospheres on pore factor is searched. The grain size shape and distribution are searched under different reduction atmospheres which consist different ratio of hydrogen and argon gases. The grain size, its distribution and the shape factor is determined with image analyzer. According to some authorities the flowing property of AUC powder is more convenient than the ADU powders. The works about the low temperature sintering temperature is with the ex-AUC derived U02 powder. The researches searched the characteristics of AUC powder in low temperature sintering. In this work it is aimed to search the possibility of using the ADU powder by sintering low temperature sintering. The pore and grain size distribution of low temperature sintering (at 1150 °C for 4 hours in CO2 atmosphere-ex-ADU derived UO2 powder) and high temperature sintering pellets (at 1700 °C for 4 hours in pure H2 atmosphere ex- ADU derived powder) were compared. It is known that the duplex grain structure which consist of fine and coarse grain develops in the AUC (ammonium uranyl carbonate) process is sintered in CO2 atmosphere. The duplex grain structure shows a bimodel grain size distribution. The purpose of this work is to identify the feature of changes in the duplex structure of U02 ex-ADU (ammonium diuranate) that is sintered at low temperature sintering (1150 °C) and CO2 atmosphere. This work describes the grain and pore growth and the grain size distribution in the duplex grain structure, and compares these items with those in a uniform grain structure and bimodel grain structure of ex-AUC powder. Factors are discussed which influence the changes on the duplex grain structure. The hydrogen in sintering atmosphere has negative effects on uranium dioxide pellets such as hydride formation zirconium by reacting with clad material, decrease in strength, decrease the density and etc. So the usability of hydrogen should be limited in sintering and reducing atmospheres. With this work, it is showed that ex-AUC derived U02 powder is usable for low temperature sintering. The density, pore size distribution, grain size distribution is in coincidence with the high temperature sintering and low temperature sintering derived from AUC powder. It was shown that by using ex-ammonium diuranate powder high theoretical densities («95%) were obtained. And also it was showed that by using low XVIlevel concentration of hydrogen (in an inert gas) in reducing atmosphere does not effects significantly the grain and pore size distribution. Also it is shown that by using low level hydrogen concentration in reducing atmosphere supplies less density decrease, increases the ratio of closed porosity The high temperature sintering needs long sintering times, so it is not economical and during the production of ex-AUC powder it lefts more uranium in its liquid waste according to ex-ADU powder. So it can be said that the ex-ADU powder is convenient for low temperature sintering. XVU