Sezyum (cs) Ve Stronsiyum (sr) İçeren Atıkların İmmobilizasyonu İçin Seramik Matrislerin Geliştirilmesi

Abstract

Radyoaktif maddelerin başta nükleer reaktörler olmak üzere, kullanıldığı her alanda radyoaktif atıklar oluşmaktadır. Radyoaktif atıkların yönetiminde çevre ve insan güvenliğinin korunması ulusal idarelerin kontrolü altında olsa da temelde uluslararası işbirliği ile oluşturulan ilkeler ve yükümlülükler altında bir denetim mekanizması söz konusudur.Nükleer reaktörlerde enerji üretimi sonucunda ortaya çıkan yüksek düzey radyoaktif atıkların bertarafı radyoaktif atık yönetiminin en önemli konularındandır. Yüksek enerjili ve uzun yarı ömürlü radyonüklitleri barındıran bu atıkların kalıcı olarak saklanması gerekmektedir. Atıkların kalıcı saklama öncesi, taşınmasının, saklanmasının ve depolanmasının kolay hale getirilmesi için katı bir forma dönüştürülmesi tercih edilen bir yöntemdir. Katılaştırma işlemi ile matris bir malzeme içine hapsedilen radyonüklidlerin çevreye salınımı ve çevreyle olası etkileşimi minimize edilmiş olur. Seramik malzemeler, nükleer teknolojinin birçok alanında olduğu gibi radyoaktif atıkların immobilizasyonu için de kullanılan malzemelerdir. Bu alanda kullanılan geleneksel malzemeler olan borosilikat camlara karşı yüksek kimyasal ve mekanik dayanım gibi üstünlükleriyle seramikler, son yıllarda yapay ve doğal olarak geliştirilmeye devam edilmektedirler. Bu çalışmanın amacı, nükleer atık yönetiminde önemli bir yeri olan fisyon ürünlerinden Sezyum ve Stronsiyum’un immobilizasyonu için temel beklentileri sağlayabilen, düşük işlem maliyetli, düşük sıcaklıkta hazırlanabilen, kimyasal kararlılığı yüksek, özütlenebilirliği düşük seramik matrisli malzemeler geliştirmektir.Bu amaçla, immobilize edilmesi düşünülen fisyon ürünlerinin kalıcı saklama alanlarında tercih edilen doğal mineraller olan zeolit ve bentonit seramiklerin ana matrisi olarak kullanılmıştır. Sezyum ve Stronsiyum’un kimyasal kararlı tuzları kullanarak üretilen atık içeren seramiklerin elastik sabitleri ultrasonik metot ile belirlenmiştir. Yaşlandırma işlemine tabi tutulan seramiklerin kimyasal kararlılığı araştırılmıştır. Atık immobilize edilmiş seramiklerin atık formu olarak performanslarına ilişkin detaylı bilgi, yapısal karakterizasyon testleri ile sağlanmıştır. Seramik malzemelerden salınan Cs ve Sr miktarlarını belirlemek için ICP-ES ile seramiklerin özütlenme hızları bulunmuştur. Atık immobilizasyonunda kullanılan bir malzeme için en önemli kriterlerden biri radyonüklit salınımını minimize etmek olduğundan, üretilen seramiklerin atık formu olarak performansını değerlendirmek için özütlenme hızları esas alınmıştır.

There are 439 nuclear reactors in the world with a total installed power of 375.9 GWe and 13.5% of the world's electricity generation. By the time of 2030 its planned to have 331 more nuclear reactors to be set into operation. From a typical 1000 MW nuclear power plant produces approximately 30 tons of nuclear waste in a year. All around the world this number is around 12000 tons for a year. With constant growth of energy consumption and needs, these numbers are unavoidablely will increase.In all fields, foremost nuclear reactors, where radioactive materials are used radioactive wastes are produced. Even though public and environmental safety against radioactive wastes are usually under control of national authorities, there is an inspection mechanism which works under the principles and obligations established by international cooperation. One of the most important aspect of the nuclear waste managment is the disposal of the radioactive wastes that produced by nuclear reactors while energy is being produced. Contain highly energized radiaonuclides with a long half-life these wastes must storage permenantly. General way of disposal these wastes are burying them under great depths on earth which makes old mines perfect locations for this purpose. Geological formations like salt, basalt and tuff, because of their resistance against underwater, are suitable locations for geological repositories. These high level wastes which are storaged to be contained for thousands of years, need couple of hundred years to lost their activities. Solidification of the wastes before disposal is a preferred method for the purpose of immobilize the high level radioactive wastes. It is both safe and economical to convert wastes containing high-energy and long-lived radionuclides into solid waste forms by mixing them with chemically compatible binding matrix materials To convert the wastes containing highly energized, long half-life radionuclides into solid waste forms by mixing them with chemically compatible binding matrix materials has both safe and economical advantages. Trapped inside the matrix material by solidification, the possibility of radionuclides effect enviroment is minimized. Ceramic materials, used for many aspects in nuclear technology, are suitable for immobilization of the radioactive wastes. Against traditonal materials like borosilicate glasses which are broadly used in waste managment, ceramics, with their higher mechanical and chemical durability, are being produced both naturally and artifically in recent years. Zeolite and bentonite minerals because of their pore sizes and cation exchange capacities in their crystal structers are effective capturing radioactive ions. These minerals show selectiveness based on radionuclides types and are very effective at capturing certain type of radionuclides.137Cs and 90Sr are the most important fision products of spent fuels. Occupying big part of the high level wastes, one of the these radionuclides 137Cs has a half life around 40 years in human body. 90Sr ,on the other hand ,because of its high level β particules, damages bones and teeths on body when exposed. Contrary to other fision products with their long half life, high activity and ability to deal biological damage, its very important in nuclear waste managment to separate cesium and strontium from high volume of wastes and storage effectively. The aim of this study is to develop ceramic matrix materials have low-cost, high mechanically and chemically durability, low leach rate, low processing temparature, and qualifications to provide fundamental expectations for the immobilization of Cs and Sr from fission products that have an importance in nuclear waste management. With this goal in mind, natural analogue minerals zeolite and bentonite prefered as main matrix for ceramics because of their concord in geological formation, for permanent storage the fision products which are considered to immobilize. Produced by using Cs and Sr’s chemical stable salts, waste immobilized ceramics were tested for their physical and chemical durabilities.Material production was performed by liquid phase sintering. After the sintering process, it was observed how the phases were formed in the waste immobilized ceramics, and how the cesium and strontium were joined to the structure in two different mineral types. Determining elastic constants of waste immobilized ceramics is performed by ultrasonic method which is a non-destructive method. With acquired velocity value of longitudinal and shear waves, ceramics elastic moduli (E), and Poisson’s ratio (µ) values was calculated. Results showed that the Poisson’s ratio is independent from amount of loading waste, and does not change significantly for similar mineral and waste type’s compositions.It was observed that Elastic moduli of Sr immobilized ceramics is three times higher than Cs immobilized ceramics. For chemical durability test, aging method test was used. Inside distillate water at 90°C the ceramics were kept for a week, while being checked periodically for mass loss and change of pH value and their unit mass losses was calculated. As a result of chemical durability tests with waste immobilized ceramics, Sr immobilized ceramics with zeolite matrix have lower leach rates by comparison with the ceramics with bentonite matrix. Materials detailed information of performance as a waste form was acquired with further characterization tests after durability tests. The elemental analysis results with XRF showed that the bentonite mineral contained strontium in its own structure. Similarly, it is observed that the zeolite mineral contains strontium in its own structure, even though it is at a lower concentration than the bentonite. However, cesium was not found in the mineral structure. Amount of Cs and Sr released from ceramic matrices determined by finding leach rates of ceramics with ICP-ES. Because it is an important criteration to assess, leach rate was being taken as a basis for produced ceramics. Sr immobilized ceramic with bentonite matrix has maximum waste loading capacity which is 55% (wt). It was observed that the process temperatures of the produced ceramics are lower than the glass matrix immobilization materials unlike their waste loading capacities. Taking all the results into consideration, it has been determined that the ceramic structures prepared using clay minerals which is the preferred barrier materials of the nuclear waste disposal areas, have usage for immobilization of radioactive wastes.