Chernobyl nükleer reaktör kazasının çay üzerine etkisi

Abstract

t ?J J. 1 / ÖZET, Bu çalışma altı holümden oluşur. Bu bölümler çok kısa ola rak şöyle özetlenebilir: 26 Nisan 1986 da Chernobyl nüklear reaktörünün 4 no.lu üni tesinde bir kaza oldu. Kaza sırasında oluşan radyoaktif bulut ların bir kolu Kafkasya üzerinden geçerek Doğu Karadeniz Bölge sine ulaşmış ve bölge üzerindeyken içeriğindeki uçucu fisyon ürünlerini (131I, lî2Te, 131tCs ve 137Cs) radyoaktif yağışla bölge üzerine indirmiştir. Yörenin ticari ürünlerinden özellikle en başta çay olmak üzere, fındık ve diğer ürünleri belirli oran larda radyoaktif olarak kirlenmiştir. îşte bu nedenle kirliliğin ve etkilerinin belirlenmesi bu çalışmanın amacı olmuştur. Genel şekilde, bir kaza sonucundaki besin kirlenme süreçle rini ve kazayı izleyen sürede bu besinlerin tüketiminden alınan iç dozu belirlemeyi amaçlayan AGRID ve RADFOOD besinzinciri mo delleri de ikinci bölümde özetlenmiştir. Üçüncü bölümde ise aktivite ve radyoaktif izotop belirleme düzenekleri tanıtılarak, çalışmanın deneysel içeriği anlatılmış tır. İzleyen bölümde ölçümlerde kullanılan `nokta kaynak geomet risi` varsayımı belirtilmiştir. Aktivite belirleme amacıyla kul lanılan düzeneğin sayma kalibrasyonu 137Cs kaynağı, radyoizotop belirleme amacı ile kullanılan 1024 kanallı çok kanallı analizö- rün enerji kalibrasyonu 137Cs e ek olarak, 60Co kaynağı yardı mıyla optimum şekilde yapılmıştır. Gerek piyasaya sürülmüş, ge rekse çay alanlarından toplanmış çay örneklerindeki ölçümler sonucu 1986 yılı ilk sürgünde ve 1985 veya 1986 paketleme tarih li birçok çay örneğinde 20-40 kBq/kg lık çok yüksek bir aktivi- teye rastlanmıştır. Aynı tarihlerde fındık üzerine yapılan ölç melerde ise 0,4-2,5 kBq/kg lık bir aktivite belirlendi, ölçümler 1987 yılı son sürgüne kadar sürdürüldü. Aynı tarihlerde piyasa daki çayların aktiflikleri ölçülerek, ölçülen değerlerin dağı nıklığının nedeni kestirilmeye çalışıldı. Diğer yandan çayın diğer ürünlere oranla bu ölçüde yüksek kirlenme nedeni araştı rıldı ve çeşitli olası faktörler öne sürüldü. Çayda kirliliğe yol açan radyoizotoplar olarak 137Cs ve 131*Cs belirlendi. Ay rıca bu izotopların örneklerdeki oranları yaklaşık olarak he saplandı. Daha sonra çayın tüketimi yoluyla alınan radyosezyum aktivi- tesinin bir ölçüsü olarak yıllık doz hesabı, yapıldı. Çeşitli varsayımlarla 0,71 mSv lik bir yıllık doz yetişkin bir kimse için hesaplandı. Bu dozun önümüzdeki yıllara yüklediği ek risk grubu ise yaklaşıkça belirlendi ve ek artışın gözlenemeyecek olduğu kestirildi. vı k-

t.J i : SUMMARY t ' This stwdy consists of six chapters. These are as follows: v Introduction, the food-chain models and the adaptabilities to this study, the experimental apparatus, measurements and calculations, the evaluation of consequences of the accident from the viewpoint of health effects, and results. These chapters can be briefly summarized respectively as below. On 26 April 1 986 an accident occured at the fourth unit of Chernobyl nuclear power station. Firstly in Sweden, it has been reported that sixteen different nuclides be observed in different proportions. Radioactive clouds, which consist of the volatile fission products (x 91l,1 ^Te,1 34Cs, and 137Cs), noble gases (85Kr,1 3lXe) and other radionuclides released from the reactor core have been reached on Aegean and Thrace regions of Turkey passing through the Balkans and also over the Caucasus through the eastern Black Sea region. The deposition of the radioactive material was variable greatly across the regions, depending mainly on whether or not it rained. Generally there was less of it in Aegean and Thrace regions during the passage of the cloud, whereas the eastern Black Sea region had continiously heavy rainfall. Thus, the eastern Black Sea region has been affected much more than all of the other regions and it was contaminated at the highest radiation level. In this manner, the determination of the radioactive contamination and its effects on the region are the purpose of this study. Therefore, all of the recent studies dealing with Chernobyl accident and its consequences are examined. But none of these publications have any results dealing with Turkey. The second chapter has three sections. In the first two. sections, several food-chain models are examined. There are many studies and food-chain models dealing with the continious routine releases from the nuclear power plants and the subsequent food contamination reports. However, the steady state approach is not suitable for the description of the chronic doses of the acute radioactivity releases, following the nuclear power plant accidents. Therefore, in addition to these which are taken into account two models are examined- briefly. The first one is the nutrition dose, model AGRID, developed by the Nuclear Engineering Laboratory of the Technical Research Centre of Finland and used to calculate the internal dose from ingestion of the contaminated food. The other is the RADFOOD model, developed by Soreq Nuclear Research Centre of Israel. In the last section, adaptabilities of the two models to this study are investigated. The third chapter has also three sections. The experimental apparatus for the determination of the activity and the identification of radionuclides has been introduced in the first two sections. Black diagrams of the measuring set of associated measures are illustrated. In this study, the activity determina tion system is called as `integral counting apparatus`. The most V Vllimportant module of the apparatus is the scintillation detector which has a Nal (Tl) scintillator. The multichannel analyzer, which has 1024 channels, is used for the identification of radioisotopes. So far many investigations have suggested the use of such a Nal(Tl) spectrometer. Today, in many countries researchers are using scintillation detectors for this purpose. However, some problems which arose from this type of detectors are given in the fourth chapter. The fourth chapter contain five sections. In the first section, it is explained that using the 137Cs and 60Co radioactive sources how the integral counting apparatus and the multichannel analyzer can be calibrated sensitively. The geometry used in the calculation of the activity measurements, is conveniently called as `the point source geometry`. Activities of dry tea samples, which are marketed by several firms and harvested directly from tea fields by the villagers, have been measured. Activities of the first shoot of the 1986 tea and the tea package dated 1985 and 1986 have been found as high as 20-40 kBq/kg. However, in the second and third shoots of 1986 tea these activities were found almost 10-25 kBq/kg less than the first shoot. In the same way activities of first and other shoots of 1987 tea were measured and found very low values of activities such as 0,5-5 kBq/kg. The reasons of this dispersion of the activity values are investigated. Of course, they are based upon very different factors. Since the tea consumption is usually via brewed tea, the activity transfer from the dry tea to the brewed tea was investigated. In the brewed tea which brewed for ten minutes it was observed that 50-65% of maximum transfer has been occured. As a result of this, the transfer rates as a function of the brewing time were determined, and an empirical function were found for the transferred activity and activities in the residue and the brewed tea were remeasured separately in order to see the initial activity is conserved whether or not. i Then, the radioisotopes which are the origin of this contamination were also investigated. The pulse height spectra of the contaminated tea samples were taken by the multichannel analyzer. Radioisotopes were identified from the results of the calculation on these spectra. These are the radiocesium isotopes (13I*Cs and 137Cs). At the end of the chapter, the interpretations of the models and experimental results are introduced. The fifth chapter divided; to five sections. There are many studies of the effects on ecosystem related with the radiocesium isotopes. It is well known that these isotopes are usually released from all the nuclear weapon tests and the reactor accidents. The first section deals with this subject very briefly. The calculation of annual dose is introduced in the second section. The necessary concepts and magnititudes are discussed for the calculation of annual dose. Thus, in addition to the physical half life, the biological half life, the effective half life as the mixing of physical and biological half lives and, viiieffective radius and the linear absorption coefficent for total body are defined. All of the numerical values of these concepts have been calculated or taken from several articles and technical reports. As a result of these, the dose conversion factors per activity of 1 Bq for 13l*Cs and 137Cs has been found respectively 1,03x10-8 Sv/year and 1,32x10-8 Sv/year. Then, the annual dose, for the adult person weighted 70 kg via the consumption of the brewed tea, has been calculated as 0,71 mSv. While calculating the dose as a measure of the intake radiocesium activity, the following was assumed. The adult person of 70 kg has tissue of 63 kg and for a day drinks five glasses of tea, each glass of it has been prepared from 2,5 gr dry tea. The tea sample considered has ah average activity of 20 kBq/kg. Concentrations of radiocesium isotopes in this tea sample are 57% for137Cs and 43% for 13tfCs, respectively. Our assumption is that the activity transfer rate from the dry tea to the brewed tea is 65%. In the literature, there are many practical formulae which have been suggested by several authors. The calculated dose values using these practical formulae were compared with the values of this study. It was found that the agreement is very good. The risk as a measure of health effects is calculated with the risk factors taken from ICRP and WHO. As a result of the annual dose of 0,71 mSv, associated health effects (lethal cancers and hereditary effects in the first two generations) were estimated for the whole 50-year period after the accident. The evaluation of the potential risk factor wouldf increase the cases to be considered. For about 585 or 709 additional cases occur with ICRP and WHO risk factors respectively. Consequently, the expected value of increases in the risk of stochastic health effects induced via tea consumption is low, and possibly couldn't be detected beside the other cases. ix