İ.T.Ü. TRIGA MARK-II eğitim ve araştırma reaktörünün nükleer güvenlik ölçümleri ile hava ve su aktivite seviyelerinin değerlendirilmesi

Abstract

Nükleer tesislerde ve radyasyonla çalışılan her tür ortamda nükleer güvenlik ölçümlerinin yapılması bir gerekliliktir. Bu gereklilik, yasal olduğu kadar radyasyonla çalışanların kendi güvenlikleri için zorunludur. Önemli bir nükleer tesis olan nükleer reaktörlerde de reaktörün gücü ve tipi ne olursa olsun, nükleer güvenlik ölçümlemelerinden ödün verilemez. Türkiye'nin tek Üniversite reaktörü olan, İstanbul Teknik Üniversitesi (İ.T.Ü.) Nükleer Enerji Enstitüsü'nde bulunan TRIGA Mark-II Eğitim ve Araştırma Reaktörü'nde nükleer güvenlik ölçümleri ulusal ve uluslararası mevzuata uygun olarak, sürekli, periyodik ve gerektiği zaman yapımaya devam edilmektedir. Bu tez çalışmasında, nükleer güvenlik ölçümleri çerçevesinde yapılan radyasyon ölçümleri farklı şekillerde ele alınmıştır. Öncelikle, dış çevre radyasyon ölçümleri verilmiştir. Bu ölçümler reaktör binası dışında yapılmaktadır. İ.T.Ü. TRIGA Mark-II Eğitim ve Araştırma Reaktörü'nün iç çevre ölçümleri kendi içinde iki farklı şekilde ele alınmıştır. Bunlardan biri; alan radyasyon ölçümleri olup, alan radyasyon monitörleri ile sürekli ölçüm olarak alınmaktadır, ikincisi ise yüzey radyasyon ölçümleridir ve portatif yüzey radyasyon ölçüm cihazları ile yapılmakta ve reaktörde farklı bölgelerde bu ölçümler gerçekleştirilmektedir. İ.T.Ü. TRIGA Mark-II Eğitim ve Araştırma Reaktörü'nde personel radyasyon ölçümleri, film dozimetreleri ve cep dozimetreleri ile alışılmış olduğu üzere yapılmaktadır. Reaktörde, şimdiye kadar alman personel radyasyon ölçümleri ve ölçümlenen dozlar istatistiksel olarak değerlendirilmiştir. İ.T.Ü. TRIGA Mark-II Eğitim ve Araştırma Reaktörü'nde numune alınarak yapılan ölçümler hava ve su numune alımları ile gerçekleştirilmiştir. Hava numuneleri sabit hava monitörü ile reaktör havuz suyu yüzeyinden emilen havanın gaz ve partikül aktivitesi olarak ve portatif hava monitörü ile alman numuneler olarak değerlendirilmiştir. Su numuneleri ise reaktör tankından ve ikinci devreden alınıp, rezidü elde edilerek yapılmaktadır. Su numuneleri değerlendirmeleri, hava numuneleri gibi reaktör çalışma sürecine bağlı olarak incelendiği gibi, birinci ve ikinci devre için karşılaştırmalı olarak da verilmiştir. Reaktörde, reaktörün kritik olduğu 1 1 Mart 1979 tarihinden bu yana alınan su ve hava numunelerine ilişkin olarak tüm veriler tablolar halinde bir araya toplanmıştır. Su ve hava numuneleri karşılaştırmalı olarak da değerlendirilmiştir. Bu şekilde reaktörün her çalışma şartı için fikir verilmiştir. İ.T.Ü. TRIGA Mark-II Eğitim ve Araştırma Reaktörü'nde yapılan tüm radyasyon ölçümleme sonuçlan ulusal ve uluslararası kabul edilmiş limitlerle karşılaştırılarak değerlendirilmiş ve nükleer güvenlik ölçüm değerlendirmesi olarak İ.T.Ü. TRIGA Mark-II Eğitim ve Araştırma Reaktörü'nün güvenilir olduğu bir kez daha bu yüksek lisans tez çalışması ile gösterilmiştir.

Nuclear safety is receiving much attention in the popular news media and technical journals and is often a topic of discussion and debate at meetings of civic clubs, professional societies, etc. The main concerns of the public are focused on; 1. Radioactivity in routine effluents and its effect on human health and the environment 2. The possibility of serious reactor accidents and their consequences 3. Transportation accidents involving radioactive wastes 4. The disposal of radioactive waste particularly high-level wastes, and 5. The possible theft of special nuclear materials and their fabrication process In response to these concerns the nuclear industry has adopted the defense-in depth approach to nuclear safety. This relies upon; 1. The careful design, construction and operation of nuclear facilities 2. The incorporation of systems to prevent such mulfunctions as do occur from turning into major accidents. 3. Built in systems to limit of maximum permissible levels doses.The defense-in-depth concept can be viewed as having some preventions; 1. Redundant sources of in-react or systems 2. Sensitive detection systems to warn of incipient events 3. Systems for automatic shutdown of reactors on a signal from monitoring instruments Here, the radiation effects are important. The radiation can have certain interactions with matter; both the matter and the radiation will be altered. If the matter undergoing the reaction happens to be a living organism, the effects of the interaction can damage the living tissue to the point of severe illness or death. Nuclear safety and health physics are concerned not only with the effects of radiation on living tissue but also with the detection of radiation that may be injurious to persons. Health physics evaluate permissible exposure levels of radiation for persons, device procedures and methods to protect indivuduals from excess exposure to radiation. It was not until the 1920s (over 25 years after X-rays were dicovered) that safety measures for handling radioactive materials were proposed. In the 1930s maximum permissible levels for exposure were set. The general acceptance of the maximum permissible exposure levels has kept down the incidence of radiation injuries. Unfortunately, radiation exposure causes no bodily sensation. This means that instruments must be used to survey the areas where radiation hazards may exist. A good surveymeter should respond in a manner proportional to tissue ionization produced. All areas around a reactor are monitored frequently to determine the radioactivity present on surfaces and in the air. Generally, two type radiation monitors are used for this purpose. These are: ¦ Area Monitors ¦ Survey Meters There is usually an instrument that continously monitors the radiation present in air. This type instruments are called as `area monitors`. In Istanbul Technical University (ITU) TRIGA Mark-n Training and Research Reactor, five area monitors are placed and radiation is monitored continously. If the radiation levels goes up to the maximum permissible levels, then they are signed the alarm.All wastes discharged from the reactor are monitored to insure that they do not cause an environmental hazard. So, environmental radiation monitoring is done outside the ITU TRIGA Mark-n Training and Research Reactor. Personnel are provided with dosimeters to measure the radition to which they are exposed. The dosimeter can be read as often as desired in an area where radiation levels high. Workers may be taken off a project before they receive more than a permissible exposure. Anyone exposed to an abnormal dose can be identified and measures must be taken to prevent further injury. Ionization chamber instruments are widely used for quantitative surveys since they respond to a wide range of energies. Geiger-Muller counter is a type of ionization chamber instrument. Geiger-Muller counters are used for area and survey measurements; since they respond to the number of ionizing events rather than energy, however they should not be used for quantitative measurements. A Geiger-Muller counter is a radition detector rather than an instrument for radiation measurement. In long-term monitoring of low levels of radiation, the dose rate is not particularly interesting. There are several different types of detectors which are suitable for personnel and environmental monitoring. Personnel monitoring devices; 1. Film-badges The effect of radiation exposure appears as a darkening of the developed film. 2. Pocket ionization chamber This type detector is direct reading type. It is particularly useful for keeping a running account of radiation exposure during work in high dose rate areas. 3. TLD dosimeter This type detectors are well suited to general personnel and environmental monitoring of X and gamma radiation. The other important safety measurement types are sample radiation maeasurements. In the reactors, two samples are taken that are air and water. In ITU TRIGA Mark-II Training and Research Reactor, air and water samples are taken and measured frequently.For the evaluation of the radiation measuremets in the nuclear safety activities, all the records of the measurements are evaluated together. Radiation measurements taken from five area monitors in ITU TRIGA Mark-II Training and Research Reactor are below the maximum permissible levels for public during the reactor operation. In addition to the area monitors, survey meters are used for evaluation of the radiation levels of the special places in the reactor, e.g., around the biological shield, on the demineralizer, on the reactor platform, on the surface of the thermal column and in the console room. Two of them (around the biological shield and in the console room) are always below the maximum permissible levels. On the reactor platform, radiation level is close to the maximum permissible level. On the surface of thermal column, the radiation levels over the maximum permissible levels, but the radiation dose goes down when it is measured far away from this surface. For the evaluation of the personnel monitoring, all the records are investigated and 98.5 % radiation workers of the ITU TRIGA Mark-II Training and Research Reactor has 0. 1 mSv during their radiation studies. Maximum level is 3.45 mSv for three mounth period that is below the 1/3 of maximum permissible level for the same period. Air samples have been taken with two different equipments in the ITU TRIGA Mark-II Training and Research Reactor and counted in the Health Physics Laboratory in the reactor. All the measurements for ITU TRIGA Mark-II Training and Research Reactor have been below the maximum permissible concentration since the reactor criticality date on 10 March 1979. Water samples have been taken from the pool surface for representation of the primary coolant system of the reactor. In addition to it, water samples have been taken from the cooling tower as a represantation of the secondary cooling system of the reactor. Variation via reactor operation time is investigated. Then it can be noticed that the secondary cooling system is not effected, but, in the primary cooling system of the ITU TRIGA Mark-II Training and Research Reactor, the radiation levels goes up via reactor operation time in full power condition. Radioactivity levels of the water samples from the reactor pool are evaluated for the energy in KWh produced in the reactor statistically and it can be seen that it is changing as a function of linearity.Results of the all radiation measurements and their evaluation according to the maximum permissible levels in the point of wiev of nuclear safety and public safety, it can be said that; ITU TRIGA Mark-II Training and Research Reactor has been operated in safe condition since the reactor criticality date on 1 1 March 1979.