Depremde hasar almış özel tasarım radyoaktif atık bertaraf tesisinin sezyum ve stronsiyum sızıntısı için modellenmesi ve risk analizi
Modeling and risk analysis of a custom designed radioactive waste disposal repository damaged in the earthquake for cesium and strontium leakage
Yazar: FURKAN ÇINAR
Danışman: PROF. DR. SEMA ERENTÜRK
İstanbul Teknik Üniversitesi / Enerji Enstitüsü / Nükleer Araştırmalar Ana Bilim Dalı / Radyasyon Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Nükleer Mühendislik = Nuclear Engineering
Dizin: Ekolojik risk = Ecological risk ; Nükleer atıklar = Nuclear wastes ; Radyoaktif atıklar = Radioactive wastes ; Risk modellemesi = Risk modelling

Özet:
Artan enerji ihtiyacının karşılanması amacıyla kaynakların çeşitlendirilmesi ve zenginleştirilmesi her geçen gün daha kritik bir hale bürünmektedir. Bu amaç doğrultusunda, Dünya çapında yaygın olarak tercih edilen çözümlerin başında nükleer enerjiden faydalanılması gelmektedir. Öte yandan nükleer enerji üretimi ve radyoaktif atık yönetimi kavramları birbirinden ayrılmaz bir ikili gibidir. Türkiye de, gerek enerji talebinin karşılanması gerekse enerji bağımsızlığı yolunda nükleer enerjiye yönelmektedir. Bu doğrultuda, Türkiye'nin hâlihazırda Mersin ili sınırları içerisinde inşaatı devam eden Nükleer Enerji Tesisi ve Sinop ilinde yapılması planlanan nükleer enerji yatırımları bulunmaktadır. Radyoaktif atık bertaraf tesisleri için risk değerlendirme çalışmalarının gerçekleştirilmesi Türkiye'nin nükleer yolculuğunun konu edildiği büyük resmin önemli bir parçasıdır.

Bütün bunlara ek olarak, son zamanlarda Türkiye ve çevre bölgelerinde küçük ve orta büyüklükteki depremlerin oldukça arttığı görülmektedir. Bu sebepten ötürü ilerleyen zamanlarda toplum nezdinde, inşa edilecek bir atık tesisinin sismik aktivite sonucu uzun vadeli güvenliğinin sağlanmasına yönelik endişelerin oluşabileceği öngörülerek bu tez çalışmasında, immobilize edilmiş radyoaktif atıkların deprem etkisi sonucunda yeraltı suyuyla temas etmesi durumu incelenmiştir. Yaklaşık 10 yarı ömre denk gelmekte olan 300 yıllık zaman dilimi boyunca gerçekleşecek Cs-137 ve Sr-90 radyonüklidlerinin salınım miktarlarının belirlenmesi ve sızıntı kaynaklı sağlık risklerinin hesaplanması amacıyla Goldsim Radyonüklid Taşınım modülü kullanılarak modelleme çalışması yapılmıştır. Modelde tasarlanan tesisten sızan radyonüklidlerin sırasıyla yakın alan, uzak alan, biyosfer şeklinde taşınması durumu incelenmiştir. Bu akış şeması, Nükleer Enerji Ajansının benzer çalışmalar için ortaya koyduğu kriterler göz önüne alınarak belirlenmiştir. 

Çalışmada incelenen bertaraf tesisinin, AFAD deprem haritasında sismik açıdan düşük risk bölgesi içinde yer alması ve Mersin'deki Nükleer Santrale olan fiziki yakınlığı sebebiyle Konya ilinin Karaman ile sınır bölgesine yakın bir noktada inşa edildiği varsayılmıştır. Bu sebeple, Cs-137 ve Sr-90 taşınım sürecindeki biyosfer bileşeni olarak yeryüzü seviyesinin 200 metre altındaki bir akifer ele alınmıştır. Ayrıca, bertaraf tesisi için beton gövde ve bentonit dolgu malzemesine dayalı yapısal bir tasarım da ortaya konmuştur. Gerçek ortamın jeolojik ve hidrolojik karakterinin temsil edilmeye çalışıldığı model üzerinde camlaştırılmış ve seramikleştirilmiş atık formları için iki ayrı senaryo simüle edilmiştir. Elde edilen verilerden yararlanarak yaşam boyu sağlık riskleri farklı cinsiyet ve yaş grupları için hesaplanmıştır. Sonuç kısmında ise her iki senaryodan elde edilen veriler birbirleriyle kıyaslanmış, tespit edilen uç değerler ilgili mevzuatlar ile kıyaslanarak ortaya çıkmalarının sebepleri üzerinde yorum yapılmıştır.

Abstract:
In order to meet the increasing energy need, the diversification and enrichment of resources become more critical day by day. For this purpose, one of the most preferred solutions worldwide is the use of nuclear energy. On the other hand, the concepts of nuclear power generation and radioactive waste management are like an inseparable duo. Turkey also turns its face to nuclear energy in order to reach its energy demand and to achieve energy independence. In this direction, Turkey already has a Nuclear Power Plant under construction within the borders of Mersin province, and nuclear energy investments are planned to be made in Sinop province. Carrying out risk assessment studies for radioactive waste disposal facilities is an important part of the big picture about Turkey's nuclear journey. The main reason for this situation is that the half-lives of the radionuclides in nuclear and radioactive wastes are in a wide range, extending up to the order of millions of years, and the effects of a possible disaster pose vital risks to public health for generations.

Deep geological burial is the widely accepted main disposal concept for long half-life wastes with radioactivity above a certain level and expected to be active for long periods. The desired result is a long-lasting, passively safe system that imposes no maintenance burden on future generations and does not cause a significant radioactivity effect on the surface environment. The main concern of this approach is the adequate assessment of geological processes and material properties to ensure that the effectiveness of the systems will be at the desired level for centuries.

In addition to all these, it is seen that small and medium earthquakes have increased considerably recently in Turkey and its surrounding regions. For this reason, in the thesis study, it is foreseen that in the future, there may be concerns about the long-term safety of a waste facility to be built as a result of seismic activity; the case of immobilized radioactive wastes coming into contact with groundwater as a result of earthquake effect was investigated. In order to determine the release amounts of Cs-137 and Sr-90 radionuclides that will occur during a 300-year period, which corresponds to approximately 10 half-lives, and to calculate the health risks due to leakage, a modeling study was carried out by using the Radionuclide Transport module of the GoldSim program.

One of the biggest challenges in such simulation studies is the inability to reflect the actual conditions exactly, and the unpredictability of the conditions that will occur due to unpredictable events such as unexpected changes in climatic conditions in the future. In the current study, the model is built on the assumptions that the climatic conditions remain constant and there is no change in the regime and chemistry of the groundwater.

In the model, the transport of radionuclides leaking from the designed facility in the form of near field, far field, and biosphere, respectively, was investigated. This flow chart has been determined by considering the criteria set forth by the Nuclear Energy Agency for similar studies.

For the analysis of radionuclide transport, it may be necessary to build subsystems to represent the hydrological and geological environment. At this point, the most important component that cannot be fully transferred to the model is the dynamic change of the environment in which the migration of Cs-137 and Sr-90 from the disposal facility to the biosphere occurs. What is meant here is the change in the structural parameters of the material such as porosity, void ratio, and hydraulic conductivity over time. In this respect, one of the uncertainties neglected in the modeling is that a homogeneous structure cannot be mentioned throughout the whole system. In particular, the part between the disposal facility and the biosphere, which constitutes the remote area representation, may contain subsystems that contain different geological environments.

It is assumed that the disposal facility examined in the study was built at a point close to the border region of Konya province with Karaman, due to its location in the low-risk zone in terms of seismicity in the AFAD earthquake map and its physical proximity to the Nuclear Power Plant in Mersin. For this reason, an aquifer 200 meters below the surface was considered as the biosphere component in the Cs-137 and Sr-90 transport process. Other parameters, other than seismic activity, that should be taken into account during the planning of radioactive waste disposal facilities are groundwater regime, rock structure, erosion, flood, volcanic activities, natural resources, and human factors such as population.

In addition to these, it is an indisputable fact that seismic activities not only cause physical damage to the facility structure but also have negative effects on the hydraulic conductivity of the geological environment, facilitating the transport of radionuclides.

Within the scope of the present study, a structural design for the disposal plant based on a concrete body and bentonite filler is discussed. In terms of material properties based on the modeling, it is predicted that the mechanical properties of the concrete, which forms the main body of the designed structure, will decrease over time, and parameters such as porosity and density have been taken below the normal values as in other studies in the literature. However, it is not possible to represent all the possibilities that may occur over a period of 300 years and have a negative impact on the material character. Another factor that is not reflected in the modeling is the fact that the activities during the construction of the facility will cause changes in the physical properties of the geological environment.

In the design of the Radioactive Waste Disposal Facility, a tunnel-shaped structure consisting of tabs containing 30 pieces of 200 L type stainless steel canister containing immobilized waste in each compartment is proposed. This design approach allowed a single segment of the facility to be considered for examining the leakage of Cs-137 and Sr-90 radionuclides as a result of the earthquake. Thus, it has been tried to represent a small part of a structure that repeats itself symmetrically more accurately by avoiding the uncertainties that may arise during the modeling of a large and complex structure.

On the model in which the geological and hydrological character of the real environment is tried to be represented, two different scenarios are simulated for vitrified and ceramic waste forms.

For the vitrified waste situation, it was evaluated that leakage occurred while 15 canisters completely contained 35SrO.65(30Na2O.70B2O3) and the other 15 canisters completely contained 20Cs2O.80(45Na2O.5P2O5.5PbO.45B2O3). For the case of ceramic form waste, it was evaluated that leakage occurred while 15 canisters were completely 0.56BN.0.44Cs and the other 15 canisters were completely containing 0.71(0.98ZE.0.02 Na2CO3).0.29Sr.

Thus, the simultaneous release of Sr-90 and Cs-137 radionuclides into the environment in both cases was reflected in the modeling. In the selection of vitrified and ceramic form wastes, it was assumed that the waste form with the least amount of extraction for both radionuclides was in the canisters.

Within the scope of the study, evaluation was made by performing separate simulations for vitrified and ceramic form wastes. Using the data obtained carcinogenic risks and lifetime cancer risk (ELCR) values were calculated for different gender and age groups. In the conclusion part, the data obtained from both scenarios were compared with each other, the determined extreme values were compared with the relevant legislation and comments were made on the reasons for their emergence.