مطالعه‌ي پايداري خصوصيات مهندسي و ريولوژيكي بنتونيت بر اثر تغييرات درجه حرارت در دفن زباله‌هاي اتمي

STABILITY OF ENGINEERING AND RHEOLOGICAL PROPERTIES OF BENTONITE DUE TO TEMPERATURE VARIATIONS IN RADIOACTIVE WASTE DISPOSAL

بنتونيت به‌دليل داشتن خصوصيات مهندسي ويژه، نظير قابليت تورم و جذب آب زياد، در طراحي مراكز دفن زباله‌هاي اتمي كاربرد دارد. پايداري خصوصيات بنتونيت، مهم‌ترين عامل در عملكرد مطلوب مركز دفن محسوب مي‌شود. در اين پژوهش با انجام يك‌سري آزمايش هاي رفتاري در دماهاي مختلف، سازوكار هاي تغيير رفتار بنتونيت مورد مطالعه قرار گرفته است. نتايج اين تحقيق نشان مي‌دهد كه خصوصيات مهندسي و ريولوژيكي (سيلان‌شناختي) بنتونيت تحت اثر افزايش حرارت تغيير مي‌كند. با افزايش گرما تا محدوده‌ي C°200، ظرفيت جذب و نگه‌داري آب بنتونيت حدود 15‌درصد و ميزان تورم خاك حدود 8 درصد كاهش مي‌يابد. براساس نتايج آزمايش هاي ويسكومتري و XRD، علت تغيير خصوصيات بنتونيت در مجاورت كوتاه‌مدت گرما را مي‌توان به تغيير آرايش پولك‌هاي رسي و جمع‌شدگي ماندگار ساختار خاك نسبت داد.

Bentonite is commonly used as a buffer material in high level nuclear waste (HLW) repositories due to its swelling and water adsorption properties. The stability of the engineering and rheological properties of bentonite is an essential factor in radioactive waste disposal projects. Such a change in properties might occur, due to the high temperature in HLW repositories. In spite of much research conducted on the stability of mechanical properties of bentonite, there has not been enough attention paid to the temperature impact on the stability of these properties. Therefore, the main objective of this paper is to focus attention on the stability of the engineering and rheological properties of bentonite, due to temperature variations in radioactive waste disposal. To achieve this objective, a series of mechanical and geo-environmental experiments were performed. These include Atterberg limit testing, suction experiments, XRD, viscosity measurement, and the swelling experiment. These experiments were performed after curing samples under different temperature conditions. The smectite soil used in this research was provided by the “Iran Barit Company”. The engineering analyses were conducted using the procedures described in the laboratory manual of the Geotechnical Research Center of McGill University and in the manual of EPA. The experimental results of this paper show that an increase in temperature will change the initial properties of bentonite. These changes mainly occur if the temperature goes above 100 C?. According to the achieved results, at temperatures above 200 C?, there will be 15 percent reduction in water retention and 8% reduction in the swelling of bentonite. In addition, based on viscosity and XRD results, the change in bentonite properties can be attributed to the change in attraction and repulsive forces among clay particles, which create a noticeable change in the microstructure of soil. Furthermore, according to experimental results, it is concluded that the impact of temperature changes on the micro-structural units of smectite will contribute to the building blocks for the macro-structure of the clay soil. Also, the influence of temperature on micropores and macropores forms the overall soil structure, which will change the permeability of heated smectite. Finally, it is concluded that the instability of bentonite properties at temperatures above 100 C? makes it necessary to control the temperature of radioactive waste under 100 C? prior to their disposal.