2nd Petrus-OPERA Conference on Radioactive Waste Management and Geological Disposal
78
Thermo-hydro-mechanical behaviour of compacted MX80 bentonite at
150°C
P. Stratos
Cardiff University, Geoenvironmental Research Centre (GRC)
Abstract
This paper presents the behaviour of compacted MX80 bentonite specimens when they are subjected at thermal and thermohydraulic gradients. The purpose of this work is to develop an improved understanding of heat and moisture movement in unsaturated clay barriers at temperatures above 100°C. Each specimen was statically compacted at 1.65 Mg/m3 and has dimensions of 300mm height and 100mm diameter. With the aid of a thermohydraulic column cell it was possible to apply boundary conditions that imitate the conditions in an underground repository. For the two on going tests undertaken in this study the applied temperatures were 25°C and 150°C at the top and bottom of each specimen. For applying the hydraulic gradient, water injected from the top of the specimen at a pressure of 600kPa. The test results show that, the temperature along the depth of the specimen stabilized in about 3-10 days whereas the relative humidity did not equilibrate. The exposure of the specimen at thermohydraulic gradient manifested a greater axial stress than that of the thermal gradient.
Introduction
The current concepts for engineered barriers in high-level radioactive waste (HLW) repositories suggest the usage of unsaturated compacted bentonite blocks as a buffer material. Main functions of the buffer are to protect the waste containers from rock movements by being ductile and retard the migration of ground water and radio nuclides in a repository [1]. Bentonite has been widely considered as an engineered buffer material for deep geological disposal of radioactive wastes because of its low permeability, good swelling capacity, chemical buffering capability, etc. The hydrothermal conditions in the repository can trigger physical/chemical processes that affect the favourable properties of the buffer and can decrease its effectiveness to isolate the waste canisters from the biosphere [2].
In order to investigate the behaviour of compacted bentonite when it is subjected at high temperatures and hydraulic pressures a multidisciplinary research programme commenced called SAFE Barriers project (a Systems Approach For Engineered Barriers). The experimental investigation of the thermal, hydraulic, mechanical and chemical behaviour of the bentonite when it is exposed at high thermal and hydraulic gradients and the improvement of existing numerical models for temperatures above the 100°C have been studied at Cardiff University.
Properties of the MX80 bentonite
Laboratory tests were performed on an industrial MX80 bentonite from Wyoming, US acquired by the TOLSA group. MX80 bentonite has a montmorillonite content around 85%. The initial water content of the bentonite was found to be 15.2% at ambient laboratory conditions. The liquid and plastic limits are 439% and 62% respectively.
Experimental device
The thermal and thermohydraulic tests were conducted in a column thermos/hydro/mechanical (THM) test device. The tests were carried out on compacted bentonite specimens. The experimental device used (Fig.1) is able to facilitate both thermal and hydraulic gradients. Three types of measurements can be taken during the testing period. Five relative humidity and temperature probes bored along the depth of the cell made possible to measure the humidity and the temperature along the depth of the bentonite specimen. The generated axial pressure can be measured at the top of the column device with the aid of a load cell and a plunger attached to it.
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to a heated p of the spec on ow the effect are describ radient (25 ° perature equi , 90, 72, 59 led to an im the areas cl er 4 days. W ed to a value yet. After 10 sure of 600 k were 110, 88 days the rela e water intru ssure to a va epresentatio mn cell nce on Radioa was statically ess took pla
extraction a lter the desir mperatures o d a constant circulating b imen with a p ts of thermal bed in this s C at the top ilibrated alon and 49°C at mmediate in oser to the With the applic
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rable dry den of 150°C at t t temperatur bath. For the pressure of 6 l and thermo section, a th end and 150 ng the depth t 40, 60, 120 ncrease of th heater (40, cation of the a for the ther rmal loading ater supply d d 45°C at 40 ity hasn’t eq elative humid 6 kPa. The a F Management a to form spe me cell body on of the spe nsity. For the the bottom e re of 25°C su tests subjec 600kPa. ohydraulic gr hermal and 0 °C at the b of the spec 0, 180 and 2 he specimen 60, and 12 rmal gradien rmal test with deionized w ecreased the , 60, 120, 18 quilibrate yet dity reached xial pressure Figure 2: Ex and Geologica cimens with y which was u ecimen in a d e first ten da end of the sp ustained with cted to hydra radients on t a thermohy bottom end o cimen within 40mm from ns’ relative 0 mm from nt the axial pr hin 21 days a water injected e temperatur 80 and 240m except of th d the 100%. e equilibrate perimental s al Disposal 300mm hei used for the different app ays only ther pecimen app h the aid of aulic gradien the compacte ydraulic test of the specim 96 hours. T heater respe humidity alo the heater) ressure at th and further d d at the top o res within th mm from the he area at 2 The hydrau after 60 day set-up ght and 100 thermal and aratus could mal gradient plied with the a circulating nt, deionized
ed bentonite . Both tests mens) for the he achieved ectively. The ong their full the relative he top end of decreased to of one of the e specimen. heater. After 240mm from ulic gradient ys at a value 0 d d t e g d e s e d e l e f o e . r m t e
2 Conclusion The behavi studied. Co boundary co gradient ach kPa. The co than the re tests. The a thermal gra Acknowled The financia Radioactive of the autho academic g References [1] Xiaodong Applied C [2] Chen Y considera 2014;72:6 Figure 3: E gradients. depths ver Figure 5: 2nd Petrus-OPE ns iour of com ompacted MX onditions we hieved with t onclusion dra lative humid axial pressur dient. dgments al support fro e Waste Man or’s PhD stu uidance is ve s g L, Prikryl R Clay Science. , Zhou S, H ation of coup 656-67. Effects of th Temperatur rsus time : Effects of t ERA Conferen pacted bent X80 bentonit ere 25°C at t the injection awn from this dity. 2) Axial re occurred a om Engineer nagement Lim udies, superv ery gratefully R, Pusch R. T 2011;52:419-Hu R, Zhou C pled thermo-h ermal and t re variations thermal and nce on Radioa tonite when e specimens he top end a of deionized s study are: pressures w at the latter
ring and Phy mited (RWM vised by Pro y acknowledg THMC-testing -27. C. Estimating hydro-mechan hermohydra s at specifie d thermohyd active Waste M 80 subjected t s with dry de and 150°C a d water was a 1) The temp were exhibit test exceede ysical Science ), is gratefull ofessor H R ged. g of three exp g effective th nical effects. aulic ed Fig the va draulic grad Management a to thermal a ensity 1.65 M at the bottom at the top of perature in th ed by both ed significan es Research ly acknowled R Thomas an pandable clay hermal condu International gure 4: Effec ermohydrau riations at s ients. Axial and Geologica and thermoh Mg/m3 were end of the s the specime he specimen thermal and tly the press
h Council (EP dged. The wo nd Dr S Trip s of potential uctivity of un Journal of cts of therm ulic gradient specified de pressure de al Disposal hydraulic gra used. The te specimen. T en with a pre equilibrates d therohydra sure recorde PSRC), UK a ork presente pathy. Their l use in HLW nsaturated be Heat and M mal and ts. Relative epths versus evelopment adients were ests’ thermal The hydraulic ssure of 600 much faster ulic gradient ed during the
and the UK’s ed forms part support and W repositories. ntonites with Mass Transfer. humidity s time t versus time e l c 0 r t e s t d . h . e.
