HALF-SCALE TEST: AN IMPORTANT STEP TO DEMONSTRATE THE FEASIBILITY OF THE BELGIAN SUPERCONTAINER CONCEPT FOR DISPOSAL OF HLW

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1 Proceedings of the 13 th International Conference on Environmental Remediation and Radioactive Waste Management ICEM2010 October 3-7, 2010, Tsukuba, Japan ICEM HALF-SCALE TEST: AN IMPORTANT STEP TO DEMONSTRATE THE FEASIBILITY OF THE BELGIAN SUPERCONTAINER CONCEPT FOR DISPOSAL OF HLW L. Areias SCK CEN/Euridice Mol, Belgium H. Van Humbeeck ONDRAF/NIRAS Brussels, Belgium A. Van Cotthem Technum-Tractebel Engineering Brussels, Belgium B. Craeye Artesis Hogeschool Antwerpen, Belgium W. Wacquier ONDRAF/NIRAS Brussels, Belgium G. De Schutter Ghent University Ghent, Belgium L. Villers Technum-Tractebel Engineering Brussels, Belgium ABSTRACT This paper presents results of a half-scale test performed by ESV EURIDICE, an Economic Interest Grouping between the Belgian Nuclear Research Centre (SCK CEN) and the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS). The primary objective of the test was to assess the feasibility of constructing the Supercontainer and to provide experimental data to validate modelling calculations obtained using the finite element program HEAT/MLS. The test focused on the early-age behaviour of the concrete matrix materials and the practical aspects of construction. Generally, the results obtained from the half-scale test confirm that it is feasible to construct the Supercontainer with currently available techniques. The results also validate scoping calculations obtained earlier with the finite element model. These findings contribute an important step to demonstrate the feasibility to construct the Supercontainer and to validate the Belgian Supercontainer concept proposed by ONDRAF/NIRAS for disposal of high level waste (HLW) in Belgium. INTRODUCTION The Supercontainer is the current Belgian reference concept designed by ONDRAF/NIRAS [1, 2] for the final disposal of heat emitting waste in Belgium. It is based on a multiple barrier system whereby every component of the Supercontainer plays a specific safety function or role requirement. In this reference concept vitrified HLW or Spent Fuel (SF), consisting of uranium oxide (UO x ) and mixed oxide (MO x ) fuel assemblies, are encapsulated in a watertight carbon steel overpack and cast in a concrete matrix. Figure 1 shows an artist's view of the Belgian Supercontainer concept. Keywords: Belgian supercontainer concept, high-level radioactive waste, half-scale test, concrete buffer, overpack, construction feasibility Figure 1 Belgian Supercontainer concept 1 Copyright 2010 by ASME

2 The Supercontainer will be constructed at the surface and disposed of in a deep geological clay layer. As shown in Figure 1, the concrete matrix comprises three buffer phases surrounding a carbon steel overpack. The long-term safety function of the overpack is to contain the radionuclides during the thermal phase, which will last several hundreds of years. The concrete buffers provide a structural confinement for the overpack as well as a radiological shielding during construction and transportation to the repository. This contributes to operational safety and minimizes handling operations in the underground repository. The concrete materials also create high alkaline conditions that favour a uniform corrosion of the metal overpack. The corrosion phenomenon of the overpack is a slow and well understood process. Optionally, a steel envelope may be installed around the outer buffers, as shown in Figure 1. An overpack will contain either two canisters of vitrified HLW; or 1 or 4 SF assemblies, as presented in Table 1. As shown, the Supercontainer concept foresees different sizes of containers depending on the type of waste it contains. Table 1 Weights and dimensions of the Supercontainer. Adapted from [3] Length Vitrified HLW 4065 mm UO X Max 6250 mm SF assemblies MO X 5550 mm OBJECTIVES One of the objectives of the half-scale test is to confirm the feasibility of constructing a Supercontainer. Another is to demonstrate that there are no potential flaws that hinder its performance. In addition, the test provides experimental data to validate modelling computations performed earlier as part of the B&C programme. This is done by monitoring the thermomechanical (TM) behaviour of the concrete buffer matrices during the different construction stages. These include casting of the concrete, insertion of a hot overpack, installing a filler and closing the Supercontainer with a lid. NUMERICAL MODELLING The finite element program HEAT/MLS [4-6] was used to study the early-age behaviour of the concrete materials during different stages of construction of the Supercontainer. This two-dimensional finite element program calculates stress and strength in a concrete structure using a state parameter approach linked to a material database. Although it is a two-dimensional tool, an accurate simulation is still possible when treating the Supercontainer as an axis-symmetrical structure. HEAT/MLS performs advanced thermal and mechanical computations and predicts expected performance with reasonably high accuracy. It calculates stresses due to temperature and humidity effects as well as strength in the concrete buffer. Values for material input parameters come from a database obtained from an extensive laboratory testing program. TEST SETUP The half-scale test [7] is a surface test performed on a scale model of a Supercontainer. The model has the same diameter and thickness but a reduced height of approximately half that of an actual Supercontainer (Table 1). Figure 2 shows a cross-section of the outer concrete buffer constructed for the half-scale test. Outer diameter 2020 mm 2150 mm 1650 mm Weight 32 t Max 70 t Max 29.5 t Canisters or SF assemblies The Belgian Research and Development (B&C) strategy [1-3] for the geological disposal of radioactive waste is a comprehensive, multidisciplinary programme based on an iterative process. It aims to develop a safe and feasible design for a geological repository and to acquire the necessary evidence to confirm it. One of the strategic issues focuses on demonstrating the feasibility of constructing the Supercontainer and integrating it in the overall repository scheme. Figure 2 Simplified section of outer buffer [dimensions in m]. Adapted from [4] 2 Copyright 2010 by ASME

3 As shown, the test model has an outer diameter of 2.11 m and a height of 3.45 m. The inner void space is 0.71 m in diameter and has a height of 2.05 m. The lid has a diameter of 1.11 m and a height of 0.70 m. When completed, the model weighs approximately 26 t. For comparison, Table 1 gives the dimensions and weights of the Supercontainer envisaged in the conceptual design. Figure 3 shows the outer and inner moulds used for the half-scale test. Both moulds are made of steel and are reusable. The outer mould consists of two half sections joined together by bolts for easy assemblage and dismantling. The inner mould is a one-piece element built with a conical shape to facilitate demoulding. The whole mould assembly rests on an elevated base to allow the structure to be moved after the test. Figure 4 shows the assembled moulds ready for casting. were inserted in small steel tubes to enable salvaging them for reuse after the test. Figure 4 Assembled moulds ready for casting Level A Z Z Level B 6 Figure 3 Outer (left) and inner mould sections X Level D X Instruments were installed in the concrete matrices to monitor temperature, strain and displacement. Sensors were also located outside, surrounding the half-scale test to monitor ambient air velocity and relative humidity. Different points were monitored, as shown in a typical instrumentation plan in Figure 5. The overpack was instrumented only to monitor temperature at various locations, both inside and outside, as shown. A metal container comprising a heat source was used to simulate the heat-emitting waste of the overpack. The container is made of carbon steel ASTM A 106 Gr B and has properties similar to the one specified in the current conceptual design of the Supercontainer. For the test, a container with an outside diameter of 508 mm and a thickness of mm was used. General details of the overpack appear in Figure 6. The heat source consists of four heater elements together capable of delivering a constant power of 300 W/m. This is equivalent to the power produced by vitrified HLW or SF assemblies when placed in an overpack. The heater elements 1 Level C Z X Level C Level B Level A Figure 5 Typical instrumentation plan [4] Z Level D The inner space in the overpack between the walls and the heating elements was filled with fine silica Mol sand to ensure a homogeneous heat distribution during the test. After filling with sand, the top of the overpack was sealed with a metal cover to prevent ingress of concrete during casting of Buffer phases 2 and 3. Figure 7 shows the overpack being installed in the half-scale test. X 3 Copyright 2010 by ASME

4 Heating element Concrete lid Filler space Heating element L=1.7 m Fill with Mol sand Overpack steel tube Stages 3 and 4 of the test were constructed together, in a single operation, using the same concrete material. In fact, the final design of the lid (Stage 4) and its interface with the other buffer materials is under investigation and will be tested in a future half-scale test. Hot cell Figure 6 Top view [left] and section through half-scale test showing details of overpack and heater elements [4] Stage1: Buffer 1 construction Stage2: Overpack insertion Stage3: Buffer phase2 Stage4: Buffer phase3 Figure 8 Cold and hot phases during the construction of a Supercontainer The test took 154 days in total to complete. This accounted for 91 days of curing for the outer buffer at ambient temperature conditions, followed by 39 days of heating and 24 days of cooling. Removal of the inner mould took place approximately 7 days after casting, while the outer mould was removed after 29 days. Figure 7 Overpack being installed in half-scale test [left] and view from top after installation CONSTRUCTION PROCEDURE The half-scale test aims to simulate the real-case construction scenario of a Supercontainer. As shown in Figure 8 and described hereunder, the Supercontainer concept is based on four distinct construction stages. Stage 1 is the construction of the outer buffer. This stage takes place outside a nuclear zone and is also referred to as the cold cell phase. Stage 2 is the insertion of the overpack containing the canisters or SF assemblies with the radioactive waste. In Stage 3 the space between the overpack and the outer buffer is filled with a concrete material, also referred to as the filler in this paper. Finally, Stage 4 is the closure of the Supercontainer with a concrete lid (Buffer phase 3). Due to the presence and handling of radioactive waste, stages 2, 3 and 4 are expected to be performed in a hot cell. The half-scale test performed for this study followed the same construction stages described above for the Supercontainer; except for the heat-producing radioactive waste, which was replaced by an electrical heat source, as described earlier. Table 2 lists the chronology of the four stages followed for the construction of the half-scale test. For practical reasons, Event Table 2 Chronology for the half-scale test Task Days Casting buffer 1 Ambient curing Stage 1 Removal inner mould Removal outer mould Stage 2 Insert hot overpack Stage 3 Cast filler Stage 4 Cast lid Heating Heating period Heat OFF End of heating Cooling Cooling period End test Stop test CHOICE OF CONCRETE MATERIALS The repository concept and geological characteristics of the reference host clay geological formations being considered for disposal of HLW and SF in Belgium necessitate well defined design specifications for the cementitious materials used in the Supercontainer in order to guarantee the safety of the disposal system. These are discussed below. The main component fulfilling the long-term containment safety function is the overpack. The three concrete buffers surrounding the overpack have to provide a favourable 4 Copyright 2010 by ASME

5 chemical environment. This environment consists of high alkaline conditions that favour a uniform corrosion of the carbon steel overpack. To create the required environmental conditions for the overpack, the cementitious materials must meet the following design specifications: Use of ordinary Portland cement (OPC) of type CEM I to prevent portlandite consumption and a subsequent lowering of ph. This cement possesses a limited heat of hydration, which reduces the risk of thermal cracking in the outer buffer; Low SO 3 and C 3 A content to avoid formation of dense hydrogarnet that can lead to swelling and cracking of the concrete; High sulphate attack resistance to offset sulphur species potentially present in the pore water of the clay host formation; Aggregates consisting only of limestone and containing not more than 2 % each of magnesium, silicon and aluminium in the form of oxides. This limits the risk of alkali aggregate reaction, which can result in expansion and cracking of the concrete; Organic additives (e.g. use of a superplasticizer) should be limited; Good workability to allow pumping; Sufficient tensile and compressive strength to resist design as well as accidental loads; Through-going cracks in the outer buffer have to be avoided since they could reduce the radiological shielding capacity of the buffer and facilitate the transport of aggressive species; Good quality, homogeneous and dense concrete is desirable; and A reasonable thermal conductivity k to sufficiently conduct heat produced by the exothermal hydration processes of the concrete and by the heat-emitting waste. As stated in the previous discussion, the principal function of the concrete buffer materials is to create a favourable chemical environment consisting of a high ph in order to limit the corrosion rate of the carbon steel overpack during the thermal phase. The OPC and limestone aggregates provide alkaline conditions with approximately a ph of 13. This high alkaline environment leads to passivation of the carbon steel overpack resulting in a reduction of the corrosion rate. The use of OPC, together with the exclusion of blending agents such as fly ash, silica fume and blast furnace slag, which tend to lower the ph, ensure the presence of a high ph. Two types of concrete are currently being considered for the buffer of the Supercontainer: a traditional vibrated concrete (TVC) and a self compacting concrete (SCC). Both are based on the composition requirements discussed later in this paper. Laboratory tests [8] showed that both types have similar characteristics and also meet all the requirements concerning strength, long-term durability and chemical compatibility for constructing the Supercontainer. For practical reasons, mainly concerning preparation and casting of the large quantities of concrete needed, the SCC composition was selected as the reference solution for the Supercontainer concept and was used to perform the half-scale test presented in this paper. CONCRETE COMPOSITION The concrete mixture used for the half-scale test is based on a reference mixture developed in the laboratory for the Supercontainer [7, 8]. This mixture was tested at a concrete plant before the start of the half-scale test. The reference composition and two test batches (TB1 and TB2) appear in Table 3. Table 3 Test batches and reference SCC composition [5] Test batch TB1 TB2 Date of batch Location 18/05/09 Concrete plant 25/06/09 Concrete plant Cement [kg/m³] Limestone 0/4 [kg/m³] Reference SCC - Laboratory WC 0/4 [%] Limestone 2/6 327 [kg/m³] WC 2/6 [%] Limestone 6/ [kg/m³] WC 6/14 [%] W 175 [kg/m³] W tot [kg/m³] Superplasticizer [kg/m³] Volume mixed [m³] The values of water content (WC) of the aggregates show significant variations between the two test batches and the laboratory SCC reference composition. The differences are due to the storage conditions of the aggregates at the plant, which stored the aggregates outside and thus exposing them to weather 5 Copyright 2010 by ASME

6 conditions. In contrast, the aggregates at the laboratory are stored inside, guaranteeing constant dry moisture conditions. Table 4 contains the compositions of three batches prepared for the half-scale test along with the SCC reference concrete for comparison. The differences in WC of the aggregates between batches are due to the storage conditions at the concrete plant, as previously discussed. Table 4 Composition of SCC used in half-scale test * [5] Date cast Location of mixing Batch 1 06/07/09 Concrete plant Batch 2 06/07/09 Concrete plant Batch 3 05/10/09 Concrete plant Reference SCC - Laboratory Cement [kg/m³] Limestone 0/4 [kg/m³] WC 0/4 [%] Limestone 2/6 [kg/m³] WC 2/6 [ %] Limestone 6/14 [kg/m³] WC 6/14 [%] W [kg/m³] W tot [kg/m³] Superplasticizer [kg/m³] (*) WC 0/4= water content of aggregate fraction; W=water; W tot=total water Batches 1 and 2 were used for casting the outer buffer concrete in Stage 1. The total volume needed for this buffer was 10.6 m 3. The plant mixed in total 12 m 3 of concrete in mixes of 2 m 3 each, the largest volume of the mixer at the plant. Two trucks, each carrying 6 m 3, transported the concrete to the laboratory where the half-scale test was performed. Travel time between the concrete plant and the laboratory took approximately 30 minutes over an asphalt road. The extra concrete was used for laboratory tests and for casting a block sample for additional testing. Batch 3 is the concrete used in Stages 3 and 4 for the construction of the filler and lid, respectively. For this batch, 2 m 3 of concrete were mixed, of which 1.2 m 3 was used to construct the filler and the lid and the rest for laboratory testing and casting of a large cylindrical sample for observation. RESULTS Properties of fresh concrete Table 5 contains the properties of fresh SCC concrete presented in Tables 3 & 4. Except for TB2 and Batch 3, all other mixes show slump values below the desired minimum of 650 mm, although the slump in these cases remains acceptable. The V-funnel values are generally smaller than the target value of 8 s, which indicate a good viscosity of the concrete. Sieve stability results, which give the segregation resistance of the SCC concrete, range between 1.7 and 14.6% and all fall under the maximum value of 15% recommended to avoid segregation. Measurements of passing ability are reported here for completeness. They are not applicable for the half-scale test since there is no reinforcement in the concrete and thus no obstacles are present that could interfere with its free passage. They are all lower than 0.8, which is the minimum value recommended to ease the passage of concrete through obstacles, such as reinforcement rebar. Table 5 Properties of fresh concrete for the half-scale test *. Adapted from [5] TB1 TB2 Batch 1 Batch 2 Batch 3 Reference SCC SF Concrete plant [mm] SF Laboratory [mm] VF Concrete plant [s] <8 <8 <8 8 VF Laboratory [s] PA [-] SS [%] Density [kg/m³] Air content [%] Density (28 days) [kg/m³] (*) SF=slump flow; VF=V-funnel; PA=passing ability; SS=sieve stability Density values vary between 2250 and 2375 kg/m 3. These values are smaller than typical values obtained for the reference concrete, which has an average density of 2390 kg/m 3. The relatively high values of air content measured for the concrete probably contributed to the observed lower density. We can conclude from the above properties that the concrete is self compacting. 6 Copyright 2010 by ASME

7 Strength Figure 9 presents compressive strength results for the reference SCC concrete together with final batches (FB1 and FB 2) delivered for the construction of Stage 1. The same figure also shows the results for test batches TB1 and TB2, for comparison. The results shown are average values of three tests performed using 150 mm cube samples (f ccub150 ) at different ages (1, 2, 3, 7, 14 and 28 days). Cubes TB1 and TB2 cured under ideal controlled conditions of 20 C and 90 % relative humidity. Samples FB1 site and FB2 site hardened at the test site. FB1 is a control sample of FB1 site cured under controlled conditions of 20 C and 90 % relative humidity. tensile strength measured from the test (f ctsp ) was corrected to give the approximate pure tensile strength (f ct ) using fct=0.9fctsp, as suggested by Taerwe [9]. The results given for TB1, FB1 and SCC confirm the trend observed earlier for the compressive strength, showing a lower mobilized tensile strength for the final batch in comparison with TB1 and the reference SCC f ct (MPa) f ccub150 (MPa) SCC TB1 10 TB2 FB1 Site FB1 FB2 Site Time (days) Figure 9 Compressive strength values for reference SCC and concrete for Stage 1 construction [5] The results show a significant strength loss of 14 % for TB1, 33 % for TB2, 30% for FB1 and 20% for FB2 compared to the reference SCC prepared in the laboratory. The high air content, together with a high degree of uncertainty in estimating water content of the aggregates, probably account for most of the observed strength loss. FB1 samples allowed to cure at the site matured faster and show higher strength at younger age and a faster strength development than the FB1 cured under controlled conditions. This is due to the ambient curing temperature, which is higher than 20 C present under controlled conditions. After 28 days, there is no difference in hardening maturity between the two samples. Compressive strength tests performed after 42 days on two samples taken from the half-scale test Stage 1 concrete at heights of 85 and 260 cm showed similar strength values of 40.6 and 38.7 MPa, respectively, both lower than the SCC values. The tests were performed on cylinders measuring mm in diameter and approximately 99 mm in height. Density measurements for the two samples gave similar results of 2265 and 2245 kg/m³, respectively. Splitting tensile strength (f ctsp ) determined for different ages for TB1 on 100 mm cubes appears in Figure 10. To account for the fixing of the failure plane in the tensile splitting test, the 1.0 SCC TB1 FB Time (days) Figure 10 Tensile strength for TB1 and reference SCC [5] Temperature Typical temperatures calculated with the program HEAT/MLS for Stage 1, together with corresponding values measured in the half-scale test, appear in Figure 11. The results shown are for point 1, located at levels B and D, as shown in Figure 5. The thin lines show the simulated results while the thick lines give the measured values. The simulated results slightly underestimate measured temperatures before and after peak hydration, although both show good agreement. Temperature ( C) Time (hours) B 1 D 1 TCB 1 TCD 1 Figure 11 Simulated [thin] and measured [thick] temperatures for Point 1 at Levels B and D during Stage 1 [5] 7 Copyright 2010 by ASME

8 The evolution of temperature during construction Stages 2 to 4 appears in Figures 12 and 13. The heating was switched off at 672 hours, as indicated by the sharp drop in temperature values in both figures. Figure 12 shows the temperature in the overpack measured at the heating elements (T heater ) and at the middle of the overpack given by thermocouple TCC 5, which is point 5 located at Level C (Fig. 5). The temperatures remain relatively stable during the test with averages of 250 o C at the heating elements and 150 o C in the middle of the overpack. As expected, both temperatures drop to ambient conditions shortly after the heating is switched off. Temperature ( C) C 1-9 C 2-8 C 3-7 TCC 1-9 TCC 2-8 TCC Theater TCC Time (hours) Figure 13 Predicted (thin) and measured (thick) temperature at Level C during hot cell stages and cooling period Temperature ( C) Time (hours) Figure 12 Measured temperature of overpack during hot cell stages and cooling period Average values of predicted and measured temperatures in the middle of the half-scale test (Level C) appear in Figure 13. The thick lines represent the measured values obtained during the test, whereas the thin lines show the simulated results calculated with the program HEAT/MLS. As shown in Figure 5, points 1-9 are located at the outer surface of the half-scale test, points 2-8 in the middle and 3-7 at the interface between the outer buffer and the filler. The trend obtained for the simulated results generally fits that given by the measurements, although their values differ in some cases. They show that simulated results slightly underestimate temperatures at the interface (points 3-7) but overestimate them at the middle of the buffer (points 2-8). Also, the initial temperature peak predicted by the simulations at the buffer/filler interface (C 3-7) is not measured by the thermocouples. Finally, the fit at the outside locations (C 1-9) of the buffer, as well as during the cooling period, is very good. In general, the model is known to be conservative, which partly accounts for observed deviations. Another reason may be the actual location of the thermocouples in the half-scale test. It is possible that they could have moved during casting operations, in which case their locations would not coincide with those used by the model. Early-age cracking The program HEAT/MLS was used to evaluate whether internal stresses created during hydration could lead to early-age cracking of the half-scale concrete structure. To do this, the program calculated tensile stresses developed as a result of thermal gradients and autogenous deformation while taking creep into account. The criteria used for evaluating potential cracking is the ratio of stress (S ii ) to tensile strength (f ct ), represented here by the relation S ii /(0.7 f ct )<1, which must remain smaller than one to prevent cracking. The safety limit factor of 0.7 can be explained by the long-term effect of tensile strength [5]. Stress-to-strength ratios S ii /(0.7 f ct ) were calculated for points at the middle as well as the inner and outer surfaces of the half-scale outer buffer. These areas represent critical regions for developing cracks. The results for Stage 1 of construction (Figure 8), using a reference ambient temperature of 20 o C, appear in Table 6. This table also shows calculated maxima for temperature, radial stress (S xx ), axial stress (S yy ), tangential stress (S zz ) and shear stress (S xy ) in the x-y plane. As shown, the highest risk of cracking occurs near the bottom, at the outer surface of the concrete buffer, where maximum values of 0.85 and 0.90 are calculated after 32 hours of hydration. These results suggest that no cracks are expected during this stage. These simulation results were confirmed by actual observations made during the half-scale test, which showed no signs of cracking during the first construction stage. However, microcracks were observed during Stages 2 to 4. These microcracks appear to be superficial and could result from desiccation of the concrete surface and/or thermal stresses due to the presence of the hot overpack. A test program is currently underway to investigate the exact nature and depth of penetration of these microcracks and their potential impact on safety issues. 8 Copyright 2010 by ASME

9 Other factors that could play a role in the formation of mircocracks, such as the thermal expansion of the overpack and its interaction with the inner buffer, or casting of the buffer under water, are being investigated. Table 6 Simulated maximum values for SCC stage 1 construction at 20 o C t max (h) * T max C S xx,max MPa S yy,max MPa S zz,max MPa S xy,max MPa S yy /(0.7 f ct ) S zz /(0.7 f ct ) (*) time in hours for reaching maximum U y (mm) E 1-2 LVE 1-2 Time (hours) Figure 14 Simulated [thin] and measured [thick] axial displacements at top of outer buffer during Stage 1 construction The maximum values in Table 6 are due mainly to the exothermal hydration reaction of the concrete during hardening. Once the hydration peak ceases, the concrete in these regions of the buffer come under compression and the risk of cracking diminishes. Similar calculations performed for the buffer phase 2 (Stage 2) indicate a maximum value of S zz /(0.7 f ct ) of 0.63 to 0.69 after approximately 277 hours of hydration, indicating that no cracking is expected in the filler. Finally, a maximum value of S zz /(0.7 f ct ) of 0.48 after 278 hours in the freshly cast lid (Buffer phase 3) also suggests that cracking will not develop in the early age. Radial and axial displacements Results of simulated (thin line) and actual (thick line) measurements of axial displacements at the top of the outer buffer during Stage 1 construction appear in Figure 14. The measurements of axial displacement started 68 hours after casting, when the concrete was hard enough to allow the installation of displacement transducers. As shown, the simulated results generally overestimate measured values, reflecting the conservative nature of the model. Similarly, simulated and measured results of radial (X) and axial (Y) displacements during construction Stages 2-4 appear in Figure 15. The results of radial displacement correspond to Levels A and C in Figure 5, whereas those shown for Level E, which is not shown in Figure 5, but which corresponds to measurements at the top of the outer buffer, are axial measurements. As noted before for temperature, the simulated results satisfactorily predict the evolution trend of measured values. They clearly show the expansion behaviour of the concrete structure during the hot phase, followed by its contraction in the cooling period after the heater is switched off. U x,y (mm) LVA LVC LVE 1-2 A C E Time (hours) Figure 15 Simulated (thin) and measured (thick) radial (A and C) and axial (E) displacements during construction Stages 2-4 GENERAL PRACTICAL EXPERIENCE GAINED FROM THE FIRST HALF SCALE TEST The first half-scale test provided valuable experience concerning the practical aspects of constructing the Supercontainer. One of these was the large scale nature of the project; and the relatively low mixing capacity of the concrete plant that supplied the buffer materials. As a result of the low mixing capacity at the plant, the concrete materials had to be prepared in several batches. This made it impossible to obtain a homogeneous mix for the half-scale test and caused delays during casting operations. Another practical aspect revealed from the test was the importance of using dry aggregates to better control the water content of the concrete mixes. An accurate control of moisture will also help adjust the quantity of superplasticizer and make it 9 Copyright 2010 by ASME

10 easier to obtain the desired slump, which contribute to a better quality of the concrete. Finally, a high capacity and a minimum level of automation at the concrete plant are needed to optimize production efficiency, quality and performance of the concrete materials. For best results the whole concrete mixing process should be automated; and the mixing capacity increased to minimize the number of batches required for casting. CONCLUSIONS Measurements and observations made during the half-scale test indicate that it is feasible to construct the Supercontainer with present day technology. At the same time, the test revealed areas that need improving to optimize construction, safety and design. The main conclusions can be summarized as follows: SCC mixtures prepared for the half-scale test showed large variations between batches, which points to difficulties in the mixing process. In particular, the limited capacity and lack of automation of mixing facilities at the plant require further consideration. Also, the difficulty in assessing the water content of the aggregates needs to be addressed; The high air content present in the different batches appeared to be related to the high amount of added superplasticizer. This could also contribute to the observed strength loss in the concrete SCC mixtures. A possible solution is to add more filler material or change the composition to reduce the amount of superplasticizer; Measurements performed during the half-scale test generally showed good agreement with simulations obtained with the help of finite element program HEAT/MLS. For the most part, the simulated results were conservative and confirmed the suitability of the HEAT/MLS program to evaluate the early-age behaviour of the concrete buffer materials for the Supercontainer; and Visual inspection of the outer buffer at the end of Stage 1 construction, after demoulding, showed no evidence of microcracks in the concrete surface. During Stages 2 to 4, however, microcracks appeared on the surface of the buffer. Preliminary investigations suggest that these microcracks are superficial and possibly developed due to desiccation and/or thermal stresses. Further investigations and testing are underway to confirm the nature of these microcracks. A possible measure to prevent desiccation is to leave the outer mould in place during the entire test. Another is to keep the surface of the outer buffer moist at all times. programme on geological disposal of high-level/long-lived radioactive waste. REFERENCES [1] ONDRAF/NIRAS, 2009, "The Long-Term Safety Strategy for the Geological Disposal of Radioactive Waste," ONDRAF/NIRAS report NIROND-TR E. [2] ONDRAF/NIRAS, 2010, "Feasibility strategy and Feasibility Assessment Methodology for the geological disposal of radioactive waste," ONDRAF/NIRAS report NIROND-TR E. [3] Wacquier, W., and Van Humbeeck, H., 2009, "B & C Concept and Open Questions," ONDRAF/NIRAS Report Rev 0, [4] Van Beek, A., Schlangen E., and Baetens, B.E.J., "2001, "Numerical model for prediction of cracks in concrete structures," Proceedings of the International RILEM Conference on Early Age Cracking in Cementitious Systems EAC'01, Haifa, Israel. [5] De Schutter, G., and Vuylsteke, M., 2004, "Minimisation of early age thermal cracking in J-shaped non-reinforced massive concrete quay wall," Engineering Structures 26, [6] Craeye, B., De Schutter, G., Van Humbeeck, H., and Van Cotthem A., 2009, "Early age behaviour of concrete supercontainers for radioactive waste disposal," Nuclear Engineering and Design 239, [7] Areias, L., 2009, "½ Scale Tests: Description," Euridice/SCK CEN internal report, Belgium. [8] Craeye, B., 2010, "Early-age thermo-mechanical behaviour of the Supercontainer for radwaste disposal," Doctoral thesis, Ghent University, Magnel Laboratory for Concrete Research, Belgium. [9] Taerwe, L., 1997, "Concrete Technology, Course material (in Dutch)," Ghent University, Magnel Laboratory. ACKNOWLEDGMENTS The authors gratefully acknowledge the technical help provided by C. Lefevre (SCK CEN) and S. De Bock (Ghent University) with instrumentation and installation during the half-scale test. N. De Smet (Ch. Kesteleyn) is thanked for providing the aggregates free of charge and for use of the concrete mixing facilities. This work was performed in close cooperation with, and with the financial support of ONDRAF/NIRAS as part of its 10 Copyright 2010 by ASME