Detection of Early Age Phenomena and Identification of Fracture Characteristics in the Second Scaled Model of the Supercontainer using combined Nondestructive Testing Techniques

The reference solution for the long term management of high-level and heat emitting radioactive waste is considered by ONDRAF/NIRAS (the Belgian Agency for Radioactive Waste and Enriched Fissile Materials) to be the geological disposal in poorly indurated clay. Before their transportation to an underground repository, the high level wastes (HLW) are post-conditioned in so called supercontainers. The supercontainers are cylindrical structures which consist of four engineering barriers that from the inner to the outer surface are namely: the overpack, the filler, the concrete buffer and the envelope. The overpack, which is made of carbon steel, is the place where the vitrified wastes and spent fuel are stored. The buffer, which is made of concrete, creates a highly alkaline environment ensuring slow and uniform overpack corrosion as well as radiological shielding. In-between the two materials the cementitious filler exists with slightly different material properties compared to the concrete buffer.
In order to evaluate the feasibility to construct such supercontainers two scaled models have so far been designed and tested by SCK/CEN (the Belgian Nuclear Research Centre). These cylindrical models have the diameter of a real supercontainer (2.11 m) but a height approximately half of the real dimension (3.45 m). The supercontainer concept comprises four construction phases that are simulated in the same way by the scaled models. The only difference is that instead of the high level and heat emitting waste, an electrical heat source delivering a constant power of 300 W/m is used. Concerning the other construction phases of the supercontainer and the scaled models, first the outer concrete buffer is cast and then the hot overpack (containing either waste canisters or heat elements), the filler and the lid are installed successively. The first construction phase is called the ''cold phase'' whereas construction phases two to four define the ''heating phase''.
The first scaled model that was performed to evaluate the feasibility to construct the supercontainer indicated crack formation on the surface of the concrete buffer. The absence of a crack detection and monitoring system precluded defining the exact time of crack initiation, as well as their origin, the penetration depth, the crack path and the propagation history. For this reason, the second scaled model test was performed to obtain further insight by answering to the aforementioned questions. In this model, reliable and inexpensive non-destructive testing techniques were incorporated. The present abstract comprises the results of three state of the art techniques; the Digital Image Correlation (DIC), the Acoustic Emission (AE) and the Ultrasonics Pulse Velocity (UPV) which were specifically adapted for this second test in order to detect and monitor the surface and through the depth fracture behaviour.
The experimental setup for the application of the DIC technique consists of three windows across the height of the scaled model. There, the so called speckle (black and white) patterns are drawn on the concrete buffer for the identification of the displacement and strain field. Two CCD cameras and a computer embedded software for the analysis of the captured images are also used. On the other hand, for the application of the AE technique four resonant sensors are forming a surface rectangular configuration on the four corners of the middle DIC window. These sensors are firstly applied to the mould of the concrete buffer and then, after its removal, directly to the buffer. Four amplifiers increase the amplitude of the detected signals that are then digitised, stored and analysed with the aid of a computer embedded software.
The paper presents results obtained from the combined use of the DIC and AE non-destructive techniques and the complementary application of the UPV technique both in-situ and in smaller scale (laboratory conditions) . The DIC and AE post processing analysis successfully identified the time of onset of cracks and the location of the first cracks formed on the surface of the concrete buffer as well as the depth and width of specific cracks and their evolution with time. Further results such as the Poisson's ratio (?) and the Young's modulus (E) and results related to the ''cold phase'' and early age phenomena (hydration, segregation of heavy aggregates, migration of gas and water, chemical reasons) are obtained with these techniques too.