Module # 1 - Buffer construction technology

Transcription

1 1. Overview Module # 1 - Buffer construction technology OBJECTIVES Definition of a common set of technical and scientific requirements for buffer composition and construction. Demonstration of the feasibility at an industrial scale of several buffer construction techniques. Demonstration of the feasibility to install and use not intrusive measurement techniques to monitor the performance of the EBS and of the host rock adjacent to the waste packages. JUSTICATION Long term safety, Robustness, Confidence Building, Monitoring. CONCEPTUAL APPROACH The module # 1 addresses two issues associated to the use of a swelling material either as an engineered barrier (in a disposal cell) or as a seal (in a drift) : 1.The definition of the technical and scientific requirements commonly shared by the European partners. 2.The development of two construction technologies based on two types of materials : Large prefabricated blocks Granular materials. In addition, the performance of the EBS and of the host rock adjacent to the (heat producing) waste packages will be monitored by non intrusive measurement techniques (wireless and/or geophysical). Such techniques will minimize the impact of the measurement (monitoring) equipment on the system performance (e.g. confinement capacity). P.S. :A future dismantling of the various buffers constructed and monitored is envisaged, but after the year WORK BREAKDOWN STRUCTURE WP(date) DESCRIPTION BUDGET(k ) WP1 (2004) INPUT DATA AND FUNCTIONAL REQUIREMENTS WP2 ( ) BASIC DESIGN OF SEVERAL BUFFER CONFIGURATIONS FULL SCALE DEMONSTRATION OF A PREFABRICATION WP3 CONSTRUCTION TECHNIQUE AND GRANULAR BUFFER ( ) INSTALLATION IN WORKSHOP WP4 ( ) WP4.1 FULL SCALE DEMONSTRATION OF THE BUFFER CONSTRUCTION IN URL(s) Task 1: In situ construction of granular/annulus buffer in URL at Mol Task 2: In situ demonstration of sealing performances in vertical borehole WP4.2 inurl at Mont-Terri WP5 ( ) TESTING NON INTRUSIVE MONITORING SYSTEMS WP6 (2008) EVALUATION AND FINAL REPORT TOTAL 5162

2 2. Module # 1 Buffer construction technology: description of technical concepts & demonstration objectives This module focuses on 3 ways of building a buffer around a waste canister and of backfilling the disposal drift. The sketch #1.1 herebelow resumes the 3 ways considered. Sketch #1.1. Basic design of Buffer Construction Prefabricated bentonite block Granular Bentonite Case 1 : Prefabricated buffer without annular backfilling Case 2 : Prefabricated buffer with granular annular backfilling Case 3 : Prefabricated buffer cradle with full section granular bacfilling The respective concepts and associated demonstration objectives are developped hereafter. a. Description of the prefabricated buffer and of the associated demonstration objectives In the French disposal concept, a disposal cell is a small dead end drift ( diameter around 3.2 m / 2.6 m) with a limited length (around 43 m / 46 m), outfitted with an engineered barrier (buffer) made of swelling clay (mixture of bentonite and sand). The main parts of the disposal cell, from the exterior to the interior are : the wall of the drift, a metal liner, an 800 mm thick engineered buffer and a permanent metal sleeve which receives the waste packages ( see sketch #1.2 ). The permanent liner is made of tube sections perforated to allow moisture from the formation to get to the engineered barrier and to saturate it ( see sketch #1.3 ). The engineered buffer is made of prefabricated rings of bentonite (70%) and sand (30%). This rings are molded with a press ( see sketch #1.5 ) and pre-assembled together in sets of 4 rings attached by a metal strap and protected by a metal corner bracing ( see figure n #1.4 ). The permanent metal inner sleeve holds the waste packages and enables their possible retrievability. The objectives of the demonstration for the prefabricated buffer are the following : to fabricate at scale one (following the design of the buffer configuration) a few rings (see figure sketch #1.6), to assemble them in sets as per the considered sketch, then to build a buffer mock-up at full scale in a workshop to check the design compatibility and proposed construction method with predefined clearances ( between the liner and the rings, the rings and the inner sleeve, the inner sleeve and the canister ) and predefined handling and guidance means, to adapt accordingly the clearances and the handling/guidance means.

3 Sketch #1.2 Conception of a disposal cell

4 Sketch # 1.3 Detailed Engineered Barrier Assembly Skech #1.4 - Set of pre-assembled bentonite rings

5 Sketch #1.5 Fabrication of bentonite rings (Picture courtesy Hydroweld - SKB/IC)

6 b. Description of the granular/annular buffer and of the associated demonstration objectives In the Belgian disposal concept,three alternative conceptual disposal designs have been developed, in line with the recommendations provided by the peer review of the SAFIR 2 report. These designs are the Super-container, the Borehole and the Sleeve design. One of the most promising options is the super-container which will be the reference design for future studies. The basic aims of this design are: to construct the different engineered barriers around the waste as much as possible in above ground conditions, thus facilitating the implementation of a QA/QC program, to enhance operational safety by separating nuclear and mining operations. The Super-container design provides radiological protection by means of a radiological shielding which permanently surrounds the waste canisters. An important requirement of the buffer material is that it be chemically compatible with the nearest components (i.e. overpack and host rock). Further, it should remain stable under high temperature conditions (up to 100 C), have about the same expansion coefficient as the surrounding metal barrel, have a sufficient thermal conductivity, and preferably have a density that can provide radiological shielding without a need to over-dimension the Super-container. The choice of the material for the buffer has not yet been made. Two options are being investigated: ordinary Portland cement (OPC) or an inorganic phosphate low ph cement (IPC). For mechanical rigidity, and in order to provide a casting form in the case of a cementitious material, the whole is packed in a metal barrel. This barrel also functions as a barrier that separates the buffer from corrosive agents resident in the Boom clay. As such, another function of the barrel is to provide chemical stability to the buffer material. Sufficient space between the outer radius of the Super-container and the concrete walls should be provided, thus allowing an easy movement of the Super-container through the length of the disposal gallery. This annular space between the super-container and the gallery lining will probably have to be filled with a backfill material. The envisaged diameter of the disposal galleries is 2.5 m. The sketch #1.6 below presents a cross-sectional representation of a Super-container, emplaced in a disposal gallery (total length probably 500 m to 1000 m). The same sketch also gives the reference choice of materials. Sketch #1.6 - Cross section of disposal cell in the Belgian concept

7 The objectives of the annular buffer demonstration are the following : to define and fabricate the annular buffer material ; to demonstrate the emplacement of this material, to verify whether this material complies with the requirements (swelling pressure, mechanical strength,..;) to demonstrate non intrusive monitoring techniques situated in the vicinity of the buffer The buffer material of the super-container (concrete) itself is not a part of this demonstrator. It is the backfill material in the annular gap surrounding the super-container that will be demonstrated. An illustration of the set-up of the demonstrator is represented in the sketch #1.7 hereafter : Sketch #1.7 Position of ESDRED Backfill test in PRACLAY

8 c. Description of the sealing performance test and of the associated demonstration objectives In a repository in low permeable host rocks such as clay (or rock salt) gas generation due to microbial degradation of waste forms or corrosion of waste containers can lead to an undesired gas generation. To avoid the build-up of higher gas pressures, seals of granular sand/clay mixtures with optimized permeability to gas (high) and liquids (low) were found in the GRS-Kenton project to be very promising. Such seals enable the gas to migrate out of the repository via the sand/clay seal and not in an uncontrolled way through the host rock which might be the pathway if the seals are made tight against fluid flow. The seals will function in drifts with irregular rough surfaces with normal cross section of about 4x4 m (sketch #1.8) as well as in disposal boreholes or other repository openings. For practical reasons, the sealing performance tests envisaged at Mt. Terri URL will be performed in vertical (and possibly inclined) boreholes (see sketch #1.9) electronic pressure gauge analog pressure gauge gas and water extraction water injection gas or water injection packer porous medium ( gas and water collection ) 1-3 m sealing with concrete or resin sealing with clay or clay mixture porous medium ( gravel or stone chips ) packer host rock 0,1-0,3 m Sketch # 1.8 : Drift seal system in a repository Sketch # 1.9 : Seal test set-up at Mt. Terri The main objective of the in-situ tests is the following : to demonstrate that the sealing potential determined in the Kenton laboratory project can also be achieved under the realistic conditions prevailing in situ.