Oral Session 1

Clay based disposal conceptsChair: Patrick Landais

International Meeting, March 14-18, 2005, Tours, FranceClays in Natural & Engineered Barriers for Radioactive Waste Confinement

O/01/1

THE JAPANESE APPROACH TO DEVELOPING CLAY-BASED

REPOSITORY CONCEPTSY. Sugita1, T. Fujita2, Y. Takahashi1, S. Kawakami2, H. Umeki1, M. Yui2,

M. Uragami1 and K. Kitayama1

1. NUMO, 4-1-23, Shiba, Minato-ku, Tokyo 108-0014, Japan2. JNC, Muramatsu 4-33, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan

The H12 repository concept for vitrified HLW was developed based on a multi barrier system with anemphasis on robust engineered barrier performance, to ensure its feasibility for a wide range of geologicalconditions typical of those observed in Japan [1]. Clay-based components (buffer, backfilling material,clay plug, etc.) play an important role in the performance of the H12 concept.

NUMO, which now has responsibility for the Japanese HLW repository programme, has adopted a volun-teering approach to the siting process. This results in special challenges for developing repository conceptswhich are tailored to the given siting environments [2]. Under such boundary conditions, maximumflexibility is required and thus a wide range of variants of the basic H12 design has been developed.These are complemented by a further range of alternative design options derived from the differentrepository development studies which have been carried out in many countries. To limit the range ofvariants considered in the “catalogue” of components which can be assembled to produce a repositorydesign, it was decided to focus on those with emphasis on the engineered barriers most studied to datein Japan – steel overpack and bentonite-based buffers, backfills & seals.

In order to evaluate the applicability of different designs to specific siting environments, NUMO hasestablished a set of “design factors” which classify the aspects which need to be considered when evaluatingthe pros and cons of different options:• Long-term Safety• Operational Safety• Engineering Feasibility / Quality Assurance• Engineering Reliability• Site Characterisation / Monitoring Requirements• Retrievability• Environmental Impact• Socio-economic Aspects

As discussed in more detail in [2], NUMO has developed two complementary approaches to the evaluationof applicability of design variants in different geological settings. A bottom-up method using specialmatrices allows individual components of a repository system (e.g. waste package, emplacement system,sealing system, …) to be assessed for relevant design factors under the range of rock mechanical, hydro-geological, geochemical and thermal conditions expected at potential sites. The top-down method, bycomparison, assesses the relative performance of complete repository systems in idealised “PotentialSiting Environments” using a form of multi-attribute analysis. Although both of these approaches havetheir own limitations, their use by teams from NUMO and supporting organisations have proved invaluablefor enhancing system understanding.

For the clay-based barriers, a Na-type bentonite from Japan is used as the reference material with addedsand content, density, water content, etc. as variable parameters. A special concern arises from compact

Page 9

International Meeting, March 14-18, 2005, Tours, FranceClays in Natural & Engineered Barriers for Radioactive Waste ConfinementPage 10

O/01/1

repository concepts, which have been developed due to possible constraints on repository layout set bylimitations on the repository footprint. This led to a study on the thermal stability of bentonite underdifferent degrees of saturation. The output of this study was integrated into a general evaluation on integrated disposal system which showed that a wide range of design options could produce highemplacement density [2, 3], including➜ Extended interim storage prior to disposal to keep temperatures below an arbitrary (but commonly

accepted) limit of 100°C➜ Conventional (“wet bentonite”) disposal systems with higher allowed temperatures (up to about 150°C)➜ Disposal in prefabricated modules which keeps bentonite dry for extended periods (with allowable

temperatures of 300°C or more)➜ Cavern disposal with delayed backfilling (requiring hundreds of years of institutional control).

With the possibility that a volunteer site could be located on the coast, the influence of saline waters on theproperties of bentonite-based barriers needs to be considered as a future task. Probably more important,however, are problems due to the interaction of hyperalkaline leachates from any cement-based engineeringmaterials with bentonite (and host rock) - as discussed in the recent international workshop co-sponsoredby NUMO and Posiva [4]. Here again a number of different approaches to solving this problem havebeen identified, including:➜ Developing models to robustly quantify bentonite alterations from such fluids➜ Minimising the problem by developing low-pH cements➜ Minimising the problem by using sacrificial buffers➜ Avoiding the problem by using designs which have no cement in safety critical areas.

Such theoretical studies are complemented by more practical investigations of performances of bentonite-based barriers as a function of the uniformity of raw material, the installed density and, in particular workingenvironment in the underground facility, e.g. high humidity. Here prefabrication concepts have beenidentified as interesting options as they may both minimise thermal problems as mentioned above butalso greatly assist operational robustness and quality assurance of the emplaced material.

Although initial focus has been on the waste package environment, the clay-based seals may also be keycomponents of the safety case. Here again practicality of implementation of idealised seals and the possibleinteractions of different engineering materials need to be considered. For seals, however, performanceneeds to be evaluated in the context of the 3-D repository layout, as illustrated in the main paper by ahydraulic analysis focusing on the intersections of the disposal tunnels and the main tunnel.

References:[1] JNC (2000): H12 Project to Establish the Scientific and Technical Basis for HLW Disposal in Japan,

JNC Technical Report TN1410 2000-001.

[2] NUMO (2004): Development of Repository Concepts for Volunteer Siting Environment, NUMO-TR-04-03.

[3] McKinley, I. Neall, F., Kawamura, H. and Umeki, H. (2004): Geochemical Optimisation of a DisposalSystem for High-level Radioactive Waste, J. Geochemical Exploration (in Press).

[4] Metcalfe, R. and Walker, C. (2004): Proceedings of the International Workshop on Bentonite-CementInteraction in Repository Environments, NUMO-TR-04-05.

International Meeting, March 14-18, 2005, Tours, FranceClays in Natural & Engineered Barriers for Radioactive Waste Confinement

O/01/2

BENTONITIC BUFFER PREFABRICATIONAND EMPLACEMENT

JL. Gaussen

Andra, 1-7 rue Jean Monnet, Parc de la Croix Blanche, 92298 Châtenay-Malabry Cedex, France

CONTEXTIn the framework of its evaluation of geologic disposal feasibility, Andra is considering the use of a pre-fabricated bentonitic buffer as a component of the disposal cell dedicated to the spent fuel. Such a conceptcould be also used as an alternative for the disposal of vitrified waste.This paper presents the justification of the buffer prefabrication method and sets forth a description ofits emplacement process. It underlines the fact that these methods facilitate the compliance with thespecifications and constitute a favorable factor in term of operational safety.

BUFFER SPECIFICATIONSThe main specifications of the buffer are the hydraulic conductivity (1.5 W/m/°C), the thermal conductivity(1.2 W/m/°C) and the gas permeability. The physical properties of the buffer (specific gravity, swellingpressure and composition) are derived from these specifications.

CHOICE OF PREFABRICATIONThe specificity of Andra’s concept rests in the fact that the buffer is prefabricated in a workshop andemplaced into the disposal cell during the construction process. The reasons which underlie theseschoices are essentially twofold. First, carrying out the necessary control of the compliance of a bentoniticblock prefabricated in a workshop is easier in comparison with the in-situ conditions. Second, Andraprefers, for operational safety reasons, to separate the construction activities from the nuclear ones.From a practical standpoint the material chosen is a mixture of sand and bentonite. The buffer consists ofa series of large diameter rings the dimensions of which are 2,30m in diameter and 0,50m in thickness.These rings are pressed until a specific gravity of 1,8 is reached.

EMPLACEMENT PROCESSThe emplacement process considered is based on the air cushion principle. A package of 4 rings is placedon a lifting pallet equipped with air cushion cells. The lifting pallet is pushed by an electric trolley suppliedin air and electricity by an umbilical. The trolley pushes the system “buffer rings + lifting pallet” intothe disposal cell and puts down the rings onto rails fixed on the lining.

DEMONSTRATION OF FEASIBILITYThe feasibility of this concept will be demonstrated through a full scale mockup fabricated in the frameworkof the European project ESDRED.

Page 11

International Meeting, March 14-18, 2005, Tours, FranceClays in Natural & Engineered Barriers for Radioactive Waste ConfinementPage 12