Gens, A. et al. (2009). Ge ´otechnique 59, No. 4, 377–399 [doi: 10.1680/geot.2009.59.4.377] 377 A full-scale in situ heating test for high-level nuclear waste disposal: observations, analysis and interpretation A. GENS  , M. SA ´ NCHEZ†, L. DO N. GUIMARA ˜ ES‡, E. E. ALONSO  , A. LLORET  , S. OLIVELLA  , M. V. VILLAR§ and F. HUERTAS } The paper describes the performance, observations and interpretation of a large-scale in situ heating test that simulates a disposal concept for heat-emitting, high-level nuclear waste. In the experiment, heaters are emplaced in the axis of a tunnel excavated in granite to simulate the heat production of radioactive waste. The test is fully instrumented, and attention is focused on the thermo- hydro-mechanical (THM) behaviour of the near-field re- gion constituted by the compacted bentonite barrier surrounding the heater and the immediately adjacent rock. Interpretation of the test is assisted by the perform- ance of a coupled numerical analysis based on a formula- tion that incorporates the relevant THM phenomena. Initial and boundary conditions for the analysis as well as material parameters are determined from an extensive programme of field and laboratory experiments. The paper presents and discusses the thermal, hydraulic and mechanical observations in the bentonite barrier and in the host rock. Special attention is paid to the progress of hydration in the barrier, to the effects of heating and vapour transport, and to the development of swelling pressures in the barrier. After five years of heating, one of the heaters was switched off and the experiment was partially dismantled, allowing the final state of the bar- rier to be observed directly. The numerical analysis performed has proved able to represent the progress of the experiment very satisfactorily. In addition, predictions concerning the final state of the clay barrier are very close to the observations obtained during dismantling. The performance and analysis of the in situ test have significantly enhanced the understanding of a complex THM problem and have proved the capability of the numerical formulation to provide adequate predictive capacity. KEYWORDS: expansive soils; full-scale tests; numerical model- ling; partial saturation; temperature effects La pre ´sente communication de ´crit le de ´roulement d’un test d’e ´chauffement in situ simulant un concept d’e ´limi- nation de de ´chets nucle ´aires fortement radioactifs a `e ´mis- sion de chaleur, ainsi que des observations sur ce test et son interpre ´tation. Dans le cadre de cette expe ´rience, des appareils de chauffage sont mis en place dans l’axe d’un tunnel creuse ´ dans du granite, en simulant l’e ´mission de chaleur de de ´chets radioactifs. Ce test est entie `rement instrumente ´, et se concentre sur le comportement thermo – hydrome ´canique (THM) de la re ´gion de champ proche constitue ´e par la barrie `re de bentonite compacte ´e entour- ant les appareils de chauffage et la roche adjacente. L’interpre ´tation du test est assiste ´e par l’exe ´cution d’une analyse nume ´rique accouple ´e, base ´e sur une formule incorporant les phe ´nome `nes THM correspondants. Les conditions initiales et limites de l’analyse, ainsi que des parame `tres mate ´riels, sont de ´termine ´s a ` partir d’un vaste programme d’expe ´riences effectue ´es sur le terrain et en laboratoire. La pre ´sente communication pre ´sente et dis- cute des observations thermiques, hydrauliques et me ´ca- niques effectue ´es dans la barrie `re de bentonite, ainsi que dans la roche ho ˆte. Elle se penche tout particulie `rement sur l’e ´volution de l’hydratation dans la barrie `re, les effets du chauffage et du transport de la vapeur, et le de ´vel- oppement de pressions d’expansion dans la barrie `re. Au bout d’une pe ´riode de chauffage de cinq ans, on a arre ˆte ´ un des appareils de chauffage et de ´mantele ´ partiellement l’expe ´rience, afin de pouvoir observer directement la situation finale de la barrie `re. L’analyse nume ´rique effec- tue ´e s’est ave ´re ´e en mesure de repre ´senter de fac ¸on tre `s satisfaisante l’e ´volution de cette expe ´rience. En outre, les pre ´dictions formule ´es sur l’e ´tat final de la barrie `re d’ar- gile sont tre `s proches des observations effectue ´es au cours du de ´mante `lement. L’exe ´cution et l’analyse du test effec- tue ´ in situ ont renforce ´ conside ´rablement les connais- sances sur un proble `me THM complexe, et ont permis de de ´montrer la fac ¸on dont la formulation nume ´rique est en mesure de fournir une capacite ´ pre ´dictive ade ´quate. INTRODUCTION In many countries employing nuclear power, deep geological storage or disposal is an intensively studied option for the long-term confinement of heat-emitting, high-level nuclear waste (HLW) (Gens, 2003). The construction of deep reposi- tories will involve the excavation of a network of tunnels in a suitable host rock a few hundred metres below the surface (Fig. 1). The canisters containing nuclear waste will be placed either in horizontal drifts or in vertical boreholes. Repository designs for HLW resort to the multi-barrier concept to achieve the required degree of waste isolation. This is based on placing several barriers, both natural and artificial, between the potentially harmful radionuclides and the biosphere. The natural barrier is fundamentally the host rock, and the artificial barriers are the solid matrix of the waste itself, the metallic canister enclosing the waste, and the backfill—the sealing material placed around the canisters to fill the cavities where they are placed (Fig. 2). This backfill, also called an ‘engineered barrier’, is often con- structed using compacted expansive clay. Bentonite has generally been chosen because of its high swelling capacity, low permeability and favourable retardation properties. The engineered clay barrier and adjacent host rock Manuscript received 8 May 2008; revised manuscript accepted 1 February 2009. Discussion on this paper closes on 1 October 2009, for further details see p. ii.  Department of Geotechnical Engineering and Geosciences, Universitat Polite `cnica de Catalunya, Barcelona, Spain. † University of Strathclyde, Glasgow, UK. ‡ Universidade Federal de Pernambuco, Recife, Brazil. § CIEMAT, Madrid, Spain. } ENRESA, Madrid, Spain. Downloaded by [ Universitat Politecnica de Catalunya] on [28/04/20]. Copyright © ICE Publishing, all rights reserved.