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Arbeitsbericht NAB 21-11

Pilot Repository Monitoring: First Concept Report

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1              Introduction

The National Cooperative for the Disposal of Radioactive Waste (Nagra) must submit an overarching monitoring concept for the general license application that covers all realisation phases of the deep geological repository (ENSI 2018). Monitoring of the pilot repository is part of the overarching monitoring concept.

This report has the aim to present the requirements and first concepts related to pilot repository monitoring. In the context of the 2021 Waste Management Programme (Nagra 2021a) this document serves as background report ("Hintergrundbericht") and answers Condition 5.2 (see Appendix) of the Swiss Federal Council attached to the 2021 Waste Management Programme (Bundesrat 2018). In addition, this report forms a basic document for the development of the overarching monitoring concept and the site-specific planning of pilot repository monitoring.

The planning and licensing steps of a deep geological repository are configured to ensure a safe, secure, sustainable, and publicly acceptable construction, operation, and closure. The pilot repository monitoring provides information on the behaviour of the early-stage evolution of the multibarrier system following waste emplacement. Therefore, these monitoring data form the input to the periodic updates of the safety case as well as for a stakeholder dialogue, confidence building, and decision making.

The present report identified the regulatory requirements on the pilot repository and its monitoring, defines the objectives of the pilot repository monitoring as developed by Nagra, and describes the next steps in development of the pilot repository monitoring concept. Moreover, a preliminary methodology for the identification of potential monitoring parameters is presented. Furthermore, ongoing research and development (R&D) and fundamental considerations about data interpretation and transferability to the main repository are discussed. In addition, selected topics of R&D projects on monitoring technology are presented.

The pilot repository

The pilot repository is an independent part of the deep geological repository and is used to monitor the evolution of the multibarrier system by means of a small representative amount of radioactive waste up to the end of the Monitoring Phase (ENSI 2020a). In the current Swiss disposal concept the pilot repository is spatially separated from the main repository (Nagra 2021a). Therefore, the pilot repository will have a certain distance to the main repository to minimize the thermal-hydraulic-mechanical (THM) impact of the main repository. Furthermore, the pilot repository will be placed in geological and hydrogeological conditions that are comparable to the main repository. The design (construction, support, emplacement) will correspond to the main repository.

A possible schematic layout for the different underground facility elements and structures of a combined repository is shown in Fig. 1-1. In the current concept an L-shaped observation drift surrounds the pilot repository on two sides. The observation drift will be used for borehole-based monitoring of the pilot repository. Access to the pilot repository and the observation drift is provided by a short branch from the operational tunnel. In the current concept (Nagra 2021a) the high-level waste (HLW) pilot repository consists of three emplacement drifts and the low- and intermediate-level waste (L/ILW) pilot repository consist of one emplacement cavern.

 

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