Measurement of the Excavation Disturbed Zone in the ZEDEX project in the Context of Repository Post-Closure Performance Assessment

Abstract

Consideration of deep disposal of radioactive wastes in the UK and France involves undertaking Post-Closure Performance Assessments of the proposed disposal system for a potentially suitable site. The results of such assessments include measures of the radiological risk that may result in the long term to members of the public, which can be compared with the relevant regulatory requirements. In France, the "Regle Fondamentale De Surete No In-2-f' produced by the French Authority for the Safety of Nuclear Installations (D.S.I.N), defines the regulations for safe disposal of medium and high level waste. In the UK, the regulatory requirements are specified under the terms of the Radioactive Substances Act 1993. An element of the Performance Assessment of a potential repository at a given site involves considering the disturbance to the host rock induced by excavation. In France, the D.S.I.N. specifies that the Excavation Disturbed Zone (EDZ) must be as limited as possible, in particular in and around access tunnel seals. It should also be verified that the evaluated excavation disturbance around the opening remains acceptable according to the quality of the confinement barriers. In the UK, Her Majesty's Inspectorate of Pollution has published, under the Radioactive Substances Act 1993, draft guidance on requirements for authorisation for disposal facilities on land for low and intermediate level radioactive wastes. This states that investigations of the site should allow the effects of disturbance to the site from construction of the disposal facility to be assessed and the consequences for radiological safety to be evaluated. In-situ measurements of excavation disturbance therefore play an important role in the design and construction of radioactive waste repositories. In order to prepare for the development of their own underground testing facilities, ANDRA, the French agency responsible for radioactive waste disposal, and UK Nirex Limited, responsible for the disposal of the UK's intermediate level and some low level radioactive waste, entered into a joint agreement with SKB (Swedish Radioactive Waste Management Company) to undertake excavation disturbance measurements within SKB's asPo Laboratory in Sweden. The project was termed ZEDEX (Zone of Excavation Disturbance Experiment). Rock mechanical, geophysical, hydrogeological and geological data were gathered around a phased data acquisition programme before, during and after a drill and blast and a tunnel boring machine (TBM) drift excavation. The primary objectives of the ZEDEX project were to understand better the development of excavation disturbance and excavation damage, and to test the methodology and instruments employed to acquire the data to enable ANDRA and Nirex to adapt the experimental configuration to their own requirements. The role of EDZ validation in the context of repository post-closure performance assessment is discussed in relation to the planning and implementation of the ZEDEX project.