Performance of THM monitoring instrumentation in FEBEX bentonite barrier after 18 years of operation under repository-like conditions

Abstract

The FEBEX (Full-scale Engineered Barrier Experiment in Crystalline Host Rock) was a research and demonstration project, which aimed to study the behaviour of near-field components in a repository for high-level radioactive waste in granite formations.Two electrical heaters were placed in a horizontal drift excavated in the granodiorite of the Grimsel Test Site in Switzerland, and surrounded by a barrier made of highly compacted bentonite blocks (FEBEX bentonite or Serrata type). A wide range of sensors were installed to track the most essential THM parameters: total pressure cells, pore pressure sensors, thermocouples, displacement sensors, moisture sensors of different types (capacitive, psychrometric and TDR) and others.The activities of the FEBEX “in situ” test started in 1994, the installation was completed in 1996 and the operational phase (heating and natural hydration) started in February 1997. A partial dismantling was carried out after 5 years of operation in 2002 and the final dismantling was performed in 2015 after more than 18 years.This paper reports the observations and analysis performed to the sensors recovered during the final dismantling operation.Qualitative and quantitative assessment of performance by type of sensor and measuring technique were done based on the results from data recorded during the operational lifetime, from the dismantling operation and from analysis carried out.As main conclusion, many sensors survived under harsh conditions and their performance was significantly better than initially expected and provided valuable information about the THM parameters for more than 18 years. The damage type/degree varied depending on sensor type/material (specific sensors failed quickly whereas others remained intact even with no need for re-calibration). This allows suggesting possible improvements. The re-calibration results for the surviving sensors indicated that their accuracy had hardly changed and that the obtained data was trustworthy.