Monitoring hydraulic conductivity with HTU at Eurajoki, Olkiluoto, Drillholes OL-KR31 and OL-KR32, in 2009

In July 2004 Posiva began to construct an underground rock characterization facility, ONKALO, at Olkiluoto. The construction of ONKALO and the subsequent construction of the repository of high-level spent nuclear fuel will affect the surrounding rock mass, the groundwater flow system and the environment in general. In December 2003 a programme to monitor these effects during the construction and operation of ONKALO was presented (Posiva 2003). The programme consists of different types of monitoring by different disciplines. HTU (Hydraulic Testing Unit) measurements are a part of the hydrogeological monitoring programme that includes measurements of hydraulic conductivity with HTU in drillholes adjacent to ONKALO. Measurements performed before and in the early stages of the excavation of ONKALO provide a baseline to which later results can be compared. The HTU monitoring measurement program was first completed in 2005 and then repeated in 2006 and in 2007 in drillholes OL-KR8, OL-KR28 and OL-KR31. Of these OL-KR31, including its extension, was measured in 2008 and OL-KR32 was added to the program as a new hole. These same holes were measured also in 2009. Double-packer constant-head method was used throughout with 2 m packer separation and a nominal 200 kPa overpressure. Injection stage lasted normally 15 minutes and fall-off stage 10 minutes. The tests were often shortened if there were clear indications that the hydraulic conductivity was below the measuring range of the system. The pressure in the test section was let to stabilise at least 5 min before injection, occasionally longer. Also, the injection and fall-off were extended when necessary to obtain a stationary state. Two transient (Horner and 1/Q) interpretations and one stationary-state (Moye) interpretation were made in-situ immediately after the test. In 2009 a total of 238 tests were accomplished covering altogether 475.24 m of drillhole in these two holes. The Hydraulic Testing Unit (HTU-system) is owned by Posiva Oy and it was operated by Geopros Oy. (orig.)