Preparation of integrated systems test progresses to pilot hole drilling phase
Posiva ensures the safety of final disposal by conducting a great number of various tests. Estimated to be carried out in 2022, the integrated systems test will see final disposal operations tested without actual spent nuclear fuel. The test will provide more information concerning the functionality of the various phases of the final disposal process.The research work for the integrated systems test, which is estimated to be carried out in 2022, started in April with the drilling of a pilot hole in ONKALO at a depth of approximately 420 metres. Spent nuclear fuel will eventually be deposited roughly at the same depth.
A central tunnel and a deposition tunnel with lengths of approximately 60 metres and 80 metres, respectively, will be excavated for the integrated systems test. The aim is to bore four deposition holes in the deposition tunnel. The tunnels excavated for the integrated systems test will be significantly shorter than those used for actual final disposal operations; the deposition tunnels can be up to 350 metres long.
“The integrated systems test is part of the test operation of the final disposal facility. The test will involve the performance of a small-scale final disposal operation under realistic conditions and using actual equipment. The only difference between the test and an actual final disposal operation is that the canisters do not contain spent nuclear fuel. The objective of the test is to ensure that all the process phases relating to final disposal function as planned,” says Chief Geologist Ismo Aaltonen.
Drilled in April, the pilot hole is of similar length as the central tunnel – approximately 60 metres – with a diameter of little over seven centimetres.
“We are currently examining a drilled sample to detect any cracks or fragmentation zones in the bedrock. Among other things, the pilot hole will be used to study the movement of water in the bedrock and to analyse the chemical quality of the water,” says Research Manager Jussi Mattila.
The site bedrock model becomes more completeAccording to Jussi Mattila, the pilot holes drilled for the test and the samples obtained in the process will provide yet new information which can be utilised in updating the geological and hydrological model of the bedrock in the Olkiluoto area. The excavation of the central tunnel to be used in the integrated systems test will begin late in the summer, when the bedrock’s suitability for the construction of the tunnel has been ensured through surveys.
More pilot holes will be drilled before the excavation of the deposition tunnel and the deposition holes to be used in the test. The purpose of the drilling operations is to ensure the suitability of the deposition tunnel for test operation and verify that the amount of water leaking into the deposition hole will not exceed the permitted value. Another motivation for the drilling operations is to confirm that there are no fragmentation zones in the bedrock at the planned location of the deposition holes.
“The integrated systems test will bring benefits not only to Posiva as the company in charge of final disposal, but the authorities performing regulatory control of final disposal as well. The test will provide all parties with information concerning the functionality of the procedures, materials and working methods utilised in final disposal,” says Mattila.
Bentonite is a powerful sealantThe deposition hole will be approximately eight metres deep and have a diameter of 1.75 metres. The diameter of a disposal canister is approximately one metre. The aim of the integrated systems test is to place copper canisters in four vertical holes which will be fitted with bentonite clay buffers surrounding the canisters.
“When bentonite clay becomes moist, its volume is multiplied several times over. As the clay expands, it will fill the space surrounding the disposal canister and seal it off, preventing the flow of water near the canister and protecting it from any bedrock movement,” says Ismo Aaltonen.
According to the current plan, no measurements will be performed in the deposition hole itself after the canister, the bentonite buffer and the backfill have been placed in the bedrock. Instead, operations such as monitoring of the flow of groundwater will take place in the vicinity of the facility.
“In actual final disposal operations, the length of the canister will depend on the origin of the spent nuclear fuel. The canisters to be used for spent fuel from Olkiluoto 1 and 2 are 4.7 metres long and accommodate 12 fuel rod bundles. The canisters for spent fuel from Loviisa also accommodate 12 bundles, but their length is only 3.6 metres. The canister designed for spent fuel from Olkiluoto 3 is 5.2 metres long and accommodates four fuel rod bundles,” says Aaltonen.
Before the integrated systems test, ONKALO will see a full-scale final disposal test known as FISST (Full-scale. In-situ System Test), which will involve the placement of copper capsules equipped with thermal resistors and buffers in two deposition holes. The demonstration tunnel used in the test will be backfilled and the mouth of the tunnel will be sealed with a cast plug. In FISST, all associated installation work will be implemented using the current prototype models.
The purpose of installing thermal resistors in the canisters is to simulate the thermal impact of actual nuclear waste on its surroundings. The underlying aim is to monitor the behaviour of the bentonite and the tunnel backfill under the heat load.
Text: Timo Sillanpää
Photography: Hannu Huovila