Finnish waste management company Posiva is to begin the full-scale final disposal test for used nuclear fuel at the Onkalo underground characterisation facility. The test, which is intended to confirm that the final disposal process will work as planned, is required before Posiva can obtain an operating licence for the repository under construction at Olkiluoto.
Posiva, jointly owned by Finnish nuclear utilities Fortum and Teollisuuden Voima Oyj, submitted its construction licence application to the Ministry of Employment and the Economy in December 2013 and the licence was granted in November 2015. The repository site at Eurajoki near Olkiluoto was selected in 2000 and won parliamentary approval in 2001. The Onkalo underground laboratory, which is to be expanded to form the basis of the repository, was used to investigate the rock at Olkiluoto. Posiva was given regulatory approval to begin construction 2016 but Posiva will need a separate operating licence for the facility, which is expected to open in 2023.
TVO said the test at Onkalo, which will last for several years, is intended to prove that the prototype processes for geological storage at the repository are “all working concepts”. These processes include placing fuel assemblies packed in copper-steel canisters inside holes drilled in the bedrock tunnels, located at a depth of 400-450 metres which will then be backfilled with bentonite clay and sealed with a cast plug. Two test canisters will be equipped with thermal resistors simulating the residual temperature of used fuel, TVO said. The temperature and pressure in the canisters, test holes and the surrounding bedrock, and the behaviour of the backfill of the tunnels, will be monitored by some 500 sensors over several years.
After the test is completed, Posiva will conduct an integrated systems test, which will see final disposal operations undertaken but without used fuel. This 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 expected to be carried out in 2022, began in April 2017 with the drilling of a pilot hole at Onkalo down to a depth of about 420m. Used fuel will eventually be deposited roughly at the same depth. A 60 metre central tunnel and a 80 metre deposition tunnel will be excavated for the integrated systems test. These will be significantly shorter than those used for final disposal operations, which can be up to 350m long.
TVO said the cost of building an encapsulation plant onsite and the excavation and equipment of two final disposal tunnels is estimated at about €500m. “So far about €1bn has been used in examining the site for final disposal and in equipment development,” Posiva's president Janne Mokka noted.
