3D modelling brings safety to disposal of nuclear waste
Text: Vesa Tompuri
GTK has performed R&D work serving the safety of nuclear waste disposal for almost 40 years. At the current estimate, the disposal of nuclear waste in the Finnish bedrock in the immediate vicinity of the Olkiluoto nuclear power plant will start in 2023.
The first two nuclear power plants were built in Finland in the 1970s. Two of the reactors constructed at the time and still in use were built in Olkiluoto located in the municipality of Eurajoki on the western coast of Finland. At the moment it is estimated that the third reactor unit will be taken into use by the end of the current decade.
The disposal of nuclear waste has naturally been in the agenda of Finnish nuclear power companies from the very start.
– Teollisuuden Voima Oy operates the power plant in Olkiluoto and Fortum Oyj operates the two reactor units in Loviisa east of Helsinki.
Teollisuuden Voima Oy and Fortum Oyj jointly own Posiva Oy established in 1995. Posiva’s primary operating rationale is the disposal of spent nuclear fuel created by the power plants of its owner companies.
Posiva has delivered enormous amounts of research materials for regulators/authorities for evaluation, in the compilation of which Posiva has also utilised the wide-ranging expertise of GTK. What is unique about this project that has lasted decades is that it combines scientifi c research looking for new knowledge with practical application and method development.
In late 1970s and early 1980s, when this research began, the first object of study was to determine the suitability of Finnish bedrock for disposal of high-level nuclear waste. It was based on international criteria applied to Finnish conditions.
– Seismically Finland is among the most stable areas in the world. For example, even eastern Sweden is seismically more active. Of course, there are many other basic factors to take into account, such as future ice ages, and the fact that groundwater becomes saline the deeper you get, says Timo Ruskeeniemi, Research Scientist at GTK.
Taking ice ages into account is significant because safe disposal of high-level nuclear waste must be targeted at hundreds of thousands of years. Consequently, when planning structures for disposal, recurring ice age stages, covering the rock by up to two kilometre sheet of ice, must be taken into account.
– In the final analysis, these kinds of details belong to the construction project designers, but we have to take them into account in our work, which must serve the decision-making of the disposal companies and authorities. It is a question of what kinds of solutions can be considered safe enough, says Ruskeeniemi.
For almost the entirety of its 40-year duration, the nuclear waste disposal project has aimed at decision-makers having enough reliable information to support their decision making. A necessary condition of selecting the disposal site was the general information about Finnish bedrock provided by the GTK. However, it was not nearly enough for the purposes of the disposal programme; what was needed now was new knowledge about subjects that had not been studied before.
Before, it was enough to know the properties of the rock, for example, at accuracy required by normal tunnel construction. For example, it was enough that fractured zones were recognised as excavation in a mine or other underground space proceeded. When it comes to safe disposal of spent nuclear fuel in bedrock a much more comprehensive understanding of fracturing is needed.
This has required best geological and geophysical research competence.
– Geophysical methods have played an essential part when studying the geological preconditions of the candidate sites in increasingly detailed manner, says geophysicist Markku Paananen, Senior Scientist for GTK.
More “general” rock study does not necessarily reveal at required accuracy the small physical differences, which may play a decisive part for disposal of nuclear waste. For example, thermal conductivity of the rock is an extremely important property when evaluating and calculating how close to each other disposal canisters producing decay heat can be placed.
3D models and modern research methods produce as accurate data as possible
In 2000s, research methods and modelling techniques have taken great strides.
– Earlier, you needed to make conclusions on the geology of bedrock largely based on what you could see on the outcrops. In this sense, it is a huge step to be able to study bedrock as a three-dimensional whole in ONKALO, the underground rock characterisation facility in Olkiluoto, instead of relying solely on limited number of drillholes, says Seppo Paulamäki, Geologist for GTK, who like Markku Paananen has worked in the disposal project for more than 29 years.
With humbleness typical of experienced geophysicist, Paananen also emphasizes the intuition of researchers in a situation when a conclusion must be drawn based on partly incomplete facts.
– Modern research methods and 3D models produce accurate data and explicit interpretations. Nevertheless, when studying very small details, such as individual fractures and their continuations, it always feels that the conclusions must be done with rather limited amount of data, says Paulamäki.
Extensive interdisciplinary research has provided the means to define the rock area suitable for disposal. 3D modelling has been an essential tool in this work.
From 101 potential disposal locations to 1
In the mid-1980s, the disposal project was at a stage during which GTK’s researchers compared the suitability of 101 potential disposal locations. From among these, Teollisuuden Voima selected five areas for preliminary disposal site studies in 1987.
Starting from 1993, detailed site studies were continued in three areas. After the new Nuclear Energy Act prohibited the export of spent nuclear fuel, Posiva Oy begun studies also in Loviisa site. In 1999, Posiva made the decision to propose Olkiluoto as the disposal site of high-level nuclear waste.