As the first ever venture of its kind, the storage of spent nuclear fuel will begin in the early 2020s on Olkiluoto, an island that also houses a nuclear power plant. The historic repository is a tour de force of Teollisuuden Voima Oy (TVO), Posiva Oy, Saanio & Riekkola Oy (SROY, currently a part of AINS Group) and many other Finnish and foreign expert organisations, which have engaged in geological studies, design work and preparation of safety cases over the past four decades to assess long-term safety.

The repository project has its roots in the early 1980s, when TVO began to seek a solution for the storage of spent fuel from nuclear power plants. Saanio & Riekkola Oy participated in the project by identifying areas that were suitable in terms of their location and ground type. In 2004, the construction of the underground research facility ONKALO kicked off with the excavation of access tunnels. The tunnel has now reached its final depth.

The next stage for this unique large-scale project involves preparing for the construction of the repository premises and the submission of the operation permission application to the Finnish government. ‘SROY is in the process of conducting tests and experiments on the performance of the clay materials to be used in the repository space and the impact of excavation on the rock bed. In addition, we are involved in the preparation of the safety case. The goal is to start using the repository for storage in the early 2020s once the commissioning permission has been granted,’ explains Pirjo Hellä, leader of the long-term safety team at Saanio & Riekkola.

Collaboration is the key to success in the repository project

We have had to assess what will happen to the repository and radiation safety over then next hundreds of thousands of years, including during new ice ages.

Pirjo Hellä

Team Leader, Assessment of long-term safety, AINS Group

SROY’s collaboration with TVO, Posiva Oy and several other Finnish and foreign expert organisations has produced a wealth of new information on how to ensure the functionality and safety of the 400-metre deep repository. For the actual commissioning, numerous instructions must be prepared to enable a rapid and appropriate response to various situations and events, such as water leaks.

‘We are currently compiling a safety case portfolio for the repository’s commissioning permission. In this portfolio, we describe the repository’s engineered barriers and factors influencing the long-term safety of the repository and their development, assess the ability of the repository, particularly its capsules, to contain the waste in the long term, and present the technologies used to protect the environment,’ explains Posiva’s Project Manager Barbara Pastina.

The preparation of the safety case, analysis of the repository location, design of the engineered barriers and assessment of the operations require multidisciplinary expertise and collaboration among various organisations. One example of the results produced by such close collaboration between SROY, Posiva and other expert organisations is the determination of requirements for the rock bed surrounding the repository. On the basis of these requirements, the repository could be placed in a location that is ideal in terms of the rock bed’s properties. The defined requirements and their management system ensure that the materials used in the repository function effectively together. Any deviations from the requirements and their impact are assessed as part of the safety case.

This is what repository operations at the Olkiluoto facility entail:

Before being placing in the repository, the spent nuclear fuel is cooled for several decades to decrease its radioactivity.

SROY’s solid expertise in underground construction and its meticulous rock bed studies, modelling of the geological development in the region, and skilled safety reporting activities played a major role in the Posiva project’s progress towards the construction permit being granted by the government.

Barbara Pastina

Project Manager, Posiva

After the period of intermediate storage, the spent fuel is encapsulated. The capsules are placed in the repository built in the rock bed 400 metres underground. The capsule, clay materials used to fill in and close the space, and the surrounding rock bed contain the spent fuel and isolate it from the environment. Even though the radioactivity of the material decreases with time, the spent fuel must be contained to ensure radiation safety.

The repository capsule for spent fuel. Photos by Posiva Oy.