POSIVA Report 2016-21



Status and Outline Planning Report for Discrete Fracture Network Concepts, Data, Methods and Models for the Olkiluoto Site 2015


Hartley, L., Baxter, S., Fox, A., Poteri, A., Aaltonen, I., Koskinen, L., Suikkanen, J.



Page count:





The description of brittle deformation structures, their properties, and representation in discrete fracture network (DFN) models provides a cornerstone for several disciplines in site description, principally: rock mechanics, hydrogeology and hydrogeochemistry. It also underpins assessment of several safety functions including consideration of mechanical, hydraulic and chemical stability of the buffer and backfill, as well as retardation of the migration of harmful substances from the repository. DFN models also provide means to assess the performance of several rock suitability criteria and their benefits for compliance, as well as supporting rock engineering evaluations. The current report begins by elucidating the roles of fracture descriptions and numerical DFN models in supporting safety assessment and repository engineering.

Although different aspects of fracture properties are of interest to particular geoscience disciplines there is also value in bringing these fracture descriptions together in a more holistic narrative to improve understanding of correlations between fracture properties and with other geological models. The current report therefore gathers together general concepts, site specific understanding and data to provide an overview of each fracture property (covering geological, hydrogeological and solute transport related properties). Essentially, the report brings together a site descriptive model (SDM) for brittle deformation structures, reflecting both present understanding and proposed developments for the overall Olkiluoto Site Description (OSD) 2018.

As of 2015, DFN models of Olkiluoto have inferred stochastic/empirical descriptions of bedrock fracturing outside deterministically modelled brittle deformation zones in a set of domains associated with the 3D ductile deformation model description. Such models were considered adequate for assessing the feasibility of the site and disposal concept for spent nuclear fuel in the CLA. However, moving toward the Operating License Application (OLA) models that can make more reliable predictions for a specific volume (e.g. a deposition tunnel) are required. This necessitates enhancements in the DFN methodology to factor other geological models, hydromechanical effects and relationships between fracture properties, as well as potentially conditioning on local fracture specific measurements, in order to meet this goal. Further, reliability of the models needs to be quantified through a series of prediction and outcome exercises ahead of the OLA, which inevitably requires that models reflect local geological, hydrogeological and in situ stress conditions as far as is practicable.

A first step in enhancing the DFN methodology is to gather conceptual models describing fracture properties from the scientific literature as suitable foundations for development of the updated Olkiluoto DFN model. It is appropriate to make the DFN methodology more sophisticated at this juncture not least because significant new information has become available from the underground investigations in and around the ONKALO underground research facility. The OSD 2011 DFN model was largely derived from surface-based drillhole data. Further toward this aim, the available data and background reports from the site are collated, current uncertainties are discussed, and methods for advancing understanding of fracture properties and DFN modelling are proposed. Plans for the validation of resulting models are also outlined.

Finally, the report presents a customised discussion of uncertainties, classifying types of uncertainty, defining other terms relevant to the processes of constraining and building confidence in models, and goes on to describe and suggest ways of managing specific uncertainties in Olkiluoto DFN models. The definitions of terminology are offered as a standardisation for communicating uncertainties and modelling processes by those involved in DFN modelling.


Geology, hydrogeology, rock mechanics, discrete fracture network, DFN, modelling


POSIVA 2016-21_web (pdf) (23 MB)


Share article:
This website stores cookies on your computer. These cookies are used to improve our website and provide more personalised services to you.


To make this site work properly, we sometimes place small data files called cookies on your device. Most big websites do this too.

1. What are cookies?

A cookie is a small text file that a website saves on your computer or mobile device when you visit the site. It enables the website to remember your actions and preferences (such as login, language, font size and other display preferences) over a period of time, so you don’t have to keep re-entering them whenever you come back to the site or browse from one page to another.

2. How do we use cookies?

A number of our pages use cookies to remember your actions and preferences (such as login, language, font size and other display preferences.)

Also, some videos embedded in our pages use a cookie to anonymously gather statistics on how you got there and what videos you visited.

Enabling these cookies is not strictly necessary for the website to work but it will provide you with a better browsing experience. You can delete or block these cookies, but if you do that some features of this site may not work as intended.

The cookie-related information is not used to identify you personally and the pattern data is fully under our control. These cookies are not used for any purpose other than those described here.

3. How to control cookies

You can control and/or delete cookies as you wish – for details, see You can delete all cookies that are already on your computer and you can set most browsers to prevent them from being placed. If you do this, however, you may have to manually adjust some preferences every time you visit a site and some services and functionalities may not work.