Pore Space Geometry (PSG) - Introduction

Finnish Flag  University of Helsinki, Dept of Chemistry – Laboratory of Radiochemistry (HYRL), Finland
Finnish Flag  Radiation and Nuclear Safety Authority (STUK), Finland
French Flag  University of Poitiers, Laboratory of Hydrogeology, Clays, Soils and Alterations (HYDRASA), France
Swiss Flag  Nagra, Switzerland
 
Virtual view outside the PSG zone

This is an in-situ resin impregnation experiment which aims to visualise connective porosity and open pore spaces available for matrix diffusion in intact rock matrices.

This experiment involves the injection of a specially formulated acrylic resin into the granitic rocks of the GTS to determine a wide range of information on the in-situ porosity. Grimsel granite and granodiorite is composed mainly of crystals of quartz, feldspars and biotite. In previous studies at the GTS (NTB 00-08), a fluorescent resin was injected into the rock matrix. After polymerisation of the resin, the matrix core could be removed for analysis without disturbing the in-situ pore distribution. The resin impregnated sites were visible under ultra-violet light. This study observed four main types of pore within the matrix.

Under crossed nicols Under ultra-violet light
 

Grain boundary pores
(image width 5mm)

   
Shear banding pores
(image width 10nm)
   

Solution pores
(image width 10nm)

   
Microfractures
(image width 10nm)

Why is the matrix important ?

Conservative estimates of the long term fate of radioactive wastes assume that some radionuclides will eventually leak from the waste into the host rock surrounding the repository. There are two main processes which lead to the retardation of radionuclides in the host rock, sorption and diffusion. When determining the safety of repositories (the repository safety assessment), the degree of radioactive retardation by diffusion is often underestimated due to uncertainties in the properties of the matrix. These over-conservative estimates of limited matrix diffusion depths may grossly underestimate retardation of radionuclides in the geosphere, especially for non-sorbing radionuclides such as 129I

Furthermore, previous studies at the GTS using resin impregnation have shown that changes to the measured porosity of the rock may occur when un-impregnated samples are removed to the laboratory for analysis. This can result in laboratory measured porosities 2 to 3 times higher being measured in the laboratory than present in situ.

Due to the great potential significance of this work to safety analysis calculations, it was decided to study this further using an alternative resin technique to the previous tests at the GTS. Over the last decade, the 14C-PMMA method has been developed for characterisation of pore space geometry for low permeable rock matrices. Impregnation with 14C-PMMA and autoradiography allows the investigation of the spatial distribution of in-situ porosity.

Further information from Andrew Martin

 

Pore Space Geometry (PSG) Experiment

The GTS underground facilities are also available to interested 3rd parties for underground testing and research. The GTS offers cost-effective access to a fully developed, well characterised underground research facility with round the year logistical support - please contact Dr. Ingo Blechschmidt, Head of the Grimsel Test Site, for further details.