2009 Report to Government on National Research and

could mix with the bulk hydrogen gas or form tritiated water, although its short
half-life (12.6 years) means that is very unlikely <strong>to</strong> reach the surface in significant
quantities.
A.61 Subsurface microbial communities are often well adapted <strong>to</strong> exploit hydrogen as
an energy resource (e.g. Nealsen et al., 2005; Pedersen, 2002) so the fate of
hydrogen is intimately linked <strong>to</strong> the evolving microbial ecology of the GDF. For
example, the hydrogen supply in a GDF will be much greater than that in a
pristine environment, with the potential <strong>to</strong> trigger a microbial bloom whose
consequences may be difficult <strong>to</strong> predict.
A.62 BGS is the lead researcher in the ~£2M EU FORGE programme on gas
migration and is supported in this by the NDA. The FORGE project is designed <strong>to</strong>
characterise and quantify the conditions during geological disposal under which
the formation of hydrogen and other gases will occur with sufficient pressure
increase <strong>to</strong> result in radionuclide migration from the reposi<strong>to</strong>ry in<strong>to</strong> the
surrounding host rock. It is predicted that corrosion of ferrous materials under
anaerobic conditions in a reposi<strong>to</strong>ry will lead <strong>to</strong> the formation of hydrogen.
Radioactive decay of the waste and the radiolysis of water will produce additional
gas. Movement of these reposi<strong>to</strong>ry gases through host rocks will occur by the
combined processes of molecular diffusion and bulk advection. If the gas
production rate exceeds the rate of diffusion of gas molecules in the pores of the
EBS or host rock, it is possible that a discrete gas phase could form and
accumulate until its pressure becomes sufficiently large for it <strong>to</strong> enter the EBS or
host rock. The FORGE project is designed <strong>to</strong> characterise and quantify the
conditions under which this may occur, enabling modelling <strong>to</strong> underpin GDF
design that lies within a safe gas generation envelope. BGS is involved in
experimental, moni<strong>to</strong>ring and modelling research contributing <strong>to</strong> three of the five
work packages comprising FORGE. These include its Large Scale Gas Injection
Test (Lasgit), fundamental issues of gas migration in ben<strong>to</strong>nite, carbonation
reactions of buffer/backfill cements and their impact upon gas and radionuclide
migration, the effect of stress field and mechanical deformation on permeability
and fracture self-sealing, validation of critical stress theory applied <strong>to</strong> reposi<strong>to</strong>ry
concepts, and baseline hydraulic and gas transport properties of Callovo-
Oxfordian argillite.
Carbon-containing gases
A.63 Methane and carbon dioxide will be produced in bulk by the various mechanisms
involved in the decomposition of organic materials in the wastes. The volumes of
methane and carbon dioxide generated will be much smaller than the volume of
hydrogen because there will smaller quantities of organic materials in a GDF than
there are of metals (perhaps a few tens of thousands of <strong>to</strong>nnes of organic
materials (Royal Society, 1994)). How much of the methane and carbon dioxide
is radioactive will depend on how much carbon-14 there is in the wastes in the
GDF and in what chemical form. This will in turn depend largely on how much
irradiated graphite is present and the method of treatment prior <strong>to</strong> disposal.
A.64 Carbon gases can be exploited by anaerobic micro-organisms either as an
energy resource (methane) or as a terminal electron accep<strong>to</strong>r (CO2), so there is
potential for a microbial community <strong>to</strong> cycle between these forms of carbon. The
chemical behaviour will also be very different, depending on whether the carbon
CoRWM Document 2543, Oc<strong>to</strong>ber <strong>2009</strong> Page 115 of 151