2009 Report to Government on National Research and

Box 9. Modelling
At a spatial scale of metres <strong>to</strong> hundreds of metres, which is appropriate <strong>to</strong> geological disposal,
given the complexity of both the engineered and natural components of the GDF, host rock
and biosphere, and given the timescale over which the safety of the GDF must be assessed, it
is necessary <strong>to</strong> use predictive computer models. These are often referred <strong>to</strong> as reactive
transport models, and many of these are now very sophisticated, incorporating multiphase
flow and detailed description of the system (Steefel et al., 2005). However, these models
inevitably incorporate considerable simplifications and approximations.
There is also a range of modelling approaches which can be used <strong>to</strong> understand interaction of
radionuclides with surfaces at the molecular level, complementing experimental studies of
sorption reactions. Density functional theory is well established and is now sufficiently
developed <strong>to</strong> describe complex elements such as actinide ions in solid matrices or adsorbed
on surfaces (see e.g. Skomurski et al., 2006). Although such modelling is far from routine,
draws heavily on very demanding experiments, and remains very challenging, it offers the
prospect of understanding and predicting reactions in complex, environmentally relevant
systems at a fundamental, molecular level.
The integration of this molecular scale modelling, which is soundly rooted in physical and
chemical principles, and prediction over much larger spatial and time scales, which is needed
<strong>to</strong> assess GDF performance, remains very difficult. Nevertheless, if the gap between these
different scales and approaches can be bridged, the product will be a powerful predictive <strong>to</strong>ol.
The USDOE has identified three Grand Challenges in the geosciences, two of which centre
around this particular question (USDOE, 2008).
Gas
A.59 The generation and movement of gases in a GDF are of concern for two reasons.
One is that sufficient gas might be generated <strong>to</strong> affect groundwater movement or
even disrupt the GDF. The other is that radioactive gases could move through
the rocks <strong>to</strong> the surface and lead <strong>to</strong> radiation exposure of humans and other
organisms. Consequently, a balance must be sought between providing sufficient
gas permeability within the GDF <strong>to</strong> prevent an excessive rise in gas pressure,
and inhibiting both fluid flow and the migration of trace radioactive gases in<strong>to</strong> the
geosphere. The principal mechanisms for generation of bulk gases are anaerobic
corrosion of metals, especially steels, and decomposition of organic wastes.
Radioactive gases can be formed by interactions of bulk gases with the wastes,
and by decay of some radionuclides (for example, radon is a decay product of
uranium).
Hydrogen
A.60 The GDF, particularly after closure and resaturation, will contain large volumes of
water, either free groundwater or bound in waste encapsulants like cement, or in
backfill. Radiolysis (breakdown by reaction with radiation arising from
radionuclides) of water generates hydrogen gas, which is vented from most ILW
waste containers. The external radiation field around ILW and HLW containers
will promote water radiolysis in groundwater and backfill. There will be several
hundred thousand <strong>to</strong>nnes of metals in any GDF, most of which will be steels,
which are a constituent of some wastes and of most ILW containers (Royal
Society, 1994). HLW containers are also made of steel and may be placed in
steel overpacks for disposal. Anaerobic corrosion of metals will generate
considerable quantities of hydrogen. Tritium will be released from wastes and
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