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Processes in the Repository

After the repository is closed, and no further operational activities take place inside the repository, it is generally acknowledged that the following sequence of events will take place within the repository.  (Note that this is a very simplified description).

1.  The repository, which was kept dry during the operational period, will begin to resaturate with groundwater, and eventually all the repository void space will be filled with water.  Repository resaturation is likely to take of the order of a few years after repository closure.

2.  When the groundwater makes contact with the steel waste containers, the containers will begin to corrode, because of the chemical reactions that take place between the steel and the incoming water.

3.  Once a container has fully corroded, after maybe a few hundred years, water can make contact with the radioactive wastes.  Radionuclides and other repository materials will then begin to dissolve in the water.  For some radionuclides, the extent to which they can dissolve in water will depend on the chemical element of the radionuclide and the local chemical conditions in the repository.

4.  The dissolved radionuclides begin to migrate in the groundwater away from their original location.  Radionuclide migration occurs because of the natural movement of the groundwater, which flows in accordance with the hydraulic pressure gradients that exist at and around the repository location.

The corrosion of steel containers results in the generation of gas (in particular hydrogen gas), which may give rise to the migration of gaseous radioactive materials away from the repository and towards the ground surface.  Significant programmes of work are currently ongoing (and have been in place for many years) to investigate the significance of this.  However, in the interests of brevity, we will not consider this "gas pathway" further.

The rate of migration of the radioactive materials depends on the nature of the dissolved radionuclides.  A major process in retarding the migration of the radionuclides is sorption.  In very simple terms, sorption is the tendency of the dissolved radionuclides to "stick" to solid materials, and then be released back into solution after a period of time.

The degree of sorption of a particular radionuclide depends on its associated chemical element.  For example, isotopes of chlorine and iodine show very little sorption.  On the other hand, isotopes of plutonium and other actinides tend to sorb very strongly.  The migration of chlorine isotopes is therefore much more rapid than isotopes of plutonium.

Travel through Rock (geosphere)

Once the radioactive wastes begin to dissolve in groundwater and migrate with it, the potential exists for those radioactive wastes to return to the accessible environment.  In this context, the term "accessible environment" refers to that region of the environment where radiation exposures to humans could occur.  Often the accessible environment is referred to as the "biosphere", and the two terms are often used interchangeably.  The ground surface and the first few metres below the ground surface are the most important regions of the accessible environment / biosphere.

The migration of radionuclides from a repository in groundwater is illustrated schematically in the following diagram:

This diagram shows a vertical section through the rocks in which the repository is located.  The radionuclide pathline shows one possible direction that groundwater could move along as it moves from the vicinity of the repository towards the ground surface.  The total length of the pathline could be a few kilometres or more.

In studying the migration of radionuclides in groundwater, detailed and complex calculations are undertaken to investigate how the groundwater in the vicinity of the repository moves, and how it is likely to affect the migration of dissolved radionuclides out of, and away from, the repository.  The result of these calculations is one or more possible pathlines, like the one shown in the figure above. The pathline calculations specify the possible directions and velocities of the groundwater movement.  Dissolved radionuclides in repository water will tend to follow the pathline through the various rock layers, as shown, and will eventually discharge at the ground surface.

The tendency of dissolved radionuclides to follow the direction of groundwater flow is referred to as advection.  (As an example, when a stick is thrown in to a flowing river, the stick follows the direction of water flow.  This motion of the stick is the same principle by which radionuclides advect in groundwater).  However, there are other transport processes that also affect the movement of radionuclides through the overlying rocks.  The two major processes are dispersion and diffusion.  Both result in a spatial spreading of the "plume" of radionuclides migrating from the repository.

Radionuclides need not discharge into the terrestrial environment.  A second possibility that is important for repositories located in coastal regions (and which has been studied extensively in the UK) is that the discharge could occur at sea (i.e. in the marine biosphere), rather than in the terrestrial biosphere.

Timescales

The time required for radionuclides to migrate from the repository through to the biosphere (either marine or terrestrial) depends on two key factors:

1.  The groundwater travel time.  This is equal to the length of the groundwater pathline divided by the average groundwater flow velocity.  For the hard rock geologies studied in detail in the UK, groundwater travel times are of the order of 100,000 years.

2.  The degree of sorption of radionuclides to rock materials.  As with sorption to backfill in the repository, the degree of sorption to rock materials varies according to the chemical elements of the various radionuclides in the wastes.

It can be seen that there are a number of processes that govern the migration of radionuclides away from the repository and back towards the biosphere.  These processes are discussed in [1], and are categorised according to the "timeframe" on which they operate.  Using this information, it is possible to construct a diagram that illustrates some of the processes and the timescales on which they operate.  This diagram is shown below.

Substantial programmes of work are ongoing to investigate how the repository and its environment will evolve over time.  It is worth noting that the timescales on which some processes act are enormous - many thousand or even millions of years.  There are therefore significant uncertainties about the timing and magnitude of some of these processes.

In assessing the safety afforded by the repository system, it is possible to address some of this uncertainty by making "cautious" assumptions about the repository and its evolution over time.  In essence, this means identifying the system evolution that corresponds to the worst case that could happen.  For example, in some safety assessments, little or no credit is taken for physical containment of radionuclides in the containers, even though in practice the containers might isolate the wastes for several hundred years.

References

[1]  United Kingdom Nirex Limited, Generic Post‑closure Performance Assessment, Nirex Report N/080, 2003.

This report is freely available from the document library at http://www.nda.gov.uk.

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