Hyperalkaline Plume in Fractured Rock (HPF) - Introduction
| ANDRA, France
JNC (now JAEA), Japan
In the case of low and intermediate level radioactive wastes (L/ILW) and many chemo-toxic wastes, most current repository designs envisage the use of large volumes of cementitious materials to immobilise the waste and to backfill the repository.
The proportions of concrete/cement in a typical L & ILW repository are shown below.
Although relative inventories are less, significant quantities of structural concrete and cementitious grouts may be used in some HLW / SF repository designs (see Waste Disposal Overview). It has long been recognised that the use of cementitous materials in an underground repository will lead to the development of a "hyperalkaline plume" in the host rock. This can be demonstrated by the following animation.
Cement pore waters are highly alkaline and cement leachates have been predicted to interact with the repository rock. This could significantly alter the original nature of the host formation, affecting the retardation qualities by changing the original geochemical and hydrological conditions. From a safety assessment point of view, the extent of such an alteration zone (often called the "hyperalkaline plume") and the influence on radionuclide retardation properties of this altered far-field must be carefully assessed.
To date, information already exists, in the form of model predictions based on laboratory and natural analogue studies (e.g. Maqarin in Jordan - see www.natural-analogues.com), on possible consequences of a hyperalkaline plume. The aim of HPF is to combine this information with data from a realistic, medium-term in-situ experiment which will characterise the development of such a plume in a fractured rock.
The HPF (Hyperalkaline Plume in Fractured rock) project integrates laboratory, modelling and natural analogue studies with the results from the in-situ studies, to gain a better understanding of the processes relevant to a cementitious nuclear waste repository.
Aims of the HPF experiment
- To assess the potential perturbations of a hyperalkaline plume on a repository host rock under realistic, in-situ conditions.
- To provide a link between existing laboratory data (well constrained, but short-term and unrealistic), natural analogue data (poorly constrained, but long-term and realistic) and modelling data with a medium term (up to four years), repository-relevant, in situ experiment.
- To test coupled (geochemistry/transport) code predictions of hyperalkaline plume interaction with fractured rocks, in particular the role played by secondary minerals (cement phases, zeolites) on potential fracture blocking and/or matrix clogging.
- To obtain a data set on time evolution of a coupled geochemical/hydraulic system.
- To examine radionuclide uptake within the hyperalkaline plume and compare this with available laboratory data on both a pristine and altered far-field.