Long-Term Cement Studies (LCS) - Introduction
Japan Atomic Energy Agency (JAEA), Japan
Nuclear Decommissioning Authority (NDA), UK
Nuclear Waste Management Organization (NUMO), Japan
Swedish Nuclear Fuel and Waste Management Co. (SKB), Sweden
The LCS (Long-term Cement Studies) project was first proposed at the HPF (Hyperalkaline Plume in Fractured rock experiment) Project Meeting in April, 2003, in Lyon. There, the basic need for further studies related to cement leaching/hyperalkaline plume production related issues was formulated with emphasis on in situ field experiments with more realistic boundary conditions and longer time scales. Furthermore, regarding the widespread requirement to use low-pH cements, this issue will be studied with high priority.
The overall aim of the LCS project is to increase the understanding of high-pH cement interaction effects in the repository near field and the geosphere in order to make confident, robust and safety-relevant predictions of future system behaviour, irrespective of repository host rock, engineered barrier system (EBS) and waste type. This requires an improved thermodynamic database and a refinement of existing modelling tools.
Incorporating as many interests of the partner organisations as possible, the following work modules were formulated.
- Field experiment(s): The HPF field experiment proved to be successful but the boundary conditions were not realistic. Therefore, The LCS in situ experiments will be set up to run under more relevant hydraulic gradients and, subsequently, enable plume observation over significantly longer time scales. In addition to using ordinary Portland cement (OPC) (pH 13.3 pore waters) several lower pH (pore waters pH 11 and less) cements will be used. Furthermore, the behaviour of both soft material (grout injection) and hard material will be studied in detail.
- Database development: Existing thermodynamic datasets for solid phases relevant to high and lower pH cementitious systems (e.g. C-S-H, C-A-S-H, clay minerals) will be combined and compared. This will provide a solid basis for the subsequent modelling task and highlight uncertainties and inconsistencies to resolve in the near future. Also, many simple solubility constants for radionuclides in alkaline to hyperalkaline conditions are highly uncertain (orders of magnitude) due to the little known metal complexation constants beyond the first hydroxide association.
- Thermodynamic modelling: A strong modelling module will be developed in order to bridge the time scales and different repository designs and materials and to enable robust and defensible predictions of the effects of a cement leachate plume on repository performance.
- Laboratory experiments are geared towards supporting process understanding, modelling efforts and in situ field experiments - particularly regarding low-pH cements.
- Alternative materials: A number of field experiments will be set up in parallel to investigate both OPC and, in particular, low-alkali cement.
The analytical program is not fully defined so far and depends, to some degree, on the available funds for the project. Generally, the planned experimental setup will enable studying a number of issues relevant for repository design. For example:
- Transport of Microbes and colloids
- Bentonite - cement interactions
- Influence of organic material (e.g. complexation, degradation), in particular the behaviour of super-plasticisers used in low pH cement
Soft and hard cement sources are emplaced in the advective flow system. The development and movement of the high-pH plumes will be monitored under near-natural and therefore repository relevant flow conditions. The basic concept of the setup of parallel LCS experiments is shown below:
- Emplacement of soft, respectively hard cement material through injection borehole into a fully saturated water conducting sheer zone. The natural flow field is in radial direction towards the tunnel wall.
- The flow field will be controlled by emplacing a recovery borehole located 1-2 metres closer to the tunnel. The flow rate (by natural outflow) can be adapted to obtain an optimal time scale of the experiment.
- Observation boreholes will be placed within the experiment flow field to obtain passive samples (circulation under natural pressure) of the high-pH plume.
Long Term Cement Studies (LCS) Concept
Long-Term Cement Studies (LCS) Experiment