Background
Resaturation processes in the bentonite buffer in a KBS-3 type repository for spent nuclear fuel are complicated and are often illustrated, analysed and modelled multi-disciplinarily as coupled thermal (T), hydrological (H) and mechanical (M) processes with multi-phase flow, elastoplastic evolution in a swelling porous medium. Previous THM-modelling showed that the re-saturation time is strongly dependent on the hydraulic conditions in the near-field of the repository. Moreover, it is difficult to predict the degree of homogeneity during the processes of resaturation.
Some of the safety functions of bentonite buffer will not start working until the bentonite buffer becomes fully water saturated and the buffer material becomes homogenised. A slow resaturation process with uneven swelling of the bentonite buffer in a spent fuel repository is an important factor that may cause creep deformation and local corrosion of the copper canister during the early period of evolution of the repository. Creep deformation and local corrosion have been identified as decisive mechanisms for the long-term integrity of the canister.
Objective
Researches of the resaturation processes make it possible to better estimate the resaturation time and to better illustrate the homogeneity of the buffer material during the resaturation processes. Deep knowledge of both the estimate of resaturation time and the illustration of homogeneity are important for SSM's judgement of the long-term safety of spent fuel repository.
Results
This report consists of three parts: (1) Research on Resaturation of Bentonite Buffer – Hydrological Modelling; (2) Research on Resaturation of Bentonite Buffer – Modelling of Resaturation, and (3) Research on Resaturation of Bentonite Buffer – Coupling between Bentonite and Rock. The aim of the first part is mainly to supply boundary conditions of groundwater flow to the second part of coupled THM-modelling. The third part focuses on modelling of the coupled processes at the interface between bentonite and the bedrock.
The hydrological modelling reveals that
- Increase of groundwater pressure in the bedrock after closure of the repository could increase the transmissivity of horizontally oriented fractures by about a factor of 3.
- Shear displacements due to themomechanical or glacial induced stresses could give significant increases in fracture transmissivity.
- Excavation-induced fractures would be encountered only in the upper 0.5 to 1 m of a deposition hole.
- Thermally induced spalling could result in a spalled zone up to 10 cm deep and 30 cm wide extending vertically along the deposition hole.
The results of the saturation modelling are as follows
- Peak temperatures in the buffer were largely insensitive to the various fracture modelling options that were considered and were well within limits specified by SKB.
- Timescales for resaturation were found to be very sensitive to the choice of whether the bentonite pellet region was explicitly considered in the model.
- Timescales for resaturation were sensitive to the representation of the fracture intersection with the deposition hole, varying by more than a factor of four depending on whether the fracture was assumed to be planar, intersecting the deposition hole along the entire circumference, or whether it was assumed to be channelled, intersecting the deposition hole over a smaller area. In several cases, remote regions of the buffer did not achieve full saturation over the simulation timescales (up to 4 000 y).
- In THM calculations, gradients along the length of the canister of normal stresses to the canister surface were found to persist for the duration of the resaturation period and were greatest when resaturation was most rapid.
- As far as the alternative conceptual models are concerned, the threshold gradient model gives slower resaturation but no changes of spatial distribution of stress. The thermal osmosis model also has little effect on the results other than an increase in drying around the canister at early times. The micro-fabric evolution model significantly increases resaturation time but the stress gradients along the canister become lower.
The modelling of coupling between bentonite and rock gives the results that
- The pellet-filled slot acts as a fast pathway to distribute the water intake from the fracture onto the outer surface of the bentonite block, and thus amplifies the water intake from the fracture.
- A rough but fully connected fracture yields the same buffer resaturation time as a homogeneous parallel plate.
- Flow channelling in the fracture near the deposition hole can affect the buffer resaturation time significantly.