Detailed Modelling for the Postclosure Safety Assessment of OPG’s DGR.

As part of the postclosure safety assessment undertaken for the
proposed Deep Geologic Repository (DGR) for Low and Intermediate
Level Waste (L&ILW) at the Bruce nuclear site, calculations were
undertaken to evaluate the repository’s potential postclosure impacts.
Impacts were evaluated for a Normal Evolution Scenario, describing
the expected long term evolution of the repository and site following
closure, and for several Disruptive Scenarios, which consider events
that could lead to possible penetration of barriers, abnormal degradation,
and/or loss of containment.
The postclosure modelling was conducted using both detailed
models with three-dimensional representation of the repository
geometry, and with an overall (system) assessment-level model. The
purpose of the detailed modelling described in this paper was to
evaluate postclosure performance in terms of repository pressures,
repository resaturation levels, mass flow rates of gas at various levels
in the shaft, groundwater flow and radionuclide transport through the
saturated geosphere and shaft seals, and capture rates of radionuclides
by a hypothetical water supply well. The results of the detailed
modelling were used to inform overall assessment-level (system)
modelling that was performed using the compartmental modelling
code AMBER and which is described in a companion paper.
Two separate detailed modelling studies were undertaken: 1) generation
of gas in the repository, repository resaturation, and transport
of gas through the geosphere and shaft sealing system was
simulated using T2GGM, a modified version of the TOUGH2 gas
transport code with coupled gas generation, and 2) transport of
groundwater and radionuclides through the saturated geosphere and
shaft seals was simulated with FRAC3DVS-OPG.
The gas generation model (GGM) incorporated within T2GGM was
developed expressly for the L&ILW waste that will be present at the
DGR. GGM calculates generation and consumption of oxygen,
hydrogen, carbon dioxide, methane, hydrogen sulphide and nitrogen
from degradation of the various organic and metallic waste streams.
Results from the Normal Evolution Scenario groundwater model
show that radionuclide transport is diffusion dominated and very
slow, with virtually no transport beyond the immediate vicinity of the
repository. Results from gas modelling indicate that the repository
will take hundreds of thousands of years to resaturate, and that there
will be no gas flow within the shaft for the Reference Case and most
Normal Evolution sensitivity cases. In the few cases where gas flow
in the shaft does occur, it is restricted to lower portions of the shaft.
No gas enters the shallow groundwater system. The gas modelling
also indicates that in most cases, repository pressures will equilibrate
near the expected steady-state in-situ pressure. In no cases do
the gas pressures exceed the lithostatic pressure.