Ontario Power Generation's Response to the Joint Review

Attachment 1 to OPG letter, Albert Sweetnam to Dr. Stella Swanson, “Deep Geologic Repository Project for Low and Intermediate Level Waste – Submission of
Responses to the Final Sub-set of Package #4 Information Requests”, CD#: 00216-CORR-00531-00143.
IR# EIS Guidelines
Section
Information Request and Response
concentrations, and are not used directly in the calculation of solubilities. Instead, the approach is to reconstruct a
model porewater composition using the measured element concentrations, charge-balance, and equilibration with the
dominant minerals present in the formation.
For safety assessment calculations, this approach was applied to porewater data for “DGR3 680.46” as a basis for
estimating in-situ water compositions (Appendix C.2.1 of the Data report (QUINTESSA and GEOFIRMA 2011) using
the geochemical code, PHREEQC. The resulting Cobourg Model 2 porewater is charge-balanced and is also
consistent with the major mineralogy of the rock and the chemical type of the water (i.e., Na-Cl dominated). The
Cobourg Model 2 porewater composition is used for the purposes given in Section c) below.
c) Use of the Cobourg Model 2 Porewater in the Postclosure Safety Assessment
The chemical composition of the water in the DGR will be affected by interaction with Cobourg porewater, by the
engineered features in the repository (e.g., concrete floors) and the bulk materials present in the waste packages, and
by any water infiltrating from the shafts. The characteristics of the resulting repository water are described in Section
3.6.2 of the Data report (QUINTESSA and GEOFIRMA 2011).
Given the uncertainties associated with reconstructing porewater compositions and the evolution of repository water
chemistry, a conservative model is adopted in the postclosure safety assessment for the release of contaminants from
the waste, and their subsequent migration through the DGR, that is not sensitive to the precise composition of the
Cobourg Model 2 porewater:
� Instantaneous release is assumed for all wastes on contact with water, except for certain ILW waste where the
contamination is present in the matrix of the materials (and so a congruent release is adopted; see Section
8.6.2.2 of the Preliminary Safety Report (PSR) (OPG 2011).
� Once released into the DGR, the contaminants are assumed not to be sorbed in the repository (see Section
8.6.2.3 of the PSR).
� No solubility limits are applied to the release and migration of contaminants in the DGR, except for C-14, which
is affected by carbon solubility limits since carbonate equilibria control is assumed due to the surrounding
limestone rock (see Section 8.6.2.2 of the PSR).
� The waste corrosion/degradation rates used in the modelling are applicable to the range of expected water
chemistries in the DGR. These are given in Sections 3.6.5 and 3.6.6 of the Data report (QUINTESSA and
GEOFIRMA 2011) and their derivation is described in Appendix E and F of the report.
Although the Cobourg Model 2 porewater is not directly used in the postclosure safety assessment’s contaminant
release and repository migration calculations, it is used as follows:
� Derivation of possible solubility limits in the repository. However, as noted above, only carbon is assigned a
solubility limit within the repository (see Section 3.6.3.2 of the Data report, QUINTESSA and GEOFIRMA
2011).
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