Thermal properties Forsmark. Modelling stage 2.3 Complementary analysis and verification of the thermal bedrock model, stage 2

This report present the results of thermal modelling work for the Forsmark area carried out during modelling stage 2.3. The work complements the main modelling efforts carried out during modelling stage 2.2. A revised spatial statistical description of the rock mass thermal conductivity for rock domain RFM045 is the main result of this work. Thermal modelling of domain RFM045 in Forsmark model stage 2.2 gave lower tail percentiles of thermal conductivity that were considered to be conservatively low due to the way amphibolite, the rock type with the lowest thermal conductivity, was modelled. New and previously available borehole data are used as the basis for revised stochastic geological simulations of domain RFM045. By defining two distinct thermal subdomains, these simulations have succeeded in capturing more of the lithological heterogeneity present. The resulting thermal model for rock domain RFM045 is, therefore, considered to be more realistic and reliable than that presented in model stage 2.2. The main conclusions of modelling efforts in model stage 2.3 are: - Thermal modelling indicates a mean thermal conductivity for domain RFM045 at the 5 m scale of 3.56 W/(mK). This is slightly higher than the value of 3.49 W/(mK) derived in model stage 2.2. - The variance decreases and the lower tail percentiles increase as the scale of observation increases from 1 to 5 m. Best estimates of the 0.1 percentile of thermal conductivity for domain RFM045 are 2.24 W/(mK) for the 1 m scale and 2.36 W/(mK) for the 5 m scale. This can be compared with corresponding values for domain RFM029 of 2.30 W/(mK) for the 1 m scale and 2.87 W/(mK)for the 5 m scale. - The reason for the pronounced lower tail in the thermal conductivity distribution for domain RFM045 is the presence of large bodies of the low-conductive amphibolite. - The modelling results for domain RFM029 presented in model stage 2.2 are still applicable. - As temperature increases, the thermal conductivity decreases. This temperature dependence tends to decrease as the thermal conductivity decreases. - Heat capacity: Domains RFM029 and RFM045 have a mean heat capacity of 2.06 MJ/(m3K) and 2.15 MJ/(m3K) respectively. - The mean in situ temperatures at 400 m, 500 m and 600 m depth are estimated at 10.5 deg C, 11.6 deg C, and 12.8 deg C respectively, and are therefore unchanged compared to model stage 2.2. - The estimates of the TRC (thermal rock class) proportions in domain RFM029 are considerably more reliable than those for domain RFM045. For the latter, the small number of boreholes in combination with the higher degree of lithological heterogeneity results in rather large uncertainties in the estimated proportions. - The aspect of the thermal model with the highest confidence is the thermal conductivity distribution of domain RFM029, because of its higher degree of lithological and thermal homogeneity compared to domain RFM045 - The aspect of the thermal model with the lowest confidence is the lower tail of the thermal conductivity distribution for rock domain RFM045. This uncertainty is related to the spatial and size distribution of amphibolite in domain RFM045.