Spatial distribution of desaturation around the tunnel predicted by three-dimensional two-phase flow modeling of the degassing process of dissolved gases in groundwater

Miyakawa, Kazuya; Yamamoto, Hajime*

JAEA-Research 2022-003, 40 Pages, 2022/05

JAEA-Research-2022-003.pdf:6.08MB

The excavation of large-scale underground facilities, such as geological disposal of high-level radioactive waste, creates an excavation damaged zone (EDZ) with cracks around the tunnel. In the EDZ, oxygen invades the bedrock through unsaturated cracks and affects environmental conditions for nuclide migration. When a tunnel is excavated in a geological formation containing a high concentration of dissolved CH, such as the Neogene marine sediments, degassed CH prevents oxygen intrusion. However, it may be promoted through gas-phase diffusion through desaturation. The purpose of this study is to illustrate the method of estimating the spatial distribution of desaturation associated with the construction and operation of underground facilities in a stratum that contains a large amount of dissolved CH. A sequential excavation analysis that reflected the actual process of 10-year excavation of the Horonobe Underground Research Laboratory (URL) was carried out along with gas-water two-phase flow analysis. The analysis results of the amount of groundwater and gas discharged from the URL were about 100 to 300 m d and 250 to 350 m d, respectively, as of January 2017. These results showed values close to the observations (100 m d and 300 m d, respectively). The analysis results of the saturation distribution were relatively high around the 250 m gallery and relatively low around the 350 m gallery, confirming that they are consistent with the in-situ observations. Although there were still technical issues of analysis regarding the conditions for groundwater drainage from the tunnel wall and the method of handling grout effects, the numerical calculation was generally appropriate. Although the results of the saturation distribution associated with the excavation were insufficient as the quantitative evaluation, they were almost correct from a qualitative point of view.

Numerical simulation of oxygen infusion into desaturation resulting from artificial openings in sedimentary formations

Miyakawa, Kazuya; Aoyagi, Kazuhei; Akaki, Toshifumi*; Yamamoto, Hajime*

Dai-15-Kai Iwa No Rikigaku Kokunai Shimpojiumu Koen Rombunshu (Internet), p.609 - 614, 2021/01

Desaturation is expected due to excavation of an underground repository, especially in the newly created fractures zone (EDZ). During the construction and operation of facilities, the air in the gallery infuses into the rock around the gallery though the excavation affected area and causes oxidation of host rock and groundwater, which increase nuclide mobilities. In the Horonobe underground research laboratory (HURL), which is excavated in the Neogene sedimentary formations, no pyrite dissolution or precipitation of calcium sulfates was found from the cores drilled in the rock around the gallery. The reason for no oxidation is estimated that the release of dissolved gases from groundwater due to pressure decrease flows against the air infusion. In this research, the mechanism of O intrusion into the rock was investigated by numerical multiphase flow simulation considering advection and diffusion of groundwater and gases. In the simulation, only Darcy's and Henry's laws were considered, that is, chemical reaction related to oxidation was not handled. The effects of dissolved gas and rock permeability on O infusion into the rock were almost identical. Decreasing humidity with relatively low permeability leads to extensive accumulation of O into the EDZ even though with a relatively large amount of dissolved gas. In the HURL, the shotcrete attenuates O concentration and keeps 100% humidity at the boundary of the gallery wall, which inhibits O infusion. Without the shotcrete, humidity at the gallery wall decreases according to seasonal changes and ventilation, which promotes O intrusion into the EDZ but the chemical reaction related to O buffering such as pyrite oxidation consumes O.