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Workreport 2017-3

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Name:

Modelling of Chemical Influences from Posiva’s Low and Intermediate Level Waste Repository on the Spent Nuclear Fuel Repository

Writer:

Höglund, L., Sidborn, M., Crawford, J., Keith-Roach, M., Hoek, J., Grundfelt, B.

Language:

English

Page count:

284

Summary:

Low and intermediate level waste (LILW) from the operation and decommissioning of Posiva’s encapsulation plant is planned to be disposed of in a repository hall to be located at 180 m depth close to the canister shaft adjacent to the access tunnel to the future spent nuclear fuel (SNF) repository at Olkiluoto. In a previous review of a preliminary safety assessment of the LILW repository, it was identified that the potential transport of LILW repository leachate to the SNF repository should be investigated quantitatively.

The current project sets out to identify, quantify and assess possible chemical disturbances from the LILW repository to the SNF repository using, among other tools, reactive transport modelling. The aim is to assess if the dilution and chemical buffering reactions in the geological barrier between the LILW repository and the SNF repository are sufficient to limit the input of detrimental substances to the SNF repository to negligible levels. The potential influences investigated are the formation of a high-pH plume by leaching of the concrete in the LILW repository, formation of isosaccharinic acid from alkaline degradation of cellulose, migration of high-density fluids due to dissolution of salts from the waste and formation of hydrogen gas from corrosion of steel in the waste and in the reinforced concrete structures.

The results of the project indicate that:

·     It is unlikely that the alkalinity in the leachate will reach the barriers in the SNF repository.

·     Using OPC-concrete in all constructions in the LILW repository increases the pH in the leachate compared to the Base case, in which the repository floor is made of high-silica (“low-pH”) concrete. However, the dilution and chemical buffering of the leachate downstream from the LILW repository appears to reduce the pH of the water reaching the SNF repository to near ambient levels.

·     Reactions between the mineral assemblies at the interface between the OPC-based concrete and the high-silica concrete potentially affect the mechanical strength and integrity of the floor.

·     Isosaccharinic acid will form in the LILW repository. The analysis indicates that the dilution along the downstream flow path is sufficient to avoid detrimental effects of the isosaccharinic acid in the SNF repository.

Keywords:

Low and intermediate level waste, low and intermediate level waste repository, chemical interaction, reactive transport, concrete leaching, buffering, dilution

File(s):

WR 2017-03_web (pdf) (67.7 MB)


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