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Water density impact on water flow and mass transport in rock fractures
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Chemical Engineering. (Nuclear Waste Engineering)
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

One way of taking care of spent nuclear fuel is to place it in a geological repository. In Sweden, a three-barrier system is planned. The system is based on encapsulating the fuel in copper canisters. These are surrounded by bentonite clay and buried under 500 m of bedrock. As a part of the safety assessment, the Q-equivalent model is used to quantify the possible release of radioactive material. This model also describes the rate at which corrosive agents carried by seeping water in rock fractures can reach the canisters, which may affect the longevity of the canisters.

The aim of this thesis was originally to develop an experimental, phys- ical model to visualize and validate the Q-equivalent model. However, the overarching theme of this work has been to study the effect of minor density differences that might be overlooked in experiments, both concentration- dependent and density-difference induced by light absorption.

In the initial diffusion and flow-experiment and associated calculations and simulations, it was found that simple Q-equivalent can describe and quantify the mass transport in both parallel and variable aperture fractures. However, this is the case only if the density difference between seeping water and clay pore water is insignificant. It was found in experiments with dyes used to visualise the flow and diffusion patterns that even minimal density differences could significantly alter the flow pattern. Density differences can result from concentration gradients or be induced by light absorption. TheQ-equivalent model was extended to account for density-induced flow. The importance of density-induced flow due to concentration gradients at the setting of a long-term repository for nuclear waste was evaluated. It was found that concentration gradients are able to induce rapid vertical up- or downward flow. This could increase the overall mass transport of radioactive material up to the biosphere or carry it downward to larger depths.

Abstract [sv]

Ett sätt att ta hand om använt kärnbränsle är att placera det i ett geologiskt slutförvar. I Sverige planeras ett tre-barriärsförvar. Konceptet baseras på att kapsla in det förbrukade bränslet i koppar kapslar som sedan placeras 500 m ner i berggrunden och omslutas av bentonitlera. Som en del av säkerhetsanalysen, används Q-ekvivalent modellen f ̈or att kvantifiera det möjliga utsläppet av radioaktivt material. Modellen beskriver också med vilken hastighet korrosivaämnen som färdas med det långsamma vattnet i sprickorna kan nå kapslarna, vilket kan påverka dess livslängd. 

Målet med denna avhandling var ursprungligen att utveckla en experimentell uppställning for att visualisera och validera Q-ekvivalent modellen. Det  övergripande temat har varit att studera effekten av små densitetsskillnader som kanske förbises i experimenten. 

I de initiala diffusion och flödesexperiementen med tillhörande beräkningar och simuleringar, framkom det att enkla Q-ekvivalent modellen kan beskriva och kvantifiera masstransport i både parallella och sprickor med oregelbunden apertur. Dock, är det endast om densitetsskillnaden mellan det sipprande vattnet och porvattnet i bentonitleran är försumbar. Det visade sig att i experiment, med färgämnen för att visualisera flödes- och diffusionsfördelningar, kan minimala densitetskillnader påverka flödesfördelningen avsevärt. Densitetsskillnaderna berodde antingen på grund av koncentrations gradienter eller ljusabsorption. Q-ekvivalent modellen utökades för att ta hänsyn till densitetsdrivet flöde. Betydelsen av densitetsdrivet flöde orsakat av koncentrationsskillnader för slutförvar av radioaktivt avfall utvärderades. Det fanns att koncentrationsgradienter kan inducera snabba vertikala upp- och nergående flöden. Detta kan öka den totala masstransporten av radioaktivt material upp till biosfären eller föra det neråt till större djup. 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2020. , p. 59
Series
TRITA-CBH-FOU ; 2020:3
Keywords [en]
Tracer experiments, solute transport in fractured rock, advection-diffusion experiments, repositories, equivalent flowrate.
Keywords [sv]
Spårämnesexperiment, ämnestransport i bergssprickor, advektion och diffusionsexperiment, slutförvar, ekvivalent flöde.
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-264940ISBN: 978-91-7873-403-0 (print)OAI: oai:DiVA.org:kth-264940DiVA, id: diva2:1375818
Public defence
2020-01-31, K1, Teknikringen 56, Stockholm, 14:00 (English)
Opponent
Supervisors
Funder
Swedish Nuclear Fuel and Waste Management Company, SKB, C7246
Note

QC 2019-12-06

Available from: 2019-12-06 Created: 2019-12-06 Last updated: 2019-12-06Bibliographically approved
List of papers
1. Diffusion in a Variable Aperture Slot: Impact on Radionuclide Release from a Repository for Spent Fuel
Open this publication in new window or tab >>Diffusion in a Variable Aperture Slot: Impact on Radionuclide Release from a Repository for Spent Fuel
2018 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 204, no 2, p. 184-194Article in journal (Refereed) Published
Abstract [en]

Diffusion experiments under stagnant conditions in a constant aperture and a variable aperture slot were made to obtain data for simulation of simultaneous flow and diffusion in fractures. This approach was necessitated by the need to avoid buoyancy-induced flow caused by density differences generated by the presence of a tracer. For this purpose, to avoid flow but negligibly influence diffusion the slots were filled with agar, which generates a 99% porous matrix, which negligibly affects diffusion but essentially stops flow. A simple photographic technique was used to follow diffusion and to determine the aperture distribution on the variable aperture slot. With the obtained data, numerical simulations were performed to illustrate how a solute diffuses from a source into the water seeping past. The results support the simple analytical solution that has been used to determine the escape of radionuclides from a damaged canister containing spent nuclear fuel in a geologic repository in fractured rock.

Place, publisher, year, edition, pages
TAYLOR & FRANCIS INC, 2018
Keywords
Solute diffusion, rock fracture diffusion, radionuclide escape
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-238167 (URN)10.1080/00295450.2018.1469348 (DOI)000447370000006 ()2-s2.0-85048357687 (Scopus ID)
Note

QC 20181107

Available from: 2018-11-07 Created: 2018-11-07 Last updated: 2020-01-17Bibliographically approved
2. Density-Driven Mass Transfer in Repositories for Nuclear Waste
Open this publication in new window or tab >>Density-Driven Mass Transfer in Repositories for Nuclear Waste
2019 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 205, no 6, p. 819-829Article in journal (Refereed) Published
Abstract [en]

In geologic repositories for nuclear waste located in crystalline rocks, the waste is surrounded by a bentonite buffer that in practice is not permeable to water flow. The nuclides must escape by molecular diffusion to enter the seeping water in the fractures of the rock. At high water-seepage rates, the nuclides can be carried away rapidly. The seepage rate of the water can be driven by the regional hydraulic gradient as well as by buoyancy-driven flow. The latter is induced by thermal circulation of the water by the heat produced by radionuclide decay. The circulation may also be induced by salt exchange between buffer and water in the fractures. The main aim of this paper is to explore how salt exchange between the backfill and mobile water in fractures, by buoyancy effects, can increase the escape rate of radionuclides from a repository. A simple analytical model has been developed to describe the mass transfer rate induced by buoyancy. Numerical simulations support the simple solution. A comparison is made with the regional gradient-driven flow model. It is shown that buoyancy-driven flow can noticeably increase the release rate.

Place, publisher, year, edition, pages
Taylor & Francis, 2019
Keywords
Radionuclide release rate, repositories, buoyancy-driven flow, channeling, equivalent flow rate
National Category
Physical Sciences Geophysical Engineering
Identifiers
urn:nbn:se:kth:diva-254128 (URN)10.1080/00295450.2018.1537460 (DOI)000470315400006 ()2-s2.0-85057240527 (Scopus ID)
Note

QC 2220190620

Available from: 2019-06-20 Created: 2019-06-20 Last updated: 2020-01-17Bibliographically approved
3. A Note on the Use of Uranine Tracer to Visualize Radionuclide Migration Experiments: Some Observations and Problems
Open this publication in new window or tab >>A Note on the Use of Uranine Tracer to Visualize Radionuclide Migration Experiments: Some Observations and Problems
2019 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 205, no 7, p. 964-977Article in journal (Refereed) Published
Abstract [en]

Uranine is a dye commonly used in tracer experiments; it is chosen for its high visibility even at low concentrations. Uranine solutions are slightly denser than water at the same temperature. However, in laboratory experiments uranine solutions have been known to occasionally show unpredictable flow behaviors. This paper investigates the possible effect of light-induced density change to explain some of these behaviors. Uranine has a wide light absorption spectrum for visible light, which can heat the dye solution and lower its density to below that of the surrounding water, which induces buoyancy-driven flow. Simulations are made in both one dimension and two dimensions to determine the extent of the effect. The results are then compared to different experiments with unanticipated flow patterns.

Place, publisher, year, edition, pages
Taylor & Francis, 2019
Keywords
Uranine, fluorescein tracer experiments, flow patterns in fractures, light-induced density-driven flow, buoyancy flow in fractures
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-255342 (URN)10.1080/00295450.2019.1573620 (DOI)000472556500009 ()2-s2.0-85063529203 (Scopus ID)
Conference
American-Nuclear-Society (ANS) Student Conference, APR 05-07, 2018, Univ Florida, Gainesville, FL
Note

QC 20190807

Available from: 2019-08-07 Created: 2019-08-07 Last updated: 2020-01-17Bibliographically approved
4. Visualisation of mass transfer between source and seeping water in a variable aperture fracture-Impact of tracer density
Open this publication in new window or tab >>Visualisation of mass transfer between source and seeping water in a variable aperture fracture-Impact of tracer density
(English)Manuscript (preprint) (Other academic)
Abstract [en]

An experiment with a vertical slot with horizontally seeping water with a dye diffusing from below was performed to help validate and visualise the Q-equivalent model, which describes the mass transfer rate from a source into flowing water such that in a repository for nuclear waste. The Q-equivalent model is used for quantifying mass transport in geological repositories. However, the tracer propagated much slower and to a lesser extent than predicted by the model. It was found that the tracer gave rise to a small density gradient, which induced buoyancy-driven flow, overwhelming that driven by the horizontal hydraulic gradient. This dramatically changed the mass transfer from the dye source into the water in the slot. For the release of contaminants, this can have detrimental as well as beneficial effects, depending on positive or negative buoyancy is induced. These observations led to an analysis of when and how density differences in a repository can influence the release and further fate of escaping radionuclides in waste repositories. This and other experiments also showed that laboratory experiments aimed to visualise flow and mass transfer processes in fractures could be very sensitive to the heating of the dye tracers by the lighting in the laboratory. 

Keywords
tracer experiments, repositories, safety assessment
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-264913 (URN)
Funder
Swedish Nuclear Fuel and Waste Management Company, SKB, C7246
Note

QC 20191212

Available from: 2019-12-04 Created: 2019-12-04 Last updated: 2020-01-17Bibliographically approved

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