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Density-Driven Mass Transfer in Repositories for Nuclear Waste
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
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. Vol. 205, no 6, p. 819-829
Keywords [en]
Radionuclide release rate, repositories, buoyancy-driven flow, channeling, equivalent flow rate
National Category
Physical Sciences Geophysical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-254128DOI: 10.1080/00295450.2018.1537460ISI: 000470315400006Scopus ID: 2-s2.0-85057240527OAI: oai:DiVA.org:kth-254128DiVA, id: diva2:1328028
Note

QC 2220190620

Available from: 2019-06-20 Created: 2019-06-20 Last updated: 2020-01-17Bibliographically approved
In thesis
1. Water density impact on water flow and mass transport in rock fractures
Open this publication in new window or tab >>Water density impact on water flow and mass transport in rock fractures
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
Tracer experiments, solute transport in fractured rock, advection-diffusion experiments, repositories, equivalent flowrate., Spårämnesexperiment, ämnestransport i bergssprickor, advektion och diffusionsexperiment, slutförvar, ekvivalent flöde.
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-264940 (URN)978-91-7873-403-0 (ISBN)
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

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Neretnieks, IvarsWinberg-Wang, Helen

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