Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions

Avner Melamed, Petteri Pitkänen

Research output: Book/ReportReportProfessional

Abstract

In the field of nuclear fuel disposal, bentonite has been selected as the principal sealing and buffer material for placement around waste canisters, forming both a mechanical and chemical barrier between the radioactive waste and the surrounding groundwater. Ion exchange and mineral alteration processes were investigated in a laboratory study of the long-term interaction between compacted Na-bentonite (Volclay MX-80) and groundwater solutions, conducted under simulated nuclear fuel disposal conditions. The possible alteration of montmorillonite into illite has been a major object of the mineralogical study. However, no analytical evidence was found, that would indicate the formation of this non-expandable clay type. Apparently, the change of montmorillonite from Na- to Ca-rich was found to be the major alteration process in bentonite. In the water, a concentration decrease in Ca, Mg, and K, and an increase in Na, HCO3 and SO4 were recorded. The amount of calcium ions available in the water was considered insufficient to account for the recorded formation of Ca-montmorillonite. It is therefore assumed that the accessory Ca-bearing minerals in bentonite provide the fundamental source of these cations, which exchange with sodium during the alteration process. X-ray powder diffraction (XRD) analyses and optical microscope observations of the initial and reacted bentonite samples were conducted. Quartz, feldspars, pyrite, calcite and minor amounts of gypsum were revealed as the primary accessories. In reacted samples, goethite and siderite were identified as secondary mineral products in association with corroded pyrite grains, while calcite and gypsum were found to disappear. From these results it is assumed that oxygen present in the water and in the bentonite pore space promotes the oxidation reaction of pyrite (dissolved) and the precipitation of goethite. As a result, the pore water pH decreases and calcite is partly dissolved. This dissolution provides a siginificant amount of calcium ions, in addition to those that arise by diffusion from the water. Some of the reaction-released bicarbonate and ferric ions are found to re-precipitate in the bentonite as siderite, while the rest (also as sulphate ions) diffuse into the water. Although the relative oxygen content in the experiment may be considered higher than that of the repository concept for nuclear fuel disposal (due to interaction in a semi-closed system with high water/bentonite ratio), the near field geochemistry predictions imply limited oxidising conditions, which are characterised by the above-described processes in sulphide-bearing bentonite and occur for some time after closure and sealing of the repository.
Original languageEnglish
Place of PublicationEspoo
PublisherVTT Technical Research Centre of Finland
Number of pages41
ISBN (Print)951-38-4961-9
Publication statusPublished - 1996
MoE publication typeNot Eligible

Publication series

NameVTT Tiedotteita - Meddelanden - Research Notes
PublisherVTT
No.1766
ISSN (Print)1235-0605
ISSN (Electronic)1455-0865

Fingerprint

bentonite
montmorillonite
water
pyrite
calcite
ion
siderite
sealing
goethite
repository
gypsum
ion exchange
calcium
chemical
nuclear fuel
mineral alteration
groundwater
secondary mineral
pore space
bicarbonate

Keywords

  • nuclear fuels
  • nuclear reactors
  • disposal
  • nuclear radiation
  • ground water
  • bentonite
  • interactions
  • chemistry
  • mineralogy
  • radioactive wastes
  • ion exchanging

Cite this

Melamed, A., & Pitkänen, P. (1996). Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions. Espoo: VTT Technical Research Centre of Finland. VTT Tiedotteita - Meddelanden - Research Notes, No. 1766
Melamed, Avner ; Pitkänen, Petteri. / Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions. Espoo : VTT Technical Research Centre of Finland, 1996. 41 p. (VTT Tiedotteita - Meddelanden - Research Notes; No. 1766).
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Melamed, A & Pitkänen, P 1996, Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions. VTT Tiedotteita - Meddelanden - Research Notes, no. 1766, VTT Technical Research Centre of Finland, Espoo.

Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions. / Melamed, Avner; Pitkänen, Petteri.

Espoo : VTT Technical Research Centre of Finland, 1996. 41 p. (VTT Tiedotteita - Meddelanden - Research Notes; No. 1766).

Research output: Book/ReportReportProfessional

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T1 - Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions

AU - Melamed, Avner

AU - Pitkänen, Petteri

PY - 1996

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N2 - In the field of nuclear fuel disposal, bentonite has been selected as the principal sealing and buffer material for placement around waste canisters, forming both a mechanical and chemical barrier between the radioactive waste and the surrounding groundwater. Ion exchange and mineral alteration processes were investigated in a laboratory study of the long-term interaction between compacted Na-bentonite (Volclay MX-80) and groundwater solutions, conducted under simulated nuclear fuel disposal conditions. The possible alteration of montmorillonite into illite has been a major object of the mineralogical study. However, no analytical evidence was found, that would indicate the formation of this non-expandable clay type. Apparently, the change of montmorillonite from Na- to Ca-rich was found to be the major alteration process in bentonite. In the water, a concentration decrease in Ca, Mg, and K, and an increase in Na, HCO3 and SO4 were recorded. The amount of calcium ions available in the water was considered insufficient to account for the recorded formation of Ca-montmorillonite. It is therefore assumed that the accessory Ca-bearing minerals in bentonite provide the fundamental source of these cations, which exchange with sodium during the alteration process. X-ray powder diffraction (XRD) analyses and optical microscope observations of the initial and reacted bentonite samples were conducted. Quartz, feldspars, pyrite, calcite and minor amounts of gypsum were revealed as the primary accessories. In reacted samples, goethite and siderite were identified as secondary mineral products in association with corroded pyrite grains, while calcite and gypsum were found to disappear. From these results it is assumed that oxygen present in the water and in the bentonite pore space promotes the oxidation reaction of pyrite (dissolved) and the precipitation of goethite. As a result, the pore water pH decreases and calcite is partly dissolved. This dissolution provides a siginificant amount of calcium ions, in addition to those that arise by diffusion from the water. Some of the reaction-released bicarbonate and ferric ions are found to re-precipitate in the bentonite as siderite, while the rest (also as sulphate ions) diffuse into the water. Although the relative oxygen content in the experiment may be considered higher than that of the repository concept for nuclear fuel disposal (due to interaction in a semi-closed system with high water/bentonite ratio), the near field geochemistry predictions imply limited oxidising conditions, which are characterised by the above-described processes in sulphide-bearing bentonite and occur for some time after closure and sealing of the repository.

AB - In the field of nuclear fuel disposal, bentonite has been selected as the principal sealing and buffer material for placement around waste canisters, forming both a mechanical and chemical barrier between the radioactive waste and the surrounding groundwater. Ion exchange and mineral alteration processes were investigated in a laboratory study of the long-term interaction between compacted Na-bentonite (Volclay MX-80) and groundwater solutions, conducted under simulated nuclear fuel disposal conditions. The possible alteration of montmorillonite into illite has been a major object of the mineralogical study. However, no analytical evidence was found, that would indicate the formation of this non-expandable clay type. Apparently, the change of montmorillonite from Na- to Ca-rich was found to be the major alteration process in bentonite. In the water, a concentration decrease in Ca, Mg, and K, and an increase in Na, HCO3 and SO4 were recorded. The amount of calcium ions available in the water was considered insufficient to account for the recorded formation of Ca-montmorillonite. It is therefore assumed that the accessory Ca-bearing minerals in bentonite provide the fundamental source of these cations, which exchange with sodium during the alteration process. X-ray powder diffraction (XRD) analyses and optical microscope observations of the initial and reacted bentonite samples were conducted. Quartz, feldspars, pyrite, calcite and minor amounts of gypsum were revealed as the primary accessories. In reacted samples, goethite and siderite were identified as secondary mineral products in association with corroded pyrite grains, while calcite and gypsum were found to disappear. From these results it is assumed that oxygen present in the water and in the bentonite pore space promotes the oxidation reaction of pyrite (dissolved) and the precipitation of goethite. As a result, the pore water pH decreases and calcite is partly dissolved. This dissolution provides a siginificant amount of calcium ions, in addition to those that arise by diffusion from the water. Some of the reaction-released bicarbonate and ferric ions are found to re-precipitate in the bentonite as siderite, while the rest (also as sulphate ions) diffuse into the water. Although the relative oxygen content in the experiment may be considered higher than that of the repository concept for nuclear fuel disposal (due to interaction in a semi-closed system with high water/bentonite ratio), the near field geochemistry predictions imply limited oxidising conditions, which are characterised by the above-described processes in sulphide-bearing bentonite and occur for some time after closure and sealing of the repository.

KW - nuclear fuels

KW - nuclear reactors

KW - disposal

KW - nuclear radiation

KW - ground water

KW - bentonite

KW - interactions

KW - chemistry

KW - mineralogy

KW - radioactive wastes

KW - ion exchanging

M3 - Report

SN - 951-38-4961-9

T3 - VTT Tiedotteita - Meddelanden - Research Notes

BT - Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions

PB - VTT Technical Research Centre of Finland

CY - Espoo

ER -

Melamed A, Pitkänen P. Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions. Espoo: VTT Technical Research Centre of Finland, 1996. 41 p. (VTT Tiedotteita - Meddelanden - Research Notes; No. 1766).