3367
Thermal influences on swelling pressure and swelling deformation of bentonites and
investigation of its factors
Effets thermiques sur la pression et les déformations de gonflement des bentonites et facteurs
d’influence
Komine H.
Ibaraki University
ABSTRACT: Because buffers for disposal of high-level radioactive wastes must have high swelling characteristics for sealing wastes,
bentonite is currently designated for that use. High-level radioactive wastes generate decay-heat, and it is has been inferred that the
swelling characteristics of bentonite decline because of the decay-heat of wastes. This study investigated swelling pressure and
swelling deformation of bentonites during some thermal experiments conducted in the laboratory. Moreover, this study discusses the
mechanism of thermal influences to bentonite-swelling by considering the above experimentally obtained results with chemical
analyses such as measurement of cation concentration of water around the specimen, a methylene blue absorption test, and X-ray
powder method for heated bentonites.
RÉSUMÉ : Parce que les bouchons pour le scellement de déchets radioactifs de haut niveau doivent avoir de fortes caractéristiques de
gonflement pour fixer les containers, la bentonite est utilisée actuellement pour cet usage. Les déchets radioactifs de haut niveau
produisent une chaleur décroissante, et ceci entraîne une baisse des caractéristiques de gonflement de la bentonite. Ce travail étudie la
pression de gonflement et les caractéristiques de déformabilité des bentonites par des expérimentations thermiques menées en
laboratoire. De plus, on discute le mécanisme des effets thermiques sur le gonflement de la bentonite obtenus ci-dessus par une
analyse chimique incluant la concentration en cations, l’absorption du bleu de méthylène ou la diffraction de rayons X sur des poudres
de bentonite chauffée.
KEYWORDS:Bentonite, Swelling, Radioactive waste disposal, Clay minerals, Chemical properties, X-ray diffraction analysis
1 INTRODUCTION
Because buffer materials for disposal of high-level radioactive
wastes (HLWs) must have high swelling characteristics and
very low permeability to seal the waste, bentonite is currently
designated for that use.Ingeological disposal of high-level
radioactive wastes, bentonite-based buffer fills spaces between
the wastecontainer and bedrockbecause it has swelling
properties and low permeability. High-level radioactive wastes
generate decayheat. It is inferred that the swelling
characteristics of bentonite declineby the decayheat of wastes.
The author assessed the swellingdeformationof one kind of
sodium bentonite thathad undergonesome thermal exposure in a
laboratory in an earlier study(Komine and Ogata, 1998) as
preliminary research. Some researchers reported changes of soil
behavior during heating (Akagi, 1994; Oscarson and Dixon,
1989). It appearspossible that bentonite receives decay heat of
high-level radioactive wastejust as other soils do (see Fig. 1). In
a disposal pit, as Fig. 1 shows, the swelling characteristics of
bentonite might declinebecause of exposure to decay heat from
wastes. It is therefore necessary to investigate thermal effectson
swelling pressure and swelling deformation characteristics of
somebentonitesfor buffer material development.
To elucidatethis problem, this study investigated swelling
pressure and swelling deformation of bentonites that had
undergone some heatexposure in the laboratory. Moreover, this
study assessedmechanisms of thermal influences to bentonite-
swelling by examiningthe experimentally obtained results using
chemical analyses for the heated bentonites such as measuring
cation concentrations of water around the specimen, methylene
blue absorption tests, and X-ray powder method.
2 BENTONITE USED FOR THIS STUDY AND
THERMAL EXPOSURE CONDITIONS
Commercial bentonitesof two kinds (Table 1) were used.
Bentonite A, called Kunigel-V1, was produced at the Tsukinuno
Mine in Yamagata prefecture, Japan. This sodium-type
bentonite contains nearly 57% montmorillonite.It is used
frequently in Japan to studyartificial barrier materials against
radioactive waste. Bentonite C, called Kunibond, is produced at
the Dobuyama Mine in Miyagi prefecture, Japan. This calcium-
type bentonite hasnearly 80% montmorillonite content.
This study produced some bentonites that had undergone
different thermal exposure by oven drying to investigate the
thermal history influence onbentonite swelling characteristics.
The heating conditions for producing the heated bentonites were
60, 90, 110, and 130°C.Heating periods were28, 120, and 365
days.Those heating temperatureswere used to simulate
decayheating of HLWsbased on analytical results of maximum
temperatures in bentonite-basedbuffers, whichare65–165°C
(Japan Nuclear Cycle Development Institute, 2000) andon
results of previous research (Japan Nuclear Cycle Development
Institute, 2000) showing that montmorillonite will not be altered
to illite at less than 130°C.
Disposal
pit
Bentonite
based buffer
Bed rock
Container of high-level
radioactive waste
Bentonite
based backfill
Buffer
Disposal pit
Bedrock
Access tunnel
（
fillingbybackfill
）
Heating
Access
tunnel
It isapprehended
that theswelling
characteristicsof
bentonite declines
by the decay-heat
of wastes.
Waste
container
:Image of
decay-heat
Figure 1. High-level radioactive waste disposal facility and impact on
bentonite base buffers during decay-heating of wastes.
