10th Asian Regional Conference of IAEG (2015)

Ferric oxyhydroxide in underground geological environments and high-level
radioactive waste disposal: influence on siting and site characterization
Hidekazu YOSHIDA(1)(1) Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8601 Japan
E-mail:dora@num.nagoya-u.ac.jp

Abstract
Ferric oxyhydroxide in the underground geological environments is an important factor
that may influence the long-term natural barrier function with respect to nuclide migration, inscenarios for the geological disposal of high-level radioactive wastes (HLW). Here, examplesof Ferric oxyhydroxide seen in the underground environment of Japan were reviewed in orderto: 1) evaluate suitable repository depths that will need to be characterized; and 2) assess how
Ferric oxyhydroxide will need to be treated in nuclide migration scenarios at a future
repository site. The Ferric oxyhydroxide examined provide the following insights into thefeatures relevant to site characterization and the rock’s barrier function: 1) Ferric
oxyhydroxide has been formed along the connective fracture network that extends from thesurface, but there is a certain depth limit of oxidation due to geochemical buffering by redoxreactions; 2) Past Ferric oxyhydroxide will remain even after the environment returns to a
reducing condition; 3) Ferric oxyhydroxide produced in the rock matrices, however, cancontribute to retarding nuclide migration by sorbing radionuclides and clogging micro-pores.
The evidence shows that Ferric oxyhydroxide formed in the geological environment may
influence on the siting as well as the geosphere barrier function in and around repository
caverns for the disposal of HLW constructed in the deep geological environment of Japan.
Keywords: Ferric oxyhydroxide, barrier function, radioactive waste, geological disposal

understanding the influence of Ferric oxyhydroxides
development into the host rock along the water
conducting fractures from the surface to repository
depth is recognized as a key issue to evaluate for thefuture site characterization (e.g. Hofman, 1999;Yoshida, 2012; Yamamoto et al., 2013; Fig.1). In
particular, to characterize the processes that control
the rate and depth of oxidizing water penetration into
the host rocks can be used to establish general criteria
for recognizing host rock bodies to evaluate thesuitability or unsuitability as host rocks for repository(Miller et al., 2000) within the orogenic environmentof Japan.

1.

Introduction
When siting and site characterization a deep
geological repository for long-lived radioactive
wastes within fractured host rocks is conducted, it has
to be characterized that oxidizing surface water will
not penetrate to repository depth along water
conducting fractures and not seriously influence on
the barrier system established (NUMO, 2002).
In Japan, presently fundamental studies are being
carried out in order to develop a technical basis for
disposal of vitrified radioactive waste (HLW),
although no host rock and specific site has been
selected (Committee for Geological Stability
Research, 2011). Within the scope of characterization,

2.

Studied materials & methodology
To evaluate long-term influence of Ferric
oxyhydroxides for the barrier function of repository
system, analogous natural evidence of Ferric
oxhydroxide formation in both crystalline rock andsedimentary rock have been investigated (e.g. Oono
& Yoshida, 2015; Yoshida et al., 2003; Yamamoto etal., 2013).
For the crystalline rock analysis, DH-5R1 deepborehole (depth of 530m) drilled by Japan Atomic
Energy Agency (JAEA) close to the UndergroundResearch Laboratory Site (MIU site) was studied1

10th Asian Regional Conference of IAEG (2015)(Akagawa et al., 2006; Yamamoto et al., 2013). This
granitic rock named Toki Granite is a stock-type
pluton and is composed of Ryoke-type granite that isrecognized as typical of Mesozoic ones distributed inJapan. Studied Ferric oxyhydroxides formed in redox
front developed along fractures was identified at thedepth of about 210m of drilled core as shown in
Figure 2. The redox front developed is seen along thegroundwater conducting fracture with the depth ofseveral cm into the matrices.
For the sedimentary rock, drilled cores excavatedin Shimanto Belt, what we call, accretionaly complex
distributed in southern part of Kyushu Island wasstudied. The site drilling was carried out toinvestigate for hydraulic dam construction. The
Shimanto belt consists of calcareous sandstone andmudstone intercalated. Redox front is seen in thefracture developed mainly in the calcareous sandstone
and the feature is quite similar of the crystalline rock(Oono & Yoshida, 2015).
Detailed morphological, mineralogical andgeochemical analysis were conducted in both type ofrocks by rock thin section, Scanning Electron
Microscopy (SEM) and X-ray fluorescence
spectrometer (XRF) respectively.

Optical microscopic observations in both
crystalline rock and calcareous sandstone revealed
that Ferric oxyhydroxides are mainly distributed at
the grain boundaries (Fig. 3) and also in the micro
fracture developed in feldspar and quartz grains. Thisfeature suggests that the Fe (Ferrous iron) content
water has been impregnated into the micro pore or
fracture connected from water conducting major
fracture by diffusion. In particular, Ferric
oxhyhdroxides are highly concentrated in micro poresof feldspar grains in crystalline rock or calcareous
matrix of sandstone probably due to high pH
conditions generated by buffering of Ca dissolution
from feldspar with penetrated groundwater as well as
diagenetic calcite filled in grain boundary. Thisgeochemical interaction is an important process of
Ferric oxhydroxides formation in host rock as
described in 3.3.

3.2 Geochemical characteristics of redox front
Profiles of selected elements across the redox
front developed in crystalline rock are shown inFigure 5. Elemental distribution clearly shows that Ca
is depleted from the redox front towards the fracturesurface. Such CaO loss commonly occurred during
low-temperaturewater-rockinteractionbypreferential leaching of Ca from feldspar andhornblend in crystalline rock and calcite precipitatedin grain boundary of sedimentary rock as well. Ca is avery common element containing all kind of rocks
and the result shows that the buffer reaction with Ca
dissolution from rock forming minerals is quite
effective process to control the impregnated Fe into
the rock matrices and also form Ferric oxhydroxides
to fill the micro pore.
Geochemical analysis revealed that uranium also
has been moved from the water conducting fracture
into the rock matrices and concentrated at the Ferric
oxihydroxide concentrated redox front. Akagawa et al.
(2006) shows that the uranium was co-precipitatedwith Ferric oxhydroxides during the uranium
migration. This strongly suggests that precipitatedFerric oxhydroxides in rock matrices could play arole of ‘barrier’ materials by its sorbtive function forraidonuclides and might also be influence on thesolubility of uranium by redox reaction (Baertschi etal., 1991; Drake et al., 2009).
These kinds of elemental migration from the

3. Results and discussions
3.1 Microscopic morphological features2

10th Asian Regional Conference of IAEG (2015)of some criteria to select suitable depth of therepository when future candidate site and host rock
will have been selected.

ground water conducting fracture surface into thevicinity rock matrices has been taken into account as
an analogue of ‘matrix diffusion’ considered as animportant process of nuclide retardation in safetyassessment of repository host rocks (Alexander et al.,
1990; Grisak et al., 1980).

4.

Conclusions
The work presented here tries to focus on theinfluence of Ferric oxyhydroxides inevitably
encountered in the certain depth of probably any
types of host rocks for HLW waste repository. The
results show several important general implicationsfor HLW disposal summarized as follows:
1) Even if in the site characterization redox front
with Ferric oxyhydroxides identified at depth of apossible HLW repository, it does not mean that thehost rock has lost the barrier function andunsuitability for repository site.
2) The pH-buffering of the host rocks(particularly Ca rich minerals) limits the extent towhich certain elements such as Fe even if conditions
do become oxidizing.
3) Ferric oxhydroxides formed in the redox front
during ancient process could contribute to the barrier
function of the biosphere by radionuclide sorbing.
This kind of analogous data can be used in thedevelopment of scenarios for the future evolution ofsite for HLW waste repositories. In particular, theobservations help to build confidence in the ability of
the geosphere to resist changes to redox conditions as
might be caused by uplift or increased recharge due
to climate change in orogenic field of Japan.

3.3 Redox front formation in rock matrices
Within both characteristics of Ferric oxhydroxidedeveloped along the water conducting fracture in
crystalline rock and sandstone (sedimentary rock), pH
buffering process of impregnated groundwater fromthe surface was a key function of development ofredox front i.e. Ferric oxyhydroxides precipitation inrock matrices. This is due to the dissolution of Ca
contained minerals such as feldspar and calcite ingrain boundary formed during diagenesis of
sedimentary rock.
Relatively acidic and reducing surface (Kamioka,1991) is readily generated with the reaction oforganic rich surface soil through and after
impregnated into the bed rock in Japan. The
impregnated surface water has been buffered
thereafter during transport through flow-path
fractures and reacted with the vicinity rock forming
minerals. This means that the reaching depth of redox
front is considered to have certain limit depend on therock types containing Ca in host rocks.
3.4 Application to HLW disposal
Natural evidence shows that redox front can be
developed inevitably in the bed rocks along groundwater conducting fractures. When developing safetycase for a future repository, it is important to provide
evidence that redox front at certain depth is notindicative of presently oxidizing conditions. It is also
necessary to show that future oxidizing environment
could not propagate downwards to the level of waste
emplacement for safety assessment timescale.
The observations and analysis carried out forunderstanding redox front development and influenceof oxhydroxides on barrier function show that there iscertain limit of redox front development throughfractures. In particular, natural feature implies that
dissolved oxygen has been consumed by water/rockinteractions as it diffuses into the rock matrices
during downward groundwater flow through fractures.
The downward depth is also related to the uplifting
rate of the area. In Japan, no candidate host rock type
has not been selected and many areas of country has arate of less than 1 mm/a. Nevertheless, it is importantto build confidence that redox fronts will not bepenetrated to repository depths in the future. Inparticular, several observations shown here revealed
that calcareous bed rock can be more effectively
buffered penetrated acidic surface water to retain
Ferric oxhyorxides in shallower depth from thesurface. This feature can be used for the knowledge

Acknowledgements
The authors are grateful to Drs. Yamamoto andTakeuchi in the Graduate School of Environmental
studies in the Nagoya University and Dr Nishimoto ofthe Nagoya City Science Center for the geochemical
data analysis and mineralogical characteristics as wellas the discussion of the process of redox frontformation.

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