JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Effect of Groundwater Radiolysis
on the Disposal System of High-level Radioactive Waste
M. Yamaguchi a) and M. Taguchi b)
a) Geological Isolation Research Unit, GIRDD, JAEA,
b) Environment and Industrial Materials Research Division, QuBS, JAEA
High-level radioactive waste (HLW) is planned to be
disposed of in a deep underground repository. In the
current Japanese design concept of the HLW disposal
system, a canister of HLW is encapsulated in a carbon steel
overpack. After the overpack finally loses its integrity and
the porewater comes into contact with the HLW (it is
typically assumed 10 3 years in performance assessment for
the disposal system in Japan), alpha-radiolysis would occur
and it is anticipated that radiolytic products such as H 2O 2
and O 2 may accelerate migration of radionuclides which are
more soluble and less sorptive to barriers in oxidized states.
Actual alpha-radiolytical process of groundwater would
be sensitive to some factors in the surrounding conditions
during system evolution. In particular, high concentrated
and dissolved H 2 which may arise due to anoxic corrosion of
overpack, would suppress radiolytic formation of oxidizing
species, since its effect is well known for low-LET
radiolysis of water. However, there are few experiments
on the effect under high-LET radiolysis of water such as
alpha-particles and model calculation could not explain the
absence of any effects by dissolved H 2 observed by 5 MeV
He 2+ irradiation 1) . Thus we have examined the effect of
dissolved H 2 by using helium ion beam from the AVF
cyclotron in TIARA and also performed model simulations on
the effect of the dissolved H 2.
Sample cells were 40 mm i.d. and 5 mm in depth and
cover glasses (0.15 mm thick) were attached with epoxy
adhesive. Gas saturated aqueous solutions were prepared
by bubbling either argon or hydrogen and they were
transferred to sample cells. H 2O 2 solutions (100 μmol
dm -3 ) were prepared by adding small aliquots of stock
solution to the cells. Samples were irradiated with 4 He 2+
ion beam accelerated to 50 MeV by the AVF cyclotron.
Incident energy was adjusted to approximately 15 MeV by
[H2O2] (mol dm -3 )
500
400
300
200 ]/
-3
m
l
d
o
m
µ
[H
100
0
0 2 4 6 8 10
2O2
Ar
H2
Time / min
Fig. 1 Hydrogen Peroxide concentration after helium ion
beam irradiation as a function of irradiation time:
(open) argon bubbled samples (solid) hydrogen
saturated samples.
JAEA-Review 2010-065
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placing an aluminum sheet (0.6 mm thick) on a sample cell.
Ion beam was scanned over 50 × 50 mm to cover the whole
area of the sample. Maximum irradiation time was
10 minutes and the accumulated doses were estimated to be
about several kGy.
Preliminary experiments were performed by using
sample cells made of Plexiglas. H 2O 2 was decomposed
almost completely in solutions saturated with H 2 by helium
ion beam radiolysis. However, experimental data were
scattered and decomposition of H 2O 2 after irradiation was
suspected 2) . New sample cells made of quartz glass were
prepared. Figure 1 shows the result with this setup. H 2O 2
concentration in degassed samples and samples saturated
with H 2 increased almost linearly with accumulated dose at
the same rate, indicating no effect of dissolved H 2 as
reported by Pastina and LaVerne 1) .
Recently Trummer and Jonsson have pointed out that the
absence of the effect of dissolved H 2 on high-LET radiolysis
of water can be explained by assuming the dose rate of the
actual irradiation volume which is about four orders larger
than the value averaged over the whole volume of the
sample 3) . Figure 2 shows this dose rate dependence also
hold for our experimental condition with 15 MeV He 2+ by
homogeneous model calculation.
This study is a part of the Project for assessment
methodology development of chemical effects on geological
disposal system funded by the Ministry of Economy, Trade
and Industry, Japan.
References
1) B. Pastina and J. LaVerne, J. Phys. Chem. A105 (2001)
9316.
2) M. Yamaguchi and M. Taguchi, Radiation Chemistry in
the 21st Century, P47 (2009).
3) M. Trummer and M. Jonsson, J. Nucl. Mater., 396
(2010) 163.
[H2O2] (mol dm -3 )
3600 Gy/s
12 Gy/s
Fig. 2 Calculated Hydrogen peroxide concentration as a
function of irradiation time at different dose rates.