Chapter 26
Environmental Transfer of Carbon-14
in Japanese Paddy Fields
Nobuyoshi Ishii, Shinichi Ogiyama, Shinji Sakurai, Keiko Tagami,
and Shigeo Uchida
Abstract It has been recognized that carbon-14 (
14 C) is one of the dominant
radionuclides affecting dose from transuranic (TRU) wastes. This radionuclide
has a decay half-life of 5,730 years, and
14
C organic materials have very low
sorption properties to clay and rock in the environment, which raises some concerns
about the releases of
14 C to the biosphere from radioactive waste repositories. For
the safety assessment of TRU waste disposal, we studied the behavior of
14 C in rice
paddy field soils. We also determined key parameters such as soil–soil solution
distribution coefficients (K d s) and soil-to-rice plant transfer factors (TFs) of
14
C in
the field soils. The TFs were obtained in laboratory and field experiments. In our
laboratory experiments, we used [1,214 C] sodium acetate as a source of
14
C
because it has been suggested that low molecular weight organic14 C compounds
are released from metallic TRU wastes. The results showed that
14 C-bearing
sodium acetate in irrigated paddy soils was rapidly decomposed by indigenous
bacteria. Although some of the
14
C was assimilated into the bacterial cells, most of
the
14 C was released into the air as gaseous compounds. The main chemical species
of
14
C gases was
14 CO 2 , and a part of the released
14 CO 2 gas was used by rice plants
during photosynthesis. Only a negligible amount of
14 C was absorbed through the
roots. Therefore, the contamination of rice plants is mainly caused by gasification of
14 C, and microorganisms are responsible for driving this process. The activity of
microorganisms is a key issue in the behavior of
14 C in paddy fields.
Keywords Bacteria • Behavior • Degradation • Radiocarbon • Rice paddy fields
• Safety assessment • TRU wastes
26.1 Introduction
Transuranic (TRU) wastes contain a variety of radionuclides, for example, Np, Pu,
and long-lived radionuclides such as
14 C and
129 I. In Japan these wastes are
categorized into four groups in accordance with their physical properties and the
N. Ishii (*) • S. Ogiyama • S. Sakurai • K. Tagami • S. Uchida
Research Center for Radiation Protection, National Institute of Radiological Sciences,
4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
e-mail: nobu@nirs.go.jp
© The Author(s) 2015
K. Nakajima (ed.), Nuclear Back-end and Transmutation Technology for Waste
Disposal, DOI 10.1007/978-4-431-55111-9_26
303
Environmental Transfer of Carbon-14
in Japanese Paddy Fields
Nobuyoshi Ishii, Shinichi Ogiyama, Shinji Sakurai, Keiko Tagami,
and Shigeo Uchida
Abstract It has been recognized that carbon-14 (
14 C) is one of the dominant
radionuclides affecting dose from transuranic (TRU) wastes. This radionuclide
has a decay half-life of 5,730 years, and
14
C organic materials have very low
sorption properties to clay and rock in the environment, which raises some concerns
about the releases of
14 C to the biosphere from radioactive waste repositories. For
the safety assessment of TRU waste disposal, we studied the behavior of
14 C in rice
paddy field soils. We also determined key parameters such as soil–soil solution
distribution coefficients (K d s) and soil-to-rice plant transfer factors (TFs) of
14
C in
the field soils. The TFs were obtained in laboratory and field experiments. In our
laboratory experiments, we used [1,214 C] sodium acetate as a source of
14
C
because it has been suggested that low molecular weight organic14 C compounds
are released from metallic TRU wastes. The results showed that
14 C-bearing
sodium acetate in irrigated paddy soils was rapidly decomposed by indigenous
bacteria. Although some of the
14
C was assimilated into the bacterial cells, most of
the
14 C was released into the air as gaseous compounds. The main chemical species
of
14
C gases was
14 CO 2 , and a part of the released
14 CO 2 gas was used by rice plants
during photosynthesis. Only a negligible amount of
14 C was absorbed through the
roots. Therefore, the contamination of rice plants is mainly caused by gasification of
14 C, and microorganisms are responsible for driving this process. The activity of
microorganisms is a key issue in the behavior of
14 C in paddy fields.
Keywords Bacteria • Behavior • Degradation • Radiocarbon • Rice paddy fields
• Safety assessment • TRU wastes
26.1 Introduction
Transuranic (TRU) wastes contain a variety of radionuclides, for example, Np, Pu,
and long-lived radionuclides such as
14 C and
129 I. In Japan these wastes are
categorized into four groups in accordance with their physical properties and the
N. Ishii (*) • S. Ogiyama • S. Sakurai • K. Tagami • S. Uchida
Research Center for Radiation Protection, National Institute of Radiological Sciences,
4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
e-mail: nobu@nirs.go.jp
© The Author(s) 2015
K. Nakajima (ed.), Nuclear Back-end and Transmutation Technology for Waste
Disposal, DOI 10.1007/978-4-431-55111-9_26
303