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1 Introduction
Carbon dioxide is one of the major causes of global warming; therefore, demands
are increasing for electricity generation technologies which emit less CO 2 than
power plants using fossil fuels. One of the possible technologies is to use nuclear
power plants which employ nuclear fuel. After the nuclear fuel has been burned for
about 3 years, the spent fuel is removed from the reactor and stored to allow shortlived radionuclides to decay and lower the amounts of spent fuel radioactive wastes
that must be disposed of and managed long-term. There are two options for the
spent fuel: direct disposal to deep underground with long-term management or recycling to extract uranium (U) and plutonium for further use by separating them from
the high-level radioactive wastes that must be also disposed of and managed longterm. In both cases, nuclear wastes containing different levels of U are going to be
disposed into deep underground. For the long-term environmental safety assessment of the radioactive waste disposal sites, mathematical models have been used.
Among the environmental transfer parameters used in these models, soil-to-crop
transfer factor (TF), which is defined as the ratio of a radionuclide concentration in
crop to that in soil, is a key parameter that directly affects the internal dose to
humans through the ingestion pathway.
To obtain the TF of U, we might be able to use naturally existing U to predict the
behavior from radioactive waste disposal sites to the biosphere. However, the physicochemical forms of the initial U in soil and added U would not be the same.
Vandenhove et al. (2014) carried out sequential extraction to identify the physicochemical forms of U in soils. One of their samples was collected from the Namur
region, Belgium, which has a high natural background level of U, and the other five
samples were contaminated with U from different sources; therefore, they used one
sample containing U initially included in the soil (taken from the high natural background level area) and artificially U added soil samples. Their results showed that
the U distribution fractions differed among the studied soil samples; for example,
ca. 30–50% of U initially included in the Namur soil are distributed in acid
soluble+residual fractions, but soil samples contaminated with UO 2 (NO 3 ) 2 ·6H 2 O
had less U distributed in these fractions (ca. 4–5% of total U). Interestingly,
Vandenhove et al. (2014) found that soil samples contaminated by the waste stream
from a phosphate industry facility showed similar results to those observed for the
Namur soil. It should be mentioned that U behavior in soil is affected by not only
the chemical forms of added or initial U but also the soil characteristics; therefore,
it is difficult to conclude that the physicochemical forms of initially included and
added U were significantly different. Another example was presented by Yamaguchi
et al. (2009); they used samples from four agricultural fields for which both nonfertilizer added soil and long-term phosphate fertilizer added soil were available
from each field. It has been reported that phosphate fertilizer contains a high amount
of U and soil to which phosphate fertilizer has been applied has more U than soil has
not had the fertilizer application (Komura et al. 1985; Tsumura and Yamasaki 1993;
Pantelica et al. 1997; Yamazaki and Geraldo 2003). Thus, an excess amount of U
K. Tagami and S. Uchida
1 Introduction
Carbon dioxide is one of the major causes of global warming; therefore, demands
are increasing for electricity generation technologies which emit less CO 2 than
power plants using fossil fuels. One of the possible technologies is to use nuclear
power plants which employ nuclear fuel. After the nuclear fuel has been burned for
about 3 years, the spent fuel is removed from the reactor and stored to allow shortlived radionuclides to decay and lower the amounts of spent fuel radioactive wastes
that must be disposed of and managed long-term. There are two options for the
spent fuel: direct disposal to deep underground with long-term management or recycling to extract uranium (U) and plutonium for further use by separating them from
the high-level radioactive wastes that must be also disposed of and managed longterm. In both cases, nuclear wastes containing different levels of U are going to be
disposed into deep underground. For the long-term environmental safety assessment of the radioactive waste disposal sites, mathematical models have been used.
Among the environmental transfer parameters used in these models, soil-to-crop
transfer factor (TF), which is defined as the ratio of a radionuclide concentration in
crop to that in soil, is a key parameter that directly affects the internal dose to
humans through the ingestion pathway.
To obtain the TF of U, we might be able to use naturally existing U to predict the
behavior from radioactive waste disposal sites to the biosphere. However, the physicochemical forms of the initial U in soil and added U would not be the same.
Vandenhove et al. (2014) carried out sequential extraction to identify the physicochemical forms of U in soils. One of their samples was collected from the Namur
region, Belgium, which has a high natural background level of U, and the other five
samples were contaminated with U from different sources; therefore, they used one
sample containing U initially included in the soil (taken from the high natural background level area) and artificially U added soil samples. Their results showed that
the U distribution fractions differed among the studied soil samples; for example,
ca. 30–50% of U initially included in the Namur soil are distributed in acid
soluble+residual fractions, but soil samples contaminated with UO 2 (NO 3 ) 2 ·6H 2 O
had less U distributed in these fractions (ca. 4–5% of total U). Interestingly,
Vandenhove et al. (2014) found that soil samples contaminated by the waste stream
from a phosphate industry facility showed similar results to those observed for the
Namur soil. It should be mentioned that U behavior in soil is affected by not only
the chemical forms of added or initial U but also the soil characteristics; therefore,
it is difficult to conclude that the physicochemical forms of initially included and
added U were significantly different. Another example was presented by Yamaguchi
et al. (2009); they used samples from four agricultural fields for which both nonfertilizer added soil and long-term phosphate fertilizer added soil were available
from each field. It has been reported that phosphate fertilizer contains a high amount
of U and soil to which phosphate fertilizer has been applied has more U than soil has
not had the fertilizer application (Komura et al. 1985; Tsumura and Yamasaki 1993;
Pantelica et al. 1997; Yamazaki and Geraldo 2003). Thus, an excess amount of U
K. Tagami and S. Uchida