Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 1, pp. 43−45, 2000

43

Present Research on the Environmental Radioactivity and Its Perspective in Japan
Noriyuki Momoshima*
Department of Environmental Science, Faculty of Science, Kumamoto University, 2-39-1, Kurokami, Kumamoto
860-8555, Japan
Received: November 29, 1999
Understanding of environmental radioactivity has progressed by investigation of undesirable events: nuclear
explosion tests and nuclear accidents. Now the importance of the research on environmental radioactivity hasrealized widely because of an increasing use of nuclear energy. One of the tops of the current research is construction of models, which explain and predict behavior of radionuclides released to the environment. Parametersof mathematical equations used in the model require reliable values, which must be determined by environmental
analysis and laboratory experiments. Development of analytical instruments and techniques would make possible
to obtain more reliable values of the parameters.

Historical Research Current of Environmental Radioactivity
Our knowledge about the environmental radioactivity hasgathered during the past century. The time course of theresearch on the environmental radioactivity would be divided
into three periods as shown in Figure 1. The first period is thedawn of radiochemistry starting with the discovery of radioactivity, the second period is the worldwide contamination by
nuclear explosion tests in 1950’s and early 1960’s, and the lastperiod is the contamination by nuclear accidents coupled withan increasing peaceful use of nuclear energy.

Figure 1. Historical research current of environmental radioactivity.

The discovery of radioactivity by Becquerel in 1896 was
carried out by investigation of the possibility that the fluorescence of uranyl compounds. Marie Curie and Pierre Curie
showed that all uranium and thorium compounds have radioactive nature independent of chemical composition and foundthat some of the minerals were much more strongly radioactive
than pure uranium itself. This observation led to the discoveryof the new elements polonium and radium in 1898. The extensive search of new elements in natural minerals, from the endof the former century to the beginning of this century, is thedawn of the environmental radioactivity. Many new stable andradioactive elements were discovered with great efforts;
researchers isolated unknown substances from various kinds ofenvironment materials under poor experimental condition. M.
Curie and P. Curie treated a huge amount of rock and isolated
polonium and radium by careful purification of pitchblende; itwould be the initiation of radiochemical analysis. However, it
had been believed that elements are unchangeable forever until
E. Rutherford and F. Soddy concluded with careful consideration for the reported observations that radioactivity was due tochanges within the atoms themselves. They proposed anepoch-making idea that transformation of the atoms of theoriginal elements to new elements. The investigations done in*Corresponding author. E-mail: momoenv@aster.sci.kumamotou.ac.jp. FAX: +81-96-342-3320.

those days are closely related to the environmental radioactivity.
The first bomb was tested successfully in 1945 and was thesignal of the following nuclear explosive races. Many nuclear
explosion tests were carried out in 1950’s and in the early
1960’s, especially those done at open air released large
amounts and various kinds of artificial radionuclides into theenvironment. The artificial radionuclides released into theenvironment, however, have brought us tremendous increase inunderstanding of environmental radioactivity. The environmental behavior of the man made radionuclides was extensively examined in the atmosphere, hydrosphere, and biospherebecause these radionuclides were the environmental tracer
introduced as worldwide scale. For example movement, mixing and diffusion process of water masses in the ocean werestudied with helps of tritium, carbon-14 and other radionuclides. Biological accumulation through food webs was wellunderstood by analysis of marine organisms at different levelsin the food chain. Analysis of transuranium nuclides, such as
plutonium, americium, neptunium, gave us an understandingof chemical, physical and biological behaviors of these
nuclides in the environment which did not exist before 1945.
Nuclear accidents at nuclear facilities related to nuclear
bomb production have taken place from the beginning of thedevelopment. However, most of the accidents were hidden fora long time as military secrets. The atomic weapons complex
called “Mayak” at Chelyabinsk province of Russia was closed
to people until 1992 in which at least two very large nuclear
accidents occurred in 1957 and 1967 which resulted in severe
environmental contamination. We experienced large nuclear
accidents related to peaceful use of nuclear energy: the Three
Mile Island accident in 1979 and the Chernobyl accident in1986. As an increasing demand of nuclear power generation,
the potential for nuclear accidents at nuclear power stations aswell as related facilities will be still high in the future. Such
accident influences a lot of people because most of the nuclear
power stations are constructed near populated areas. In thecase of the Chernobyl accident people have been evacuatedfrom the contaminated area to the safety area. The contamination spreads out to the forested area; thus, the behavior ofradionuclides in forest ecosystem is the major concern ofresearchers after the accident. The knowledge about thebehavior of radionuclides in the forest ecosystem has accumulated significantly for these ten years; however, it would takes
time to understand the whole picture.
The research on environmental radioactivity has progressedin connection with nuclear explosion tests and nuclear accidents, those are undesirable events for the development of© 2000 The Japan Society of Nuclear and Radiochemical Sciences

Momoshima

44 J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000
nuclear energy, and this situation may still continue in near
future as the JCO accident recently occurred at Tokaimura,
Japan.
Present Research Current of Environmental Radioactivity
Present direction of the study on the environmental radioactivity can be evaluated by screening of scientific programs inrecently held symposia. Domestic and international symposia,
which included issues on the environmental radioactivity, wereheld in Japan every year that a total number of eight symposia
except regular annual meetings were opened for these twoyears. The number of the recently opened symposia is considered to be not small, suggesting active research circumstancesfor environmental radioactivity in Japan. The scientificresearch contents in the symposia distributed in a wide range
from natural radionuclides to artificial radionuclides including
nuclear explosion tests and nuclear accidents. A remarkablefeature extracted from the recently opened symposia is that anincreasing number of presentation concerning models.
Prediction of environmental behavior of materials using aproper mathematical model is sometimes required in the fieldof environmental sciences because of difficulty in doing experiments that reproduce the real environment.
The symposium of “Improvement of Environmental
Transfer Models and Parameters” held in Tokyo in 1996 is agood representative of recent research trend for modeling ofenvironmental radioactivity.1 The sessions arranged in thesymposium are listed in Table 1, which well documents thepresent situation of the model research. The sessions consistedof environmental analysis, model construction for different
environmental compartments and interaction between differentcompartments. The behavior of radionuclides is expressedusing mathematical equations in the model and parameters in
the equation determine and control results. To construct a
good mathematical model, understanding of each environmental process of radionuclide is necessary and also a reliablevalue of the parameter must be given as shown in Figure 2.
Construction of models is impossible unless matching of environmental processes and values of parameters. The values ofparameters in the mathematical equations are only obtainable
by environmental analysis or laboratory experiments.
However, the values thus obtained would have some uncertainties. Even if the laboratory experiments were carried out at avariety of conditions, it does not reproduce the real environment. The values deduced form the environmental analysis are
originated from unintended events such as accidents that wereuncontrolled or unexpected release; it was not designed for
determination of parameter values of the specific environmental processes.
The number of parameters, which should be included in theequation, depends on the purpose and preciseness of model.
For a simple model a few number of the parameter would beused. To increase the preciseness of the model, the number ofparameter inevitably increases. Chemical, physical and biological processes involved in each radionuclide in the environment are numerous, and an interaction between processes iscomplicated and is changeable by place, season, climate, etc.
It, therefore, seems to be almost impossible to make a perfectmodel that represent real environment; there is some limitation
to evaluate the behavior of radionuclides by model. In spite of
the circumstance mentioned above, it would be true that usinga model is the only possible way for prediction and evaluationof a whole picture of radionuclides after introduction into avery complicated environmental system. The fate of theradionuclides from the Chernobyl accident is still unclear
because movement and distribution of radionuclides in a forest
ecosystem is a dynamic process; the results obtained from theenvironmental analysis revel a cross section of the dynamicprocess at the moment.
Latest Research and Prospect of Environmental Radioactivity
The recent development in modeling owes to an increasing
scientific understanding of environmental processes and also to
the social requirement for safety assessments of radionuclides
that are released accidentally to the environment. Developmentof analytical instruments and techniques progresses understanding of environmental processes. Inductively coupledplasma mass spectrometry (ICP-MS) enables us to measurevery low level concentrations of elements. Long-lived
radionuclides such as uranium and thorium are sometimes
detectable for water samples without pre-concentration.
Technetium, plutonium, neptunium are able to determine by
ICP-MS and 239Pu/ 240Pu ratio is obtained by ICP-MS that isimpossible to measure by conventional alpha spectrometry.
Microwave induced plasma mass spectrometry (MIP-MS) is
under consideration for 129I measurement. However, the detection limits of the instruments are sometimes not enough toFigure 2. Construction of model based on environmental and laboratory experiments.

TABLE 1: Contents of the International Workshop of “Improvement ofEnvironmental Transfer Models and Parameters” 1 Held in 1996, Japan
Session 1General topics
Session 2
Transfer parameters
Session 3
Behavior of radionuclides in the terrestrial environment
Session 4
Atmospheric dispersion model
Session 5
Tritium behavior in the terrestrial environment (1)Session 6
Tritium behavior in the terrestrial environment (2)Poster session
A: Transfer of radionuclides from soil to plants
B: Behavior of radionuclides in soil
C: Uncertainty of radionuclide behavior in terrestrial environment
D: Deposition and absorption of radionuclides from atmosphere to plants

Present Research on the Environmental Radioactivity
determine radionuclide concentrations involved in each environmental process. Speciation is difficult for some radionuclides by direct measurement, indicating a limitation of reliable
parameter values from environmental analysis.
The parameter values obtained from laboratory experiments
are often used for the model. Most of the laboratory experiments are designed focusing on a certain environmental
process such as change in oxidation state, adsorption on soil,
complex formation, etc. Researches related to the interaction
between processes are small compared to that of each process.
It is almost impossible in Japan to do field experiments using
radioactive tracers; such experiments carried out under controlled condition would give us useful information about parameters, including that for interaction of processes. A large
experimental facility is under construction at Institute forEnvironmental Sciences in Rokkasho, Japan, in which quasi

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000 45
field experiments are possible using radioactive tracers undercontrolling climate.2 Experimental circumstances to determine
more reliable parameters for the model are seemed to be readysoon in Japan.
References(1) M. J. Frissel, R. M. Brown, and S. Uchida (eds.),
Proceeding of international workshop on Improvement ofenvironmental transfer models and parameters (Nuclear
cross-over research, Japan, 1996).
(2) Y. Ohmomo and N. Sakurai (eds.), Proceeding of international meeting on Influence of climatic characteristics
upon behavior of radioactive elements (Institute for environmental Science, Japan, 1997).