A high-level radioactive waste will be disposed of into deep geological formations, to isolate it from the human environment. Since the waste contains significant amounts of long-lived radionuclides, the safety of its disposal, particularly as regards the requirement for the protection of human and his environment even in the far future, is one of the essential subjects of all countries engaged in nuclear power production. The radiation protection system has long been established and applied to regulate radiation exposures to the public associated with a relatively short-term release of radionuclides from nuclear installations. Radioactive waste disposal, which potentially offers a long-term impact to the public, inevitably institutes a specific requirement that individuals and populations in the future should be accorded at least a current level of the protection. This requirement has caused a serious debate on how to establish radiation protection standards and criteria, and how to demonstrate compliance with them. This paper intends to discuss specific items such as numerical guides to indicate the consequences, time frames over which safety assessment is to be carried out and uncertainties to be involved in the assessment.
Deposition of 134Cs, 137Cs and other fallout nuclides was observed at PNC O-arai Engineering Center in May 1986 after Chernobyl accident. The fallout radionuclides, especially 90Sr and 137Cs in lake water, sediments and fish have been measured in Lake Hinuma located 2km north-west of the Center since 1986 in order to investigate migration behavior of these nuclides in lake ecosystem. In August 1986, the 137Cs in sediments was irregularly distributed at each sampling point in Lake Hinuma. It was different from distributions of 239, 240Pu and 241Am, which showed that concentrations at center of the lake were higher than that near the shore in 1986. Horizontal distributions of 137Cs in sediments then shifted to similar distribution of 239, 240Pu by 1989. The 137Cs concentrations in sediment showed a tendency to increase slightly in 1987 and 1988 and then to decrease. An apparent half-life of 137Cs in sediment was derived from field data as approximately 3 years. Distribution coefficient (Kd) for 90Sr in sediments, a ratio of the concentration of 90Sr in dry sediment to that in water, was approximately 90(l/kg) as 50% value in cumulative probability distribution of observed KdS. Concentration Factors (CFs) for 90Sr were from 4 to 45(l/kg) in edible parts of fishes (Haze, Ugui and Wakasagi).
Concentrations of thorium are measured for the front glass of CRT (cathode ray tube) of television sets and other kinds of glass such as a laboratory dish with the radioactivation analysis. Samples are prepared from specimens of glass used in the previous experiment for uranium concentrations. Use is made of two types of sample: wafer and powder. The results measured for the thorium concentrations are shown to be 0.7-15ppm, which are generally higher than the values of uranium concentrations of 0.2-12ppm obtained previously. This trend is consistent with that of natural abundance for thorium and uranium in the earth's crust. Moreover, the thorium concentrations are generally higher in the glass of CRT than in the other kinds of glass. This fact accords with the tendency shown in the experiment for the uranium concentrations in the glass.
Recently, there have been several reports of increases of leukemia, especially childhood leukemia, in populations resident in the vicinity of nuclear installations. However, there is no information on cancer mortality of the people living near nuclear power stations in Japan. Therefore, we have studied the standardized mortality ratio of malignant neoplasms of lymphatic and hematopoietic tissue near nuclear power plant sites in Japan. Data on the cancer mortality were obtained from the magnetic tapes of National Vital Statistics, Ministry of Health and Welfare, for years of 1973-1987. Standardized Mortality Ratio (SMR) of all leukemia, lymphoma, non-HODGKIN'S lymphoma, multiple myeloma and acute non-lymphatic leukemia was calculated for various age groups (0-14, 0-24 years and all ages) and different period (1973-1977, 1978-1982, 1983-1987 and 1973-1987) for each nuclear power plant site. The results showed that some of the studied population had higher rates of certain malignant neoplasms, and some had lower rates, either before or after the facilities came into operation. Our statistical analysis provided no evidence of any association between particular sites and increased cancer mortality rates.
It is necessary to know the dose distributions not only inside but also outside the region exposed by the useful X-ray beam, to evaluate doses in a patient during the radiographic procedures. In this paper, we calculated systematically three dimensional absorbed dose distributions outside the useful beam of diagnostic X-rays in a rectangular prism water phantom using the Monte Carlo method. The isodose curves, which showed the scattered dose distribution, were in turn moved further away from the useful field as simultaneously increasing the X-ray tube voltage and/or the field size. But, this movement of isodose curves in the low dose rates tends to gradually decrease with the increase of the tube voltage as a result of the probability density of photon scattering angle dependence on the photon energy. When the width of the X-ray field is larger than the free path length in the water of the scattered photons, the distance of the isodose curves from the edge of the field is almost constant. This systematic data of dose distributions is very useful to estimate doses at various locations when patients undergo radiologic examinations.
In radiological protection, the study of optimization and decision-making techniques is rather scarce compared with the importance of its concept. Taking cognizance of this, International Commission on Radiological Protection (ICRP) presented the decision-aiding techniques based on conventional optimization method in ICRP Publ. 55. This study aims to introduce other decision-aiding techniques to comprehensively support the analysis of optimization process by the use of the goal programming and goal vector methods. The analytical process for the problem of selecting options among given protection alternatives is illustrated with the data of the uranium mine example. The techniques studied are considered to provide a useful tool for the decision maker with respect to their intelligibility and simplification in the choice of optimal alternative associated with radiological protection.
The dose assessment study following the Chernobyl nuclear accident by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) was reviewed. A discussion on the advantageous feature of its methodology was made with emphasis on the simplicity in the determination of effective dose equivalent commitment through the ingestion pathway. The transfer factor of 137Cs from deposition to total diet: P23 which was the key parameter in the Committee's calculation model was derived by a regression analysis of the radioactivity survey data reported for Tokyo and Hokkaido. The effective dose equivalent commitment was given as 15 and 23μSv respectively for the population in Tokyo and Hokkaido as a result of the unit deposition density (1kBq/km2) of 137Cs. The contribution of the first year transfer to the total was significantly less in both cases than those reported for European countries and the United States, presumably owing to less consumption of dairy products and meat which could reflect radioactive contamination in the environment shortly after the release.
A brief historical review of the studies on the natural reactor and the occurrence of 244Pu in the early solar system is presented. Results from recent studies on the occurrence of ‘excess’ heavy isotopes of xenon in carbonaceous chondrites and lunar fines reveal that they formed when the ratio of 244Pu to 238U in the early soalr system was in the range of 0.0153 to 0.262 (atom/atom). While the carbonaceous chondrites seem to have all formed from a parent body (4, 890±8) to (4, 908±8) million years ago, ages of lunar fines varied from (4, 649±7) to (4, 992±8) million years.