Plutonium Released by the Nagasaki A-bomb: Mobility in the Environment

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• 作者：Mahara ; Yasunori ; Kudo ; Akira
• 刊名：Applied Radiation and Isotopes
• 出版年：1995
• 期刊代码：6_09698043
• 类别：ce
• 出版时间：November, 1995
• 卷：46
• 期：11
• 页码：1191-1201
• 文件大小：795 K

摘要

The first and second environmental releases of man-made ^{239 + 240}Pu came from nuclear explosions at Alamagordo and Nagasaki in 1945. The release at Nagasaki was more serious than at Alamagordo, because it happened in an area with a high population density. Unfissioned ^{239 + 240}Pu and various fission products (e.g. ⁹⁰Sr and ¹³⁷Cs) have been interacting here with various environmental materials (soils, sediments, and plants) under wet and temperate conditions for more than 45 years. To assess the environmental mobility of ^{239 + 240}Pu, the distributions of radionuclides from this release were investigated at Nishiyama (3 km east of the hypocenter) where heavy black rain containing unfissioned plutonium and fission products fell 30 minutes after the nuclear explosion. The vertical distributions of ⁹⁰Sr, ¹³⁷Cs and ^{239 + 240}Pu were determined in unsaturated soil cores up to 450 cm deep. Most radionuclides were found in the soil column 30 cm from the ground surface (95%of ⁹⁰Sr, 99%of ¹³⁷Cs and 97%of ^{239 + 240}Pu). However, ⁹⁰Sr and^{239 + 240} Pu were detected in the groundwater as well below a depth of 200 cm. No ¹³⁷Cs was found below 40 cm from the ground surface or in groundwater. These observations reveal that about 3%of the total ^{239 + 240}Pu has been migrating in the soil at a faster rate than the remaining ^{239 + 240}Pu. Sharp peak of ¹³⁷Cs and ^{239 + 240}Pu, indicating heavy deposition from the Nagasaki local fallout of 1945, were found in sediment cores collected from the Nishiyama reservoir. On the other hand, since ⁹⁰90Sr is mobile in fresh water sediments, there was no 1945⁹⁰ Sr peak in the sediment cores. ^{239 + 240}Pu peaks were unexpectedly discovered in pre-1945 sediment core sections. Although ⁹⁰Sr was found in these sections, no ¹³⁷Cs was found. By contrast to the distribution in sediment cores, ¹³⁷Cs in tree rings had spread by diffusion from the bark to the center of the tree without holding a fallout deposition record. Most of the ^{239 + 240}Pu was distributed in the tree rings following a similar deposition record to that found in sediment cores. Furthermore, a very small amount of ^{239 + 240}Pu (about 1%) was found unexpectedly in pre-1945 tree rings. The only reasonable explanation for these unexpected discoveries is the existence of mobile ^{239 + 240}Pu in the environment.